Hi, I'm John from the Facebook's London security team, where I work on some privacy preserving products, and primarily end-to-end encryption and messenger. So this is something that we built and launched in July 2016 when we call it secret conversations, which enables you to talk end-to-end encrypted between one device of yours and then one of the recipient of these messages with optional ephemerality as well. So in this talk I'm going to give a bit of an overview of why this feature looks the way it does, and then take a deeper look into some of the cryptography that we chose. So to first disfraim this discussion, let's start with a higher level model of how regular message conversations work. So first and foremost, Messenger is indexed by your Facebook account, so as opposed to many messaging systems where you might be talking to essentially a phone number, with Facebook you're talking to an identity, a name, a profile picture, a friend graph, and this is an identity that's cloud-based. It's administered by Facebook essentially, and it's accessible across all of your devices. You can log on anywhere to access Messenger from your smartphone, your laptop, your friend's laptop, an internet cafe, an old feature phone, and you'll have all of your message history available. So a totally valid use case is you've forgotten your computer and you log on on your friends, you want to check your messages, which leads me to the final point of this model where devices can be shared between people. So this is particularly notable with web browsers, but also our mobile apps, say in Android you're able to switch between different users too. So that's a sort of overall model of how Messenger tends to look. Let's look at what an E2E Messenger would typically offer. So first and foremost, you wouldn't really be E2E if the provider could read your messages. It would completely defeat the object. You ideally want perfect forward secrecy to make sure, of course, that if your keys are compromised at some point or your overall device state, that earlier messages don't leak. Ideally you'd also have future secrecy so that you can be safe post-compromise as well. You want to be able to verify that the person you're talking to truly is who you think they are. It's not that much use having an encrypted connection to someone if you don't know who the other endpoint is. And then a slightly subtler point is that you want to make sure if your device is shared that only the person using the device at the time or the person who's meant to be using the device at the time messages are available and anyone else who may have used that device at some other point, their messages still remain secure. So with this sort of list of prerequisites for what we'd want from an E2E map, how would we expect this to look at a sort of slightly higher level? Well to start with you've got to have a well-defined set of endpoints. If you're messaging someone E2E you've got to know which keys you're encrypting to. You've got to have some level of consistency in identity so that you know that say if you checked someone's key yesterday you're still messaging the same person. If it's constantly changing then that's not as useful because again you could just be swapped out and talking to some completely different man in the middle. You're not going to be able to have message history in a reliable manner so if you switch to a new device we couldn't find any way to really marry this with perfect forward secrecy and still reliably have your message history available. This one sort of goes without saying but your client needs to be able to encrypt and store crypto state and messaging data. So this excludes for example some basic browsers without JavaScript support. And finally because you might have shared devices you're going to need some mechanism by which to protect the data from one account from another account on a shared device.  WhatsApp, SQL and Viber are some of the most obvious ones. Allo is also pretty close. And notably these are all indexed by your phone number so this already provides that well-defined set of endpoints that you need. They do have some sort of model of secondary device each of them but they all share the same identity still so this consistently verifiable identity isn't broken and you've still got message history not being available when you use a new device. Then also these clients are all apps provided by the vendor so they can make sure that this binary has the necessary crypto, the necessary storage capabilities in. But messenger really doesn't fit this model at all. The endpoints will constantly change but still have full message history. We've got messaging over a number of services including ones which don't support JavaScript on our basic mobile site. So as we want to add end-to-end encryption, one option is to try and do this by default and in doing so totally change the product and actually remove a bunch of features that people use messenger for, a lot of the things that people rely on. Or we can build it as an optional mode and not force people to use it and this is of course what we did. So this is the model for messenger and how we would undertake this forward. Let's take a look at some of the high-level choices. There were still many degrees of freedom in how this can look. So from the start we wanted to use off the shelf crypto. We didn't want to roll our own where we could avoid it and there is a small aspect in the message franking which is slightly novel and I'll cover that later but in general we were using off the shelf tools. One of the earlier decisions we had to make was how many devices could be in any given thread. So we knew from the start that it would have to be a subset of devices for messenger because there are always some particularly the no JavaScript versions which won't have access to secret conversations. So we had to have some sort of device management model. Multi device in this world would be somewhat complex to come up with and potentially more difficult for a user to understand. So we ended up just saying each thread has one device per person in. Very simple to understand what's going on and also somewhat simplifies the engineering as in general the multi device protocols that we could use would bootstrap off a one-to-one protocol so we'd have to have created this one-to-one version in the first place. So with one device per thread per person the next option is well is that your only device that you can use for all secret conversations or can you actually use different devices for different threads and we did actually experiment with the latter model a bit but this ended up adding extra complications for starters to get this well-defined set of end points you now actually had to have a symmetric setup sorry not symmetric synchronous so I actually send a request from my device which defines well defines which device I'm using but I don't know which you are until you respond. So this was far less good user experience and so we ended up going with the single device per account meaning your model is really simple particularly with the previous point that any device that you can use for secret conversations is the only one that you can use for secret conversations you know exactly who you're messaging you know exactly where your messages are going then finally we had to decide which services were going to support here we just went for our main apps so iOS and Android web presents a number of challenges not least trust in the JavaScript cryptography and we didn't want to address this in the initial version so that's what we aim to build let's take a look into some of the deeper specifics of the crypto so the first main decision was what end-to-end protocol should we use and we quickly settled on the signal protocol here or the axolotl ratchet at the time that we started designing this and pretty much because it is the best in class it's what's apps using signal of course uh google alo uh viber I think with their own implementation so it's pretty much an industry standard I won't go too deeply into this as I think there are two talks on this protocol later on but suffice to say it's really good another feature that we wanted to support was abuse reporting so we care a lot about people feeling safe on Facebook meaning that having an ability to address abusive behavior in some degree is is important for us to maintain so in an end-to-end encrypted model this is much more limited compared to channels where we do have the plain text available but we did want people to at least be able to report these messages to us now naively you could have a simple model where when Alice sends something to Bob over the encrypted channel in this diagram denoted by the the green channel and Bob sends it up to Facebook saying this is what Alice sent me but the problem here is we've no way of verifying that Bob's being honest because because we're not permanently storing the ciphertext server side just during the delivery process and we didn't want Bob to have to store this either so he ends up building a mechanism that we called message franking which allows us to verify that messages truly were sent over our infrastructure as claimed but without leaking the message contents to us in less they are explicitly reporting and without us having to store anything about this server side so I'll cover this more deeply in the next few slides if this diagram isn't particularly clear but so at a high level you've got Alice is adding an HMAC over her contents over her plain text to be sent up to Facebook and then we add our own HMAC on top of this so that when Bob reports it we have enough data to verify that the plain text that he sent is matches what Alice sent him because of the HMAC and we can verify that the HMAC truly was accurate because of the one that we added so Alice has to send a message it's called this M and she generates a random nonce and appends this to the message or contains it within the message structure she then generates an HMAC over this which we're going to call the franking tag keyed with this random nonce then encrypts the structure containing the message and the nonce using lib signal and sends up to Facebook I want to send this message to Bob here's the franking tag and here's the cipher text on our end we compute what we call the authentication tag which is an HMAC over this franking tag combined with some basic metadata so went from Alice to Bob at this time and then we forward all of this to Bob so that it came from Alice here's Alice's franking tag here's our authentication tag and here's the actual cipher text on Bob's side he ignores the authentication tag because this is meaningless to him because he doesn't have Facebook's secret key but having decrypted the cipher text from Alice is able to verify that Alice was honest with the franking tag because he has both the plain text and the secret nonce then this verification ensures that the authentication tag that Facebook has sent is computed over the correct data because he knows that Alice was honest in her initial franking tag so if Bob then wants to report the message he just needs to send the plain text up with the original nonce the timestamp the fact that it came from Alice and then this authentication tag which we can receive from this we can recompute the franking tag and then we compute the authentication tag verify that it was correct and then we've now proved that it truly was sent from Alice to Bob at the same time that we thought and we've got a similar level of assurance on this as with any normal message sent of a messenger in plain text in our regular mode so next we have a mechanism to send attachments we used a separate data store for attachment