Study On Energy And Cavitation Characteristics Of S-shaped Airfoil Bidirectional Axial Flow Pump

With the development of national economy,the standards of flood control and environmental management have continuously improved.Many large pumping stations along the Yangtze River(lake)and river network have proposed a two-way operation to satisfy the requirements of irrigation and drainage.The application of a bidirectional axial flow pump impeller in large-scale irrigation and drainage dual-purpose pump station has been more and more extensive.At present,the energy and cavitation characteristics of the bidirectional axial flow pump are not well understood,and the hidden dangers of a safe and stable operation are buried in the pump station project.Through numerical calculation and experimental studies on the energy and cavitation characteristics of "S"-shaped two-way airfoil and bidirectional axial flow pump impeller,the parameters such as airfoil lift resistance,impeller external characteristics,and flow field structure are obtained to determine the trend of cavitation generation and development of "S" shaped two-way airfoil and bidirectional axial flow pump impeller,master the cavitation characteristics of "S"shaped two-way airfoil,and bidirectional axial flow pump impeller,and determine the different pressure pulsation characteristics of“S”shaped two-way airfoil and bidirectional axial flow pump impeller,and determine the cavitation correlation of "S" shaped two-way airfoil and bidirectional axial flow pump impeller.The research results can guide the design optimization of a two-way airfoil and two-way impeller,further ensuring efficient,safe,and stable operation of a two-way pumping station.This paper combines numerical calculations and experimental studies to obtain the following results:(1)As the inlet pressure decreases,the lift coefficient,drag coefficient,lift-to-drag ratio,and torque around the Z-axis decrease gradually.When the inlet pressure drops to 6 m at different angles of attack,the lift and drag coefficient decreased.At negative angles of attack,the lift is negative,and at positive angles,the lift is positive.At the negative angle of attack,the bubble first appears on the Sup side of airfoil.At the positive angle of attack,the bubble first appears on the Sdown side of airfoil.As the inlet pressure decreases,the bubble extends from the inlet side to the outlet side until the entire calculated water body is filled.The negative angle of attack cavitation is more intense than the positive angle of attack.Under the same inlet pressure conditions,the negative angle of attack bubble increases more than the positive angle of attack.Under different inlet pressures,the bubble is basically distributed on the Sup side of airfoil at the negative angle of attack.At the positive angle of attack,the bubble extends from the Sdown side of airfoil to the Sup side of airfoil as the inlet pressure decreases.The numerical calculation results of "S"-shaped airfoil are consistent with the experimental lift resistance characteristics and flow field structure,indicating that the numerical calculation method is reliable.Through numerical calculations and experiments,it is reasonable to design the bidirectional axial flow pump impeller with a positive attack angle.Because the energy performance of the bidirectional axial flow pump is reflected in the lift-to-drag ratio of airfoil,the lift-to-drag ratio is large,the energy performance of designed the bidirectional axial flow pump impeller is good.The static pressure distribution of positive attack angle is more uniform,and the cavitation performance is more excellent.Once again,the rationality of designing the bidirectional axial flow pump impeller using a positive angle is demonstrated.In the range of positive lift,the smaller the attack angle,the better the cavitation performance.No cavitation hopper phenomenon is reflected in the bidirectional axial pump impeller.In the positive lift range,the larger the flow rate,the better the cavitation performance.(2)The bidirectional axial flow pump impeller based on the two-way symmetric "S"-shaped airfoil design has theoretically symmetrical hydraulic performance.In actual use,because of the configuration of rear guide vanes during forward operation,the reverse performance is poor.The forward operation performance of the bidirectional axial flow pump is better than that of reverse operation.The performance difference under large flow conditions is large at each angle.The efficiency difference between the two operation conditions is 10%,and the flow-head curve is different by 2°.The main reason for this difference between the flow-head is that the angle of impeller inlet water flow is different,and the absolute flow angle of impeller inlet is larger when the forward operation than the reverse operation.The flow efficiencies of forward and reverse operations under high-efficiency zone conditions have the