Knudsen number

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

Lattice Boltzmann method at finite Knudsen numbers

Abstract: A modified lattice Boltzmann model with a stochastic relaxation mechanism mimicking "virtual" collisions between free-streaming particles and solid walls is introduced. This modified scheme permits to compute plane channel flows in satisfactory agreement with analytical results over a broad spectrum of Knudsen numbers, ranging from the hydrodynamic regime, all the way to quasi-free flow regimes up to Kn ~ 30.

Experimental investigation of rarefied gas flow in different channels

Abstract: An experimental investigation of flows of a large number of inert and polyatomic gases in various channels, a non-ideal orifice, flat slits with different surface roughnesses and wall materials, capillary packets with molten walls and a capillary sieve, has been made.The unsteady flow method and a highly sensitive capacitance micromanometer were used (the sensitivity being ∼ 3 × 10−4N/m2Hz). Measurements were made in a range of Knudsen numbers 5 × 104–10−3 at ∼ 293 °K, and some measurements for flow through a non-ideal orifice were carried out at 77.2°K.It was found that, both in the viscous slip-flow and free-molecule regimes for the channels with molten walls, the experimental conductivities were higher (by ∼ 15%) than theoretical ones calculated assuming diffuse molecular scattering by the walls. We have also observed that the channel conductivity essentially depends on the channel surface roughness and on the kind of gas. The larger the roughness height, the lower the conductivity. From the experimental data the tangential momentum accommodation coefficients were calculated.

Reconstruction and Effective Transport Properties of the Catalyst Layer in PEM Fuel Cells

Abstract: In this study, a stochastic method for the microstructural reconstruction of catalyst layers in proton exchange membrane (PEM) fuel cells is presented, and empirical expressions for the effective transport coefficients in the reconstructed catalyst layers are obtained for use in macroscale fuel cell simulations. The proposed reconstruction method is based on simulated annealing, where a reconstruction problem is formulated as an optimization problem. By exploiting information from the manufacturing process and adopting a trial statistical function, the proposed method performs three-phase reconstruction, consisting of platinum/carbon, electrolyte, and gas pores, with limited experimental information on the microstructure of the catalyst layer. The lattice Boltzmann (LB) method is used to evaluate the effective diffusivity in the reconstructed catalyst layers. Because the characteristic size of pores is of the order of the mean free path of gas molecules, both molecular and Knudsen diffusion are of importance in the catalyst layer. A validated higher-order LB method is employed to consider the finite Knudsen number (Kn) effects in the catalyst layer. Results show strong effects of Knudsen diffusion on reactant gas transport, especially at near-atmospheric pressures. An empirical expression for the tortuosity of the proton transport in the electrolyte phase is also obtained.