【作者】 孟宇；

【导师】 李志信；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2004， 硕士

【摘要】 随着微加工技术和MEMS技术的快速发展,当量直径在微米/毫米量级的微细、微小通道在现代工业中被广泛使用,微细/微小管内的对流换热成为当前重要的研究课题之一。由于微加工技术的限制,微细管的壁厚相对于管内径而言较大,常规尺度下被忽略的固壁轴向导热对管内对流换热的影响,在微尺度下需要给以充分的重视。本文采用实验研究和数值分析相结合的方法,研究了固壁导热对微细管内对流换热的影响。实验采用内径为924和952的厚壁不锈钢圆管作为实验管,其外内径之比分别为2.1和2.4。通过对不锈钢管进行直接通电加热产生均匀内热源的方式,实验研究了以去离子水为工质的管内不可压层流换热。当采用文献中常用的算术温差法整理数据求总传热系数时,结果与常规认识一致;当采用忽略固壁轴向导热影响的一维热阻模型整理实验数据时,扣除固壁导热热阻后获得的努谢尔数明显与常规尺度下的理论预测值不同。论文采用FLUENT通用软件对实验工况下的管内对流换热与固壁导热的耦合问题进行了数值模拟。表明管内换热已经进入充分发展阶段,用壁温与当地流体的截面平均温度之差定义换热系数获得的努谢尔数接近4.36, 与常规尺度下等热流边界时的努谢尔数相近。但用忽略管壁轴向导热的一维热阻模型整理数据时,努谢尔数明显大于4.36, 且并且随雷诺数的增大而增大。研究结果表明,管壁轴向导热的作用与大尺度下厚壁管的对流与导热耦合问题相同,采用忽略固壁轴向导热影响的一维热阻模型整理微细/微小管内对流换热可获得偏离常规尺度下理论预测的努谢尔数,取决于条件的不同,所得出的努谢尔数既可大于(固壁有均匀内热源)也可小于(外壁为等温或对流条件)常规尺度下的理论预测值,且随雷诺数的增大而增大,但不能称为微尺度对流换热的新现象。更多还原

【Abstract】 With the rapid development of micro-fabrication technique and MEMS technology, so micro/mini tubes with an equivalent hydraulic diameter of micron/millimeter are widely used in the current industry and study of convection in such tubes are becoming more and more important. Because of the limitation with the micro-fabrication technique, a micro/mini tube usually has a great wall thickness relative to the tube inner diameter. So the effects of axial heat conduction in the tube wall, which can be neglected for the normal-sized tube case, must be considered when studying the convective heat transfer in a micro/mini tube.The effects of heat conduction in tube wall on single-phase laminar convection of deionized water in a mini-tube having a comparatively great wall thickness were investigated experimentally and numerically in this research. The two tubes were used in the experiments. One had a 0.924 mm inner diameter and a 2.1 outer-to-inner diameter radio; the other had a 0.952 mm inner diameter and a 2.4 outer-to-inner diameter radio. In the tests both tubes were made of stainless steel. The tube was heated by applying a D.C. voltage directly to the two tube ends. The overall heat transfer coefficients (K) and Nusselt number (Nu) based on the arithmetic mean temperature difference between the fluid and wall were obtained. The results of K and Nu are consistent with that for normal-sized tubes. However, discrepancies exist between the Nusselt number based on the one-dimensional model that neglects the axial heat conduction and the standard value given by conventional theory, where the temperature difference between the wall temperature and local bulk fluid temperature is used to calculate the heat transfer coefficient.Numerical calculations for the conjugate problem of in-tube convection and tube wall conduction were conducted on the basis of the experimental measurements using the FLUENT code. The results showed that the flow were fully developed and the Nusselt number based on the classic theory was close to 4.36 while the Nusselt <WP=4>number based on one-dimensional model was greater than 4.36 and increasing with the Re increasing.The effects of tube wall heat conduction on convection in mini-tube were the same as that in a normal-sized thick wall tube. The Nusselt numbers obtained from the one-dimensional model based on the experimental measurements may be either greater the standard value given by the conventional theory (uniform heat source in the tube wall) or smaller than it (iso-thermal or iso-flux at the outer tube wall), depending on the conditions. This was not a fresh phenomenon in a micro/mini scale.更多还原