| The new type dishwasher pump is the core component of a new type sink dishwasher,which innovatively realizes the function of open cleaning with a pipeless structure.Due to the leakage flow between the compound impeller and the wall,the leakage vortices interfere with each other and block the flow path,leading to key technical problems such as large energy loss and low efficiency.Based on the research on the hydraulic performance of new type dishwasher pumps,this dissertation uses particle image testing(PIV),numerical simulation(CFD),and pressure fluctuation testing techniques to study the gap leakage vortex and its unsteady flow characteristics in the compound impeller and twin-tongued volute of the new type dishwasher pumps,and analyzes the effects of impeller geometric parameters on the evolution of the gap leakage vortex and energy loss characteristics.The generation,development,and shedding processes of gap leakage vortices are clarified,and the correlation characteristics between gap leakage vortices and pressure fluctuations are revealed.The research content and conclusions of this dissertation are as follows:(1)In response to the key technical issue of the inability to test the performance of new type dishwasher pump,a simplified hydraulic characteristic test bench was built to obtain the performance curve of the new type dishwasher pump.Using PIV technology,the average velocity field and vorticity field in the volute flow channel under different operating conditions and impeller speeds were compared and analyzed,and the effect of single and double channels on the flow field in the volute was studied.Taking an improved new type dishwasher pump as the research object,the RANS method was used for numerical simulation.The accuracy of the numerical simulation was verified by comparing the hydraulic characteristics and internal flow field obtained from the experiments.The results show that the performance curve of the new type dishwasher pump is consistent with the law of the centrifugal pump,and the flow rate condition will change the distribution of the flow field in the volute.Under the high flow rate condition,the velocity gradient in the volute flow passage increases,and a vortex is generated in the near wall region of the volute inner wall surface corresponding to the negative vorticity region in the flow passage.The rotational speed of the impeller does not change the law of velocity distribution and vorticity distribution in the volute flow passage,and the rate of change of the velocity and vorticity in the flow passage is basically linear with the rotational speed.The velocity distribution and vorticity distribution in the volute flow passage are determined by the total flow rate of the pump and are independent of the flow rate in a single flow passage.(2)Based on the flow characteristics,pressure fluctuation characteristics,and energy loss characteristics of the tip clearance leakage flow in the axial flow of a compound impeller were studied.The evolution mechanism of the tip clearance vortex in the axial flow of the impeller was identified using the Q criterion,and the correlation characteristics between the vortex distribution in different directions and the tip leakage vortex in the rotational coordinate system were analyzed.Based on the Proper Orthogonal Decomposition(POD)method,the correlation characteristics between tip leakage vortex and frequency are analyzed.The results show that the axial flow tip leakage vortex of a compound impeller includes two primary tip leakage vortices and a secondary tip leakage vortex.The primary tip leakage vortex is evolved from two corner vortices originating at different locations in the tip clearance.The generation of primary tip leakage vortices causes pressure fluctuations at the impeller inlet,and the shedding of primary tip leakage vortices is the main reason for local energy loss in the flow channel.The POD mode of the tip clearance vorticity field exhibits a positive and negative alternating distribution,and the trajectories of the two coherent structures are consistent with the position of the tip leakage vortex.The dominant frequency of the mode is blade frequency and a multiple of the blade frequency,which successfully proves that the coherent structure of the POD mode can reveal the evolution law of the tip leakage vortex.(3)A novel dynamic static interference mechanism between a compound impeller and a twin-tongued volute was studied based on the POD method,and the relationship between the leakage vortex in the volute and pressure pulsation was obtained.In response to the complex flow characteristics of the outlet of compound impeller and inlet of twin-tongued volute,indepth research was conducted on the energy loss characteristics in the inlet of volute channel and the influence of the tongue on dynamic and static interference.The results show that the rotor-stator interference between the outlet of the compound impeller and the volute causes leakage flow,and the leakage vortex exhibits periodic oscillations at both the impeller outlet and the volute inlet.The energy loss in the tongue region is the largest,and under the interference of the tongue,there are backflow and secondary flow in the impeller flow passage.The gap leakage vorticity in the volute is mainly contributed by the circumferential and axial vortices.The trajectories of the two coherent structures of the POD mode at the inlet of the volute are consistent with the positions of the vortex bands at the top and bottom of the volute,and the main frequency that affects the leakage vortex oscillation at the inlet of the volute is the blade frequency;The main frequency in the volute flow channel is the same as at the inlet of the volute,but the proportion of low-order modal energy in the volute flow channel decreases,indicating that the flow field in the volute flow channel is affected by multi frequency characteristics;The flow field at the volute tongue is mainly affected by the shaft frequency,so the energy oscillation at the tongue has little effect on the gap leakage vortex.(4)Through parameter design,the effects of three parameters,namely,the inlet diameter of the guide ring,the outlet diameter of the impeller,and the number of blades,on the hydraulic performance of a new type dishwasher pump were studied.The correlation characteristics between the gap flow changes caused by the parameters and the pump performance were compared and analyzed,and the effects of the parameters on the evolution of the gap leakage vortex and energy loss were revealed.The results show that the increase in the inlet diameter of the guide ring leads to a decrease in the pressure difference between the inlet and outlet of the open pump,resulting in a decrease of the head.The increase in the inlet clearance of the compound impeller leads to an increase in leakage and energy loss,resulting in a decrease in the efficiency of the pump.As the diameter of the impeller outlet increases,the velocity difference and pressure difference between the impeller inlet and outlet both increases,so the head of the pump increases,but the increased energy loss at the tongue leads to a decrease in efficiency.When the number of compound impeller blades decreases,the secondary flow and axial vortex in the impeller flow passage interfere with each other,forming a complex flow field,resulting in increased energy loss in the flow passage.As the number of blades increases,the vortices in the flow passage decrease.Due to the narrowing of the flow path,the shape of the shedding vortex becomes elongated,the leakage amount in the flow path decreases,and the area with high energy loss also decreases,resulting in an increase in pump efficiency.(5)A new type dishwasher pump pressure pulsation test was built.Based on pressure pulsation testing technology,the characteristics of pressure pulsation in the axial flow gap under different guide ring inlet diameters were studied,and the effects of water level,impeller speed,and rotation of volute on the pressure pulsation in the pump were analyzed.Then,the new type dishwasher pump prototype was modeled,and numerical simulations were conducted for different schemes.The axial flow gap of the compound impeller and the pressure pulsation characteristics in the inlet passage of the twin-tongued volute were analyzed.The pressure pulsation characteristics of the new type dishwasher pump were further verified based on the numerical simulation results.The results show that the lower pressure region caused by the gap vortex also shifts upward with the increase of the gap,and the law of pressure fluctuation varying with the gap at different chord is consistent.Through the time-frequency analysis of continuous wavelet transform,it is found that there is a secondary frequency of 0.25 f BPF at the impeller inlet,causing the pressure oscillation at the inlet.The pressure fluctuation at the top of the volute mainly occurs downstream of the tongue area,and the pressure value at the tongue area rapidly decreases.During the 1/4rotation cycle after passing through the tongue area,the pressure fluctuation at the inlet of the volute is severe,and during the subsequent 1/4 rotation cycle,the pressure value at the top of the volute increases,and the pressure fluctuation gradually stabilizes. |
PDF Full Text Request