data rather than inlining this within the signal protocol it's this better suited the messaging transport that we're using and gave shorter latency so here's a diagram of how this works and the next slide will again cover this slightly more clearly but you're generating a random key alongside the image you're encrypting it using the concealed library if you're on android or a re-implementation of the same protocol if you're on ios then this cipher text is being uploaded to facebook stored for 30 days in our back end and facebook returns to us an id and authentication string for us to download it later on we also then include in the message packet this key that decrypts the message a checksum of the cipher text and potentially a thumbnail of the image to improve the ux on the initial delivery so here's it laid out i think i'm pretty much covered covered that so i'll just leave that up for a sec and i'd note here that this authentication string alongside the id is just there to prevent people from scraping through the attachment id space so that they can't just iterate over it until they find random cipher text to download uh which should be meaningless to them because they're cipher texts but we wanted to avoid that anyway and so the received path here is fairly simple you've uh you decrypt your cipher text giving you the thumbnail the key um the id the authentication string and the checksum you displayed this thumbnail you download the cipher text check the checksum decrypt it display it so finally we wanted to address this shared device issue that we have um and protect the on device secret conversations database when you might be logged out um so this is partly because good practice for sensitive data being stored but uh largely motivated motivated by the this shared device model so to isolate data between users we simply provide an author we provide a key from facebook at the point of login um and then we store all of our secret conversations database for a given user under that key um and then as soon as you log out it's wiped from memory on the phone and so you don't have access to it anymore um this looks roughly like this um each arrow donating a layer of encryption um so you've got a storage key on the device which is encrypted by this server provided key from facebook uh that stores first your private keys so your lip signal identity key your signed pre key etc and also your thread storage keys each threads being encrypted under a separate key which then contains the messaging data and the session keys for that thread so that's the product let's and why it looks as it does let's finish up with just some of the challenges we're looking into and hoping to get good solutions for in future in which this community may have some thoughts on so a huge one here is that of actually authenticating your pairs um we wanted to be able to provide some simple model whereby you can trust the person you're talking to truly is who they claim to be um this is our current solution um it's uh probably a fairly familiar type of screen to most people so you can just compare your public identity keys people but we're fully aware that the vast majority of people won't do this um so we want Alice to really be able to verify she's truly talking to bob without this having specialist knowledge without knowing that you need to go check your keys without having sort of any certificate authority and ideally come up with this sort of ux that browsers have sort of perfected well maybe not perfected yet but uh browsers are doing pretty well on with tls and require very little little knowledge from the actual user next uh we would like to extend this feature beyond a single device per account to multiple to support multiple devices um to better reflect how regular messenger is used by many people um so our current model is really quite simple and very easy to understand this is your secret conversation device this is your identity key you can message someone else's designated device you compare with their identity key and you know exactly where the channel for these messages is but we'd like to remove this restriction um there are engineering challenges there but ultimately we had some higher level ux concerns so we've got to manage how devices get enrolled for your accounts um how they then get subsequently disenrolled um whether this disenrollment can happen automatically at some points and how that will affect the overall experience because of course you don't want people again to have to understand how will the crypto works in real life and so they shouldn't have to know that they need to disenroll a device at some point and also once you're dealing with multiple devices it makes the authentication problem trickier um so you're no longer verifying a single key but you could be verifying a collection of keys for a given account uh and a collection which is more likely to change more frequently so a common option used in multi party chat uh such as with whatsapp and signal is conversation codes which provides a fingerprint over all participants of the thread um but in our case we feel it's not quite that simple as that relies on you knowing that you have all all of the devices within the thread so when you when you're comparing um this is this phone number this is this phone number and this one and between them we know that there are three devices in this thread and we can see all three are sharing the same key you know that that's okay but for us if you're unlikely to have a large number of all of your devices in the same place at once verifying that you share the same key isn't sufficient as you don't know how many other people out there share the same key as you well people devices sorry so it's probably not the a complete solution for us and finally I've said that we didn't want to address this initially but we would eventually like to add web support now this compounds the multi device concerns significantly as with web sessions you're more likely to have many more of them um over time you're much more likely to say be logging in in incognito mode in your browser in very frequently as opposed to logging in on your phone once and leaving it there until you get a new phone so this means there'll be a lot lot many more keys on any device it's uh risk significantly scaling issues particularly with client side computation and then combines this problem of well how do we disenroll these devices particularly given that with lots of single-use browsing sessions there's a risk of forward secrecy that suddenly you're creating a lot of states out there which are okay need to be capable of decrypting all future conversations until until that device is disenrolled and then finally we have to overcome the general trust issues with JavaScript crypto being provided by a remote provider and potentially changed every web request and how can you trust that this code is actually correct and it's an unsolved problem as far as I'm aware this is fundamentally different from our mobile apps where we're actually serving a binary through a third party and you've got a pretty good idea that you're seeing the same binary that everyone else in the world is so it's a big difference there that's all I've got thanks very much for listening I'm not sure if I have time for questions we can take one or two questions oh great so the end to end encryption is protecting against wire tapping it doesn't protect against civilians which is like who is talking to who so if facebook doesn't add end to end encryption right people then know that it's not good to say to talk to a journalist over facebook so they will self-censor and instead will use signal that doesn't collect metadata as we know from the documents from the court right so maybe why guys you bother and adding end to end like I mean honestly it will just people will get in trouble so yes as you've observed protecting metadata wasn't really part of our overall model it's very difficult to achieve within our world and we get a lot of benefits from it for example being able to perform better spam protections in general yes if you're if you're trying to protect your metadata from your provider then this perhaps may not be what you're trying to use but what you need to use but we wanted to provide something which at least can protect your content which is what we were really focused on and provide this to as many people as we can and so with this update we were able to ship end to end encryption as an option to a billion people without them having to do anything which yes doesn't protect their metadata but does give them significant increase in messaging security I had a question about the web or the storage encryption it looked like the key was to encrypt the storage was provided by facebook is that is that are you able to be compelled to decrypt the storage on the device then or I guess I'm not clear on the why that why this choice is made so I'm probably not the best person to answer policy questions as I um I largely had handled the technical side of things um so if there were a physical device um then like it would be possible for facebook to access that storage key um but um in general this is this attack this is to prevent the attack where you have a device um which you know someone else has been using but they're logged out of now and you as someone um without access to that storage key I want to read that those messages and so it's to protect I get it's to protect against other people accessing your physical device and to raise the bar yeah I mean I get that I was just wondering if there was a usability or some other reason for that control of the key by facebook um no it was it was just just to raise the bar when when switching devices hi thank you for this presentation and thank you for rolling this out um I agree it doesn't solve all the problems but I'm happy to see it being pushed into the real world at least we can raise the bar and um leave you with some data that you can that you don't have immediate access to um if you want to try to solve this I recommend thinking about mixing the user's password into this I'm assuming you don't store a clear text of the user's password if you mix the user's password into the account key then you can potentially protect this against both you're being able to recover and everyone else um yes that so that that could work aside from in cases of say account recovery where someone has actually lost access to their password um so right so if you're doing this as an optional opt-in thing you can say hey if you want us to be unable to have access to this check this extra box during opt-in this is just an option for you thank you thank you is it quick because we have to actually move on uh sure yeah just on the last point about making this work on the web um I work on firefox I'm very keen to have this work on the web uh so do you have any thoughts on things we could do to make the web more friendly to this stuff things we could add to the browser to to make make it more amenable to this sort of thing that sounds like probably a slightly longer point when we have time for but should we talk after we take it offline yep great thank you I think we should actually move on so uh John thank you very much this is a great thank you


