The temperature and bulk flow speed of a gas effusing or evaporating from a surface into a void after reestablishment of collisional equilibrium

Molecules effusing, evaporating, or sublimating from a surface into a void are initially not in equilibrium and therefore do not obey the Maxwell velo city distribution function. Starting with a Maxwell transmission distributi on of speeds, we calculate from conservation of momentum and energy the tem perature and center-of-mass speed of a drifting Maxwell distribution after equilibrium has been reestablished by collisions in the limit of negligible return flux. The drift (bulk flow) speed corresponds to a Mach number larg er than 1, As opposed to effusion, vaporization and sublimation are two-pha se processes that restrict the pressure and thus the molecular Aux, but wit hout influencing the temperature or bulk flow speed. (C) 2000 Academic Pres s.