smallest total hydraulic loss.The difference between forward and reverse operations is 6%,and the impeller inlet flow uniformity is 10%.In the forward operation,the total pressure distribution inside the impeller is obviously higher than the reverse operating condition.In the forward and reverse operations,the maximum pressure pulsation of impeller inlet and impeller outlet is calculated using the same distance from the center of impeller,the impeller inlet is large.The pressure pulsation between the impeller and guide vane is affected by the dynamic and static interference,the value is also larger,but smaller relative to the impeller inlet.The maximum amplitudes of pressure pulsation of four measuring points in the same section are not much different.At the multiple of leaf frequency,the raise and drop of pressure pulsation amplitude of different amplitudes appear,and the main frequency of larger amplitude is at the low frequency.A peak is present in the high-frequency portion,but no large peak occurs.The maximum pressure pulsation amplitude of the reverse operation of the bidirectional axial flow pump is greater than that of forward operation.(3)The forward operation energy performance of the bidirectional axial flow pump is equivalent to that of conventional axial flow pump.The reverse-operation energy performance of the bidirectional axial flow pump is worse than that of conventional axial flow pump,but it is better than the direct reversal performance of conventional one-way axial flow pump.The forward operation cavitation performance of the bidirectional axial flow pump is superior to the reverse operation,and the critical NPSH predicted by the numerical calculation is smaller than the test value.This is mainly related to the numerical calculation and the experimental test,the cavitation nucleus of the medium is different,the instability of the flow state and the instability of cavitation are related.The numerical calculation of pressure pulsation is similar to the magnitude of the test pressure amplitude.The amplitude of test is larger.The numerical calculation is not accurate in the prediction of small amplitude,but the accuracy basically satisfies the requirements of scientific research,indicating that it is reliable to study the pressure fluctuation of the bidirectional axial flow pump under different flow rates and measuring points by numerical calculation.(4)The forward operation of the bidirectional axial flow pump is better than that of reverse operation.A lot of vacuoles are present on the suction side of blade in both forward and reverse operations,and the bubble volume on pressure surface is less.The inlet pressure is reduced,and the vacuole volume is gradually increased.The change in bubble volume in the axial direction is basically the same in forward and reverse operations,but the bubble is extended to 43%of the chord length of blade during the forward operation.In the reverse operation,the bubble extends to a position 50%of the chord length of blade.The total gas-phase volume of impeller domain in the forward operation is smaller than that in the reverse operation,and both of them show an increasing trend with the decrease in inlet pressure.The cavitation performance of conventional axial flow pump is superior to that of the bidirectional axial flow pump.Under the optimal working condition,the static pressure in the two-dimensional(2D)flow field,and the 3D flow field in the direction of chord length is similar.The positions of convergence point of suction surface and pressure surface are the same;the positions of static pressure abrupt change are also the same.The static pressure fluctuation of 3D flow field of suction surface is relatively large,and the static pressure of 2D flow field changes smoothly,this is related to the flow doping of 3D flow field.When the cavitation is born,the 2D and 3D volume distributions are similar.With the development of cavitation,the 3D flow field bubbles move to the airfoil outlet to a greater extent.In general,the 2D flow field cavitation position is the same as 3D flow field.The2D flow field area of bubble is small.Under cavitation,the inlet pressure pulsation amplitude of the bidirectional axial flow pump at each measuring point is the largest,and the pressure pulsation occurs at 4 times rotation frequency.The pressure pulsation characteristics under cavitation and noncavitation are similar,but under the noncavitation pressure pulsation,the peak frequency shifts to a low-fold frequency.Under noncavitation,there is basically no peak value at less than 1 time rotation frequency,but the peak value of components less than 1 time rotation frequency at the cavitation is obvious.The inlet,impeller,and outlet parts exhibit this characteristic,which is mainly related to the periodic generation and collapse of bubbles under cavitation.Overall,under cavitation conditions,the pulsation amplitude is greater than that under noncavitation.