Experimental characterization of boiling two-phase flow structures under BWR core operating conditions

Abstract

Local two-phase flow parameters have been measured at the Westinghouse FRIGG facility in a test fuel bundle under Boiling Water Reactor (BWR) core operating conditions, over a wide range of flow and power (up to critical power). The measurements were performed at end of heated length using sapphire optical probes and Pitot tubes placed in various radial positions of the bundle. By using both instruments simultaneously, the void fraction, mixture velocity, dispersed phase (bubble or drop) characteristic lengths and volumetric interfacial area can be measured. In addition, the identification of two-phase flow regimes can be attempted. The measurements are of particular interest since there is very limited available data in the literature at high pressure relevant to BWR operation.Experimental measurements are reported in this paper covering a range of inlet flow and bundle power relevant to BWR operation, at a system pressure of 70 bar. The dynamic void fluctuations are used to establish two-phase flow regime maps, one based on global parameters (for 1-D applications) and the second based on local parameters (for CFD applications). However, it is discovered that transition flow regimes typically observed at low pressure (cap, slug, etc) are not similarly observed at high pressure. In addition, the measured characteristic lengths of the dispersed phase are reported in bubbly and annular two-phase flow regimes. In view of the results, recommendations are provided for further experimental work and modeling improvements.