【作者】 张先锋；

【导师】 陈义良； 刘明侯；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2007， 博士

【摘要】 作为MEMS技术的一个重要分支，微流体系统已得到了很大的发展和广泛的应用，但其内部的流动和传递过程的理论仍不成熟。为此，本文着眼微尺度流动与传热传质，采用理论分析和数值模拟的方法，分别就微喷管内的流动特性及推进性能、微尺度下强化混合技术进行了较为系统的研究，并设计和制作了微喷管和微混合器，用实验的方法来验证理论分析。针对微槽道中气体流动的特征，分别采用连续介质模型和分子运动模型(DSMC方法)简要分析了连续介质模型中不同滑移边界条件的适用范围，并总结出稀薄效应对槽道中压力的的影响和可压缩效应的作用趋势是相反的。通过DSMC方法和求解无滑移和有滑移的Navier-Stokes方程，对微喷管中的流动特性和推进性能进行了数值模拟，分析了喷管的外形、工作压力及温度边界条件对流场结构和推进性能的影响，提出相应的优化方案，并以此为参考设计和制作了微喷管。研究表明，在喷管扩张段气体的非平衡效应和稀薄效应增强；雷诺数是表征喷管性能的特征参数，调节喷管尺寸和工作压力获得合适的雷诺数，以改善推进效率；增大推进工质的滞止温度将提高比冲，但以牺牲推进效率为代价。微尺度下强化混合技术即是增大混合流体间的接触面积，加强分子扩散的过程。其中，被动式混合过程中，雷诺数(Re)和施密斯数(Sc)是它的相似准则数，而在主动式中，可以采用雷诺数(Re)、斯托拉赫数(St)和佩克莱数(Pe)或沃姆斯莱数(Pe)和佩克莱数(Wo)为相似准则数。从微尺度下混合过程的机理出发，根据不同应用场合，提出了基于合成射流的主动式混合器和带挡板的被动式混合器。利用Navier-Stokes方程和组分扩散方程研究了主动式混合器中进口条件以及射流参数对混合过程的影响，采用了相似分析方法分析了被动式混合器中不同槽道结构以及进口条件下的混合特征，研究表明，非对称的结构有利于混合的发生，被动式混合器中雷诺数表征了混合的发展状况。通过实验手段研究了不同槽道形状的被动式混合器中混合过程，利用流场显示方法验证了被动式混合器中混合效果随雷诺数的发展过程以及挡板高度对混合过程的影响的理论分析。更多还原

【Abstract】 As one of the main branches of MEMS, micro flow systems have achieved a great development and wide application. However, there are many unsolved academic problems and challenges in the flow and transport processes of the micro fluidic systems. Focusing on the microscale flow, heat and mass transfer, in the present dissertation, the flow characteristics and thrust performance of the micronozzles, mixing enhancement technique for micro channel flows were studied in detail. Based on analytically and numerically results, micronozzles and micromixers were designed and fabricated. Their performances were evaluated numerically and experimentally.Numerical methods based on the continuum hypothesis and molecule kinetic theory are performed to investigate the gaseous flow in the microchannels. The validity of different boundary conditions based on continuum model is examined by DSMC method under different Knudsen number. It is found that the impact of rarefied and compressibility effects are different.DSMC method and Navier-Stokes equation (with no-slip and slip conditions) are applied to study the effects of configuration, operating parameters and temperature boundary conditions on flow characteristics and thrust performance of micronozzles in detail. The optimization proposals for micronozzle design are put forward based on the numerical data and analytical results. Numerical results show that the rarefied effects and non-equilibrium effects are strengthened due to larger Mach number in the divergent section of micronozzle. The Reynolds number is the key parameter for the micronozzle performance. To improve the propulsion efficiency, the appropriate Reynolds number could be obtained by either regulating the operating pressure or nozzle size. Special impulse is increased by the higher propellant temperature, which results in the loss of thrust efficiency.Due to the laminar nature of the flow, the mixing rate is strongly diffusion limited and long mixing channel or time may be required to attain full mixing. As a result, the rapid mixing is difficult to achieve in these micro channels. Strategies to increase stream-stream mixing can be broken into two main categories: active and passive. For passive mixing, the Reynolds number and Schmidt number are the similarity parameters. On the other hand, Reynolds number, Strouhal number, peclet number or Womersley number and Peclet number are the similarity parameters for active mixing.For different application purposes, active mixer based on synthetic jet and passive mixer with baffles at the inlet of confluent channel are proposed. To investigate the effect of the inlet velocity and the jet parameters on the mixing efficiency, Navier-Stokes equations with species diffusion equation are solved to study the mixing process in the active mixer. The similarity theory is applied to study the mixing performance of the passive mixers with different channel configuration and inlet conditions. It is found that active mixer with asymmetric structure has better mixing effectiveness; For passive mixing, as the baffle height increased, the mixing effectiveness improved significantly, while the pressure loss due to extra baffles increased a lot accordingly. The mixing performance improvement and pressure loss depended strongly on the operating Reynolds number. The mixing performances of passive mixer are also experimentally studied by the flow-field visualization. The results validate the numerical data.更多还原