I wasn't sure that I was going to talk about this, but I have to. I can't resist. So we've got to look at Susan Collins She recounts what happened on that really horrifying day when a bunch of MAGA chuds stormed the US Capitol in an attempt to I guess install Donald Trump as president permanently because they believed that the election was stolen from Trump You know, I don't doubt that that was horrific and terrifying like if you're a lawmaker and you were like Sectioned off in some portion of the Senate or the House and like you you heard all of these lunatics like running through the halls of Congress I have no doubt that would be terrifying and a lot of them feared for their lives AOC said that she feared for her life So it's scary But Susan Collins reaction as she recounts the events of that day is Really Interesting I should say it So Susan Collins says that as the Capitol was attacked her first thought was that the Iranians had followed through on Their threat to strike the Capitol.  We're getting invaded by Iran Even if She had no idea what was going on and she heard zero Maniacs screaming and they just said senator Collins. You've got to go. There's a threat currently To think that Iran Would be capable or have the balls to carry out a ground invasion of the United States. I mean That's so detached from reality Irrational that I can't not laugh at her like I I get that she was worried and when fear takes over You're thinking really irrationally, right? You're not thinking logically, you know You have flight or fight triggered and and you don't know like your adrenaline's pumping But I think Iran I'm assuming she saw at least like some of them, you know with maga flags to think this was around like I just I don't know I Don't know how to respond to that you think this was a wrong Susan Susan Jesus. I She thought this was Iran Let me get the exact quote again Iranians followed through on their threat to strike the capital There's not much to say about this Susan Collins is a dumbass and She shouldn't be a United States senator She should not be a United States senator and like even if let's say, you know, you had that moment of I Don't know irrational thinking and you were just fear had taken over and you thought it was Iran or North Korea or whatever Would you say that though afterwards like wouldn't you keep that to yourself? Like if you have an embarrassing thought in a moment where there's no clarity and there's only confusion like Wouldn't you keep that to yourselves like to say Iran? followed through on their threat She just she doesn't She doesn't understand like Anything about the world and it's just sad. Oh my god There's nothing left to say that this is not a substantive segment. I just had to point this out because Susan Collins is a She is a unique Unique person very unique person will say that to be polite


Hey, you guys so in this video, I'm gonna be showing you how I do a protein treatment on my hair now Everyone is different, but some things I do want to tell you is that I am low porosity I have 4b a mix of 4b 4c maybe some 4a and the boomy will discuss her But primarily 4b 4c. I've been natural for nine years. Okay? I've been doing protein treatments often on for the last five years and now I do a protein treatment every two months For me it may you know vary depending on the person I don't have all the scientific science background to explain to you when you need a protein treatment So I would be leaking in the cards over here somewhere a link to the green beauty channel She has several different videos On protein treatments. Okay, so I'm gonna show you what I typically use And I have a lot of different things in front of me. So bear with me So for this particular video, I will be using Africa's best hair mayonnaise In conjunction with the green beauty channel real protein treatment I love this stuff. I've been using it ever since she brought it out. Absolutely love it Using these together. So with this hair mayonnaise with this particular Hair mask hair treatment, whatever protein treatment in conjunction with this You have to use a moisturizing deep conditioner afterwards. If not, your hair is going to feel dry So I have this hair mayonnaise. I also have Or s's hair mayonnaise and be on the lookout because I will be doing a comparison video of these two. Okay I do have different cholesterol here. I have tons of different cholesterol I have hallowed beauty. I have about five different types of this And then I have queen Helen now. This is where it gets a little blurry There's just a little this is like a light protein treatment in my opinion with the cholesterol So sometimes I may do a diy Moisturizing deep conditioner using the cholesterol as the base sometimes um But again for this video, I'm going to be doing Africa's best with the protein. That's the protein treatment for my Moisturizing deep conditioner. I'm going to be using Misha's Moisturizing deep conditioner as my base now She was okay. I did a formal review of this particular product and it was just okay But I said I believe I said in that video. This will make a great base for a diy and that's what we're going to do So again, this is indulge And to this I have added a package of or s replenishing conditioner I added two tablespoons of honey Um about one fourth of a cup of this swabs conditioner And about two tablespoons of oil now again other I've done other diy Moisturizing um deep conditioners using cholesterol. I mainly use queen helen cholesterol But I also love to use eating body works jojoba natural deep conditioner Now, I know several you have complained that This product has dried out your hair because I was going back looking at some of the comments because I do have a formal review Of this particular product it dried out your hair. They just didn't work. So for this one We wanted to stick with this. All right, you guys. So okay, I'm going to go ahead and Cleanse my scalp and come back on cam with soaking wet hair. Okay All right, y'all I have stepped out of the shower I'm going to go ahead and mix several huge spoonfuls of this african pride hair mask To the green beauty channel's real real protein treatment I think I'm going to do one to one and a half tablespoons of this liquid protein treatment Now one thing I do want to say you want to ensure that your hair is soaking wet when you apply the hair mask My hair has started to dry, but that's okay. I'm going to go ahead and apply it to my hair And allow it to sit on for about 30 minutes Ryan little It's wet and warm cloth to my hair and with my big head it will generate enough heat for the deep conditioner Going to go ahead and rinse that out and this is what my hair looks like completely soaking wet And it feels like I've done a protein treatment. My hair feels very strong. It feels healthy But I definitely need to follow up with a moisturizing deep conditioner Ryan little Rinsed it out and look at my hair. It feels soft. It feels good Now I'll get at my leaving conditioner and that is it everyone. Thank you so much for watching. Take care. Bye


The Germany of Siegfried, of mythical heroes, and the barbaric splendor of all that goes with war, but the foretaste of defeat. Was this myth that Hitler had given his people all that they now had left to back their pledge to him in return? My Fuhrer, the West Wall stands and will hold come what may. The 58th volume of the United States Army's History of World War II is entitled The Siegfried Line Campaign by Charles B. McDonald. It covers the fierce fighting along the German frontier near the city of Aachen in the fall and early winter of the year 1944. This volume was prepared by the Office of the Chief of Military History as a part of the most comprehensive historical program ever attempted by any nation. It will provide the American people with the full report of the U.S. Army's role in World War II and a permanent record of the accomplishments of the men who fought its battles. In his preface, the author says, This period may eventually be regarded as one of the most instructive of the entire war in Europe. A company, a battalion, or regiment fighting alone and often unaided was more the rule than the exception. In nuclear war or in so-called limited war in underdeveloped areas of which we hear so much today, this may well be the form the fighting will assume. It has been said that those who ignore their history are condemned to repeat it. The story of the Siegfried Line Campaign takes place in northern Germany. It is the story of savage combat by small units against a tenacious enemy, formidable defenses, inhospitable terrain, supply problems, and weather as hostile as any enemy threat. There was no mistaking that frontier. Like those legendary dragon's teeth that sprang from the earth as armed men, its anti-tank obstacles stretched in a ribbon of mute sentries uncoiled across the quiet countryside. Built in the late thirties, the myth of the Third Reich was built into those dragon's teeth, drawing the face of Germany in a grimace of defiant scorn for a world at peace. Now the years of war were passing elsewhere, and the neglected line with its empty fortifications was becoming part of a landscape where earth and myth merged into one, as that summer now merged into fall. And with fall, the Allied threat came at last. Its armies sweeping the enemy before them across Europe like leaves are swept before a vengeful wind. But confidence, not vengeance, was what lit the faces in General Courtney Hodges' First Army. What was the myth to men who had landed at Normandy on D-Day, made the break from the beachhead and liberated Paris? After moving at unprecedented speeds, all three corps of the First Army readied the thrust into Germany, assured that even the frontier would scarcely check the enemy's retreat before them. It was the Seventh Corps under General J. Lawton Collins that would set the First Army's pace and make the main effort. In order to breach the Siegfried Line's fortifications before the Germans had a chance to man them, Collins mounted a reconnaissance in force the day after the first patrols had penetrated the frontier. To the north, with only their own tanks to cover them, they attacked across open field where every step forward was a gamble, overroads alive with mines and villages alive with snipers. Fences and backyards were the only shovers. And too often, no shover at all. Within five days the Seventh Corps had seized the high ground around Aachen, pierced the forward band of the Siegfried Line along an 11-mile front and penetrated the heavier line of fortifications in two places. Now the closest enemy was Delay. Every day, every hour of it, General Collins knew, must bring the Germans closer to their defenses. Since July, Hitler himself had taken personal charge of Germany's war. His strategy for the Western Front could now be summed up in two words, stand fast. And backing his edict with reinforcements, 15,000 men of the German 12th Infantry Division had reached the Rohr River the night of the 16th and were pouring into the defense positions of the Siegfried Line. They were waiting as the Seventh Corps reconnaissance burst into a full-scale attack on the line's main fortifications, extending to the south, into the gloom of a forest straight from the German folktales of the brother's grin. The stifling embrace of its densely interwoven trees, obscuring the sky itself, the Hurtgen Forest, a name that would come to symbolize the long-drawn heartbreak and despair of war, as surely as its fairytale counterparts had enshrined the nightmares of childhood. On the same day, Hitler's heavy reinforcements appeared to dispute the Seventh Corps passage of the Siegfried Line. To the north, over the Netherlands, the whole sky blossomed with parachutes in the war's largest airborne assault. The Operation Market Garden had succeeded. The Siegfried Line would have been outflanked, ending the Hurtgen Forest nightmare almost as soon as it began. But the weather, combined with unexpectedly firm German resistance, to prevent the airborne troops from keeping the foothold over the Rhine that would have set the stage for the drive to Berlin. Again, there was no alternative. The