Study on inducer and impeller of a centrifugal pump for a rocket engine turbopump

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1 Original Article Study on inducer and impeller of a centrifugal pump for a rocket engine turbopump Proc IMechE Part C: J Mechanical Engineering Science 227(2) ! IMechE 2012 Reprints and permissions: sagepub.co.uk/journalspermissions.nav DOI: / pic.sagepub.com Soon-Sam Hong, Dae-Jin Kim, Jin-Sun Kim, Chang-Ho Choi and Jinhan Kim Abstract A hydraulic performance test is conducted for a fuel pump of a liquid rocket engine turbopump. The pump driven by an electric motor is tested in a water environment. Experimental results indicate that the inducer has a negligible effect on the head and efficiency of the pump but a significant effect on the cavitation performance. Additionally, an autonomous inducer test is carried out to investigate the effect of the inducer on the pump performance in more detail, and it is found out that the pump reaches a critical cavitation point when the inducer head is dropped by 55%. A reduction of required net positive suction head of the centrifugal pump by attachment of an inducer is also calculated considering the flow interference between the inducer and the centrifugal impeller, and it is found that the calculation shows a reasonable agreement with the test. Keywords Inducer, impeller, centrifugal pump, cavitation, turbopump Date received: 21 March 2012; accepted: 3 May 2012 Introduction In typical pumps used in space rocket engines, an inducer is employed upstream of a main centrifugal impeller in order to avoid unacceptable cavitation, improve the suction performance, and reduce the propellant tank pressure and weight. Inducer design is focused on obtaining sufficient cavitation margin rather than high efficiency. Therefore, compared with a centrifugal impeller, a typical inducer has a lower flow coefficient, a smaller inlet angle, a sharper leading edge, fewer blades, and higher blade solidities. Many inducers will operate under slightly cavitating conditions. The head rise of an inducer has to be just large enough to suppress cavitation in the main pump impeller. Many publications have been devoted to the design of turbopump inducers. 1 8 A design manual by Jakobsen 1 provides general background information and some publications 2 4 apply the rules of the design manual 1 to the inducer design at the turbopump system level. A book by Brennen 5 contains many examples of inducer performance data. Japikse 6 reviews some of the design methods, flow observations, and design practices for commercial pump inducers. Some papers 7,8 address the inducer design criteria from the perspective of both steady performance and instability in cavitating flow. Inducers often suffer from cavitation instabilities 9 11 such as rotating cavitation, cavitation surge, and alternating blade cavitation which sometimes cause serious shaft vibration and strong flow fluctuation. With regard to the steady performance of inducers, inducer head rise and inducer head breakdown are important issues. The required net positive suction head (NPSH) of a centrifugal pump is reduced by employing an inducer upstream of a centrifugal impeller, but the amount of reduction of the required NPSH is less than the head rise produced by the inducer. Takamatsu et al. 12 derived a formula semi-empirically to estimate the required NPSH of a centrifugal pump with inducer from individual cavitation performances of the inducer and the centrifugal impeller. They 12 also proposed a graphical procedure for the estimation of Korea Aerospace Research Institute, Daejeon, Republic of Korea Corresponding author: SS Hong, Korea Aerospace Research Institute, Daejeon , Republic of Korea sshong@kari.re.kr

2 312 Proc IMechE Part C: J Mechanical Engineering Science 227(2) the required NPSH of a pump when the required NPSH of pump without inducer and the cavitation performance curves of inducers are known (Figure 1). They 12 used the following equation NPSH re ¼ ½H i Š mþi h ð1þ where NPSH re is the amount of reduction of the required NPSH of a centrifugal pump by attachment of inducer, ½H i Š mþi the inducer head rise at critical condition, and h the head reduction due to the occurrence of flow interference between the inducer and the centrifugal impeller. The first objective of this article is to show experimentally the effect of an inducer on a centrifugal pump, and therefore, a series of tests are carried out. Conventional hydraulic and cavitation performance tests are performed with a fuel pump for a liquid rocket engine, 13 and then, pump tests without the inducer are performed. Additionally, autonomous inducer tests are carried out to evaluate the performance of the inducer itself and to discover the correlation between the inducer and the impeller in terms of cavitation. The second objective of this article is to estimate the reduction of the required NPSH of the centrifugal pump. The reduction of the required NPSH is estimated considering the flow interference between the inducer and the centrifugal impeller and it is compared to the test result. The estimating method is presented here to give a tool for the calculation of the required NPSH of centrifugal pumps with inducer. Test facilities The pump under investigation in this study has a specific speed of (760 US units of gal/min, ft, r/min) and its rotating part is composed of a three-bladed axial flow inducer (inlet tip blade angle 10.4, outlet mean blade angle 17, and tip solidity 2.6) and a seven-bladed centrifugal flow shrouded impeller with inlet mean blade angle 19. The inducer and the impeller are shown in Figure 2. The pump is designed to operate under the nominal condition of mass flow rate 29.1 kg/ s, inlet pressure 0.28 MPa, outlet pressure 13.2 MPa, and rotational speed 20,000 r/min. 13 A full-scale impeller and inducer are used in the tests. Both a pump test with the inducer and a pump test without the inducer are carried out to investigate the effect of the inducer on the pump performance. The nominal rotational speed of the pump is 20,000 r/min, but the pump and inducer are tested at reduced speed due to the power limit of the electric motors. In the pump test without the inducer, a dummy spacer with the same hub shape of the inducer is installed in the place of the inducer. Tests are conducted in two test facilities, that is, a pump performance test facility and an inducer performance test facility. The measurement accuracy is estimated to be 0.3% for the head, 0.4% for the flow rate, 0.6% for the efficiency, and 0.3% for the NPSH. Pump head With inducer Without inducer ΔNPSH re NPSH Inducer head H + [ i ] m i 45 Δh NPSH Figure 1. Estimation of the required NPSH of the pump. 12 NPSH: net positive suction head. Figure 2. Inducer and impeller.

