Research On Effect Of Pressure Gradient On Impeller Performances Of Centrifugal Pump

The pressure gradient along the flow path inside a centrifugal pump impeller is an important factor affecting the distribution of velocity field, which directly determines the performances, especially the efficiency of impeller. Therefore, the influence of pressure gradient is needed to be considered during the course of hydraulic design of impeller. In this paper, the effect of pressure gradient on the impeller performances of centrifugal pump was studied using theoretical and numerical calculation methods.First of all, the impact of zero pressure gradient, favorable pressure gradient and adverse pressure gradient on the flow was expounded in theory. Theoretical analysis showed that the relationship between different forms of pressure gradient and flow was summarized as the interaction between liquid endure surface force and itself inertial force. Under the environment of adverse pressure gradient, surface force and inertial force was in the opposite direction, not only the flow was inhibited, but also the separation of boundary layer was accelerated.Secondly, the internal flow field of several different geometries diffusion tubes was calculated using CFD method. In the perspectives of velocity distribution and hydraulic loss, the influence of distribution of pressure gradient on diffusion tubes internal flow and its hydraulic performances were illustrated. Study showed that the distribution trend of cross-section area was a key factor to affecting the distribution of pressure gradient inside a diffusion tube, the smaller the pressure was, the better hydraulic performances of diffusion tube were.The pressure gradient inside an impeller is related to both the length of flow path and the area of cross-section along the flow path, and the path length and cross-section area are related to the geometric parameters of impeller. In this part, a centrifugal pump with the specific speed of 63 was taken as object of study. By adjusting the blade warp angle and the setting angles at inlet and outlet of blade, the main factors that influence the pressure gradient distribution along the flow path inside a impeller were analyzed, and the relationship between the pressure gradient and impeller hydraulic efficiency was studied. Three processes were developed about the influence of pressure gradient on the performances of impeller:(1)Adjust the blade wrap angle to 150°, 160° and 170°, and remain the original setting angles at inlet and outlet of blade. Study showed that with the increase of blade warp angle, the flow path became longer, and the ratio between the areas of outlet and inlet of impeller passage increased. In this case, the pressure gradient with larger wrap angle was not obviously different from the original, and the impeller efficiency decreased, which means that only adjusting the wrap angle can’t increase the impeller efficiency, because the small difference of pressure gradient between the new design and original can’t effectively improve the flow field structure, and more hydraulic friction loss occurs because of the longer flow path.(2)Adjust the blade wrap angle to 150°, 160° and 170°, and adjust the setting angles at inlet and outlet of blade correspondingly in order to control the variation trend of cross-section area along the flow path. The study showed that the flow path became longer at the same wrap angle by adjusting the setting angles, and with the increase of wrap angle, the ratio between the areas of outlet and inlet of impeller passage decreased. In this case, pressure gradient obviously decreased with the increase of wrap angle, and the impeller efficiency increased correspondingly.(3)Remain the wrap angle at 160°, and further adjust the setting angles at inlet and outlet of blade. The study showed that with the decrease of setting angles at inlet and outlet of blade, the flow path remained almost the same length, but the ratio between the areas of outlet and inlet of impeller passage decreased constantly. The inlet area of impeller passage differed slightly from each other, but the outlet area decreased obviously, which means that the setting angle at outlet of blade has much greater influence on the cross-section area than that at inlet of blade. With the decrease of blade setting angles, the pressure gradient decreased, and the impeller efficiency increased.In summary, the distribution of pressure gradient along the flow path inside a impeller is a key factor to restrict the performances of impeller. The distribution regularity of pressure gradient is determined by the distribution of cross-section area along the flow path. Blade wrap angle and blade setting angle at outlet are the main factors influencing the distribution of cross-sectional area. Within a certain range,increasing the blade wrap angle and decreasing the blade setting angle at outlet will effectively reduce the pressure gradient in the middle and rear parts of flow path,which can not only postpone or prevent the boundary layer separation, but also improve the velocity distribution, reduce the velocity gradient and decrease thehydraulic friction loss in impeller, as well as improve the impeller hydraulic efficiency.Finally, the chief and secondary impellers for a multistage centrifugal pump were designed by decreasing the pressure gradient along the flow path, and carried on a experiment. Results showed that the multistage centrifugal pump met the design requirements and had a wide high-efficiency range. The computational and experimental performances of multistage centrifugal pump were highly fit, which verified the accuracy of the numerical calculation method as well as the reliability and feasibility of research results about the influence of pressure gradient on impeller performances of centrifugal pump. The conclusion of this paper enhances comprehension of the internal flow mechanism within the fluid machinery, provides an important reference information for hydraulic design of centrifugal pump impeller,and has a high value in engineering application.