Preliminary Experiments With an Underexpanded Gas Jet Into Water

When a heat exchanger in a Fast Breeder Reactor cracks, highly pressurized water or steam escapes into the surrounding liquid sodium. A sodium-water reaction then occurs, forming disodium oxide (and hydrogen gas). It can cause secondary damages to the heat exchangers by the reaction heat and erosion corrosion. The released flow of steam from the cracks of the heat exchanger is an underexpanded jet because the ambient pressure outside the tubes is lower than the critical pressure. When the pressure of a jet released at high pressure cannot be reduced to the low pressure of the ambient fluid, the flow is said to be underexpanded. Because this expansion causes a reduction of pressure and the pressure is lower than the critical pressure, the velocity of the flow can reach supersonic speed. Several studies have examined the underexpansion of the gas-gas phase. However, there have been few studies on the underexpansion of gas-liquid two-phase flows. The flow characteristics of the gas-liquid two-phase flow differ from the gas-gas flow because breakups of the bubbles appear in the gas-liquid two-phase flow. Therefore, in this study qualitative measurement was carried out for the purpose of revealing the flow with the underexpanded gas jet injected into water. The gas jet distance L and the expansion angle θ were then obtained from averaged image of a high-speed camera. L and θ increased approximately linearly with increasing pressure. The entrainment velocity and the velocity of entrained water droplets into the gas jet were obtained by PIV. Images of unstable expansion near the jet nozzle were captured for the first time.