Knudsen number

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

A volume-based hydrodynamic approach to sound wave propagation in a monatomic gas

Abstract: We investigate sound wave propagation in a monatomic gas using a volume-based hydrodynamic model. In Dadzie et al. [Physica A 387, 6079 (2008)], a microscopic volume-based kinetic approach was proposed by analyzing molecular spatial distributions; this led to a set of hydrodynamic equations incorporating a mass-density diffusion component. Here we find that these new mass-density diffusive flux and volume terms mean that our hydrodynamic model, uniquely, reproduces sound wave phase speed and damping measurements with excellent agreement over the full range of Knudsen number. In the high Knudsen number (high frequency) regime, our volume-based model predictions agree with the plane standing waves observed in the experiments, which existing kinetic and continuum models have great difficulty in capturing. In that regime, our results indicate that the “sound waves” presumed in the experiments may be better thought of as “mass-density waves,” rather than pressure waves.

Knudsen diffusion through ZIF-8 membranes synthesized by secondary seeded growth

Abstract: Herein we demonstrate the synthesis of ZIF-8 membranes via secondary seeded growth on tubular stainless steel porous supports. The membranes were characterized and evaluated for the separation of CO2/N2 and N2/CH4 gas mixtures. The adsorbate polarizability correlated with the adsorption capacity on ZIF-8, and the amounts of gases adsorbed were in the order: CO2 > CH4 > N2. The CO2/N2 separation selectivity’s for the ZIF-8 membranes were close to the Knudsen selectivity, suggesting that Knudsen diffusion through non-ZIF pores dominated the separation. On the other hand, the separation selectivity’s for N2/CH4 were slightly higher than the Knudsen selectivity, indicating that the flow contribution from the ZIF pores favored the transport of N2 over CH4.

2D Navier–Stokes Simulations of Microscale Viscous Pump With Slip Flow

Abstract: In this paper we provide numerical solution of the Navier―Stokes equations coupled with energy equation for gaseous slip flow in two-dimensional microscale viscous pumps. A first-order slip boundary condition was applied to all internal solid walls. The objectives are to study the performance of the pumps and to study the effect of velocity slip on its performance. Mass flow rate and pump efficiency were calculated for various pump operation conditions when an external pressure load is applied at the pump exit plane. Geometric parameters were held fixed in this work. Microviscous pump performance was studied in detail for several values of the Reynolds number, pressure load, eccentricity, and slip factors. Our numerical results for no-slip were compared with previously published experimental and numerical data and were found to be in very good agreement. Slip values and eccentricity were found to be major parameters that affect the performance of pump. Pump head decreases with increasing slip factors. Maximum pump efficiency increases with increasing slip factor up to Kn approaching 0.1. However, the maximum value of pump efficiency is found to experience a steep degradation for Kn approaching 0.1. The values of moment coefficient always decrease as both slip factor and distance of the rotor from the lower wall increase. Also, as slip factors and distance of the rotor from the lower wall increase, less net ffow rate is predicted. For a given fired driving force at the rotor surface, there is an optimum value for the behavior of pump efficiency with distance of the rotor from the lower wall. Future research should be conducted to modify the current design to make this concept work for higher Knudsen number.

Surface stress, initial stress and Knudsen-dependent flow velocity effects on the electro-thermo nonlocal wave propagation of SWBNNTs

Abstract: This paper investigates the electro-thermal nonlocal wave propagation of fluid-conveying single-walled Boron Nitride nanotubes (SWBNNTs) using nonlocal piezoelasticity with surface stress, initial stress and Knudsen-dependent flow velocity effect. SWBNNT is embedded in a vicsoelastic medium which is simulated as visco-Pasternak foundation. Using Euler–Bernoulli beam (EBB) model, Hamilton׳s principle and nonlocal piezoelasticity theory, the higher order governing equation is derived. A detailed parametric study is conducted, focusing on the combined effects of the electric parameters, viscoelastic medium, initial stress, surface stress, Knudsen number ( K n ) and small scale on the wave propagation behaviour of the fluid-conveying SWBNNT. The results show that for smaller values of wave number the dispersion relation for different fluid viscosities seems to be similar. At the higher values of wave numbers, increase in the wave frequency values is remarkable due to increase in fluid viscosity. The electric field as a smart controller, surface effect, initial stress, temperature change and slip velocity effect have significant role on the wave frequency. The results of this work is hoped to be of use in design and manufacturing of smart MEMS/NEMS in advanced medical applications such as drug delivery systems with great applications in biomechanics.