Wall superheat prediction in narrow rectangular channels under fully developed boiling of water at low pressures

Paper in proceeding, 2015

The modeling of two-phase heat transfer is a crucial issue in the safety analysis of nuclear reactors. The thermal-hydraulic correlations employed in this kind of simulations are usually derived from experiments that were carried out over limited ranges of parameters and for specific geometries. Therefore their applicability to systems with different characteristics has to be carefully scrutinized. In this paper, an assessment study of wall superheat correlations under fully developed boiling is presented. This is a contribution to the validation and improvement of the thermal-hydraulic modeling of the Jules Horowitz Reactor, which is a research reactor under construction at CEA-Cadarache (France). The SULTAN-JHR experiments are used. These tests were performed at CEA-Grenoble with upward water flow in a vertical uniformly heated narrow rectangular channel with gap of 2.16 mm. The experimental conditions ranged between 0.2 and 0.9 MPa for the pressure and between 0.5 and 4.4 MW/m2 for the heat flux. It is shown that the correlations of Thom and Jens-Lottes significantly overestimate the wall superheat. The correlation of Belhadj and Qiu, which were developed for narrow channels at low heat fluxes, cannot accurately predict the experimental data. On the other hand, satisfactory results can be obtained with Gorenflo (standard deviation of 11.9%) and a simplified version of the Forster-Greif (standard deviation of 10.1%) correlations. In conclusion, considering the validity range of the above correlations along with the outcomes of the current assessment, the simplified Forster-Greif correlation is thus recommended for the analysis of the JHR.