Research On Flow Separation In The Low Specific Speed Centrifugal Impeller

The performance characteristics of high head and low discharge associated with low specific speed pumps imply that this type of pump embodies some special features different from those of other centrifugal pumps. The large impeller discharge diameter will result in significant mechanical losses, making the efficiency of the impeller very low. The high head produced by the impeller means high adverse pressure gradient within blade passage and because of this the flow often separates from the blade surfaces even at the design point. In addition, the ratio of the maximum power consumed by the shaft to the shaft power at the design point can be as high as 1.6, which often results in frequent failures of prime movers.The low specific speed centrifugal impeller widely uses cylinder blade, the employed contour of cylindrical blade has the extremely essential influence to the power performance. The adverse pressure gradient is very high in low specific speed impeller, thus the flow separation was generated easily in the blade surface. Thus it is essential to compare the different kinds of contour of cylindrical blade to prevent effectively flow from separating and evolution of flow separation. Moreover, the number of blade relates with the proliferation degree of inner flow field, thus there is a optimum number of blade in each kind of contour to prevent effectively flow from separating and evolution of flow separation. Moreover the total area should be smallest to reduce effectively the friction loss.Based on characteristic of the low specific speed centrifugal impeller and its request of improving efficiency target, especially on the frequent surface flow separation caused by the excessive adverse pressure gradient in the impeller, author analyzed flow passage stream movement rule for three kind of different blades contours, and based on hydrokinetics theoretic, also designed each kind of blade contour with different number blade. Through the numerical simulation by using Fluent software, based on Navier-Stokes equations and the realizable k-e model is applied to calculate the change in relative velocity field of impeller and the distribution of pressure in the flow field, the analysis obtained the best cooperation the most advantageous blade contour to avoid flow separation with the blade number.