Mechanism Analysis Of Three-dimensional Separated Internal Flow In A Centrifugal Impeller With Slotted Blade

The transient nature of the internal state and the complexity of pressure pulsation make low specific speed centrifugal pump operation under transient conditions a dynamic and complex process.With a change in flow rate,the centrifugal pump’s internal flow shows obvious hysteresis characteristics.Under off design condition,there is flow separation,vortex,secondary flow and other unstable flow phenomena in the pump passage,and flow separation is by means of all kinds of vortex movement apparently.Therefore,new method shall be come up with to suppress flow separation.In this paper,a low specific speed centrifugal impeller as the research object,using the slotted blade flow control technology,orthogonal experiment design method combined with numerical simulation methods to analyze that slotted blade affect the performance of low specific speed centrifugal pump impeller,moreover,according to the results of numerical simulation further to investigate the internal basic flow characteristics of slotted blade model,and constrained streamline blade wall separated vortex formation and development is discussed.The main research contents of this paper are as follows:1.Numerical simulation and orthogonal experimental design methods were used to study and analyze the selection of blade slot parameters in the low-flow condition,design flow condition and large-flow condition.From the results of range analysis,the primary and secondary order of the influence of the slot factors on the performance of the centrifugal pump with low specific speed can be obtained.Under the condition of small flow rate,the primary parameter of influence of blade slotted on head is slotted deflection Angle;the primary parameter of influence on efficiency is slotted shrinkage ratio.The primary parameter of influence on the critical NPS is deflection Angle.Through comprehensive analysis,the optimal test scheme is A₁B₂C₂,denoted as Model1.Under the design condition,the primary parameter of influence on the head is the deflection angle of the slotted;the primary parameter of influence on efficiency is slotted diameter;the primary parameter of influence on the critical NPS is deflection Angle of the slotted.After comprehensive analysis,the optimal test scheme is A₁B₃C₂,denoted as Model 2.After comprehensive analysis,the optimal test scheme is A₃B₁C₁,denoted as Model 3.The head of model 1 is increased by 7.4%,the efficiency is improved by 8.1%,and the critical NPSH is increased by 3.2%,indicating an obvious optimization effect.The head of Model 2 decreased by 0.062m,the efficiency decreased by 0.8%,and the critical NPSH decreased by 0.064m.The head of No.3model decreased by 0.057m,the efficiency decreased by 0.62%,and the critical NPSH decreased by 0.057m.The optimization effect of Model 2 and Model 3 is relatively limited,and the performance is slightly reduced.However,only discussing a single condition can not reflect the optimization effect of the optimal test scheme.2.Based on the steady calculation results of the original model and the three slotted models,the mechanism of the slotted blade improving the flow in the impeller was discussed,and the effects of the jet flow and entrainment of blade slotted on the velocity field,pressure field and turbulent kinetic energy in the impeller passage were analyzed.The results show that under the low flow rate condition,when the slotted is near the leading edge of the blade,the low pressure area at the impeller inlet and the flow separation area near the suction surface can be effectively reduced,and the velocity distribution becomes relatively even.A larger slotted shrinkage ratio means a stronger entrainment,so the flow condition of the boundary layer is improved and the flow separation phenomenon is inhibited.Moreover,Model 2 obviously improves the non-uniform distribution of turbulent kinetic energy in the original model.In the design condition and the large flow condition,the static pressure at the impeller inlet area is relatively large,so the entrainment effect of the slot is not obvious.At the same time,in the design condition,a region of high turbulent kinetic energy appears in the suction surface near the slotted,which indicates that a new turbulent vortex region is formed here,but the efficiency and head of the slotted model are obviously improved.Under the condition of large flow,the distribution of turbulent kinetic energy in the impeller of the slotted model does not change much.3.On the basis of the steady calculation results,the unsteady calculation of the slotted blade centrifugal impeller model was carried out to further study the influence of the slotted blade on the velocity field,the radial force,the vortex structure characteristics and the generation and development of the separated vortex on the blade boundary layer,etc.The results show that the velocity field in the corresponding region can be changed by slotting at different positions,and the distribution of the velocity field becomes uniform,and the size of the vortex is weakened to a certain extent.In the low-flow condition,as the slotted position approaches the trailing edge of the blade,the pressure surface and suction surface of the blade are subjected to less radial force.In the frequency domain diagram,the main frequencies of the slotted model occur at the