Experimental Investigations on the Effect of Pressure on CHF in a Uniformly Heated Vertical Tube at Low Flow Conditions

Abstract

The importance of investigating the Critical Heat Flux (CHF) situation at low flow and different pressure conditions has been of particular interest due to the situation related to the accident condition when predicting the nuclear reactor core behavior during the loss of the coolant accident (LOCA) situation. Therefore, a series of tests to determine the CHF in forced convective subcooled water up flow has been conducted in a uniformly heated vertical tube of 18 (mm) inner diameter and 3660 (mm) length at three different system pressures of 5, 69, and 138 (bars). The experiments were performed for four low mass fluxes, ranging from 0.027 (kg/ m² s) to 0.22 (kg/ m² s). The inlet water subcooling was varied up to 85 (K). The onset of the CHF was determined by the sudden rise of the wall temperature of the electrically heated tube. The results showed a clear dependence of CHF on mass flow rate and system pressure. The experimental results were compared with two acknowledged empirical correlations of different approaches and a CHF look up table. The Bowring (1972) correlation was found to be the most effective to correlate the experimental results with excellent similarity in trends and behavior, and with RMSE of 0.97%. A comparison with the Shim and Lee (2006) correlation yielded RMSE of 1.12%. When the CHF look up table of Groeneveld et al. (2007), was used, RMES of 1.44% was obtained.