where R g is the gas constant (= N A k B, N A : the Avogadro constant) and M is the molecular mass (kg/kmol). (For the polyatomic ideal gas, N o is the number of atoms in a molecule.) In gas, constant-pressure specific heat capacity c p,f has a larger value and the difference depends on the temperature T, volumetric thermal expansion coefficient β and the isothermal compressibility κ [ c p,f – c v,f = Tβ /(ρ f κ), ρ f : the fluid density]. For dense fluids that the interactions between the particles (the van der Waals interaction) should be included, and c v,f and c p,f would change accordingly. The net motion of particles (under gravity or external pressure) gives rise to the convection heat flux q u = ρ f c p,f u f T. Conduction heat flux q k for ideal gas is derived with the gas kinetic theory or the Boltzmann transport equations, and the thermal conductivity is k f = 1 3 n f c p , f ⟨ ⟨ u f 2 ⟩ ⟩ τ τ f - f , where ⟨ u f ⟩ is the RMS (root mean square) thermal velocity (3 k B T / m from the MB distribution function, m : atomic mass) and τ f-f is the relaxation time (or intercollision time period) [(2 π d n f ⟨ u f ⟩) from the gas kinetic theory, ⟨ u f ⟩: average thermal speed (8 k B T / πm), d : the collision diameter of fluid particle (atom or molecule), n f : fluid number density].