Siegfried Line campaign would have to continue. Only deceptively open country and the villages to the north, down to the shadowy heart of the Hurtgen Forest itself. The foot soldiers of the Seventh Corps carried their assaults. But for every yard advanced, the next came that much harder. The line had been penetrated, pillboxes captured and destroyed. But of the enemy, an enemy that had fled halfway across Europe and now suddenly turned to fight with the resistance as inexplicable as it was fierce, an enemy that seemed to be everywhere and nowhere all at once in the confusion and uncertain shadows of the thick trees, the only proofs were the prisoners and the dead laid out at peace on the forest floor. Nineteen days had passed since General Lightning Joe Cullins had pierced the first dragon teeth of the Siegfried Line. But now as September closed around his Seventh Corps and those on either side of him, the entire First Army for the first time since the breakout at the Normandy Beachhead was stopped. The author of the official Army history from which our narrative is drawn, Mr. Charles B. MacDonald, has been discussing this critical period of World War II with General J. Lawton Cullins, retired Army Chief of Staff. General Cullins, the First Army Seventh Corps actually had broken through the Siegfried Line before coming to a halt. But it was not only the fortifications that had stopped them. At that time, you were the Seventh Corps commander what was the situation? Yes, after the breakthrough in Normandy in late July, we had the Germans on the run and we wanted to keep them on the run. We didn't want to give them a chance to stop and organize themselves. But I had hoped to get through the Siegfried defences before we had to stop. But just as we had crossed the German frontier we began really to run out of gasoline and ammunition, and unfortunately, out of good weather. And the loss of the good weather caused our close-upport aviation practically to stop. And this was a real blow. Men were still in good shape and were anxious to keep going, but they were tired. And even more important, our tanks began to need replacement and repairs. For example, the Soviet Union, which had spearheaded our attack, normally had some 230-odd tanks. But when we hit the Siegfried defences, only about 75 of them were fully operating. Now, you remember also that the Siegfried defences east of Aachen were very powerful defenses and from the high ground overlooking our positions they were looking right down our throats. Now, an attacker in order to break through has to move and when he does that, he gets out into the open and then he becomes vulnerable to enemy fire. The next step now was to build up supplies, bring up reinforcements and reorganize in order to exploit your breakthrough with a new drive by the First Army to the Rhine River. Yes, that's right. We had to do all of these things, but we still had unfinished business right where we were. In fact, there were three attacks that had to be launched. To the north of Aachen, the First Army had to get its 19th Corps through the Siegfried defences and then south of Aachen and the Hurtgen Forest, the 5th Corps, the right wing corps of the First Army had also to get forward. But in order for them to move, the 7th Corps in the center had to clear the Hurtgen Forest. These things were necessary to exploit the breakthrough that we'd already made through the first set of the Siegfried defences and then in addition, we had Aachen, the city of Aachen on our left and while we dominated the high ground overlooking Aachen, we couldn't go past the city and leave a pocket of resistance there on our left rear. So therefore the 7th Corps had to capture Aachen. Now all of these things, these three attacks had to be coordinated and my judgment, never has received proper public recognition for the great job that he did in leading the First Army all the way from our breakthrough in Normandy to our meeting with the Russians on the Elba River. Aachen, ancient capital of the Holy Roman Empire was attacked on October 8th. For 13 days, this shrine to Germany's resplendent history was convulsed in the tumult of modern war. All the vain hopes of the 20th century were crowded through streets that had witnessed the serenity of medieval culture and learning and the coronation of 32 emperors and kings. To the Americans, the surrender of Aachen meant another concentration of enemy resistance wiped from their path. To the returning refugees evacuated when the order had come for the city's last ditch defense. In these crumbled ruins lay the still warm ashes of the myth of the Third Reich. Waiting now not the thousand years into the future that Hitler had pledged it but more than a thousand years back to the day when the city had given birth to Charlemagne, Emperor of the West. In terms of war, Aachen now belonged to the past. For the man of the 19th Corps north of Aachen it was the same yard by yard man by man struggle on the Roar River and the Cologne plain that opened to the Rhine. But below Aachen, still lying a thwart the entire south wing of General Hodges First Army the Hurtgen Forest remained unclear. November came and with it the staggering assault delivered by the 28th Division of General Giraud's 5th Corps but the merciless forest absorbed that wholesale tragic heroism and asked for more. It had become a nightmare from which there was no awakening even as a new enemy joined the old. The worsening weather was an enemy the men could come to grips with but to almost as little avail. And like the old one it seemed to become more tenacious as the days passed. The October attack had broken into a series of minor engagements when General Eisenhower determined upon the crushing new offensive that would sweep the entire western front. The 9th Army under General William H. Simpson was moved in north of Aachen. Today the weather delayed its opening blow until at last November 16th dawned upon the heaviest air bomb apartment in direct support of ground troops to be launched in all of World War II. The British 2nd Army to the north the American 1st and 9th Armies were to drive for the Rhine. Again the main effort would be made in the Aachen sector as the most direct route to the vital rural industrial area Germany's arsenal and there again it would be made by the 7th Corps the onslaught did not miss its mark. By now Germany's war losses and weapons and equipment were so enormous as to defy instinct. The summer months alone had taken over a million German casualties and now new and uncounted numbers pressed the total to near 4 million since the war began. Yet they fought back their communications shattered their supply lines smashed they returned the attack with a ferocity that seemed whetted by their losses. This was no fairytale this was no myth 10,000 tons of bombs had been dropped on the German defences but still the Germans replied on the Alliance Supreme Command down to the exhausted GI it had not even the fairytales it was war war demanding new men new courage and giving nothing in return but those demands pressed harder hopes for the sweeping success of the great November offensive had been swallowed just as the Hurtgen Forest swallowed new reserves and asked for more it was war that demanded casualties any way it could get them and the winter became as vicious an enemy as any lurking the remains were still measured in yards in feet in depth still they were gains and while battle strategies might founder the inexorable rules of war would demand that either advance or retreat must eventually prevail even in the Hurtgen Forest one of those must prevail it was advance each man advanced on his own terms trapling the bleak fragments of a myth that fled before every step he took forward existed though nothing but cold and mug had ever existed beyond the Hurtgen Forest war became human again but that did not repeat it ready enough desolation and waste and sheer loss of everything to be valued there was always another engagement another pocket of desperation defending what was already lost clinging in desolate wastes of roads and towns and people in ruin the hollow entreaties of defeat dragon teeth the barbed wire and hidden guns of the secret line were behind where the Hurtgen Forest nightmare lay at last splintered and torn open to what winter sunlight there was three months had passed since the first patrols had pierced that frontier but even now these two American armies were barely 22 miles inside Germany the November offensive was three weeks old the armies ready to flood the plane if either side had burst the dams hidden high in the Hurtgen Forest a week of hard fighting because no one could have named its end the First Army broke through to the Roar River and the plane opened out toward the Rhine where hopefully that war, even this war might end the Siegfried Line campaign was a part of history but was it a part and perhaps after all the real part of war General Collins the dash of long sweeping drives was conspicuously absent from this campaign but that is what much of war is like long periods of dogged determined fighting yes that's correct as a matter of fact I think the chief of military history has put it very well in his foreword to your book when he says that you germies waged the campaign described in this book but the individual soldier pitting his courage and stamina against harsh elements as well as a stubborn enemy emerges as the moving spirit of these armies as a matter of fact no part of the fighting in Europe tested the American soldier quite like the Siegfried campaign did and the fighting in the Hurtgen Forest the American soldier there proved that he could take some pretty hard knocks and still slug it out and I might say that if this story lacks the smashing climax this is because the historian unlike the novelist is never the master of his material ordinarily of course the campaign would have continued with a big push across the roar river to the Rhine yes but as a matter of fact while we were slugging it out in the Hurtgen Forest Hitler was planning to move to the south of us that he hoped would end the war and there about 25 miles south of Aachen he hit a very thinly held part of our line made a breakthrough there and it led into what became known as the battle of the bulge and this goes a long way toward explaining the fierce resistance put up by the Germans on the roar plain and in the Hurtgen Forest Hitler headed to his counter offensive Hitler had ordered that no Allied soldier was to be allowed to set foot beyond the roar river yes but fortunately in the battle of the bulge we used up all of the German reserves of men and material west of the Rhine River and the attacks of the first and the ninth armies through the Siegfried defences prior to the bulge had chewed up all the local reserves available back north north of the bulge to renew our attack across the roar river and the Cologne Plain things moved along very rapidly and we soon had Cologne as a matter of fact within two weeks and we were firmly on the Rhine and thereby had guaranteed the ultimate success of our war in Europe


We're so focused on the displacement of residents but what's also being lost is a history, a culture that has been in these communities for decades and what's being lost as well is even sort of the mutual aid that a lot of these folks relied on in the face of urban and social policies that have vastly neglected these neighborhoods and as a result mutual aid has stepped in to help in terms of providing them with a healthy life. You know having access to community gardens or having access to community child care for folks who you know might not be able to afford child care in a more professional setting.


