Heat Transfer in Rarefied Gases

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This chapter presents the results of the recent analytical and experimental investigations of heat transfer in rarefied gases. It concentrates on the heat transfer through the electrically neutral nonreacting gases. The effects of ionization, dissociation, etc., that may arise in high-speed high temperature gas flows are not considered. The term “rarefied” means that the molecular mean free path is not small compared to a characteristic dimension . The parameter that describes the degree of rarefaction is the Knudsen number. In defining the Knudsen number, it is important to select the appropriate characteristic mean free path and length. When the Knudsen number is very small, then, in the vicinity of the body, the number of collisions between the molecules is large compared to the number of collisions between the molecules and the body. In this case, the usual continuum concepts are applicable and the Navier-Stokes equations and the Fourier heat conduction law are valid. When the Knudsen number becomes sufficiently large, then the continuum concepts must be modified for calculating the heat transfer. At very high Knudsen numbers where the number of collisions between the molecules and the wall is much larger than the number of collisions between the molecules, the flow is termed “free molecule.”