Design And Flow Field Analysis Of The Screw Waterjet Propulsion

With the rapid development of waterjet propulsion technology,waterjet propeller has been widely used in new types of marine system because of its compact structure,low vibration and noise,and wide range of working conditions.However,under the small flow conditions,due to the backflow and cavitation caused by the instability of the flow,the performance of the propeller is reduced and adverse effects such as vibration and noise are generated.In response to this shortcoming,hydraulic components have been designed for flow-through components of the new screwwaterjet propeller that can satisfy both the hydraulic performance and the cavitation characteristics.And the systematic analysis of the energy characteristics and internal flow characteristics of the waterjet pump model,the influent flow channel model,and the overall model of the waterjet propeller was performed by using computational fluid dynamics method.The main research work and achievements of this paper are as follows:1.Summarize the research status of waterjet propulsion and propose the main research content of this paper.This paper introduced the design method of blade used in waterjet pump and selected the streamline method to design the impeller and guide vane of the new screwwaterjet pump.In order to solve the problems such as the pre-rotation phenomenon at the inlet of the impeller of the pump and the reduction of the hydraulic performance caused by the excessively large blade inlet distortion under the small flow conditions,the blade angle at the inlet and outlet of the impeller was modified.Parametric design method is used to carry out preliminary design of the inlet flow channel.2.Based on the theory of computational fluid dynamics,the calculation process is described in detail in terms of basic control equations,discrete methods of control equations,establishment of three-dimensional models,meshing,calculation of mesh independence,and setting of boundary conditions.The external characteristic curve obtained by the steady flow calculation of the whole flow field of the waterjet pump model agrees well with the experimental data,which verifies the accuracy of the numerical simulation.The flow characteristics of the waterjet pump under different flow conditions were analyzed.The results show that there were severe pressure changes at the inlet and outlet sides of the blade.The external characteristics and internal flow characteristics of the water jet propulsion pump under different parameters were analyzed.The results show that an appropriate increase in the rim airfoil placement angle would widen the range of the high-efficiency zone,but an excessively high tipping angle would lead to a significant reduction in efficiency,demonstrating that the correction of blade angle is necessary;and choose proper number of blades and axial length can improve the cavitation performance of the water-jet pump.The observation and analysis of the inlet and outlet flow of the impeller of the propulsion pump have been carried out.The results show that the inlet and outlet backflow generated at a small flow rate has a negative impact on the internal flow of the impeller.3.The parametric design method was used to simulate the flat inlet flow channel model under different flow channel parameters.Through the analysis of flow channel performance under different geometric parameters,the law of flow channel performance changes with geometric parameters are found.Finally,the flow channel parameters with relatively uniform outflow,high flow path efficiency,and excellent resistance to cavitation are obtained.The inlet channel with different speed ratios are simulated and analyzed from the aspects of outflow quality,pressure distribution and vortex distribution,to provide references for finding out the best working conditions of inlet flow channel and improving the hydraulic efficiency.4.Perform steady-state calculations on the whole flow field of the waterjet propeller model and use the wall area method to simulate the thrust of the waterjet propeller model.From the aspects of the cross-section velocity and the distribution of the turbulent kinetic energy in the cross-section,the internal flow field of the waterjet propeller under different speeds and rotational speeds was simulated and evaluated.The results show that both the impeller inlet upstream transition section and the guide vane body have a certain improvement in the outflow quality;the main causes of the turbulent energy dissipation near the impeller inlet are:(1)vortices generated by the interaction between the inlet pre-rotation and the main flow;(2)the extension of the reverse vortex pair in the inlet channel;(3)the impact loss of the suction surface and the pressure surface.In this paper,the internal flow of waterjet propeller at different speed and rotational speed was simulated,and the vortex generation and development of inlet flow and impeller inlet were observed.The results show that although the simulation of the inlet channel model alone can predict the generation of vortices in the flow channel,the prediction of the vortex development process is not accurate enough;the vortex pair fusion produces a secondary flow with large turbulent kinetic energy,which is detrimental to the inflow characteristics in the inlet flow channel.