Influence Of Splitter Blades On The Internal Flow Charateristics In Low Specific Speed Centrifugal Pumps

The low specific speed centrifugal pump is provided with low flow rate and high head, whose ns is between 30 and 80. The mainly prominent problems are those including that low efficiency, instable internal flow and unsteady operation characteristics. Most of all, the stability of pump is the precondition. As the unsteady flow structures bring about the instability of pump in operation and the variation of flow and energy structures give rise to the internal unsteadiness, it is necessary to investigate internal flow characteristics at different conditions for the essence of unsteady operation. In addition, the design method of adding splitter blades in traditional impeller is popular in improving the internal flow of low specific speed centrifugal pump. For the purpose of studying the influence of splitter blades on internal flow, two schemes, including the impellers with and without splitter blades are adopted during the investigation, which are labeled 4+4 and 6 respectively.So far, computational and experimental fluid mechanics are the principal approaches to research internal flow characteristics in centrifugal pumps. The paper investigates the influence of splitter blades on internal flow and pump performance by means of CFD and PIV, which aims to reveal the internal reasons of splitter blades impacting on flow and then go for the optimization design of low specific speed centrifugal pumps.The main research contents and conclusions of this paper are as follows:1. The internal flow fields in two impellers for various flowrates and phases are measured by PIV to analyze the law of unsteady flow occurrence and development and the effect of unsteady flow in centrifugal pump performance, as well as comparing the performance curves and internal characteristics of two schemes to study the influence of splitter blades. The research results show that there are humps in the head performance curves of two schemes and the head of 4+4 is always higher than that of 6 at different conditions. However, the efficiency of 4+4 is lower than that of 6. The absolute velocities in two scheme impeller passages are not changeable obviously with flow rates. The absolute velocity in 4+4 is higher than that in 6, while the velocity distribution is more uniform in 6. Both relative velocity distributions in two impellers are uniform and almost along the blade shape at the design condition. Due to the blocking of tongue for fluid flowing under mass flow rate, the fluid gathers in the flow channel near tongue forming the high velocity region in 4+4 and 6. However, compared with that in 6, the relative velocity is more even and the low velocity area is smaller, meanwhile the velocity gradient is lower in 4+4 under part-load condition. The phenomenon indicates that the unsteady flow structure occurring more easily in 6, which accounts for the hump tend to appear in 6 more easily. The radial velocity is larger in 4+4 than that in 6 at the same radius. Moreover, the splitter blades can improve the structure of jet-wake. The interaction between blades and tongue is most intense at low flow rate.2. The numerical simulation of two schemes for various flowrates is applied to study the internal flow characteristics in different fluid domains, which is aimed to reveal the internal reasons of hump formation at low flow rate and the influence of splitter blades on the hump. The results illustrate that the hump phenomenon is more obvious in 6 and the reasons are multiple during the flow process from inlet to outlet. Firstly, the backflow appears in impeller inlet at part-load condition and it could be back to suction blocking the flow passage. The backflow leads to the tangential velocity in impeller inlet, which decreases the head at low flow rate, as well as resulting in the hump. Secondly, the vortexes, which are at the pressure side near impeller outlet and at the suction side near impeller inlet and their rotational directions are opposite, cloud keep developing and block the whole flow channel in 6. It accounts for the backflow area in the blade inlet and then the head decreases. To compare the internal flow in 4+4 and 6, the splitter blades can restrict the development of vortexes in impeller channels when closing to tongue and the hump phenomenon is alleviated at part-load condition. The last one is about the double vortexes in the volute section. The double vortexes are symmetrical under the design condition and the intensity is small. However, the double vortexes become asymmetrical and the development of the one near shrond dominates at part-load condition. The predominant squeezes the other one and affects the interal flow near impeller outlet in 6, leading to the additional energy loss and then the hump appearing more easily. The different characterizations of instability at low flow rate, which contain the static pressure distribution, the turbulence kinetic energy distribution and the energy gradient distribution, show that the energy gradient K is more distinct to illustrate the unsteady flow area and the increasing value of K indicates the flow tending to be unstable.3. The differences in two schemes, including torque, radius force, pressure distribution near outlet and the interaction between blade and tongue, are compared by the unsteady simulation. The research results show that the torques in two schemes change with time like sinusoidal vibration and the fluctuation is greatest at low flow rate. However, the fluctuations under the design and overload conditions are more flat. Meanwhile, compared with that at low flow rate, the vibration is opposite at mass flowrate. The radius force at different flow rates in 4+4 is larger than that in 6, which is caused by the stronger interaction and the hydraulic imbalance. In addition, the pulsation of radius force in 4+4 is also stronger that in 6, especially under the part-load condition. The pressures along the impeller periphery are even at the design condition, but the pulsations become larger when deviating from the design condition. At the same time, the number of pulsations is related with the number of blades. As the number of impeller outlet in 4+4 is more than that in 6, the pulsation is more obvious in 4+4 and the largest vibration is largest in the region near tongue, which illustrates that the interaction is strongest. The pulsation in the region near tongue is influenced by the unsteady flow structure and the interaction between blades and tongue. The main effects of flow pulsation in tongue region are the structure of jet-wake and the secondary flow at design and overload condition, while the dominant factor is the vortex near impeller outlet at low flow rate. Because of the increasing number of blades in 4+4, the pulsation is stronger in 4+4 than that in 6.