Knudsen number

About: Knudsen number is a research topic. Over the lifetime, 5052 publications have been published within this topic receiving 104278 citations.

Gas flows caused by evaporation and condensation on two parallel condensed phases and the negative temperature gradient: Numerical analysis by using a nonlinear kinetic equation

Abstract: A rarefied gas between its two parallel plane condensed phases with different temperatures is considered, and the steady gas flows caused by evaporation and condensation on the condensed phases are investigated numerically by using the (nonlinear) Boltzmann–Krook–Welander equation. The flow properties are clarified for a wide range of the parameters (the Knudsen number, the ratio of the temperatures of the condensed phases, and the ratio of the saturation gas pressures at these temperatures). In particular, the phenomenon of the negative temperature gradient, which has extensively been studied for weak evaporation and condensation, is confirmed to occur also for nonweak evaporation and condensation. In the course of the numerical analysis, it is demonstrated that the linearized Boltzmann equation does not give the correct description of the problem for very small Knudsen numbers (i.e., Knudsen numbers comparable to or smaller than the flow Mach number), however weak the evaporation and condensation may be. In addition, a high‐speed evaporating flow toward a perfectly absorbing wall is investigated as an extreme case of strong evaporation and condensation.

Area and edge effects in radiometric forces.

Abstract: The radiometric force on several configurations of heated plates placed in a stagnant gas is examined experimentally, with a high-resolution thrust stand, and numerically using the direct simulation Monte Carlo method and a discrete ordinate solution of a model kinetic equation. A wide range of pressures from 0.006 to 6 Pa was examined, corresponding to Knudsen numbers from 20 to 0.02, in argon and helium test gases. The radiometric force, important in a number of emerging micro- and nanoscale applications, is shown to be mostly area dependent in the transitional regime where it reaches its maximum at $\text{Kn}\ensuremath{\sim}0.1$.

The Effect of Vacuum Core Boundary Conditions on Separation in the Gas Centrifuge

Abstract: A vacuum exists in the central region of the cylindrical rotor of a high-speed countercurrent gas centrifuge when operated with UF/sub 6/ for the enrichment of uranium. Since solutions of the Navier-Stokes equation are used to determine the isotopic distribution in the rotor, the location of the vacuum core boundary has a direct effect on the predicted separative work of the gas centrifuge. Because criteria for terminating the continuum region based on the Knudsen number are somewhat arbitrary, an approximate model developed by Onsager, which yields an analytical solution, has been used to evaluate the location of the boundary of the vacuum core more correctly. The results show that the location of this ''top of the atmosphere,'' in density scale heights, changes with the peripheral speed of the centrifuge. Using this location in the calculation of separation performance parameters of the gas centrifuge reduces, at the higher peripheral speeds, the contribution of axial diffusion to the effective stage length of a theoretical stage in the centrifuge. The correction due to imposing the top of the atmosphere limitation on axial diffusion becomes significant at high speeds and low countercurrent circulation rates.