摘要
微细通道在微机电系统、微电子、生物、通讯,尤其是航空航天上的广泛应用使得微尺度流动及换热特性研究成为当前重要的研究课题。近二十年来,大量的实验及分析结果说明微细通道内流动和换热与常规尺度通道的结果不同,具有明显的尺度效应,同时,不同研究者实验得到的结果大相径庭,甚至相互矛盾。本文以揭示微细通道内流动的机理为目的,以局部阻力特性为着重点,对微细通道内的单相和两相流动进行了实验研究及理论分析。
本文首先针对微细通道流动的实验需要,发展了缝隙测压的压力测量技术,就缝隙测压的可行性以及缝隙宽度大小对压力测量的影响进行了实验研究。首先采用缝隙测压技术(缝隙宽度分别为33μm、67μm和132μm)测量不同的流体流过内径为336μm的不锈钢管时的摩擦阻力;然后将所得的实验数据与经典公式进行比较。实验结果显示,应用缝隙测压这种压力测量技术于微管内流体压力的测量是可行的。当取压缝隙宽度与管径之比ξ≤0.2时,测量误差很小,且随着流体Re数的增大变化很小,因此为了减小测量误差,取压缝隙宽度最好小于0.2倍管径。
其次,采用缝隙测压的方法,分别以去离子水、甲苯、无水乙醇和氮气为实验工质,首次测量了内径在330μm～850μm的微管内单相以及氮气-水气液两相流体的突扩及突缩局部阻力损失。实验是在室温和大气压力下进行的。单相流体的雷诺数范围为589～8520,气液两相质量干度为2.6×10-3～1.63×10-1。实验结果表明,1)对于单相流体突扩流动来说,当较小管内流体处于层流阶段时,微管内流体突扩局阻系数稍大于常规管内的实验结果;当较小管内流体处于湍流阶段时,微管内液体突扩局阻系数和常规管的对应值基本一致,但微管内气体突扩局阻系数远远大于常规管的对应值,其原因可能是由于气体的局部马赫数大于0.3,气体的可压缩性不能忽略,因此常规管内局部阻力系数的数据处理不再适用于微管内可压缩流体。2)对于单相流体突缩流动来说,当较小管内流体处于层流阶段时,微管内流体突缩局阻系数远远大于常规管内的实验结果;当较小管内流体处于湍流阶段时,微管内流体的突缩局阻系数和常规管的对应值基本一致,K_c = 0.5×(1-σ)~(0.75)能较好地预测微管内的突缩局部阻力系数。3)对于气液两相流体突变(突扩及突缩)流动来说,均相流模型远远低估了微管内突扩局部阻力损失,而远远高估了微管内突缩局部阻力损失;滑移比S = (ρ_L/ρ_G)~(1/3)的滑移流模型能够较好地预测微管内突变局部阻力损失,说明在突变区域流体间出现了速度滑移。4)得出了单相突缩阻力预测关系式,并采用洛克哈特-马丁内利的实验数据处理方法以及该预测关系式,得出了两相突缩阻力的预测关系式及使用范围。
第三,由于尺度的减小使得微细通道通常具有较大的相对粗糙度。二维粗糙元模型预测及数值计算结果显示,由于粗糙元的存在,3%的相对粗糙度将对管内突缩流动阻力产生较大的影响。粗糙元存在导致涡流区的增大及涡流损失的增加可能是导致微细粗糙管内突缩流动阻力增加的原因。
最后,采用高速摄像仪以及压差波动相结合的方法,以氮气-去离子水为实验工质,对水力直径为0.99mm的矩形小通道内的两相流流型及摩擦压降变化进行了实验研究。实验结果表明,除了由于表面张力增强而引起的泡状流流型(此流型中圆形气泡占据整个通道)不同以外,小通道中的流型和常规通道中的流型大致相同。两相流摩擦压降预测关系式的选用与流型具有密切关系。从总体来说,L-M关系式比其它预测关系式的偏差都小,能更好地预测了两相流压降变化。
Due to the widespread applications of microchannels in MEMS, microelectronics, bioengineering and biotechnology, communication, specially in aerospace,the study of characteristics of microscale momentum and heat transfer becomes an important subject of recent investigation. Recently twenty years, a large number of research results show the characteristics of momentum and heat transfer in microchannels are different from those in conventional one and with scale effect, and the conclusions and the results have obvious confusions or contradictions each other. The main purpose of the present study is to clarify some mechanisms about fluid flowing through microchannels. The experimental study and theoretical analysis on single- and two-phase flow in microchannels is performed in this work with emphasis put on the minor pressure drop in microchannels.
