Experimental and theoretical investigation of reflux condensation phenomena in a vertical U-tube

Abstract

Experimental and theoretical investigations of the reflux condensation phenomenon in the steam generator U-tubes, which is an important heat removal mechanism in a certain phase of small break loss-of-coolant accident (LOCA) in nuclear power plants, have been carried out. The flow pattern and the mode of the reflux condensation are the most important factors in estimation of decay heat removal capability of the steam generators during a small break LOCA. In the present work, a series of experiments have been performed to establish the basic modes of the reflux condensation. Basically, three different modes have been identified which agrees with the previous findings by others. The three modes are (1) the filmwise reflux condensation mode, (2) the complete reflux condensation mode and (3) the two-phase thermosyphoning mode. The experimental results of the filmwise reflux condensation mode are compared with the modified Nusselt condensation theory. Major experimental data obtained to compare with the theoretical predictions were (1) the filmwise reflux condensation length(L) for a given steam flow rate($m_g$) and the surface temperature of the tube wall($T_w$) where the filmwise reflux condensation was taking place. To examine the effect of the cooling water temperature on the reflux condensation length, three series of tests were conducted at three different temperature levels. Results showed that the Nusselt condensation theory slightly overestimated the filmwise reflux condensation length. In order to investigate and to establish the transition criteria between the three different reflux condensation modes another series of experiments were carried out while changing the inlet steam flow rate. The transition point from the filmwise reflux condensation mode to the complete reflux condensation mode coincided with the flooding point. The two-phase thermosyphoning mode, on the other hand, is largely determined by the total heights of water column and two-phase col...