Study On Heat Transfer Mechanism And Calculation Method Of Passive Residual Heat Removal Heat Exchanger

Passive Residual Heat Removal Heat Exchanger (PRHR HX) is a critical equipment of the 3 rd generation AP set nuclear power plant. The heat transfer mechanism of PRHR HX is very complex, which is different from the common heat exchanger. At present, there are few published literatures of the heat transfer mechanism of PRHR HX. The literatures about the heat transfer calculation method are rarer.Supported by the’Heat Transfer Analysis and Design Research of the CAP 1400 Passive Residual Heat Removal Heat Exchanger’project, a series study was conducted to understand the heat transfer mechanism of the PRHR HX. The experimental apparatus was set up to simulate the heat exchanger progress of the PRHR HX. The mechanism of the heat transfer transition region between the saturation boiling region and the natural convection region was investigated. Then the heat sink three section heat transfer model was proposed. The experiment and PRHR HX heat transfer progress was simulated using the FLUENT software. The fluid temperature and velocity distribution was obtained and the reason of temperature stratification in tank was analyzed. Based on this, the PRHR HX heat transfer calculation method was developed and the PRHR HX heat transfer calculation software was then compiled. The main conclusions are as follows:(1) The natural circulation convection caused by the temperature difference and height difference between the primary loop of the nuclear power plant and the PRHR HX is the main heat transfer pattern in the heat exchange tube. In the tank, the heat transfer is mainly relied on the nucleate pool boiling and the natural convection.(2) The natural convection and boiling experiment in tank was conducted through the self-built experimental equipment. During the heat transfer process, the fluid temperature in the upper part of the water tank increased firstly. An obvious temperature stratification phenomenon appeared in the axial direction of the tank. In the radial direction, the fluid mixed uniformly, so the temperature difference was small. This phenomenon appeared in both natural convection and boiling experiment in tank.The concept of heat transfer transition section was put forward and the transition region between the saturation boiling and natural convection region was experimentally found. According to the experiment, the heat transfer mechanism of the heat transfer transition region was subcooled boiling. Based on these research works, the three section heat transfer model in the heat sink was proposed. Along with the time, natural convection, subcooled boiling and saturated boiling would appear in tank according to proper order and the three heat transfer patterns would coexist at some time. The correctness of the three section model was verified by the experiment.The heat transfer calculation correlation for the transition section was then revised based on the experimental data. The new heat transfer correlation could be used to calculate the heat flux in the transition region. This correlation successfully solved the discontinuity of heat transfer parameters on the interface between the saturation boiling region and natural convection region.The applicability of the different heat transfer empirical correlation was also investigated. The best-estimate fit correlation for tube inside convection was Dittus-Boelter correlation. For the tube outside heat transfer, McAdams correlations and Rohsenow correlation provided a better applicability for natural convection and saturated boiling.(3) The heat transfer process of the experimental model and the full scale model was numerical simulated. The temperature distribution and the flow pattern of the tank fluid were obtained. The temperature stratification was caused by the natural flow in the tank according to the simulation results. The vapor volume fraction distribution was also obtained. The numerical simulation provided an easier method for the heat transfer study of PRHR HX.(4) The heat transfer calculation method of the PRHR HX was then developed and the correctness of it was confirmed using the data in the published literature. Based on the calculation method, the PRHR HX heat transfer calculation software was compiled. It could be used for the heat transfer design and checking calculation. The software had been applied in the nuclear industry.