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.
This paper systematically reviews the mathematical modeling based on the computational fluid dynamics(CFD)method of equilibrium and nonequilibrium hypersonic flows.First,some physicochemical phenomena in hypersonic flows(e.g.,vibrational energy excitation and chemical reactions)and the flow characteristics at various altitudes(e.g.,thermochemical equilibrium,chemical nonequilibrium,and thermochemical nonequilibrium)are reviewed.Second,the judgment rules of whether the CFD method can be applied to hypersonic flows are summarized for accurate numerical calculations.This study focuses on the related numerical models and calculation processes of the CFD method in a thermochemical equilibrium flow and two nonequilibrium flows.For the thermochemical equilibrium flow,the governing equations,chemical composition calculation methods,and related research on the thermodynamic and transport properties of air are reviewed.For the nonequilibrium flows,the governing equations that include one-,two-,and three-temperature models are reviewed.The one-temperature model is applied to a chemical nonequilibrium flow,whereas the two-and three-temperature models are applied to a thermochemical nonequilibrium flow.The associated calculations and numerical models of the thermodynamic and transport properties,chemical reaction sources,and energy transfers between different energy modes of the three models are presented in detail.Finally,the corresponding numerical models of two special wall boundary conditions commonly used in hypersonic flows(i.e.,slip boundary conditions and catalytic walls)and related research,are reviewed.
