Experimental study of the influence of geometrical parameters on the cavitation of a small centrifugal pump

Transcription

1 Fluid Structure Interaction and Moving Boundary Problems 89 Experimental study of the influence of geometrical parameters on the cavitation of a small centrifugal pump A. Nemdili Laboratory LRTTFC, Department of Marine Engineering, Technical University of Oran, Oran, Algeria Abstract Small centrifugal pumps are largely used for automobile cooling systems. This study presents investigated NPSH-curves of a small centrifugal pump working with a semi-open and a closed impeller. The influence of the impeller blade number, the front clearance and the cover, and the influence of the impeller width on the cavitation are investigated. It was found that: - with decreased blade number, the NPSH-curves have higher values, due to the change of the pressure distribution along the blades. - The cover of the impeller has an unfavorable effect on the cavitation sensitivity. This last increases with increased values of the clearance, due to the variable clearance flows for covered and uncovered impellers. - Narrowed impellers are characterized with higher NPSH-curves, due to the fact that for the smaller flow cross-sections the speed at the inlet of the narrowed impellers is higher than for wider impellers, when the risk of cavitation increases. Keywords: centrifugal pump, semi-open impeller, closed impeller, blade number, front clearance, impeller width, cavitation, NPSH curve. 1 Introduction Small radial pumps over the last few years have become simpler and produced with favorable costs. They are largely used for automobile cooling systems. Small centrifugal pumps with semi-open impellers (i.e. without cover plate or with half shrouding), because of the low exactness production, are operated with greatly different front clearances between 0.2 and 1 mm. These impeller types

2 90 Fluid Structure Interaction and Moving Boundary Problems are produced with only a simpler blade curvature and efficient pouring technique. Tip clearance flow is an important source of turbomachinery energy loss. The function of a sealing gap disposed at impeller inlet between impeller shroud and casing is to reduce the leakage rate and its disadvantageous effects. Leakage losses in ring clearances can only be calculated approximately using empirical methods obtained from measurements on model test stands with simplified flow conditions (Engeda [1], Stoffel [2] and Csaba [3]). For semi-open impellers, tip clearance loss by different clearances is for great interest. It is not possible to find a direct and reliable measuring method of the leakage rate under normal operating conditions without to disturb the flow conditions (Hergt et al [4]). A claim of three conditions in the design of pump impellers are named in all papers engaged with the optimization of water pumps: - a highest possible efficiency, - low number (good suction capability), - and a high delivery head for the available small space. For low cost pumps in cooling systems these points could enhanced by two further conditions: Least sensitive in the changing of the front clearance: Cooling systems are designed for special operating points for all conditions of the engine operation according a volumetric flow. A change in the front clearance can be one of the reasons of the changing in these operating points of the cooling system, which can not be correctly predicted every time. Flat characteristic H-Q: Cooling systems for combustion engines are thermostatically controlled, and the operating point of the whole system is changing during the car driving. This could be the reason for pressure shocks, which are high load for any part of the cooling system. These pressure shocks could be prevented by choosing an impeller with a flat characteristic H-Q, i.e. Head = f (Flow). This study presents the investigated NPSH characteristics of the same pump working with a semi-open or a closed impeller. The influence of the front clearance, of the cover, of the blade number and of the width of the impeller on the NPSH characteristic are investigated. 2 Test facility and experimental work The experimental setup consists mainly on a test rig and a data acquisition system (Figure 1). The fluid ( Ethylen-Glycol Mixture) is sucked by the test pump (6) from a fluid tank (13) through a valve (9), a 3m long pipe, a suction pipe to the test pump and the discharge pipe. The test pump is driven by a variable speed motor ().

3 Fluid Structure Interaction and Moving Boundary Problems ,5m 0, ,1 0, Ventil to fill the test rig with fluid 9 Service ventil 2 Flow meter 10 Heater 3 Outlet pressure measurement 11 Electric temperature control 4 Pressure difference measurement 12 Vacuum pump 5 Inlet pressure measurement 13 Compensating tank 6 Test pump 14 Flow control Variable speed motor 15 Ventilation 8 Purge of the test rig Figure 1: General layout of the test rig. Experiments were conducted for 5 radial impellers with and without cover. The blades of all impellers are of single-curvature shape with same blade angles β S1 and β S2, and different blade number Z. They have the same inlet radius r 1, the same outlet radius r 2, and different outlet blade width b 2. Design features of the used impellers are shown in Table 1 and Figure 2. For each pump NPSH curves were investigated for both values of the front clearance. Table 1: Impeller geometry data. Pump Nr r 1 (mm) r 2 (mm) β S1 ( ) β S2 ( ) Z (---) b 2 (mm) Front clearance FC (mm)

4 92 Fluid Structure Interaction and Moving Boundary Problems Z = 5 Z = Z = 9 r 1 r 2 b 2 Cover Closed Impeller Semi-open Impeller Figure 2: Test impeller configurations. 3 Experimental results and discussion On the test rig, the characteristics = f (Q) are measured. 3.1 Influence of the blade number Characteristics = f (Q) for the tested impellers by varying the blade number are shown in Figure Experiments for impellers with cover and b 2 = 10mm : Z = 5, FC = 0,3mm : Z =, FC = 0,3mm : Z = 9, FC = 0,3mm : Z = 5, FC = 1mm : Z =, FC = 1mm : Z = 9, FC = 1mm Figure 3: Influence of blade number on the curves for both values of the front clearance FC.

