Knudsen number

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

Porous ceramics for multistage Knudsen micropumps—modeling approach and experimental evaluation

Abstract: This paper describes the evaluation of four types of porous ceramics for use as thermal transpiration materials in Knudsen pumps that operate at atmospheric pressure. Knudsen pumps are motionless gas pumps that utilize thermal transpiration along a channel or a set of channels; the channels must have a temperature gradient and must constrain the flow to remain within the free molecular or transitional flow regimes. Of the ceramics evaluated, a clay-based, 15 bar synthetic ceramic (15PC) presents the most favorable properties for Knudsen pumps. For an input power of 3.4 W, a 25 × 25 mm 2 nine-stage Knudsen pump that uses this material provides a maximum pressure head of 12 kPa and a maximum gas flow rate of ≈3.7 μ Lm in −1 . Reliability tests demonstrate more than 11 750 h of continuous operation without any deterioration in their gas pumping capabilities. A fitted model suggests that the temporal evolution of pressure at the sealed outlet of a Knudsen pump can be captured adequately using four parameters. These parameters correspond to various nonidealities that play dominant roles in the transient response of these pumps. (Some figures in this article are in colour only in the electronic version)

Surface impact of seeded jets at relatively large background densities

Abstract: The large inertia of heavy molecules suspended in a light carrier gas is exploited in the impingement of seeded free jets against surfaces at near‐continuum conditions. It emerges that the kinetic energy at impact can be close to the free stream convective kinetic energy, even at near‐continuum densities, i.e., at Knudsen numbers of 10−2 or less. Thus, one can simulate collision energies previously obtained only under molecular beam conditions but with the much higher flux densities characteristic of free jets upstream of the transition to free‐molecule flow. The principal design parameter in such ‘‘high density seeded free jet’’ experiments is the heavy molecule Knudsen number, analogous to the Stokes number governing the inertial impaction of aerosol particles. When the ratio of masses mp/m for the heavy and the light gases is large, the interspecies transfer of momentum and energy is rather slow, and the Stokes number is of the order of mp/m times larger than the light gas Knudsen number. Thus, there i...