Prediction of nucleate boiling heat transfer on horizontal U-shaped heat exchanger submerged in a pool of water using MARS code

In advanced nuclear power plants, a horizontal U-shaped heat exchanger submerged in a pool is under development as a key equipment of a passive safety system. For the successful design of the heat exchanger and the safety analysis of the nuclear power plant incorporating this passive safety system, the reliable prediction of the nucleate boiling heat transfer on the horizontal parts of the U-shaped tubes is one of the important factors. At present, the best estimate thermal hydraulic analysis codes such as RELAP5 and MARS are used to analyze the nucleate boiling heat transfer on the horizontal U-shaped heat exchanger submerged in a pool; however, it is still not known how to physically model the heat exchanger pool, and which correlations are suitable among the pool boiling and forced convective boiling correlations. To secure the applicable correlation for the heat exchanger, this study assessed 15 nucleate boiling correlations using MARS. To improve the prediction capability of the best estimate code, this study investigated the main heat transfer mechanisms on the horizontal U-shaped heat exchanger submerged in a pool, then proposed a prediction method, and finally developed a nucleate boiling model. From the validation of the proposed model against PAFS (passive auxiliary feedwater system)-related experimental data of PASCAL and ATLAS-PAFS, the proposed boiling model predicted the experimental heat transfer coefficients much better than the default nucleate boiling model by Chen (1966, Ind. Eng. Chem. Process Des. Dev. 5 (3), 329) in MARS. A total of 366 data points were predicted with a mean deviation of 8.1%. Hence, the proposed nucleate boiling model appears to be usable to the reliable design and the safety analysis of the passive safety system using the best estimate codes.