Experimental study of water subcooling effect on steam-water flooding in a large-diameter vertical tube
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An experimental study is being conducted at Texas A&M University to investigate countercurrent flow and "flooding"phenomena in light water reactor systems, in order to improve the safety of current and future reactors. In a countercurrent, two-phase flow system, flooding can be defined as the onset of flow reversal of the liquid component which results in co-current flow. To better understand the occurrence of flooding in the surge line geometry of a PWR, experiments have been conducted in a 76.2 mm (3-inch) diameter tube with subcooled water and slightly superheated steam as the working fluids at atmospheric pressure. The authors have previously reported the conditions for flooding with water flow rates that vary from 0.00022 to 0.00076 m3/s (3.5 to 12 GPM) and a water inlet temperature of about 70C (158F). The steam flow rate needed to achieve flooding decreased with water flow rate up to some transition water flow rate. Above this water flow rate, the required steam flow rate increased with water flow rate. For certain combinations of liquid and steam flow rates, flooding was not observed until the local water temperature reached the saturation temperature. Once saturation was achieved, the steam no longer condensed and could transfer enough momentum to carry the liquid upward. This paper focuses on the effect of water subcooling on flooding conditions. Data for a range of water inlet subcooling are presented. The degree of subcooling in these tests was observed to affect the steam flow rate needed for flooding by affecting the amount of phase change heat transfer. The effect of subcooling on phase change and on energy balances within the test section are explained from a phenomenological perspective and implications for reactor safety analysis are discussed.
