Velocity Distribution Measurements in a Near-Free Molecule Orifice Flow

Abstract

Measurements have been made of the velocity distribution of the flow of cesium gas from an orifice in the near-free molecule regime. The apparatus design is based on the time-of-flight method using a single disk chopper and a surface ionization gauge. Measured velocity distributions are compared with the corresponding Maxwellian distribution adjusted to have the same peak amplitude. Although the two distributions agree closely at high Knudsen numbers (i.e., Kn>10), the comparison at smaller Knudsen numbers indicates that the measured distribution is deficient in low-speed molecules. This deficiency increases with decreasing Knudsen number and with decreasing angular position from the centerline of the orifice. The experimental results are compared with the predictions of existing analyses of near-free molecule flow through an orifice.