Numerical simulation on critical heat flux of downward-facing surface with modified wall boiling model

Abstract

In this work, a modified wall boiling model was developed based on the prevailing Rensselaer Polytechnic Institute (RPI) wall boiling model, the wall heat flux was divided into four parts: sliding heat flux, evaporative heat flux, convective heat flux, and quenching heat flux. Based on the modified model, numerical simulations of the critical heat flux (CHF) of the downward-facing surface with different dimensions and scales were carried out. The CHFs at five angles of 30°, 45°, 60°, 75°, and 85° toward the heating surface under the 2-D model were calculated. Compared with the experimental data, the deviations are all within ±20%, which is better than the RPI model. In the wall flux distribution, the sliding heat flux at the CHF position accounts for more than 30%, and the proportion increases with the increase of angle, which indicates that bubble sliding has a great influence on the downward-facing surface. The influences of dimension and scale were analyzed based on the 1:1, 1:5, and 1:10 3-D numerical simulations. Compared with the 2-D model, the outlet area of the 3-D model is significantly increased, resulting in a 42% decrease in the maximum velocity in the 3-D flow channel under the same inlet velocity. Due to the decrease in flowability, the CHF of 3-D is lower than 2-D under the same condition. By comparing the results of the 1:1, 1:5, and 1:10 models, it can be found the trends of void fraction and wall temperature are similar. With the decrease of scale, the velocity at the critical point increases gradually, and the CHF also increases gradually.