Experimental Study of the Effect of Blade Configurations on the Performance of Centrifugal Pumps

In the present paper, the effect of change the blade configurations of the centrifugal pump impeller on its performance has been investigated. The pump under investigation has been designed and manufactured to be able to change its blades configurations in a way that the performance of the pump could be measured. The blades configurations have been tested for conventional blades, as well as split to two and three parts. The split blades were tested for clockwise and anti-clock-wise shifting. The experimental test rig has been carried out and the pump performances, (speedheadflow ratepower and efficiency) have been measured for conventional impeller blades configuration, as well as for eight iterations of different impeller configurations according to the blades arrangement for different rotational speed ranged from 700 to 1450 rpm. The main blade configuration parameters are the splitting angle (forward or backward) and splitting diameter. The results showed that, for the same pump rotational speed, the pump head, efficiency and design flow rate, may increase or decrease by 12 to 16 %, compared to the basic impeller configuration. 1Introduction Centrifugal pumps are widely used because of their design simplicity, high efficiency, wide range of capacity, head, smooth flow rate, and ease of operation and maintenance. The development of centrifugal pumps was very rapid due to its relatively inexpensive manufacturing and its ability to handle voluminous amounts of fluid. The blade number of the impeller and their configurations are considered as important design parameters in centrifugal pumps design. They may affect the pump performance in a way that the existence of the back flow and flow separation in the vane passage which may lead to pressure fluctuations and efficiency drop. The pressure coefficient increases when the splitter is placed closer to the suction side of the main blade. Increasing the length of the splitter can raise the pressure coefficient with little or no effect on efficiency [1]. The outlet velocity profile is improved by splitting vanes; relative velocity increases evidently with the increase of flow rate [2]. The complex internal flow patterns were acquired and the relative velocity varying law near the suction surfaces and pressure surfaces of long blades and the operation sides and opposite sides of short blades were also revealed[3]. The suction performance was * Egyptian Armed Forces, Egypt. † Ain Shams University, Egypt. Paper: ASAT-15-191-HF 2 improved mainly at partial flow rates if the leading edges of splitter blades were located at 20% of the meridional chord length and improved mainly at larger flow rates if the leading edges of splitter blades were located at 40% of the meridional chord length [4]. The head of the backward centrifugal pump (specific speed =63.61) has been increased by 5% to 18% when slotted vanes are used with width to blade length is from 18% to 22%, and the ratio between the slot position and blade length is from 35% to 50% [5]. With increasing the cut piece of blade edge the pump head exceeded the original head [6]. The back flow in the impeller has an important influence on the performance of pump and a more-blade complex impeller with long, mid and short blades can effectively solve low flow rate instability of the low-specific-speed centrifugal pump [7]. The experimental findings showed that the high vibrations at the blade-passing frequency are primarily raised by high pressure pulsation due to improper gap design [8]. Adding splitters had a positive effect on the pressure fluctuations which decreased at the casing duct [9]. The most remarkable result is that the optimized geometry has an advantage in the amplitude of pressure pulsation reducing considerably the amplitudes in the outlet pipe [10]. In the present paper, as an exploring work to enhance the centrifugal pump performance, the effect of changing the blade configuration from conventional curved blade to a new blade arrangement with splitters have been highlighted. This arrangement will be compared with the conventional one to study their effects on the pump performance. 2. Experimental Test Rig Experimental work is usually necessary as a bench mark for investigating the flow field in centrifugal pumps as well as a tool for the verification of the theoretical study. In the present study, an experimental test rig has been carried out to investigate the effect of new splitter / blades configurations in centrifugal pump impeller, either adding splitters in curved or staggered arrangements, on the centrifugal pump performance. The test rig has been commissioned at the hydraulic lab in the Department of Mechanical Power in the Faculty of Engineering at Ain Shams University. A schematic drawing of the experimental test rig arrangement has been introduced in Figure 1. The experimental test rig consists of, a centrifugal pump of designed flow rate of 0.015 m 3 /s and head of 21 m running at 1450 rpm derived by an electric motor of 10 HP (7.5 KW). The pump impeller of radial type has 5 backward blades with outer diameter of 0.28 m and 0.1 m eye diameter. The pump is supplied from a tank of 5 m 3 capacity and the pipeline has a control delivery valve. The pump is driven by means of a variable speed electric motor with a controller allows the pump speed to vary from zero to 1450 rpm. 2.1 Different Impeller Configurations The centrifugal pump has been manufactured with five blades. The modified pump impeller is designed and manufactured to fit a pump in the hydraulic laboratory. The original impeller configuration with its principal dimensions has been shown in Figure 2, where the blades have their own curved flow path. Impeller configuration has been modified to introduce staggered arrangement with different iterations as shown in Figures 3. Every blade divided into three equal parts so that each part can be fixed separately and the shroud divided into 15 parts (at angle 24 o ) to give an opportunity to change the blade configuration into four stages. Table 1 shows the staggered angle of the impeller blade parts. Paper: ASAT-15-191-HF 3 Fig. 1: Schematic diagram of the Experimental test rig arrangement Fig. 2: Original impeller configuration. Paper: ASAT-15-191-HF 4 First and Second iterations Third and fourth Iterations Fifth and Sixth Iterations Seventh and Eighth Iterations Fig. 3 Modified Blades of the Impeller Table 1: Angle of rotation of the impeller blades parts. Iterations Relative angle of rotation Inner part Middle part Outer part -24 o 0 24 o -24 o 0 24 o -48 o -24 o 0 24 o 48 o Conventional impeller * * *