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

Sonic point position and critical pressure ratio in non-uniform microchannels

Non-uniform microchannels are used in the micro space propulsions and micro gas turbine generators. The performances of these applications are determined primarily by the performance of the microchannel used. Based on the micro-fabrication technology, three-dimensional rectangular cross-section straight-convergent-divergent-straight (SCDS) microchannel was fabricated. Its width in the throat cross-section was 16μm, the depth was 20μm and the contraction ratio was 1.625∶1 For different inlet-to-outlet pressure ratios, the volume flow rate characteristics of nitrogen were measured experimentally. The pressure ratios ranged from 1.0 to 4.0, outlet volume flow rate ranged from 0 to 0.12 mL/s and outlet Reynolds number was less than 350 The gas flow characteristics were analyzed by numerical simulation based on finite-volume method. Good agreements between computational results and experimental data were found. The simulation results indicate that two novel behaviors are different from that at conventional scales. One is the first sonic point position, the other is the critical pressure ratio（the critical pressure ratio is defined as the local-to-inlet pressure ratio , here the local pressure corresponding to the pressure at the first sonic point in the internal flow of SCDS microchannel）. These two novel behaviors should be attributed to the surface effects. Due to the larger surface-to-volume ratio (S/V) for microchannels, the factors related to the surface effects have more impact on the microflow. So the relationships between novel behaviors and the value of S/V are discussed further.