5 Fluid Structure Interaction and Moving Boundary Problems 93 It can be seen from the curves = f (Q) that when the blade number increases, the NPSH decreases and improves. This is observed for the both values of the front clearance. 3.2 Influence of the impeller width The characteristic curves = f (Q) for the tested impellers by varying the impeller width are shown in Figures 4 and 5. Experiments for impellers with cover (C) and Z = : b 2 = 10mm, FC = 1mm : b 2 = mm, FC = 1mm : b 2 = 10mm, FC = 0,3mm : b 2 = mm, FC = 0,3mm Figure 4: Influence of the impeller width b 2 on the curves for both values of the front clearance FC. Experiments for impellers without cover (NC) and Z = : b 2 = 10mm, FC = 1mm : b 2 = mm, FC = 1mm : b 2 = 10mm, FC = 0,3mm : b 2 = mm, FC = 0,3mm Figure 5: Influence of impeller width b 2 on the curves for both values of the front clearance FC.

6 94 Fluid Structure Interaction and Moving Boundary Problems From the Figures 4 and 5, following conclusions can be explained: When the impeller width decreases the values of NPSH increase for the smallest value of the front clearance. But the values of NPSH decrease when the impeller width increases for the highest value of the front clearance. This is observed in the both cases for the impellers with and without cover. 3.3 Influence of the cover and the front clearance Figures 6 and show the influence of the front clearance and the inset of the cover on the characteristic curves = f (Q) Experiments for impellers with b 2 = 10mm and Z = 5 : C, FC = 0,3mm : C, FC = 1mm : NC, FC = 0,3mm : NC, FC = 1mm Figure 6: Influence of the front clearance and the inset of the cover on the curves Experiments for impellers with b 2 = 10mm and Z = 9 : C, FC = 0,3mm : C, FC = 1mm : NC, FC = 0,3mm : NC, FC = 1mm Figure : Influence of the front clearance and the inset of the cover on the curves.

7 Fluid Structure Interaction and Moving Boundary Problems 95 The obtained results show that: With the use of the cover, the variation of the front clearance does not have a high influence on the NPSH values. For Impellers without cover, this influence of the front clearance is very small. Pumps with full shrouded impellers have higher values of the NPSH as pumps with semi open impellers. 4 Conclusions The paper presents investigated NPSH characteristics of small centrifugal pump working with a semi-open or a closed impeller and with different geometrical parameters. In principle the efficiency, the delivery head increases through the inset of the impeller shroud. From the experiments and the analysis of the obtained results, it was found that: - With decreased blade number, the NPSH-curves have higher values, due to the change of the pressure distribution along the blades. - The cover of the impeller has an unfavorable effect on the cavitation sensitiveness. This last increases with increased values of the clearance, due to the variable clearance flows for covered and uncovered impellers. - Narrowed impellers are characterized with higher NPSH-curves, due to the fact that for the smaller flow cross-sections speed excessive at inlet of narrowed impellers are higher than for wider impellers, when the risk of cavitation increases. During the design of impellers it appears that supplementary requirements to the conventional requirements concerning the construction of pumps for the automobile area are necessary. These concern the insensibility of the clearance adjustment and the flatness of the characteristic H-Q. Through the variation of the width, the number of blades, and the use of the cover, these requirements can be best possible fulfilled. Here it appears that the use of the cover best fulfills the above-mentioned requirements for all impellers. As the cover expensive is, a semi-open impeller can be used under fixed claims certainly with a variable geometry (particularly the outlet impeller width) in order to obtain the better NPSH curve. The clearance sensibility does reduce only through the enlargement of the impeller. References [1] Engeda, A., Correlation and prediction of efficiency of centrifugal pumps due to tip clearance effects, Proc Instn Mech Engrs Vol 209, pp , 1995.

8 96 Fluid Structure Interaction and Moving Boundary Problems [2] Stoffel, B., Der Dichtspalt in Kreiselpumpen - ein einfaches element mit sehr komplexen Auswirkungen (in German), Strömungsmechanik und Strömungsmaschinen 46 s , [3] Csaba, F., The clearance geometry of pump impellers affects their cavitation performance,. Conference Fluid Machinery, Budapest, pp , [4] Hergt, P.; Brodersen, S.; Stoffel, B. and Ludwig, G., The influence of prerotation on the leakage flow through sealing gaps in pumps, Second International Conference on Pumps and Fans, Tsinghua University, Beijing, Oct. -20, [5] Shiels, S., The importance of running clearances in centrifugal pumps, World pumps, June, pp. -3, 199. [6] Shiels, S., Centrifugal pump application - Key hydraulic and performance criteria, Proceedings of the Twelfth International Pump Users Symposium, pp , [] Decker, H., Untersuchungen des Betriebsverhaltens von Kreiselpumpen mit halboffenen Laufrädern (in German), Dissertation Universität Karlsruhe (TH), Dezember [8] Fandi, T., Beiträge zur Sicherung der funktionellen Austauschbarkeit von KFZ-Kühl-mittelpumpen (in German), Dissertation Technische Universität Chemnitz-Zwickau, Stuttgart, 1996.