double blade frequency,and decay rapidly at the sub-frequency until there is basically no response at the high frequency.Under the rated flow condition,whether the blade is slotted or not,there is little difference in the radial force.The dominant frequencies of the pressure surface and suction surface in the X and Y directions of Model 1 both appear at the rotating frequency of the impeller,and the amplitude of the dominant frequencies is the largest.Under large flow conditions,Model 2 can effectively reduce the radial force on the blade pressure surface and suction surface.The main frequency amplitude of the blade pressure surface and suction surface in the x and y directions are the smallest,and still located in the impeller rotation frequency.With regard to the unsteady evolution of the vortex structure in the impeller,the double-layer band-shaped vortex in the flow passage of Model 1 evolves into multi-layer band-shaped vortex under the condition of small flow,and the tubular vortex structure is constantly formed in the outlet area of the impeller.In the flow passage of Model 2,the overlapping banded vortex structure is stretched into a concave-convex shape and finally broken into a lamellar vortex structure.Model3 continues to form a banded vortex structure from the suction surface to the pressure surface.Under the rated flow condition and large flow condition,the change of vortex shape structure of Model 2 and Model 3 is not obvious,because the entrainment effect of the slot is very limited.According to the flow spectra of the pressure surface and suction surface of the blade,the separation flow patterns such as separation point,spreading separation zone and spiral flow group were obtained on both the slotted pressure surface and suction surface.When the slotted is near the leading edge of the blade,the formation of separated flow patterns such as separated points on the pressure surface and suction front edge of the blade and spiral flow groups can be effectively suppressed.A larger slotted shrinkage ratio will reduce the number of spiral flow groups and eliminate the spreading separation zones.When the slot position is close to the trailing edge of the blade,the spanwise separation zone in the trailing edge of the blade can be eliminated.On the basis of the steady calculation results,the unsteady calculation of the slotted blade centrifugal impeller model was carried out to further study the influence of the slotted blade on the velocity field,the radial force received by the impeller,the vortex structure characteristics and the generation and development of the separation vortex on the blade wall,etc.The results show that the velocity field in the corresponding region can be changed by slotting at different positions,and the distribution of the velocity field becomes uniform,and the size of the vortex is weakened to a certain extent.In the low-flow condition,when the slotted position is near the trailing edge of the blade,the pressure surface and suction surface of the blade are subjected to less radial force.In the frequency domain diagram,the main frequencies of the slotted model appear at the double blade frequency,and decay rapidly at the sub-frequency until there is basically no response at the high frequency.Under the rated flow condition,whether the blade is slotted or not,there is little difference in the radial force.The dominant frequencies of the pressure surface and suction surface in the X and Y directions of Model 1 both appear at the rotating frequency of the impeller,and the amplitude of the dominant frequencies is the largest.Under large flow conditions,Model 2 can effectively reduce the radial force on the blade pressure surface and suction surface.The main frequency amplitude of the blade pressure surface and suction surface in the x and y directions are the smallest,and still located in the impeller rotation frequency.In terms of the unsteady evolution of the vortex structure in the impeller,the double-layer band-shaped vortex in the flow passage of Model 1 evolves into multi-layer band-shaped vortex under the condition of small flow,and the tubular vortex structure is constantly formed in the outlet area of the impeller.In the flow passage of Model 2,the overlapping banded vortex structure is stretched into a concave-convex shape and finally broken into a lamellar vortex structure.Model 3 continues to form a banded vortex structure from the suction surface to the pressure surface.Under the rated flow condition and large flow condition,the change of vortex shape structure of Model 2 and Model 3 is not obvious,because the entrainment effect of the slot is very limited.According to the flow spectra of the pressure surface and suction surface of the blade,the separation flow patterns such as separation point,spreading separation zone and spiral flow group were obtained on both the slotted pressure surface and suction surface.When the slotted is close to the leading edge of the blade,the formation of separation flow patterns such as separation points on the pressure surface and suction front edge of the blade and spiral flow groups can be effectively inhibited.A larger slotted shrinkage ratio will reduce the number of spiral flow groups and eliminate the spreading separation zones.When the slot position is close to the trailing edge of the blade,the spanwise separation zone in the trailing edge of the blade can be eliminated.