The Thermal Accommodation Coefficient: A Critical Survey

Abstract

Abstract : Accommodation coefficient (alpha) values which can be confidently applied to a solution of satellite-drag and heat-transfer problems must be obtained from experiments with gas molecules whose velocities match those of satellites. The problem of generating in the laboratory a nearly monoenergetic beam of molecules in the range 1 - 10 eV with adequate flux has not been solved. Experimental facilities such as shock and arc tunnels which may produce molecular flows of nearly adequate velocities are not readily adaptable to alpha measurements. For these measurements the transient flow obtained in a shock tunnel is extremely inconvenient, while the composition and energy distribution of the gas obtained in an arc tunnel is not known with sufficient accuracy. The composition of a satellite surface in orbit, and hence the accommodation coefficient, will most probably differ appreciably from that of an untreated surface of the same material exposed to a laboratory vacuum of, say only, 10 to the -7th power mm of Hg. Furthermore, in orbit, satellite surface composition may change with changing surface temperature and ambient gas pressure, resulting in corresponding changes in alpha.