Experimental study on boiling heat transfer in a three-rod bundle at near-critical pressure

Heat transfer experiments of flow boiling heat transfer in a three-rod bundle with spacer grids are performed at Shanghai Jiao Tong University. The test section consists of three heated rods and three unheated rod-like elements with an outer diameter of 15 mm and a pitch to diameter ratio of 1.2. The effective heated length is 1960 mm. The test conditions are as follows: pressure ranging from 20 to 21.5 MPa, mass flux varying from 800 to 1600 kg/(m(2).s), heat flux between 400 and 700 kW/m(2) and inlet quality ranging from -0.49 to 0.2. Circumferentially non-uniformed wall temperature distributions are observed and analyzed respectively in both subcooled and saturated boiling regions. Parametric effects of mass flux, heat flux and system pressure on heat transfer are discussed in detail. The heat transfer coefficient increases with the rise of mass flux and system pressure. However, due to the occurrence of DNB, the heat transfer coefficient decreases with the increase of heat flux, which is opposite to the nucleate boiling heat transfer at low pressure. Several correlations developed for the film boiling are selected for assessment against the experimental data. In the subcooled boiling, the Yagov and Dedov correlation gives the best prediction accuracy. In the saturated boiling, the prediction of Collier correlation agrees favorably with the test data. (C) 2019 Elsevier Ltd. All rights reserved.