Direct simulation Monte Carlo study of orifice flow

A study of the flow properties of argon through an orifice ha s been performed with the direct simulation Monte Carlo method. The study covered the full extent of the tra nsition 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 ge ometry with argon gas. The variation of molecule flow r ate through the orifice and the variation of mass-thickness of t he gas downstream of the orifice are given in the complete transition range. The molecule flow rates computed herewit h show good agreement with previously published experimental measurements. The isentropic equations of a perfect gas are shown to reproduce the expected relationsh ip 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 fr ee- molecule flow. However, the computed flow rate is less than the flow rate of a nozzle in the continuum flow. Emp irical equations for both the characteristic speed and normalized num ber thickness have been derived and shown to predict well the values of any gas, for all practical purposes.