ABSTRACT
This chapter examines molecular diffusion, viscosity, and transport of thermal energy individually at all pressure conditions. Transfer phenomena at low vacuum represent a group of effects associated with self-compensation of differences in molecular densities, momentum, and energy at existence of gradients of molecular densities, velocities, and temperatures, respectively. The chapter focuses on the transfer of thermal energy by gas conductivity because this phenomenon may dominate at some vacuum conditions. The transport equation for the diffusion of gases is already derived at low-vacuum conditions with assumptions of the same mass of molecules and temperature over the entire considered system. The major problem of thermal conductivity of gases is in assuming a constant coefficient of thermal conductivity and taking into account only a temperature gradient as the parameter driving the heat transfer by thermal conduction. High-vacuum or free-molecular conditions are attained in vacuum systems where the direct collisions of molecules with walls predominate over the volume molecular collisions.
