变截面微管道内声速点位置及临界压比

等直-收缩-扩张-等直（SCDS）变截面微管道主要应用于微型空间推进器和微型气体涡轮机等微器件中，研究其内流动特性对指导上述两种微器件的设计和性能的提高具有非常重要的意义.采用硅微加工技术在硅片上制作出矩形截面三维SCDS微管道，喉部宽度为16μm，深度为20μm，收缩比为1.625∶1实验测量了不同进出口压比条件下微管道内氮气的流量特性.实验设定进出口压比值范围为1.0—4.0，由此出口体积流量范围为0—0.12mL/s，流动的特征雷诺数小于350在实验研究的基础上，采用有限体积法对其内部流动特性进行了数值模拟，数值模拟结果与实验结果相符合.数值模拟结果发现两点不同于常规流动的特异现象：其一为最早出现声速点不在最小截面喉部附近，而是移到出口截面附近，即声速点的位置发生变化；其二为在进出口压比达到4.0时，SCDS微管道内部流场才出现声速点，即临界压比值（这里将管道内部首次出现声速点时对应的当地点压力与进口压力的比值定义为临界压比）发生变化.将其原因归结为表面效应（表面积与体积比值S/V）的影响，并进一步研究了这两点特异现象与S/V值之间的关系. 关键词： 等直-收缩-扩张-等直变截面微管道 声速点位置 临界压比 表面积与体积比值     

Multi-relaxation time lattice Boltzmann simulation of inertial secondary flow in a curved microchannel

The inertial secondary flow is particularly important for hydrodynamic focusing and particle manipulation in biomedical research.In this paper,the development of the inertial secondary flow structure in a curved microchannel was investigated by the multi relaxation time lattice Boltzmann equation model with a force term.The numerical results indicate that the viscous and inertial competition dominates the development of secondary flow structure development.The Reynolds number,Dean number,and the cross section aspect ratio influence significantly on the development of the secondary vortexes.Both the intensity of secondary flow and the distance between the normalized vortex centers are functions of Dean numbers but independent of channel curvature radius.In addition,the competition mechanism between the viscous and inertial effects were discussed by performing the particle focusing experiments.The present investigation provides an improved understanding of the development of inertial secondary flows in curved microchannels.     

Calculation of the gas flow rate in a microchannel

Approximation formulas are obtained for calculation of the gas flow rate in long isothermal microchannels. The quasi-gas-dynamic equations with Maxwell’s slip conditions are shown to predict a minimum in the flow rate within a channel that is called the Knudsen minimum. Corrections are proposed enabling derivation of approximate formulas for flow rates valid for any Knudsen number.     

Modeling of methane steam reforming in a microchannel with a heat flow distributed in length

Numerical modeling of a flow of a high-temperature mixture of methane with water vapors in a two-dimensional plane microchannel with activation of chemical conversions on the channel wall has been performed. The modeling was performed within the framework of Navier-Stokes equations for a laminar flow of a multicomponent compressible gas. The influence of the external heat flux supplied to the gas mixture and its distribution along the channel length on the properties of the methane steam reforming have been investigated. It has been shown that not only the amount of heat supplied to the reaction zone but also the method of heat supply along the channel length are important. All the reactions with the residence time of the mixture on the order of tens of milliseconds terminate several centimeters downstream from the channel inlet, which makes it possible to optimize a compact reactor for obtaining a synthesis gas.     

固体激光微通道冷却器内流动特性的数值模拟

建立了固体激光微通道冷却器数学模型,运用商业软件Fluent进行求解计算,并与文献中数据进行对比,验证了模型的可靠性。分析微通道尺寸及结构对转捩雷诺数影响,结果表明:微通道当量直径对于转捩雷诺数影响甚微,收缩比的大小是引起转捩雷诺数不同的关键因素,最终确定了不同收缩比下的转捩雷诺数。     

固体激光微通道冷却器内流动特性的数值模拟

建立了固体激光微通道冷却器数学模型,运用商业软件Fluent进行求解计算,并与文献中数据进行对比,验证了模型的可靠性。分析微通道尺寸及结构对转捩雷诺数影响,结果表明：微通道当量直径对于转捩雷诺数影响甚微,收缩比的大小是引起转捩雷诺数不同的关键因素,最终确定了不同收缩比下的转捩雷诺数。     

微通道冷却器内流动和传热特性的数值模拟

为满足固体激光器用微通道冷却器的换热要求, 根据冷却器结构分别建立了二维和三维物理模型, 利用计算流体力学方法首先对比研究两者的流动特性, 然后考察雷诺数和玻片生热量对微通道流动和传热特性的影响。结果表明：对于类似大平板间的矩形微通道层流流动区域, 其流动及传热特性可直接采用二维简化模型进行模拟分析;对于重点关注的转捩区, 采用三维模型模拟分析更好;当雷诺数增大到转捩点, 流体的传热效果得到明显增强;随着雷诺数的增大, 玻片生热量对通道内最低压力需求的影响逐渐减小;不同玻片生热量对微通道流动影响不可忽略, 对努赛尔数和通道总压降基本无影响。     

Liquid film and gas flow motion in a microchannel with evaporation

The problem of stabilized combined motion of the gas flow and liquid film in a microchannel under the action of local heating with consideration of evaporation processes was set in selected variables. The exact solutions to the linearized problem were derived. The analytical formulas for calculation of the film thickness stabilized below the heater and the total rate of liquid evaporation were obtained. The technique of approximate calculation of total heat transfer is shown. Exemplary calculations are presented.     

Numerical study of heat transfer enhancement of counter nanofluids flow in rectangular microchannel heat exchanger

This paper reports a numerical analysis of the performance of a counter-flow rectangular shaped microchannel heat exchanger (MCHE) using nanofluids as the working fluids. Finite volume method was used to solve the three-dimensional steady, laminar developing flow and conjugate heat transfer in aluminum MCHE. The nanofluids used were Ag, Al₂O₃, CuO, SiO₂, and TiO₂ and the performance was compared with water. The thermal, flow fields and performance of the MCHE were analyzed using different nanofluids, different Reynolds numbers and different nanoparticle concentrations. Temperature profile, heat transfer coefficient, pressure profile, and wall shear stress were obtained from the simulations and the performance was discussed in terms of heat transfer rate, pumping power, effectiveness, and performance index. Results indicated enhanced performance with the usage of nanofluids, and slight penalty in pressure drop. The increase in Reynolds number caused an increase in the heat transfer rate and a decrease in the overall bulk temperature of the cold fluid. The increase in nanoparticle concentration also yielded better performance at the expense of increased pressure drop.