Research On Internal Flow Characteristics Of Francis Waterjet Under Turning Conditions

Waterjet is an important strategic equipment for the national defense industry and the national economy.It is widely used in high-speed ships,concealed submarines,and large and medium-sized transport ships.Affected by the structure of the water inlet channel and the disturbance of the drive shaft,the uneven flow of the water jet thruster will occur during operation,which will have a serious impact on the internal flow characteristics of the water jet propulsion pump.Especially under turning conditions,affected by the steering angle,the circumferential unevenness of the velocity and pressure distribution at the inlet surface of the pump is aggravated,and the flow state is further changed.Therefore,it is of great significance to study the inlet flow patterns of waterjet thrusters under different turning conditions and reveal the influence of inlet flow conditions on the internal flow and cavitation characteristics of the pump.This paper is funded by the National Natural Science Foundation of China project "Dynamic behavior and instability mechanism of mixed-flow pump rim leakage flow under near-stall conditions(51679111)",with a mixed-flow water jet as the research object,based on numerical calculation methods.The internal flow characteristics and cavitation characteristics of the Francis waterjet under different turning conditions are discussed.The main work content and research results are as follows:(1)A three-dimensional calculation model was established for the mixed-flow water-jet thruster as the research object.Based on the RNG k-ε turbulence model,steady calculation of the water-jet thruster under different turning conditions was carried out,and the performance and inflow state of the water-jet thruster under different turning conditions were analyzed.And quantitatively analyze the quality and quality of the inflow surface of the pump.The results of the research show that under turning conditions,the pre-rotation of the pump inlet surface is intensified,and the inlet flow quality is relatively low.Compared with the straight travel condition,the unevenness of the inlet flow surface is reduced,and the inlet flow quality is relatively better.Under the condition of right turning,as the turning angle increases,the head and thrust of the water-jet thruster gradually decrease,and at the same time the internal vortex of the inlet flow channel is deflected,which causes the speed and pressure distribution of the pump inlet surface to change,which affects the area of the pump inlet surface.Over-current capability and the location where backflow occurs.Under the condition of left turn,the head and thrust show an upward trend with the increase of the angle,and the inlet surface shows the opposite speed and pressure distribution of the right turn.(2)The vortex change mechanism of the inlet flow passage under different turning conditions is analyzed,and the relationship between the inlet flow state and the internal flow characteristics of the water jet propulsion pump under different turning and angles is established.The research results show that: under the condition of right turning,the vortex inside the inlet flow channel gradually deflects along the flow direction,and the vortex gradually evolves from counterclockwise rotation to clockwise rotation.Because the pre-rotation direction of the liquid flow inlet is the same as the impeller rotation,the impeller passes The flow capacity decreases,the flow separation phenomenon on the hub side is weakened,and the rim leakage vortex area is smaller.In the left-turn condition,the vortex evolution of the inlet flow path is opposite to that in the right-turn,the flow capacity of the impeller is enhanced,the flow separation phenomenon on the hub side is intensified,and the rim leakage vortex area is relatively large.(3)Based on the unsteady calculation of the internal flow of the Francis water jet under different turning conditions,the axial direction of the inlet flow passage,the pressure pulsation characteristics at the inlet and outlet of the impeller and the change trend of the radial force of the impeller under different turning angles are compared and analyzed.The research results show that the amplitude of the axial pressure pulsation of the inlet flow channel is reduced under the turning condition,the amplitude of the blade frequency is reduced,and the pulsation amplitude and the amplitude of the blade frequency are smaller when turning to the right.At the impeller inlet,the pressure pulsation amplitude decreases under the condition of right turning.From the wall side to the drive shaft,the main frequency is changed from the blade frequency to the shaft frequency.Under the condition of left turning,the pressure pulsation does not show regular changes,but the main frequency is always at Ye frequency office.At the same time,the change of the steering angle causes the pressure pulsation at the exit of the impeller to increase,and the flow becomes more unstable.Under turning conditions,the radial force of the impeller is reduced but the pulsation is strengthened.The main frequency of the radial force pulsation is at the blade frequency,and the amplitude of the main frequency also increases.Under the condition of left turning,the radial force of the impeller is smaller but the pulsation is more obvious.The main frequency amplitude is higher than that under right-turn conditions.(4)Based on the Zwart cavitation model,a numerical simulation of the internal cavitation characteristics of the waterjet at different steering angles was carried out.The cavitation distribution inside the impeller under different steering angles was studied,and the internal connection between the steering angle and the cavitation distribution was revealed.The research results show that the cavities on the blade surface are concentrated on the front edge of the back.Due to the influence of non-uniform inflow,the difference in the distribution of cavitation on the blade surface is obvious.Compared with the straight-line sailing condition,the cavitation inside the impeller is relatively reduced and the cavitation effect is weakened under the right-turn condition.In the left-turn condition,the internal cavitation of the impeller is relatively increased,and the cavitation is more severe.It is necessary to appropriately control the turning angle under the maneuver condition.