Development of Heat Transfer Model for Horizontal U-Shaped Heat Exchanger Submerged in Pool

Abstract

A horizontal U-shaped heat exchanger submerged in a large pool has been developed as a key equipment of passive safety systems such as PAFS, PCCS, and ECS. The reliable prediction of the heat exchanger heat transfer performance has been an important issue for the optimum design of the heat exchanger and the safety analysis of the nuclear power plants installed with these passive safety systems. In order to obtain a reliable prediction of the local heat transfer coefficients at the inside/outside tube wall and the heat removal performance of the heat exchanger in the PAFS, this study performed the heat transfer analysis using MARS-KS 1.2 and developed the heat transfer model package for the horizontal U-shaped heat exchanger submerged in a pool. The heat transfer model package consisted of the horizontal in-tube condensation model and the natural convective nucleate boiling model on the horizontal U-shaped heat exchanger submerged in a pool. For the horizontal in-tube condensation model, this study assessed the predictive capability of the previous horizontal in-tube condensation heat transfer models for annular and stratified flows using various horizontal in-tube steam condensation experimental data. From the assessments of nineteen annular flow- and eleven stratified flow condensation models, it was found that the annular flow condensation model by Dobson and Chato (1998) and the stratified flow condensation model by Cavallini et al. (2006) were the most applicable models to the heat exchanger of the passive safety system. By replacing the models by Shah (1979) and Chato (1962) in the original MARS code with the models by Dobson-Chato (1998) and Cavallini et al. (2006), this study improved the predictive capability of MARS for the horizontal in-tube condensation heat transfer in the heat exchanger of the passive safety system. For the natural convective nucleate boiling model, this study first investigated the predictive capability of the previous nucleate boiling models for the horizontal U-shaped HX submerged in a pool using the PASCAL data. From the assessments of seven nucleate pool boiling and eight forced convective boiling models, it was found that, among previous nucleate boiling models, there was no model applicable to the horizontal U-shaped heat exchanger submerged in a pool. Thus, this study investigated the nucleate boiling heat transfer mechanism on the horizontal U-shaped heat exchanger submerged in a pool, taking into account the PASCAL experimental data, MARS simulations and literature survey comprehensively. Furthermore, this study developed the nucleate boiling heat transfer model on the horizontal U-shaped heat exchanger submerged in a pool. From the validation results of the proposed nucleate boiling model against the PASCAL and ATLAS-PAFS data, it was found that the proposed nucleate boiling model predicted the experimental heat transfer coefficients well on the upper and lower parts of the U-shaped tube within a deviation of ±19 %. For the natural convection model, this study proposed the natural convection heat transfer model on the horizontal U-shaped HX submerged in a pool based on the PASCAL data. The proposed natural convection model satisfactorily predicted the heat transfer coefficients of the PASCAL within a deviation of ±33 %. Finally, this study developed the natural convective nucleate boiling model on the horizontal U-shaped heat exchanger submerged in a pool by combining the proposed nucleate boiling model and the natural convection model. The proposed heat transfer model package on the horizontal U-shaped heat exchanger submerged in a pool was validated with the PASCAL, ATLAS-PAFS, and NOKO experimental data. The validation results revealed that the proposed model package could provide the improved prediction of the local heat transfer coefficients at the inside/outside tube wall and the heat removal performance of the heat exchanger in the passive safety systems, especially PAFS, compared to the default models in MARS-KS 1.2. It is expected that the proposed heat transfer model package on the horizontal U-shaped heat exchanger submerged in a pool is applied to the best-estimate thermal-hydraulic analysis codes and thus contributes to the reliable design and the safety analysis of the passive safety system with this type of heat exchanger.