Some Properties of a Gas Flow Submitted to a Temperature Gradient

The flow through a channel generated by a temperature gradient along a channel surface is analyzed.This flow is characterized by the steady state Thermomolecular Pressure Difference (TPD) and by the transient stage characteristic, the relaxation time, which indicates how fast the flow reaches the steady state.From a simple analytical description, the expression to estimate TPD is proposed.The experimental data with rectangular and circular cross-sections are analyzed to estimate TPD, showing good agreement with each other.Two normalizations of TPD are suggested which allow to eliminate the dependency on gas and cross-section, allowing for application to a channel with unknown cross-section. Then, quasi analytical expression for the relaxation time is proposed.This expression allows to calculated explicitly the relaxation time for the thermal creep flow in the case of a known cross-sectional shape using the numerical results obtained from the kinetic theory.In the case of channel with unknown cross-section the relaxation time is related to their conductance.The proposed expressions are compared to the experimental data through channels of rectangular and circular cross-sections and the good agreement are found.