A Monte Carlo analysis of the NASA / ESA Spacelab vacuum vent valve

The objective of this thesis is to determine the conductance of the National Aeronautics and Space Administration (NASA) / European Space Agency (ESA) Spacelab vacuum vent valve for gases in the collisionless molecular flow regime. Since this valve is the only vacuum access afforded to experiments residing inside the pressurized spacelab module, the value of its conductance is necessary in calculating the pressures and flow rates available to these experiments. While the conductance of the pipes connecting the experiments to the valve can be determined using previously published results, the conductance of this valve can not. For the purposes of this thesis, the gases are assumed to have a Maxwellian speed distribution. The Monte Carlo method of numerical approximation is utilized to determine the valve conductance in this flow regime. This is done by calculating the probability that a particle entering the valve will exit it through the valve exit plane and not be reflected back to exit through its inlet plane. This probability is approximated by generating a large number of particles at random locations on the valve inlet plane with velocity vectors conforming to the cosine law. The particles are then followed, one at a time, to determine the opening through which they exit. If the velocity vector of a particle crosses a solid boundary, it is assumed that the particle is absorbed by that solid surface. Further, an absorbed particle is assumed to be ejected from the same location in the solid surface with a velocity vector whose direction follows the cosine law. Flow visualization is employed to help better under stand the flow phenomenon and to help isolate programming errors. To model the solid surface absorption and desorption of each particle, the ray tracing principles and approaches used in high resolution computer graphics modeling are used.