Fluid particle refers to matter in a gas or liquid phase, with the particle being the smallest unit (made of atoms or molecules) in it, for which further breakdown would change the chemical identity of the particle. For a fluid in motion, the convection heat flux vector qu = ρ f cp, f u f T and the surface-convection heat flux vector qku (which describes the interfacial heat transfer between two phases in relative motion, in which at least one phase is a fluid) are influenced by the specific heat capacity cp, f of the fluid particle (whereas qku also depends on the isotropic fluid thermal conductivity kf, viscosity μf, and velocity). The fluid (gas or liquid) velocity u f can be subsonic or supersonic, and for contained gases at low pressures or in small spaces, it is possible for fluid particle–surface collisions to dominate over the interparticle collisions. In this chapter, we examine energy storage and transport in fluids, as well as fluid interactions with surfaces (and the associated fluid flow regimes).

Fluid particles can have five types (forms) of energy: potential, electronic, translational, vibrational, and rotational (Figure 1.1). The electronic energy is part of potential energy, however, here we use the potential energy for interparticle interactions only. Each form of energy can be considered separately, and the total energy is the summation of these five energies. The maximum vibrational and rotational energies a molecule can attain are limited by the dissociation energy ϕe.