Investigations On Working Performance Of Two-Phase Underwater Ramjet

As a crucial part of marine crafts,propulsion system is always a hot research area in maritime industry.This paper presents both theoretical anlysis and numerical simulation of the two-phase underwater ramjet,and the working process and performance characteristic of this engine were investigated.First,by analyzing the engine's working process,an ideal power cycle model for a two-phase underwater ramjet was established.Four performance parameters were defined to evaluate overall performance of the two-phase underwater ramjet systems: transmission efficiency,propulsive efficiency,overall efficiency and specific impulse.Subsequently,the influences of cruise velocity,air/water ratio and cruising depth on the theoretical performance of the two-phase underwater ramjet were discussed.The results indicate that one of the most outstanding advantages of two-phase underwater ramjet is its high proplusive efficiency with the order of 50%-95%.As a result,the overall efficiency magnitude are as high as 30% at cruise speed of 100m/s.Furthermore,regarding rules of specific impulse versus cruise velocity under a certain air/water ratio,the occurrence of peak specific impulse of order 400 s is observed.Like other underwater engine with open system,increasing back pressure will cause serious performance drop,which means that the two-phase underwater ramjet is only efficient for less-than 10 m shallow depths.Secondly,unlike giving a fixed initial bubble radius in previous studies,the more advanced numerical model,which can calculate the initial bubble radius on the basis of the flow field parameters was developed by considering the process of bubble formation.Then,the accuracy of the present model was verified by comparing experimental data with numerical results.Subsequently,the performance of the two-phase underwater ramjet was predicted through present model.The results indicate that the initial bubble size has a strong dependence on the operating condition,such as vessel velocity and gas mass flow rate.During the bubble formation process,the influence of gravity and buoyancy was negligible.Moreover,with other condition fixed,there is an optimum vessel velocity for the ramjet,in which maximum thrust is generated.And a smaller orifice diameter always promotes the engine performance,while this promotion is negligible when orifice diameter is smaller than 1 millimeter.Thirdly,the performance of a typical two-phase underwater ramjet and its inner flow field were investigated by the application of computational fluid dynamics method and the effect of cavitation was taken into account.The research shows that the injected air forms a gaseous annulus along the inner wall of mixing chamber and nozzle,and there is an obvious interface between gas and water,which means that they are not well mixed.When the engine is cruising at a relatively high speed,cavitation will accur occur at the inlet of the engine and cause serious total pressure loss,which leads to flow separation at the middle of the diffuser and serious drop in engine performance.With the increasing of mass flow rate of the injected air,the mixing chamber pressure increases noticeably and the cavitation disappers.Moreover,the thrust and specific impulse can be promoted by increasing the temperature,but the overall efficiency will decrease dramatically.