Yeah, yeah, it's a huge win for us Yeah, we've been playing a lot of really tough teams and you know things haven't gone our way in a lot of those matches So it just feels great for the guys to finally go out and kind of show what they're capable of And go out and you know really take it to a good team and win the match So just feel great for them for going out and showing up in front of a lot of people here and really playing great ball Yeah, it's always an emphasis. We're always trying to be strong at the net You know stopping the ball before it comes on our side and the guys just did a great job of kind of executing What we were talking about in our scouting plan and really picked it up in the you know from that first set and just got after them Yeah, that we've been talking about how we can sustain, you know high level of play throughout an entire match and a big point of that is just communication and energy and The guys really responded to I think, you know the crowd in the moment playing, you know High-ranked team and really just enjoyed themselves for one of the first times. I think this year playing some volleyball


Playoffs beginning tonight in girls basketball as Nevis hosted Bertha Hewitt. Early on Nevis up two they swing it around to number three Sadie Therner. She buries the tray and they're up five. Back the other way bears pushing the tempo. Michaela Hinsman down low dances around and drops. With home team fires back Mackenzie Mitchell in the paint gets the bucket to drop. Nevis wins a barn burner they take it 44 to 42. If you enjoyed this segment of Lakeland news please consider making a tax deductible contribution to Lakeland PBS.


So, good morning. Let's continue our lecture and this will be on boiling heat transfer as well. So, as a quick recap on what we looked into yesterday, first we started off on the basics of phase change. Then we looked into what is boiling and what is evaporation, what is the difference. Essentially boiling is the bulk phase change process. Then we followed it up with looking at what is required to ensure boiling. So, some amount of super heat is required essentially to try and maintain a bubble of radius r and that liquid scan exists in a metastable state and heteroflates fluctuations are required to disturb them and become vapor. Then we looked into what are the conditions required for bubble inception and how to calculate the nucleation size. We looked into an ideal case of bubble growth dynamics, inertia control growth, heat transfer control growth and then we looked at bubble growth at the wall. Essentially that a range of cavities can be activated and there's something called the bubble evolution cycle where the bubble grows, departs and a new bubble forms. So, in today's lecture we will just start with one or two simple problems on what we looked at yesterday. Then we look at a study that I did which is on DNS of bubble growth. It just gives you a perspective of what is being done today in bubble growth and follow it up with discussion on pool boiling curve and the correlations involved therein. Parametric dependence of this pool boiling curve to different operating conditions, some problems on pool boiling correlations. We'll then follow it up with two phase flows and look at pressure drop calculations and one or two problems on that and then we'll start on flow boiling. So, let's begin with the problems one or two from yesterday's study. So, this is the question. Consider a spherical bubble of pure vapor suspended in a superheated liquid form. On a PV diagram, can you tell me what kind of phenomenon can occur so that this bubble either grows or collapses? So, let's look at this from the PV diagram point of view. So, you have the saturation curve and if you remember an isotherm follows this trend where between here to here is an unphysical form. So, several such isotherms will exist. So, now we have a stable vapor in a state of superheated liquid say TL and PL. So, say your liquid is superheated. So, this is your metastable region you know. So, this is located say here and the corresponding pressure of the system is PL. So, this is TL, this is PL. And the saturation pressure corresponding to TL is this. So, your vapor pressure is going to be somewhere in between this and we know that your PV is somewhere closer to PSAT. So, this system exists. Now, what do you think should happen for the stable bubble to be able to either grow or collapse? So, I would like you to think in terms of the temperature at the interface, the temperature of the vapor right. So, a scenario what is your pressure and temperature? So, you look think of in terms of the hetero phase fluctuations. So, some form of fluctuation occurs so that your pressure and your temperature locally drops. So, essentially you fall on this curve a curve below the pressure drops. So, then what happens is the temperature locally drops. So, here is your T SAT for the previous case. Now, the temperature of the liquid locally has dropped. So, essentially the vapor interface sees as if it is at a higher temperature. So, it wants to lose heat. So, the minute it begins to lose heat it is going to collapse. So, essentially a drop in pressure is going to trigger a bubble collapse. Now, what do you have to do to make the bubble grow? Obviously, if there is going to be a jump in pressure, a violent shaking which causes an increase in pressure, then and it has to the temperature has to jump above your saturation temperature over here. So, the minute it jumps to a higher level say from here to this line over here, then the saturation temperature at this location is actually higher. So, what it is going to do? It is going to see that it has access to heat. It has access to extra heat. So, it is going to grow. So, essentially it is your pressure and temperature variations locally to the interface for it to grow or collapse. So, going on to the next problem for bubble growth in water at atmospheric pressure and 120 degree Celsius, estimate the bubble size corresponding approximately to your transition between your inertia and heat control growth. So, you remember that there are two forms of growth which is your inertia control growth which is a function of T and your heat transfer control growth which is a function of T power half. So, this is quite a simple problem. So, for a bubble growth in water 120 degrees, estimate the bubble size which lies in the transition zone. So, if you remember I gave you a correlation by Mckick which talks about modelling growth right from inertia control growth to heat transfer control growth. So, all you need to go is look at this graph that I have given you. I think in the previous lecture the graph is a little bigger and much clearer. So, what I want to point out here is that your inertia control growth is for a time T plus non-dimensionalized time T plus 0.01 below that. And for T plus greater than 100 it is heat transfer control growth. So, for transition control growth you can pick any T plus and corresponding R plus on this non-dimensionalized graph. So, in my case I have just picked R plus 1. You do not have to do that you can do something in this range. So, if you take R plus as 1 you plug it in on that R plus. So, your R transition size essentially is B square by A and then it is just a matter of plugging in the properties. So, essentially I want you to be able to use this graph and try to work out transition inertia control growth and heat transfer control growth. I think you will get after plugging in the values a transition size of 0.185 mm. So, you can just work this out and if you have some problem you can get back to me. Now, moving on to DNS of high pressure bubble growth. Why are we talking about this now? You are looking at trying to model a bubble growing. But an important assumption that you have made over here is the bubble is spherical all the time. Made be at the wall or in an infinite pool of liquid. But in reality your bubble is nowhere close to being a sphere. So, it is going to have random growth sizes. So, essentially when you are trying to model bubble growth it makes sense that you track the interface locally. So, nowadays computational techniques actually track your interface at every localized point. Now, why is this useful? A for the size, B I would like you to consider that you have only looked at atmospheric pressures up until this point. You have not looked at high pressure conditions and say for nuclear reactors or industrial systems. It is high pressures at say 100 bar, 150 bar and I have no idea how my bubble is growing. I cannot conduct experiments to look at it. So, I do not know how much heat is being transferred. And later on you will understand why this is such an important thing to be able to study what size your bubble is going to be. So, essentially I can only depend on a computational framework to be able to try and predict. So, that is the goal of trying to do a simulation. So, DNS stands for direct numerical simulation wherein you are trying to solve a form of the Navier stokes and you are also trying to track the interface. And you will be surprised to see that the equations are similar to what you have looked at at the core. So, that is what I want to highlight some of the similarities and the dissimilarities. So, when you are looking at first trying to predict growth for 150 bar pressures you want to be able to first validate the system. So, as a first exercise here I am just showing you the work that we did taking an experiment which was available for 45 bar a bubble growing they were able to visualize it and to be able to apply the DNS computations to observe the bubble growth and understand what the bubble growth is. Is it inertia control? Is it heat transfer control? Is there something else additionally involved in it? So, let us find out. So, first looking at the methodology like I told you we are looking at tracking the interface locally at the grid point level. So, how do we calculate the heat transfer? So, if you look at it you have the interface which is divided across the cells. So, essentially there is a vapor phase and a liquid phase. So, what we are trying to do is we are trying to calculate the gradient of temperature in the vapor phase and the gradient of temperature in the liquid phase. So, conduction to the interface. So, that is your QL and QV and your net transfer either it is going to be condensation or evaporation is going to be a sum of these two. So, essentially we determine m dot as a summation and then what do we do with that? We know say some amount of vapor is being produced. So, essentially we need to track the interface. What do I mean by tracking the interface? I am starting at a level 0 at time t and I have produced this much more. So, I am going to move my interface forward by a velocity. If you remember you push the bubble interface with u equal to you know you have an expression for u that you determine. So, something similar to that. So, you are pushing your void interface a little bit more with that m dot. So, that is what your second equation there does. And once you moved it your temperature has to restructure itself correct. So, you are solving a temperature equation after that. And if you remember this temperature equation is similar to the heat controlled differential equation that you saw yesterday. Where you derived r is a function of square root of t. So, there are some similarities in the method. So, we set up a simulation of a container with an initial bubble size and we try different grid sizes. Essentially because if you are trying to make this finer you are trying to calculate it more accurately. You