| Water-jet propulsion has the advantages of strong cavitation resistance,low noise,good maneuverability,and the main engine is not easy to overload.It gradually replaces the propulsion to provide propulsion power for all kinds of high-performance ships under variable working conditions.Due to the complex environmental factors such as hull wave,sea wave and shallow draft of high-performance ship,the water-jet propulsion often draws air from the inlet of the inlet passage.As a hydraulic machinery,the water-jet propulsion is very sensitive to the gas in the working medium.Under the condition of gas content in the incoming flow,the internal flow pattern of the water-jet propulsion will change due to the influence of gas properties,which will easily cause the performance degradation of the water-jet propulsion,induce the insufficient thrust,and threaten the navigation safety of ships on the sea.Supported by the National Natural Science Foundation of China(NSFC)project "Research on unsteady flow and thrust characteristics of gas-liquid two-phase flow in and around a water-jet propulsion under cavitation and free surface suction conditions"(No.51879120),this paper studies the energy performance and thrust characteristics of an axial-flow water-jet propulsion under different speeds and gas-liquid two-phase media with different gas parameters The main research contents and results are as follows:1.The numerical model of single-phase flow in the whole flow field of water-jet propulsion is established.At the same time,the hydrostatic thrust performance test of water-jet propulsion is carried out.It is found that the experimental thrust value is in good agreement with the simulation thrust value at the design speed,which proves the accuracy of the simulation model of water-jet propulsion,The conclusions are as follows:(1)With the increase of the speed,the flow of the water-jet propulsion increases gradually,the thrust and the lift decrease gradually,and the efficiency increases first and then decreases.The distribution of pressure and velocity on the inlet surface of impeller is very uneven at different speeds.With the increase of speed,the distribution of high and low pressure areas on the inlet surface changes significantly,accompanied by asymmetric vortices.(2)Influenced by the geometric structure and speed of the inlet passage,the distribution of high and low pressure areas in the longitudinal section of the inlet passage is very obvious,and the pressure gradient changes greatly,resulting in a large number of v ortices in the inlet passage,which greatly affects the flow quality of the impeller inlet surface.(3)With the impeller rotating,the turbulent kinetic energy of the impeller blade changes obviously,and the vortex distribution in the longitudinal section of the inlet passage does not change significantly.2.Based on the Eulerian-Eulerian heterogeneous flow model and the BC profile function of CFX 2019R3 software,the data of inlet pressure field and velocity field of single-phase flow were derived as the inlet boundary conditions of gas-liquid two-phase flow.Gas phase was added to the simulation,and different inlet void fraction and bubble diameter were set to study the energy performance and flow characteristics of gas-liquid two-phase flow in water-jet propulsion,At the same time,compared with some single-phase flow results,the conclusions are as follows:(1)The existence of gas will lead to the decrease of thrust,head,shaft power and efficiency,which is related to the inlet void fraction and bubble diameter.(2)Compared with the pressure load on both sides of the blade in pure water flow,it is found that the pressure load on both sides of the blade is more affected by gas at high speed than at low speed;At different speeds,the volume fraction of gas on the suction surface is significantly larger than that on the pressure surface;In the axial section of the impeller,the gas-liquid two-phase velocity increases gradually from the hub to the flange.Affected by the change of speed,there is flow separation phenomenon in each channel of the middle section.The degree of flow separation in the gas-liquid two-phase flow field is greater than that in the pure water flow field at each speed,and the degree of flow separation increases with the increase of speed.(3)With the increase of inlet void fraction,the pressure difference on both sides of blade decreases gradually;The volume fraction of gas phase on the suction surface is obviously larger than that on the pressure surface,and the fluctuation range of gas phase velocity on the suction surface is larger than that on the pressure surface;When the void fraction is higher than 15%,the effect of inlet void fraction on the flow separation is not obvious.(4)With the increase of bubble diameter,the pressure difference between the two sides of the blade decreases gradually,and the work capacity of the blade decreases;The gas accumulation on the blade surface increases,and the gas distribution on the pressure surface tends to move towards the blade outlet;With the increase of flow separation in the axial section of impeller,the flow field becomes more unstable.(5)At low speed,the gas in the inlet channel is mainly accumulated in the lower elbow of the channel,and at high speed,it is mainly accumulated in the slope of the channel and near the upper elbow of the drive shaft.The flow nonuniformity in the inlet channel is positively correlated with the speed,inlet gas content and bubble diameter,and the flow nonuniformity at the outlet of the inlet channel is smaller than that at the inlet channel under different gas parameters.3.Through the transient numerical calculation,the relationship between the pressure fluctuation on the inlet and outlet surfaces of the impeller and the radial force of the impeller with different speed and gas phase parameters is studied,the conclusions are as follows:(1)In the case of single-phase flow and gas-liquid two-phase flow,the influence of ship speed on the pulsation of impeller inlet and outlet surfaces in time domain and frequency domain is similar.(2)With the increase of inlet void fraction,the oscillation frequency of time-domain curve of each monitoring point on inlet surface accelerates,the oscillation amplitude of monitoring point on hub decreases,and the amplitude of main frequency first increases and then decreases;The oscillation amplitude and dominant frequency amplitude of the middle monitoring point first increase and then decrease,while the oscillation amplitude and dominant frequency amplitude of the monitoring point at the flange decrease.The fluctuation regularity of time-domain curve of each monitoring point on the outflow surface is weakened,and the main frequency amplitude is reduced.(3)With the increase of bubble diameter,the oscillation frequency of time-domain curve of each monitoring point on the inlet surface does not change significantly,but the oscillation amplitude increases significantly.The main frequency amplitude of the monitoring point at the hub and middle position of the inlet surface first increases and then decreases,and the variation rule of the main frequency amplitude of the monitoring point at the rim is opposite;The time-domain curve of each monitoring point on the outflow surface is the same as that on the inflow surface,but the main frequency amplitude is different.(4)At the same speed,the radial force of two-phase flow impeller increases compared with that of pure water.The magnitude,pulsating waveform and amplitude of radial force of impeller are basically consistent with that of speed,inlet void fraction and bubble diameter.That is to say,with the increase of speed,inlet void fraction and bubble diameter,the radial force increases,the oscillation frequency and amplitude of radial force pulsating curve increase.4.ZGB cavitation model is used to study the influence of water-jet cavitation flow field and air content in the incoming flow on the cavitation flow characteristics of water-jet propulsion at different speeds,the conclusions are as follows:(1)In the case of pure water cavitation,with the increase of the inlet velocity ratio IVR,that is,the increase of the speed,the cavitation number and the flow field pressure of the water-jet propulsion,it shows that cavitation is less likely to occur,that is,the anti cavitation ability of the water-jet propulsion is stronger at high speed.(2)With the increase of inlet void fraction,the cavitation number and flow field pressure increase,which indicates that the increase of inlet void fraction is helpful to enhance the anti cavitation performance of water-jet propulsion to a certain extent;At the same time,the peak value and oscillation frequency of pressure fluctuation are less than that of pure water cavitation flow field.The addition of gas helps to slow down the instability of cavitation flow. |
