Experimental investigation on heat transfer to supercritical water in a three-rod bundle with spacer grids

Abstract

Accurate prediction of the heat transfer performance in tight rod bundles is important for the design and safety analysis of supercritical water-cooled reactors (SCWRs). However, the error of the current correlations for predicting supercritical heat transfer coefficient in rod bundle and grid spacer is large under certain conditions. In the present paper, an experimental study of heat transfer to supercritical water in a three-rod bundle with spacer grids is carried out at Shanghai Jiao Tong University. The rod bundle is composed of three heated rods and three unheated rod-like fillers with an outer diameter of 15 mm and a pitch-to-diameter ratio of 1.2. Honeycomb grid spacers with a blockage ratio of 0.158 are equipped in the bundle. The system pressure is 23 MPa and the mass flux ranges from 800 to 1600 kg/(m2·s). Moreover, the heat flux varies from 400 to 700 kW/m2 and the fluid temperature is in the range of 285–382 °C. Wall-temperature gradients are observed along the circumference direction of heated rods. The wall temperature is higher in the gap area between heated rods and it is lower in the cold-wall area. When approaching the pseudo-critical point, the non-uniformity of the wall temperature is significantly reduced. Six existing correlations for fully-developed heat transfer at supercritical pressure are assessed against the experimental data, among which the Bishop correlation gives the best predictions. A new correlation is developed based on the experimental data and most of them are predicted within ±15% with a mean absolute deviation of 6.0% and a root-mean-square deviation of 7.3%. The enhancement ratios of heat transfer downstream of spacers are substantially under-predicted by the commonly used correlations. Based on the observation that the maximum enhancement ratio increases linearly with the Reynolds number in pseudo-liquid regions, a new correlation is proposed to predict the heat transfer augmentation downstream of spacers, correlating most of the data within ±20% with a mean absolute deviation of 6.2% and a root-mean-square deviation of 8.4%.