Numerical Research On Underwater Gas-liquid Two-phase Jet Propulsion

With the development of structural optimization design and drag reduction technology, the velocity of ship increase considerably. The traditional underwater propulsion devices aren't suitable for the high speed environment due to liquid cavitation, which is the challenge to the underwater propulsion technology. The underwater gas-liquid two-phase jet propulsion has attracted extensive attention in recent years, for the potential to provide higher thrust and efficiency under high speed environment.The underwater gas-liquid two-phase jet propulsion can be classified into bubbly water ramjet and mist jet propulsion according to the working medium and principle. Bubbly nozzle flows is the most important aspect of the bubbly water ramjet inner flow, and also the key for performance analysis of the engine. In this paper, numerical simulation of bubbly nozzle flows and performance analysis for bubbly water ramjet are carried out at first. Subsequently, the mist nozzle flows and the performance of mist jet propulsion are discussed. In the end, the merits and drawbacks of bubbly water ramjet and mist jet propulsion are presented. The main contents are as follows:Based on homogeneous model, bubbly nozzle flows are investigated by steady and unsteady numerical simulation. Rayleigh-Plesset equation is included in the model to describe the variation of bubble size. Influences of initial bubble radius, inlet velocity and void fraction on the flow field are obtained. Critical bubbly nozzle flows are investigated by unsteady numerical simulation and influences of initial bubble radius, inlet velocity and void fraction on the flow field are obtained.Based on two-fluid model, bubbly nozzle flows are investigated by steady numerical simulation. Heat transport equation and bubble dynamic equation (modified Rayleigh-Plesset equation) are included in the model. Comparison of numerical results with experimental data validates the rationality of mathematical model and numerical method. Influences of inlet velocity and temperature difference between gas and liquid on the flow filed are analysed, and the results show minor influences.The mathematical model for bubbly water ramjet engine is constructed by separately analyzing the physical processes occurring in each part of the engine. Influences of inlet area, the ratio of mixing chamber area to inlet area, air mass flow rate, vessel velocity, initial bubble radius, and nozzle parameters on design performance of the engine are emphatically investigated. These analytical and numerical results are useful for optimal design of bubbly water ramjet engine. The off-design performance is analyzed, and results indicate that the bubbly water ramjet engine can work normally under off-design working environment.Based on two-fluid model, mist nozzle flows are investigated by steady numerical simulation. Heat and momentum transport equations are included in the model. Subsonic flows within converging nozzle and supersonic flows within converging-diverging nozzle are simulated separately, and influences of inlet void fraction and droplet diameter on the flow filed are obtained respectively.The mathematical model for mist jet propulsion engine is constructed by separately analyzing the physical processes occurring in each part of the engine. The design performance of the engine is analyzed based on the model. Influences of vessel velocity, inlet pressure, droplet diameter, air mass flow rate, water mass flow rate, and nozzle parameters on the engine design performance are obtained. These analytical and numerical results are useful for optimal design of mist jet propulsion engine. The off-design performance is analyzed, and results indicate that the mist jet propulsion engine can produce higher thrust and efficiency under off-design working environment.