are removing all the errors. So, essentially you try and go to higher cell sizes. So, you can try even up to 2 million we have tried it. But the catch here is the computational power. How long it is going to take? When we simulated it. So, this is the sample of the kind of contours that you get. And what do you see here? You see here that as the bubble grows it seems to take away heat from the nearby zone. So, there is some kind of heat removal from the wall. And as it grows and departs you will actually see the quenching take place. So, the evolution cycle the complete heat transfer can be characterized. More importantly our predictions were able to match the experimental values. And further investigation actually showed us. Now, this is the important part. For high pressure boiling the growth rate follows the trend that you saw for heat transfer control growth. So, the formulation that you learn for heat transfer control growth is applicable for high pressure conditions. So, when you are trying to design for nuclear reactors you are trying to go in for r in terms of square root of t. Now, the coefficient is the important part. So, is that all? No, actually there are some more intricacies involved in heat transfer. So, when you are trying to design for boiling at a surface till now we have just approximated it to be a sphere. And your heat transfer was how much evaporation takes place into the vapor. But the kind of heat transfer that you are saying is conduction across the thermal boundary layer and that is the important part. What we find out is that this is not all there is to it. There is actually a micro layer that is involved. What do I mean by the micro layer? So, by the micro layer I mean that your bubble forms at a cavity and is in contact with the wall. It is not able to move as fast of the rest of the interface. So, it forms a thin micro layer across which there is very high heat transfer. So, that is a heat transfer that we are not calculating in this expression. So, some people have gone ahead and developed a structure similar to this, but they given some empiricicity in the beginning. Why? Why can't you just do these DNS simulations to be able to predict? Because for example, to give you a timeline to make that bubble grow accurately, it took us at least 10 days to 14 days for that simulation to run. So, you are looking at a single bubble which is growing at a slow space. So, when you are trying to do atmospheric pressure with a bubble like this, you will have high resolution requirements with at least three weeks of runs going on for a single bubble. So, empiricism has its perks. I mean I know that people should complain that two phase flows is filled with empiricicity, but essentially this is what it helps you do. It actually puts in an empirical correlation C over here and you can calculate the rate at which the radius will change in within 15 minutes. So, essentially what this gives you is the rate, but you do get some input on the heat transfer magnitude, some amount of verification. Going on to scales of boiling. What do I mean by scales of boiling? Boiling we have looked at till now is just a single bubble, but we have several such bubbles forming. For example, let me just show you this video by Evelyn Wong in MIT. So, this is a pool boiling experiment. Look at how closely the bubbles are growing and how fast. So, it is hardly just that single bubble nicely growing at one corner. Eventually when you crank up your heat, you are going to have bubbles forming at very high frequency of different sizes. And when you come into flow boiling kind of approaches, can you see with increasing heat fluxes, this middle one has the highest heat flux. You have bubbles of different sizes and different moving velocities, different shapes going on. So, this is what you have to eventually try and capture. So, how do you extrapolate from a single bubble R of T to something that you are trying to model here? Series of bubbles growing at the wall. So, this they achieve by using the wall heat flux partitioning. So, you have an input heat flux and what they do is they divide it into the heat flux. So, they partition it into single phase convection to liquid, then there is an evaporation. After the bubble departs, there is a quenching phase. So, it is this that you will mostly be dealing with. So, when you try and calculate the heat flux on the surface, you will have to account for all of this. And in built into this is the RT term because your RT term will give you the amount of area, duration at which it overlaps the surface and the extent to which it grows. So, this still talks about heat flux locally on a surface. It does not talk about industrial scale problems where you are trying to model draw bundles. So, there we have to come to correlations again. Correlations which take some amount of this physics like a force balance or some amount of this growth rate relations, but essentially are fully correlations. So, that brings us to the pool boiling and the pool boiling curve. So, what is this pool boiling curve? Like you have looked at in that video, as they cranked up the heat, the bubbles that formed were closer, more rapid and more fast in their formation.  So, let us just reproduce or deconstruct that graph more carefully. So, this is a curve that was generated by Nukiama in the 1930s. So, what he essentially did was he started with a low super heat and he cranked the surface temperature up and he visualized how boiling changed. So, essentially in the first portion as he heated up the surface as expected the natural convection curves were set up. So, till this point on there was just natural convection involved that is your first portion of the curve and then at one particular point boiling started. So, this boiling where just individual bubbles were formed in a very isolated manner. So, this was the isolated boiling region. With increasing heat more heat flux was ensured and higher number of bubbles were being formed. So, this continued on till a particular maximum heat flux was possible. Now, this region is the most favorable it is called the nucleate boiling region. Because you can achieve just for a small increase in wall super heat a very high increase in the wall heat flux. So, it reaches a top maximum point after which there is so much of bubbles around that they begin to coalesce. So, they start forming small vapor thin films. And when small thin films like these form what happens is that there is no further well there is lesser heat transfer that is possible across these vapor films. So, essentially your heat transfer or heat flux is going to drop and at one particular point over here and beyond. So, this portion is called the transition boiling. Then you go on to the final part where instead of sporadic vapor films you actually see a continuous vapor film formed and that is called the film boiling regime. So, essentially you see that your heat flux peaks at a particular point and is minimum at a particular location right. So, we know that we are only interested in maximum heat transfer possible. So, that maximum heat flux possible is an interesting location for us and we want to be able to predict that right. But it is actually more important than we actually thought. So, in the nuclear reactor case they have huge projects being funded to be able to just predict that maximum heat flux and that is known as the critical heat flux location. So, why is that of so much important? So, let us look at trying to instead of controlling the wall temperature let us look at trying to look at controlling the wall heat flux. So, I am increasing the wall heat flux and your surface temperature. So, this is for this graph here you see is for atmospheric water. So, I reach the maximum peak location point which needs a super heat of 30 degree Celsius right. Now, imagine I just increase my heat flux beyond that by just a little bit. You directly jump to the pool boiling regime where the surface temperature is almost 1000. So, what does that mean? You will appreciate this video I think. So, this can you see it because the temperature is increasing so high that at one particular point it is going to snap. So, essentially that is why critical heat flux is very important. In nuclear reactors they are scared that if that rupture happens because of such a high heat flux or heat surface temperature. Your nuclear reactor can have some break where your you know your radioactive fluid can slip out. So, what causes that? Essentially it is boiling. You have series of boiling happening with closed bubbles and just after your critical heat flux this immediately forms a film and there is a spike in the wall temperature and causes that rupture. So, you need to be able to predict from this bubble size growth rates to a film formation and that is why this is very important. So, people try so with that motivation in mind people try and develop correlations for different regions to be able to predict the heat flux that will put you in different regions and design systems accordingly. So, for the first region which is your natural convection region it is pretty much what you have learnt in your natural convection classes. Essentially all those correlations are only going to be used. So, you know that qv is equal to h delta t right. So, then you know this is given by your natural convection correlations as h t by k. So, all you need to do is take your natural convection correlation and get your h and plug it in here and that is all that is been done over there. So, all your nu correlations are what you see over there. So, essentially to calculate heat flux in this region you just need to use those correlations. So, going on to your nucleate boiling which is your most interesting most efficient region what you need to do is use you can use any correlation, but the most popularly used one is the Rosenhoff's correlation. So, Rosenhoff in his model comes up with at least he tries and incorporates some amount of physics. So, I like to bring your attention to the first term in the brackets there that is nothing but a balance between your buoyancy force and your surface tension force that is holding the liquid at the wall the bubble at the wall. So, that is a force balance takes care of the mechanical thing for the heat transfer you can see the next term that is c p delta t by h l v that is nothing but your Jacob number. So, if you remember that is a measure of how good your boiling is that is a number that you it will feature quite frequently in your correlations. So, he used these two let us call it physics in the equation and proposed some numbers in and around it going on to trying to predict the critical heat flux location. So, you have Zuber's correlation. So, I am just presenting one or two correlations you are free to use any correlation that you want for any specific reason. But I want you in your paper to just mention whichever correlation and why you have chosen that and I am I am flexible with what correlation you are going to use. So, in Zuber's correlation as far as calculating your CHF location is concerned he proposes this correlation, but if I like to draw your attention to what is in the brackets right. So, he tries to capture something over there and that is different from your buoyancy versus surface tension ratio. So, that brings us to how they conceptualized CHF there were many theories for CHF one of them is by Cole. So, he saw it that CHF essentially means a vapor fill in some cases right. So, there is essentially constant presence of vapor at the wall Cole saw it slightly differently. He said that it is because of a lack of refilling of