Direct simulation Monte Carlo study of orifice flow

A study of the flow properties of argon through an orifice has been performed with the direct simulation Monte Carlo method. The study covered the full extent of the transition regime between free-molecule and continuum flow, both in the upstream and downstream regions. Results for Mach number, number density, velocity and temperature are shown for three representative cases for a specified geometry with argon gas. The variation of molecule flow rate through the orifice and the variation of mass-thickness of the gas downstream of the orifice are given in the complete transition range. The molecule flow rates computed herewith show good agreement with previously published experimental measurements. The isentropic equations of a perfect gas are shown to reproduce the expected relationship between properties as a function of the computed Mach number in the continuum regime, but they clearly deviate elsewhere, as expected. The theoretical density function and flow rate agree well with the computed values in the free-molecule flow. However, the computed flow rate is less than the flow rate of a nozzle in the continuum flow. Empirical equations for both the characteristic speed and normalized number thickness have been derived and shown to predict well the values of other gases, for all practical purposes.