Study On Boiling Heat Transfer Coefficient In Liquid Nitrogen Bath By Inverse Heat Conduction Method

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 .