A Preliminary Examination of the Effects of an Orifice on the counter-current Flooding Limit in Vertical Tubes

Abstract

In CANDU reactors, during a postulated loss of coolant accident (LOCA) , the emergency core cooling (ECC) water coming from the inlet and outlet headers enters the fuel channels through feeder pipes. These pipes consist of vertical and horizontal runs; in some feeders, orifices and/or venturi flow meters are installed for flow adjustments and measurements. The steam coming from the fuel channels may flow in the direction opposite to that of the water therefore creating a vertical or horizontal counter-current two-phase flow. Under these conditions, the rate at which emergency water can enter the fuel channels is limited by the flooding phenomena. At the flooding point, the liquid is partly entrained in the same direction as the steam flour. This point is greatly affected by the geometry of the feeder pipes, shape and number of fittings, flow area restrictions and the way the feeder pipe is connected to the header. With the above in mind the overall objectives of this research project are to gain greater insight into the flooding mechanism in pipes containing flow area restrictions representative of some of the feeders in a CANDU nuclear reactor and to aid in the improvements of' the models for countercurrent flow and flooding as they are applied to CANDU feeder pipes under refilling conditions. This paper describes the test facility recently constructed at the Institut de Genie Energetique for the study of counter-current two-phase flows. Experiment al data on the counter-current flooding limit is presented for a 3/4" I.D. tube both with and without an orifice. It is of great importance to first examine the CCF behaviour and the CCFL for a given test facility due to the fact that it is widely recognized that the particular entrance and exit geometry of the test facility greatly affects the point at which flooding occurs. For this reason the CCFL is determined both with and without the orifice (B == 0.83) in the test section for a wide range of inlet liquid flow rates using two different water delivery methods: injection through a porous walled tube and liquid overflow. The results for the CCFL both with and without the orifice in the test section are then compared to some of the most commonly used flooding correlations seen in the open literature.