Three-dimensional simulation of a solid–liquid flow by the DEM–SPH method

Developing cross-linked poly(ethylene oxide) membrane by the novel reaction system for H2 purification

The Knudsen pump (KP) is a kind of micro-pump that can form thermally induced flows induced by temperature fields in rarefied gas environments. It has the advantages of having no moving parts, simple structure, easy construction and extension, a wide range of energy sources, and low energy consumption. With the development of Micro/Nano Electro Mechanical Systems (MEMS/NEMS), extensive studies have been conducted on KPs, and the applications of KPs have widened. In order to obtain efficient flow fields in KPs, it is necessary to adopt modern computational methods for simulation and analysis. In many circumstances, the simulation and experimental results have good agreement. However, there seems to be no comprehensive review on KPs at present. In this paper, KPs are first defined and classified according to the flow mechanisms of the thermally induced flows. Then, the three aspects of configurations, performance, and applications of KPs in the current state of research are reviewed and analyzed. Finally, the current problems of KP are discussed, and some suggestions are provided for future research and applications. Knudsen pumps are micro-sized, non-mechanical gas pumps that can be useful in a range of biomedical and mechanical fields. Zhang and his team from China’s North-eastern University analyzed Knudsen pumps’ configurations, performance, applications, and current problems. In Knudsen pumps, gas flow is induced by temperature gradients along the walls of the device. They benefit from low energy consumption and ease of manufacture. Different configurations, realized through the placement of variably heated walls, offer multiple mechanisms to transport rarefied gases. Knudsen pumps can be used, amongst other applications, to generate a vacuum, deliver fluids, and separate gases. Micro-channels play a key role in Knudsen pump efficiency and gas flow operation; however, there is a lack of research into efficiency which the authors say should be addressed in future research.

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Knudsen pumps: a review

This work investigates the species separation in the rarefied flow of the argon-helium mixture through convergent-divergent micronozzles. Imposing a molecular mass ratio in the order of 10, the flow of this mixture can lead to the formation of serious nonhomogeneous phenomena such as the species separation. This study is performed in the ranges of 2.0–4.0 for the geometrical expansion ratio, 200–400 K for the wall temperature, and 0.003–1.454 for the inlet Knudsen number. The effects of these parameters are examined on the separative performances of micronozzle. The direct simulation Monte Carlo method is selected as the solution method because it can provide reliable solutions in the current rarefied flow regime study. The current study reveals two important separative effects in the mixture flow through micronozzles. The first effect is the lateral species separation, which results in the enrichment of heavier species near the centerline. The second effect is the streamwise separation, which leads to the enrichment of one species, mostly the lighter one, as the mixture passes through the micronozzle. The current results show that increasing the expansion ratio will enhance the lateral separation monotonically. However, there are specific wall temperature and Knudsen values, which can result in optimum lateral separative effects. In addition, it is observed that the expansion ratio has little effect on the streamwise separation. However, increasing either the wall temperature or the Knudsen number will enhance the streamwise separation, albeit with a limiting value at very high Knudsen numbers.This work investigates the species separation in the rarefied flow of the argon-helium mixture through convergent-divergent micronozzles. Imposing a molecular mass ratio in the order of 10, the flow of this mixture can lead to the formation of serious nonhomogeneous phenomena such as the species separation. This study is performed in the ranges of 2.0–4.0 for the geometrical expansion ratio, 200–400 K for the wall temperature, and 0.003–1.454 for the inlet Knudsen number. The effects of these parameters are examined on the separative performances of micronozzle. The direct simulation Monte Carlo method is selected as the solution method because it can provide reliable solutions in the current rarefied flow regime study. The current study reveals two important separative effects in the mixture flow through micronozzles. The first effect is the lateral species separation, which results in the enrichment of heavier species near the centerline. The second effect is the streamwise separation, which leads to th...

Numerical study of species separation in rarefied gas mixture flow through micronozzles using DSMC

We investigate effects of wall heat transfer on the structure of pressure driven flow in micro/nanochannels using the Direct Simulation Monte Carlo method. The effects of non-zero wall heat flux on the pressure distribution, velocity profiles, heat flow patterns, and the mass flow rate are reported. The simulation results show that cooling decreases slip at the wall and pressure along the channel. Cooling changes the heat flow direction along the channel while heating does not. At higher degree of rarefaction, the direction of the heat flow is mainly axial along the channel. An existence of cold-to-hot heat transfer process is demonstrated in the cooling wall case. Cooling can also create a heat singularity point in the domain. There is a critical Knudsen number about unity for which heating or cooling does not affect the mass flow rate through the channel. Below the critical Knudsen number, heating decreases and cooling increases the mass flow rate. Above it, heating increases and cooling decreases the mass flow rate.

https://pure.hw.ac.uk/ws/files/7492130/Wall_hear_transfer_effects_on_the_hydrothermal_behaviour_of_Poiseuille_flow_in_micronanochannels.pdf

Wall heat transfer effects on the hydro/thermal behaviour of Poiseuille flow in micro/nanochannels

A multi-scale flow analysis in hydrogen separation membranes using a coupled DSMC-SPH method

The prediction of the multiscale flow in the Knudsen pump is important for understanding its pumping mechanism. However, there is little research on such interesting multiscale phenomenon in the Knudsen pumps. In this paper, a novel numerical analysis method combining the direct simulation Monte Carlo (DSMC) method with the smoothed particle hydrodynamics (SPH) method is presented for simulating the multiscale flow, which is often encountered in the application of the Knudsen pumps. Validity and accuracy of the new method are given by comparing its results with that of the previous research. Using the coupled multiscale approach, the rarefaction and the temperature drive are studied, which are two main factors on the performance of the Knudsen pumps. To investigate the effect of rarefaction on the performance of the Knudsen pump, various pump operation pressures are compared. The flow characteristics and pumping ability at different rarefaction are analyzed, and the phenomenon of the multiscale flow is also discussed. Several cases with different linear or nonlinear temperature gradients are set to investigate the effect of temperature gradient on the performance of the Knudsen pump. The flow characteristics of the Knudsen pump such as the velocity, pressure increase, and the mass flowrate are presented. A unique phenomenon, the reverse transpiration effect caused by the nonlinear temperature gradient is studied, and the reason of the significant pressure increase in the pump channel is also analyzed. Since the multiscale gas flow is widely encountered in the microflow systems, the above method and its results can also be greatly beneficial and provide significant insights for the design of the MEMS devices.

Rarefaction and temperature gradient effect on the performance of the Knudsen pump

Thermal transpiration effect on the mass transfer and flow behaviors of the pressure-driven hydrogen gas flow

Multiple tests of a complex system are conducted by applying varying combinations of input parameters from a pool of input parameters. Results of the tests are fitted into a model of the complex system by using multi-dimensional fitting. Using the model of the complex system, identification is made of at least one optimized combination of input parameters to yield a desired response of the complex system.

Xianting Ding

Papers

A multi-scale flow analysis in hydrogen separation membranes using a coupled DSMC-SPH method

Rarefaction and temperature gradient effect on the performance of the Knudsen pump

Thermal transpiration effect on the mass transfer and flow behaviors of the pressure-driven hydrogen gas flow

Rapid identification of optimized combinations of input parameters for a complex system