EXPERIMENTAL STUDY OF UNDEREXPANDED SONIC AIR JETS IN WATER
水下欠膨胀高速气体射流的实验研究

The structure and mixing properties of underexpanded turbulent air jets in still water were experimentally investigated in this paper. The experimental system, which consists of compressor, gas tank, injector, water tank as well as pipe, regulator and mount, was built to simulate horizontal sonic jets submerged in water and to measure flow properties of the jets. In order to determine static pressure distribution in the nearfield of the underexpanded jet, two types of horizontally movable, static probes were developed and tested. Static pressures along the axis of jets were obtained for the ratios of static pressures at nozzle passage exit to ambient water being 1.0, 1.2, 1.4, 1.6, 1.8 and 2.0, were illustrated. Using gamma-ray absorption method, where the Gamma-Ray probe is vertically movable, mean void fraction distributions, for the ratios being 1.0 and l.4, were measured at 3 different profiles. Among them, as a reference, the ratio of 1.0 represents the full expansion case. The flow structure and mixing properties were analyzed through the distributions of static pressure and mean void fraction. The results are useful for the analysis of the flow in the external-expansion region near the passage exit. Static pressure measurements show that a shock-wave-containing external-expansion region exists for Underexpanded sonic air jets in water, which is similar to that of underexpanded air jets in air. Underexpanded air jets in water damp faster than underexpanded air jets in air due to different mixing condition. Mixing entrainment layer surrounding the external-expansion region for air jets in water involved a bubble region near the liquid side and a drop-containing region near the gas side.