【作者】 洪剑平；

【导师】 金滔；

【作者基本信息】 浙江大学 ， 制冷及低温工程， 2008， 硕士

【摘要】 深冷处理技术作为一种有潜力的热处理工艺,已经在各行各业中得到较为广泛的应用,然而其对材料的作用机理问题还存在争议,需要进一步进行研究,而分析其中的传热问题是机理探索的基础之一。深冷处理通常以液氮为冷却介质,此时工件在液氮浴中的沸腾换热系数是一个非常重要的参数。目前沸腾换热系数求解方式有很多,包括经验公式法、集总参数法、给定热流法和反传热法等。与其它求解方式相比,反传热法种类多且求解精度较高,从而在沸腾换热系数的求解中采用反传热方法成为一种选择。本文首先介绍了课题背景及意义、沸腾换热经验公式及Westwater沸腾实验结果、反传热方法的研究进展,以及本课题的研究基础等背景。第二章基于反传热方法原理及考虑显式有限差分法的应用限制,编写了用于计算一维导热条件下沸腾换热系数的程序。考虑到程序需要调用时间间隔极小的温度数据,设计了一套快速采集温度的反传热测量装置,并对自制热电偶响应时间进行了估测。对不锈钢块不同朝向时各测点的温度变化情况进行了测量,并利用反传热程序对实验数据进行分析,以了解不锈钢块朝向对沸腾换热系数的影响情况。鉴于利用反传热方法得到的换热系数数据存在波动且存在个别不合理数据点,采取对换热系数数据进行分段拟合以及修正后再分段拟合等方法获得相关表达式。最后为验证上述换热系数的准确性,对两个不同测试对象的整体温度场分别进行了实验测量和采用不同方法所得换热系数的数值模拟,并将两者的结果进行了对比分析。结果表明,根据五种不同方法得到的换热系数的模拟结果与实验测量结果之间都存在一定的偏差,利用反传热计算并去除个别不合理数据点后的修正拟合所得换热系数更加贴近实际测量结果。更多还原

【Abstract】 As a promising heat treatment technique, cryogenic treatment is widely used in many fields, however, there is still controversy about the mechanisms for cryogenic treatment in material. Further investigation has its necessity and the analysis of heat transfer problems in cryogenic treatment is one of key points in mechanisms exploration. Cryogenic treatment usually uses liquid nitrogen as cooling medium, where the boiling heat transfer coefficient of workpiece in liquid nitrogen bath is an important parameter. Many methods have been developed to obtain the boiling heat transfer coefficient, including experiential equation method, lumped parameter method, inverse heat conduction method and so on. Comparing with other methods, inverse heat conduction method has the advantages of variety and high precision, so it can be considered as a choice in obtaining the boiling heat transfer coefficient.An introduction will be firstly made on the background and importance of research, the experiential formula for boiling heat transfer and Westwater’s results on boiling experiments, the advances in inverse heat conduction method, and our previous efforts on this research topic. Following the fundamentals of inverse heat conduction and the requirement of explicit finite difference approach, Chapter 2 will be mainly devoted on the compilation of a program for solving boiling heat transfer coefficient under one-dimension conduction condition. Considering that the program requires temperature data with small time intervals, an inverse-heat-conduction measuring system was designed for fast temperature acquisition and also for assessing the qualification of the thermocouples’ responding time. Making use of inverse heat conduction program and multi-directional experimental data of test points in stainless steel block, the block’s orientation was found to have no apparent influence on boiling heat transfer coefficient. The repeatability of heat transfer coefficient data is also fairly good. To obtain a mathematical expression for numerical simulation, data-fitting procedure was carried out to clear the fluctuation in heat transfer coefficient data. We also notice that there exist some unreasonable data in the calculation result of heat transfer coefficient from the inverse-heat-conduction program, a corrected fitting of the data (by eliminating the unreasonable data points from original calculated heat transfer coefficient data) is also suggested to improve its accordance to experimental results.Finally, in order to verify the accuracy of the heat transfer coefficient obtained from different methods, the comparison was made to the temperatures from experimental measurement and from numerical simulation (with those different heat transfer coefficients), respectively. The comparative results demonstrate that there always exits some discrepancy between simulating results by adopting the above heat transfer coefficients and measuring results from experiments. However, compared with the results with other heat transfer coefficients, those from the simulation with the heat transfer coefficient by corrected fitting shows best agreement with experimental results .更多还原