Development of a correlation for mixed convection heat transfer in rod bundles

A correlation is developed to predict mixed convection heat transfer of water flow in rod bundles. The rod bundles are 5 x 5 and the Re number ranges from 1 x 10(3) to 3 x 10(4) in the experiment. In the forced convection region, the Dittus-Boelter correlation predicts well with the experimental data. However, in the mixed convection region, the correlations reported in the literature are all deviated from the measured data. Moreover, based on the classical form of buoyancy parameter, Gr*/Re-3.425/Pr-0.8, the experimental data of circular channels extracted from literature are compared with the measured data. The comparison shows that the buoyancy parameters of the onset point of heat transfer impairment (HTI), turning point of HTI and onset point of heat transfer enhancement (HTE) in rod bundles are very close to that in circular channels, but the maximum degree of HTI in bundles is significantly weaker than that in circular channels. The experimental data indicate that the circumferential heat transfer is significantly uneven in HTI region, which result in the differences of buoyancy parameter of turning point in different circumferential directions and eventually lead to the weaker overall degree of HTI in bundles. Considering the derivation method of the classical form of buoyancy parameter proposed by Rouai, a new buoyancy parameter form for rod bundle channels is derived based on the Dittus-Boelter correlation for forced convection heat transfer and the Cheng and Todreas's correlation for friction factors in rod bundles. Based on the new buoyancy parameter form, a new correlation to predict mixed convection heat transfer in rod bundles is proposed and provides good prediction accuracy. (C) 2021 Elsevier Ltd. All rights reserved.