Heat is thermal energy associated with temperature-dependent motion of particles. The macroscopic energy equation for infinitesimal volume used in heat transfer analysis is ∇ ∇ ⋅ ⋅ q = − − ρ ρ c p ∂ ∂ T ∂ ∂ t + ∑ ∑ i , j s ˙ ˙ i − − j , where q is heat flux vector, − ρc p (∂T / ∂t) is temporal change of internal energy (ρ is density, c p is specific heat capacity at constant pressure, T is temperature and t is time), and s ˙ ˙ is the energy conversion to and from thermal energy (i and j are for principal energy carriers). So, the terms represent energy transport, storage and transformation. Heat flux vector q is composed of three macroscopic fundamental modes, which are conduction (q k = − k ∇ T, k : thermal conductivity), convection (q u = ρc p u T, u : velocity), and radiation ( q r = 2 π π ∫ ∫ 0 ∞ ∞ ∫ ∫ 0 π π s I p h , ω ω sin ⁡ ⁡ ( θ θ ) d θ θ d ω ω , ω : angular frequency, θ : polar angle, I ph,ω : spectral, directional radiation intensity, s : unit vector), i.e., q = q k + q u + q r.