Analysis of Viscous Slip at the Wall in Gas Flows of R134a and R600a through Metallic Microtubes

摘要

The leakage of gas between the reed valve and the valve seat may significantly affect the efficiency of compressors adopted for household refrigeration. Such leakages are induced by differences of pressure between the compression chamber and the suction/discharge chamber and occur through small gaps in the order of micrometer, formed as a result of surface imperfections. Given the small dimensions of these clearances, a rarefied flow of refrigerant can occur under some operating conditions of the compressor. In this case, non-equilibrium phenomena, such as viscous slip between the fluid and solid boundaries, appear and they cannot be predicted by the classical fluid-mechanics continuum assumptions. The tangential momentum accommodation coefficient (TMAC) is a flow parameter that must be considered in order to correctly describe these rarefied gas flows, characterizing the exchange of momentum between the gas molecules and the surface. Many studies provide the value of TMAC, but usually the results are limited to nitrogen and noble gases for glass and silicon microchannels. The present paper reports measurements of mass flow rates and TMAC values for R134a and R600a through metallic microtubes (stainless steel and copper). The results show that viscous slip can occur even in flows of heavy polyatomic molecules typical of gases used in the refrigeration industry.