Effect Of Nozzle Aeration On Performance Of Water-Jet Propulsion With A Screw Mixed-Flow Pump

Water jet propulsion device is the most widely used propulsion device in modern ships.It generates thrust by ejecting accelerating water flow in the opposite dir ection to the movement.Spiral mixed-flow water jet propulsion pump is the core power component of water jet propulsion.Two-phase flow propulsion is a process of adding gas jet into water to make the gas jet carry out physical processes such as mixing diffusion,fluid enrolling and momentum transport in the water flow,so as to improve the velocity of the water flow and thus the thrust.In this paper,a spiral mixed-flow water jet propulsion pump was designed according to the underwater vehicle model,and underwater gas jet was mixed into the nozzle of the spiral mixed-flow pump.Numerical calculations were carried out with CFX under different aeration positions and different aeration rates to find the aeration positions and aeration rates of the spiral mixed-flow pump with better propulsion performance.Specific research contents and conclusions are as follows:(1)The influence of aeration position on the performance of the helical mixed-flow pump was studied from the aspects of aeration length and aeratio n depth while the aeration volume remained unchanged.With the aeration depth unchanged,three aeration lengths were set respectively in the initial and main jet sections.The thrust of the helical mixed-flow pump increased monotonically with the increase of aeration length as a negative exponential function,the head decreased first and then increased as a quadratic function,and the efficiency increased monotonically as a negative exponential function.The aeration length remains unchanged and two aeration depths are set.When the jet is in the center of the nozzle,the axial thrust,propulsion efficiency and head are maximum.At different aeration positions,with the increase of flow,thrust increases,head decreases,and efficiency increases.After aeration of the nozzle,the water velocity increases significantly,and the velocity distribution at the outlet is uneven,which leads to uneven pressure distribution,changes in outlet vorticity distribution,and unstable flow.(2)Keeping the aeration position unchanged,six types of aeration were selected to study the propulsion performance of the helical mixed-flow pump under different working conditions.With the increase of aeration,the axial thrust presented a monotone increase of a primary function,the head of the helical mixed-flow pump presented a quadratic function,which first decreased and then increased,and the propulsion efficiency presented a negative exponential function.With the increase of flow,thrust increases,head decreases,and efficie ncy increases.With the increase of air content,the flow velocity increases significantly,the nozzle outlet pressure decreases,the high vorticity area becomes larger,and the flow becomes more unstable.(3)The transient characteristics of high-speed gas jet aerated in spiral mixed-flow pump nozzle are studied,and the phenomenon of gas jet return is emphatically analyzed.Due to the shear instability of gas jet,gas return will occur intermittently,and the gas returns in the nozzle,which is easy to ca use cavitation erosion to materials.