Research On Design Method Of Non-overload And Low Cavitation For Medium Specific Speed Centrifugal Pump

Centrifugal pumps in special areas,such as naval ships,aerospace,aviation and so on,are often proposed to meet the requirements of both non-overload and low cavitation centrifugal pumps.The non-overload of centrifugal pump means that the maximum axle power of the pump should be less than the rated power of the motor in the full flow range（from zero flow to maximum flow）.The low cavitation of centrifugal pump means that the centrifugal pump has a higher specific speed of cavitation（or a lower cavitation margin）.In this paper,aiming at the research of non-overload and low cavitation of medium specific speed centrifugal pump,the main work is as follows:1.For the centrifugal pump with medium specific speed,under some assumptions such as no pre-rotation at the impeller inlet,according to the calculation formula of the power reserve coefficient K（i.e.the ratio of maximum shaft power to rated power of the centrifugal pump）,it is deduced that the K value of the centrifugal pump with medium specific speed is the correlation function of the specific speed of the pump,the outlet angle of the impeller blade and the number of blades.Based on this function relationship,the three-dimensional graph of K value under different blade numbers and the contour map of K value about blade outlet angle and specific speed are drawn by using software.It can be seen from the contour map that the outlet angle of impeller blade can be obtained according to the number of impeller blades and the power reserve coefficient.2.Assuming that there is no pre-rotation at the inlet of the impeller,according to the formula of the specific speed of cavitation,it is deduced that the specific speed of cavitation C_NP（subscript NP denotes no pre-rotation）can be expressed as the flow angle at the inlet of the impeller and the function related to the coefficients K₁ and K₂.Based on this function relationship,the three-dimensional C_NPP values and the contours of C_NPP values with respect to the inlet flow angle and coefficient K₂ of impellers under different coefficients K₁ are plotted by using software.The contour plot shows that when the coefficients K₁ and K₂ are given a certain value,there will always be only one inlet flow angle of impeller to maximize the C_NPP value and have better cavitation resistance.Under given conditions,the inlet liquid flow angle can be found by contour plot of C_NPP value.3.According to the above functions and contour diagram,a centrifugal pump A（n_s（28）845.）with medium specific speeds is designed to meet the requirements of both non-overload and low-altitude design.A centrifugal pump B（n_s（28）1253.）with medium specific speeds needs to meet the requirements of non-overload design.Through numerical simulation,it is found that the difference between the reserve coefficient of power and the set value is small,which can meet the requirements of no-overload design;the difference between the specific speed of cavitation and the predicted specific speed of cavitation obtained by numerical simulation of centrifugal pump A is small,and the effect of cavitation performance is better.4.Examples show that the research theory in this paper can not only quickly and accurately design the specific speed centrifugal pump without overload requirements,but also improve the cavitation performance.