cold liquid it is not that vapor is constantly at the wall. So, he was saying that your bubbles form depart in quick succession one after the other. So, they rise with the terminal velocity. So, essentially the force balance that he was interested in is the buoyancy versus the drag force. For him surface tension the bubble is already left the wall. So, he does that force balance and he comes up with that correlation there. I am not even going to try and pronounce that name, but that guy proposed a correlation which takes into account CHF for sub-cooling cases. So, in his case you can see that instead of proposing the standard Jacob number which is CP superheat by HLV. He takes into account that you are trying to raise the temperature from sub-cool liquid. So, essentially that is the kind of sensible heat that he is interested in. So, that is the formulation that he has tried to include in addition to your buoyancy versus drag force term. Coming to film boiling and this was I think another good video by captured by Professor Mudaga in Purdue and that is film boiling. So, that is just a wire and you have vapor forming and departing from the wire. So, essentially what you see is that there is a thick vapor film involved right. This is relatively thin, but essentially you will see a slightly thicker film. So, to model heat transfer there they say that film boiling has such huge heat that there is radiation component involved. So, you have a convection heat transfer component and a radiation heat transfer component. And your convection heat transfer component also has a phase change because your water is converting into steam there right to vapor. So, you have a HLV component and you have a radiation component and because it is bubbles leaving the surface, he is characterized it by the surface tension versus buoyancy force right. So, let us talk about how this curve will change for different conditions. Now this is a curve for saturated water at atmospheric pressure. So, when we try and look at how this is going to change for the first case which is with sub-cooling. Now imagine this is not a saturated curve, but for sub-cooling conditions what happens in different cases. We know that with sub-cooling natural convection will ensure more heat transfer. So, your curve is going to shift slightly a bit up. Then for nuclear boiling experimentally it is found that sub-cooling does not really help heat transfer significantly. Because essentially it in itself is quite efficient in forming bubbles and departing it is only bothered about heat transfer from the vapor. So, the nuclear boiling case experimentally they found does not help this pool boiling curve in the nuclear boiling region. But at the critical heat flux zone it seems to increase the ability. So, a system like for example, you are designing a nuclear reactor and instead of operating it at a saturated condition. Instead of sending water at saturated condition you send in water at sub-cool conditions. Then you are actually increasing the capacity of the system. You want the critical heat flux will happen at a much higher heat flux than at a lower heat flux. The target is essentially to increase the critical heat flux so that it never reaches that region. So, you find that most of the nuclear reactors will have a sub-cooled temperature inlet. So, your liquid is going to be sub-cooled and this is essentially the reason why. Because you have increased the critical heat flux. Then transition boiling again there is not much understanding because it is such a quick phase. It quickly changes to film boiling but people poichlate that it does increase the heat flux. And when you go towards film boiling the increase is there but it slowly decreases with sub-cooling. Because after a point yes there is sub-cooled liquid over here. It might cause some condensation locally but beyond a particular temperature it is the radiation which will take over. So, that is going to govern the case. So, your parametric dependence is that it essentially shifts it by a little bit. But you know not significantly in the nucleate boiling region and in the last portions of film boiling. First convection pretty much the same effects as that of sub-cooling. But only in the film boiling region it is able to ensure more transport of vapor. Because a forced convection is able to carry the vapor on forward. So, that helps the heat transfer coming to wettability of the surface. So, essentially when we were talking about hydrophilic and hydrophobic surfaces what we saw was that. So, when you have a hydrophilic surface this is vapor and this is liquid. Once the vapor forms and departs the liquid quenches the surface. But when you talk about a hydrophobic surface meaning liquid doesn't want to come into contact with the surface. Your vapor film will directly form it won't be a bubble anymore. So, this vapor film essentially is going to push you into the film boiling region directly. So, what people have seen is that this region disappears. And essentially you are looking at a film boiling region directly. Surface roughness essentially talks about you know trying to refill cavities. So, it is just an extension it is a minor point that I would like to point out. That once you have a lot of roughnesses it has a capability to trap air. So, that is going to promote nuclear boiling more heat transfer right. And in radiation people are not really sure because it helps heat transfer. But it can also affect because of its emissivity changes the radiation. But nobody has quantified it. So, with this background of pool boiling curve and pool boiling correlations. Let's just work out one or two problems in using these correlations. So, let's look at the first problem here. Water at one atmosphere is heated in a pan. The bottom of this pan is of a specified diameter and is maintained at 115 degree Celsius. Estimate the power required to boil the water in this pan. Determine the evaporation rate. What is the ratio of the surface heat flux to the critical heat flux. And what is the pan temperature that is required to get critical heat flux. So, let's just tackle this problem one at a time. So, looking at the first thing. You are given water at one atmosphere. So, your saturation temperature is 100 degree Celsius. Superheat is 15 and you have the properties of the liquid. So, I am not going to go into the properties of the liquid. And then you are asked to find out first the power required to boil the water in this pan. So, the first thing that you are trying to say is that I assume you want to initiate new create boiling. So, the first thing would be to come up with the Rosenov's correlation. So, you take the Rosenov's correlation plug in the values all of which which is it is here. And you have been given that it is a copper pan. So, in this table there is a better picture again in your textbooks I think. So, for water, copper say polished surface you have the value of CSF. Which is one of the coefficients that you need is 0.013 and n is 1. So, I think you know all the terms that are involved in that expression.  So, it is the bottom side of a pan. So, you know the area. So, you have the power required and you will get something to the likes of 8.16 kilowatt. The evaporation rate. So, it says determine the evaporation rate. It is nothing but m dot h of g. So, you can just plug that in and you will get something to the amount of 13 kgs per hour if you are expressing it in terms of the hour. The next thing is what is the ratio of the surface heat flux to the critical heat flux. So, that means you take the correlation of Zuber's say which is the critical heat flux correlation. And you have all the values that is there plug it in and the ratio is 0.42. Now, it asks you to find out the surface temperature at which this critical heat flux is there. In this case it is water at one atmosphere which pretty much is just your pool boiling curve and you I have told you that the critical heat flux is at 30 degrees Celsius. So, in a later case you will be calculating the surface temperature in the next problem. But essentially it is 30 degrees Celsius here. So, going to the next question. Horizontal copper tubes 25 mm in diameter and 7.75 meter long are used to boil saturated water at one atmosphere. If the tubes are operated at 75% of the critical heat flux, how many tubes are needed to provide a vapor production rate of 750 kilograms per hour? What is the corresponding tube surface temperature? So, again it is just pretty simple. So, you have the critical heat flux which is Zuber's correlation and you plug that in solve that. So, you have 0.75. So, from the previous problem it is water at one atmosphere. So, in the previous problem this we get it to be 0.99 megawatt per meter square. So, your actual heat flux is 75% of this value. So, you have times this. Now, what do you ask to calculate the number of tubes? So, essentially this is into area which is n times pi TL. And that can be equated to m dot h of g and your m dot is given to be 750 kilograms per hour. So, it is just a matter of solving this relation and you get the number of tubes. I think I get something around 10 and you calculate the surface temperature. So, you have your heat flux known. So, go back to your Rosenhoff's correlation. You have the left hand side known. You can just calculate the super heat that is required. So, that is this problem here. The third problem, what is the critical heat flux for boiling water which is kept at a pressure of one atmosphere, but on the surface of the moon? Now, you know that the gravity of the surface of the moon is 1 sixth that of the earth. So, how does that lower gravity or reduce gravity condition what happens to your critical heat flux? So, when you take a look at, I wonder if that correlation is your, yes. So, it is your best correlation. If you take your, so essentially this becomes a comparison between your critical heat flux at on earth to that of moon. This relation pretty much holds because your QCHF is found to be proportional to one fourth. So, for the case of moon keeping everything else the same you have and what you will find that, so here we have already calculated it in the previous problem we were about 1.1 megawatt per meter square. So, you would actually find that the critical heat flux decreases in a reduced gravity condition. So, why does that happen? If you think about it, it is pretty much your buoyancy force which is pulling your liquid away from the surface and preventing and you know actually facilitating the quenching of the surface, but in reduced gravity conditions this will not happen so efficiently. So, your vapor is pretty much going to be lying around on the surface. So, essentially that means that your critical heat flux is lower. It is going to happen quicker because your vapor is going to be closer, your threshold is much lesser. So, essentially this is a topic of interest for ISRO because they are trying to do cooling of, cooling in their space stations. So, they are trying to you know try boiling is an efficient form of heat transfer for them, but in reduced gravity conditions they have to be very careful and they would like to investigate how boiling is occurring in reduced gravity conditions. So, coming to two phase flows right. So, two phase flows essentially is a more complicated form of pool boiling. You have looked at various regimes in pool boiling. So, you started with natural connection, nuclear boiling regime, transition boiling and film boiling. Essentially it is how these vapor phases are there in relation to your liquid. So, in the case of flows again the flow rate in itself complicates the flow pattern quite a lot. So, if you look at this video it basically shows you how the flow pattern changes with increasing glass flow rate and can you see that. So, you can see slugs actually going up slugs of at intermittent points. Then you actually have vapor coming down water falling on the side and then there is annular flow. So, people actually have seen these in experiments and they have tried to characterize them into different categories.  