Firstly, a novel pressure measurement technique, the tiny gaps on the tubes, was developed. In order to verify the feasibility of pressure measurement though a gap, the experiments of water and nitrogen flow through the stainless microtube with inner diameters 336μm were performed. The relationship between friction factor and Reynolds number is obtained by measuring the pressure drop and the flow rate. The experimental results show the pressure measurement through gaps for microtubes is a feasible method. The error of pressure measurement is very small for the ratio of gap width and tube diameterξ< 0.2 and doesn’t change with the increasing of fluid velocity, so the width of gap should be smaller than 0.2D.
Secondly, this thesis investigated single-phase and gas-liquid two-phase flow across the abrupt expansion and contraction in microtubes with the diameter from 330μm to 850μm, using de-ionized water, toluene, ethanol and nitrogen at room temperature and atmospheric as the working fluids. The experimental results on pressure drop with measurement though gaps were used to characterize the minor losses in microtubes. The ranges of single-phase flow Reynolds numbers and mass quality in two-phase flow were 589～8520 and 2.6×10~(-3)～1.63×10~(-1) in the smaller tube respectively. In single-phase flow experiments, the expansion loss coefficients were slightly larger than the experimental results from conventional tubes in the laminar flow; while in the turbulent flow, the expansion loss coefficients were roughly consistent with those from conventional tubes with exception of expansion loss coefficients for nitrogen showing a linear change with Reynolds number in the smaller tube, the reason may be that the local Mach number is more than 0.3, the effect of compressibility should be fully considered, so the data deduction method for flow area change in conventional tube doesn’t suitable for microtubes. The contraction loss coefficients were larger than those from the conventional tubes in the laminar flow; while in the turbulent flow, the contraction loss coefficients were slightly smaller than those from conventional tubes and predicted well by Kc = 0.5×(1-σ)~(0.75). In two-phase flow experiments, the two-phase flow pressure drops caused by expansion were significantly lower than the homogeneous flow model and those by contraction significantly higher than the homogeneous flow model; the slip flow model with a velocity slip ratio S = (ρ_L/ρ_G)~(1/3) showed a good prediction that reveals the occurrence of velocity slip. An empirical correlation for two-phase flow pressure drops caused by the sudden contraction was developed based on the proposed contraction loss coefficients correlation for single-phase flow and Lockhart-Martinelli correlation. Thirdly, microtubes are commonly characterized by higher relatively roughness.
Computions are carried out to investigate the effect of two-dimensional roughness elements on the sudden contraction pressure drop in microtubes and a preliminary theoretical model is brought forward. It can be concluded that higher pressure loss for incompressible fluid flowing through sudden contraction in rough microtubes can be partly attributed to the roughness elements.
Finally, by the means of combination of high-speed camera and pressure signal fluctuation, two-phase frictional pressure drop and flow regimes in small horizontal rectangular channels with 0.99mm inner diameter were experimentally investigated, using nitrogen and water. The flow regime map and the characteristics of frictional pressure drop in different flow regimes were obtained. Comparisons between the experimental data and some predictions indicate that the L-M model shows a better predictive ability than the other empirical correlations.
引文
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