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The rarefied gas flow through circular tubes of finite length has been investigated computationally by the direct simulation Monte Carlo method. The reduced flow rate and the flow field have been calculated as functions of the gas rarefaction, the length-to-radius ratio, and the pressure ratio along the tube. The gas rarefaction, which is inversely proportional to the Knudsen number, is varied from 0 to 2000, i.e., the free-molecular, transitional, and hydrodynamic regimes are embraced. A wide range of the length-to-radius ratio, namely, from 0 corresponding to the orifice flow up to 10 representing a sufficiently long tube, has been considered. Several values of the pressure ratio between 0 and 0.7 have been regarded. This pressure range covers both gas flow into vacuum and into a background gas. It has been found that the rarefaction parameter has the most significant effect on the flowfield characteristics and patterns, followed by the pressure ratio drop, while the length-to-radius ratio has a rather modest impact. Several interesting findings have been reported including the behavior of the flow rate and other macroscopic quantities in terms of these three parameters. In addition, the effect of gas rarefaction on the choked flow and on the Mach disks at large pressure drops is discussed. Comparison of some of the present numerical results with available experimental data has shown a good agreement.

A rarefied gas flow into vacuum through a tube of finite length is investigated over the whole range of gas rarefaction by the direct simulation Monte Carlo method. The nonequilibrium effects at the inlet and outlet of the tube have been considered by including in the computational domain large volumes of the upstream and downstream reservoirs. Results for the dimensionless flow rate and for the flow field are presented for a wide range of the gas rarefaction and for various values of the length to radius ratio in the range from 0 to 10. The influence of the gas-surface interaction model, as well as the effect of the intermolecular potential model on the gas flow, is examined. A good agreement has been obtained between the present numerical results and the corresponding experimental ones available in the literature.

Rarefied gas flow into vacuum through short tubes at variable wall temperatures was studied numerically with the direct simulation Monte Carlo method. A wide range of tube wall temperatures varying from 120 to 1200 K was considered, as well as several rarefaction parameters and tube aspect ratios. The mass flow rate, specific impulse, thrust, and characteristics of the gas flow field were analyzed. The results showed that the tube wall temperature, as well as the aspect ratio and rarefaction parameter, had a significant influence on the characteristics of the tube flow. The mass flow rate was found to decrease with increasing wall temperature, while the specific impulse increased. This paper reveals the combinations of rarefaction parameter and aspect ratio whereby the wall temperature has the strongest influence on the flow field of the gas, as well as on the overall macroscopic parameters.

For the torus of the nuclear fusion project ITER (originally the International Thermonuclear Experimental Reactor, but also Latin: the way), eight high-performance large-scale customized cryopumps must be designed and manufactured to accommodate the very high pumping speeds and throughputs of the fusion exhaust gas needed to maintain the plasma under stable vacuum conditions and comply with other criteria which cannot be met by standard commercial vacuum pumps. Under an earlier research and development program, a model pump of reduced scale based on active cryosorption on charcoal-coated panels at 4.5 K was manufactured and tested systematically. The present article focuses on the simulation of the true three-dimensional complex geometry of the model pump by the newly developed ProVac3D Monte Carlo code. It is shown for gas throughputs of up to 1000 sccm (∼1.69 Pa m3/s at T = 0° C) in the free molecular regime that the numerical simulation results are in good agreement with the pumping speeds measured. Me...