It is only that there is increasing amount of vapor and in different forms. Why does this matter? Because we are trying to calculate heat transfer in all these systems and the heat transfer in the first is going to be drastically different from the last. So, in any industrial case we need to know first what kind of flow pattern forms to be able to apply the requisite correlations. So, it is highly dependent on the flow pattern and the flow pattern is dependent on the input conditions of glass flow rate and the liquid flow rate. And why is this important? Because the first important calculation that you do is your pressure drop calculation. You need to know interfacial area because you need to know bubble breakups. Heat transfer involved there on and there. So, the first task is to be able to try and predict the flow pattern. So, what people have done is they have conducted many experiments and tried to develop maps like these called the flow regime maps. So, what are these flow regime maps? They have tried to chart out by you know varying your velocity of the liquid and velocity of the gas what different flow patterns can be observed. So, if you want to try and develop say flow through a tube you just have to go and see what is my mass flow rate for gas mass flow rate for liquid and find out the kind of flow pattern that is possible from one of these flow regime maps and then devise your equations accordingly according to the flow pattern that you will be observing. Going on to the mathematical modeling that is involved. So, once you have determined the kind of flow pattern that is there then you have the mathematical modeling of these faces. Essentially how people tackle these various flow patterns is that they try and average out the vapour. So, you will see complex vapour and liquid forms. So, essentially what they try and do is that they assume some amount of vapour and some amount of liquid in a segregated form. So, essentially these are such kind of numbers. So, first volume fraction or void fraction essentially is the volume of vapour occupied to the total volume. So, volume fraction of liquid or volume fraction of vapour synonymous with void fraction is an expression where which gives you amount of in a cross sectional area amount that is the volume that is occupied. The next three are quality. So, quality essentially can be of three types equilibrium quality, flow quality and static quality. So, let us the first is static quality is the simplest form in a cross section the mass of vapour to the total vapour. Flow quality talks about the mass flow rate of vapour to the total flow rate and equilibrium quality basically talks about the averaged enthalpy across a system to the total enthalpy. And slip obviously is the velocity ratio between the two phases. All these expressions that you will be seeing further will require some form of being able to transfer from say a void fraction to a quality and to a slip. So, this is just the interrelationship between all these terms which you will require in your calculations, right. So, to be able to model the flows that you saw essentially there are three popular models that are preferred one is your homogeneous equilibrium model the second is your separated flow model and the third is your two fluid model. So, when you are talking about your homogeneous equilibrium model essentially the assumption is that your liquid and vapour are moving at the same velocity they are just a mixture and you use properties that are mixture properties. So, that simplifies any kind of slip between the velocities. So, this probably would be useful in a place where you are trying to model highly dispersed flows in a sense very little vapour lot of liquid then you can actually assume it to be one medium and get a mixture property involved. The second is your separated flow model wherein essentially they try and model the liquid and the vapour phase separately the momentum equations and they try and capture this the slip between the phases. This is important if they are significantly equal amount of vapour and liquid the separated flow is preferred. And finally, the two fluid model in this case it solves two equations of mass two equations of momentum and two equations of energy and there will be interfacial interaction terms. So, your two fluid model is kind of the most up to date model which is being used by all your commercial cold softwares and is preferable but obviously it is computationally intensive. So, right now we will probably in this part of the talk we will just restrict ourselves to the first two models and for the subsequent lectures we will talk about the two fluid model. And the next part is where we go into the pressure drop calculations and I think I will pick it up from the next class and we will start with how to calculate the frictional acceleration and static pressure drops in a tube. Thank you.


From Santa Clara, in the heart of Silicon Valley, it's theCUBE, covering The Churchills 2019. Brought to you by SiliconANGLE Media. Hey, welcome back, everybody. Jeff Frick here with theCUBE. We're in Santa Clara, California at The Churchills. It's the ninth annual awards banquet put on by The Churchill Club, and this year is all about leadership. And we're excited to be joined by our next guest who knows a little bit about leadership. He's Carl Guardino, the president and CEO of the Silicon Valley Leadership Group. Carl, great to see you. Great to see you too, Jeff. So what is the Silicon Valley Leadership Group all about? The Silicon Valley Leadership Group is an association of about 360 primarily innovation economy employers that wanna make a positive proactive difference here in the region, as well as in our state and across the United States. So what are some of the hot topics that are on top of the plate right now? Because there's a lot of craziness kind of going on here in Silicon Valley. There is, but what we try to do is impact those issues that are as important to families in their living rooms as they are to CEOs in their boardrooms. And here in the Bay Area, we call those the issues. An acronym, T-H-E-E, the T, traffic, H, housing affordability, E, education, and the fourth E, the economy. And we try to bring together a diverse points of view for those areas that unite us and where we can actually solve some of those challenges. Right, and those are big, big challenges. And it's, you work both with public as well as private groups to try to bring them together to make movement on those things. We're a bridge. And the first thing about a bridge is that you try to bring folks together to cross the bridge and work together. The second most important thing about a bridge is that you build them, you don't burn them down. And that's the role that we try to play with 360 highly engaged CEOs and C-suite officers. And it's only appropriate because tonight you'll be sitting down in a conversation with the mayor of San Jose, Sam Liccardo, to kind of get into some of these issues. San Jose seems to be on a roll right now, on a positive roll. A lot of positive news coming out of San Jose. Yes, and that always starts with leadership rather than luck. San Jose mayor, Sam Liccardo, 10th largest city in the United States has been able to strike that balance of being pro-innovation economy, while also caring deeply about his citizens, the residents of San Jose, 1,053,000, in how we make sure that we have a strong and vibrant economy, but also a great quality of life. So how do you even begin, we'll start with traffic, the T and the, to address that issue. It's so multifaceted, right? It's so tied to jobs, it's tied to housing, it's tied to the growth of the economy. Unfortunately freeways are slow to build, public transportation is expensive, but we continue to see growth there. How do you kind of eat that, eat that elephant one bite at a time on something like traffic? Well, the role of the leadership group is again, by bringing people together to solve complex problems in a democracy with winning solutions. So we'd rather win than win. And when it comes to traffic, one of our core competencies is actually to lead and run ballot initiatives to fund transportation improvements throughout the region and the state. In fact, in the last 30 years alone, I've had the pleasure of going on loan from the leadership group to run ballot campaigns for transportation improvements that have totaled $30 billion in revenue through those measures approved by voters to reach into our own wallets rather than our neighbors to build improvements that this Christmas, in time to go into your stocking, will be bringing BART to San Jose and working on the electrification of Caltrain, linking transit and better road improvements, making it better for all of us trying to travel throughout this region. Right, good, we need it. We do. And on the housing, you know, because the housing is also very closely tied to traffic.  They just can't support it in higher density areas like San Francisco, San Jose, to bring those jobs next to that. So we're seeing a huge transformation in the housing space as well. And we need a huge transformation, both in transportation and in housing. And it's really the flip side of the same coin. T for tails or transportation, H for head or housing. And you have to make sure that you keep those linked. In fact, one of our initiatives right now is to work with all six fixed rail transit operators throughout the nine Bay Area counties. What are the current and future uses of those half miles around every fixed rail transit stop that you have? How do we maximize those uses? Here's a great example. What Google wants to do in downtown San Jose at the Deeradon SAP station is only because of the Silicon Valley leadership group's work to bring BART to that station, Electrify Caltrain, Light Rail is there, Amtrak, ACE, et cetera. And they want to have 20 to 25,000 future Google employees there within the next 10 to 12 years. Why?  I can't wait to see what you guys do to El Camino. That's the next one that's going to, as somebody once said at one of these traffic things, it's just a bunch of old retail stores with empty parking lots being displaced by Microsoft, or excuse me, by Amazon. So I think we'll see a big transformation with housing and jobs along that quarter, which happens to parallel the Caltrain and it's near and dear to my heart. So a lot of good opportunities, I think, to make improvements. Jeff, there is, and as hard as transportation and traffic solutions are to put into place, housing is even tougher. And while Bay Area residents think housing is the bigger crisis, the solutions are tougher to come about because the community isn't as united on those solutions. So the role that a group of employers like ours play is how do we bring people together around solutions that make sure that we build homes that are good for everyone in our society? Well, Carl, I like your positive attitude, a lot of winning and no whining, so I wish you nothing but success and we'll be watching. Thank you, Jeff. You're welcome. He's Carl, I'm Jeff. You're watching theCUBE. We're at the Churchill's in Santa Clara, California. Thanks for watching. We'll see you next time.