3 Hong et al. 313 In terms of non-cavitating performance, the test speed lower than the one used for the design seems to have a negligible effect on the applicability of the results because Reynolds number has a negligible effect if it is higher than a million. 14 Reynolds number, which is based on the wheel tip speed and the rotor diameter, for the pump and inducer tested here is much higher than a million. However, the cavitating performance from the lower speed might be a little different from that of the design speed. Nevertheless, in terms of cavitation, the matching of the inducer and the impeller in this study seems to be useful because the speed difference between the inducer and impeller tests is small. Pump performance test facility Hydraulic and cavitation tests for the pump are conducted in a pump test facility, 15,16 where the pump is driven by an electric motor (Figure 3). The working 0 m 1 m Test pump 2. Regulating valve 3. Turbine flow meter 4. Water tank 5. Torque meter 6. Gear box 7. Motor Figure 3. Plane view of pump performance test facility. 7 fluid is water at room temperature and measurement parameters are pump head, flow rate, power, efficiency, etc. The facility is composed of a water tank, an electric motor, a gearbox, a torque meter (Magtrol, TMHS 212 model), and a turbine type flow meter (Hoffer, Ho 4 4 model). The water tank has a volume of 3 m 3 and its pressure is adjusted using a vacuum pump and compressed air. The pump is driven by a variable-speed electric motor with a capacity of 300 kw. The rotational speed of the pump is set to 8300 r/min in this study. Inducer performance test facility Hydraulic and cavitation tests of the inducer are carried out at an inducer test facility, 17,18 which has a structure similar to that of the pump performance test facility. The working fluid is water at room temperature. The facility (Figure 4), which is similar to the pump test facility, contains a water tank with a volume of 0.9 m 3, an electric motor, a torque meter (Magtrol, TM 208 model), and a turbine type flow meter (TRIMEC, TP100 model). The inducer is driven by a variablespeed electric motor with a capacity of 10,000 r/min and 37 kw. The inducer is followed by a collector with a rectangular cross section whose area is constant along the circumference (Figure 5). In this study, the rotational speed of the inducer is set to 6000 r/min. Results and discussion Pump performance Head characteristics of the pump are presented in Figure 6. The head decreases with increasing flow rate 0 m 1 m 1. Water tank 2. Turbine flow meter 3. Settling chamber 4. Test inducer 5. Collector 6. Torque meter 7. Motor 8. Booster pump 9. Regulating valve 10. Heat exchanger Figure 4. Plane view of inducer performance test facility.

4 314 Proc IMechE Part C: J Mechanical Engineering Science 227(2) within the tested flow rate range. It is shown in Figure 6 that the head from the pump test without the inducer is almost identical to the value of the pump with the inducer, which means that the effect of the inducer on the head characteristic of the pump is negligible for the pump. Efficiency characteristics of the pump are presented in Figure 7. The efficiency increases with flow rate within the tested flow rate range. Throughout the tested flow range, the efficiency of the pump without the inducer is relatively about 1% higher than that of the pump with the inducer, which implies that the hydraulic efficiency of the inducer is lower than that of the impeller. Moreover, this result shows that the effect of the inducer on the efficiency characteristic of the pump is still small for the pump. A cavitation performance test is conducted near a design flow coefficient condition, and the results are shown in Figure 8. The dimensionless required NPSH (based on the critical condition of 3% head drop), or NPSH re =,nom is about for the pump with inducer. Without the inducer, however, NPSH re =,nom increases up to This means that the cavitation performance without the inducer deteriorates remarkably compared to the pump with the inducer. Additionally, the shapes of the cavitation performance curves are different from each other: with a further decrease in the NPSH after the critical COLLECTOR with inducer without inducer FLOW INDUCER η pump / η pump, nom Q/Q nom Figure 5. Schematic of test section of inducer performance test facility. Figure 7. Effect of inducer on pump efficiency with inducer without inducer with inducer without inducer Q/Q nom NPSH Figure 6. Effect of inducer on pump head. Figure 8. Effect of inducer on pump cavitation performance.

5 Hong et al. 315 condition, the head decreases slowly for the case without the inducer, while the head drop is very sharp for the pump with the inducer. The difference between the cavitation performance curves may result from the fact that an inducer can function even on cavitating condition. Once the cavitation at the inducer reaches the threshold, then even a slight decrease in the suction pressure might result in a serious cavitation at the main impeller. During the cavitation test, the pump vibration is measured by attaching an accelerometer on the pump volute casing in the radial direction. The acceleration signal is acquired at a sampling rate of 50 khz throughout the cavitation test. The characteristics of the pump vibration are shown in Figure 9 together with the pump head, which are identical to the results in Figure 8. No filter is applied to the acceleration signal, and the root mean square (RMS) value is presented. The vibration level increases when the head decreases due to the pump cavitation, and it increases sharply at the region of head breakdown. It is interesting that the vibration level in the case without the inducer is higher than that with the inducer even at the NPSH above the critical cavitating condition. The cavitation bubble at the impeller should have a larger effect on the vibration level than that of the inducer because the power consumed at the impeller is much higher than that of the inducer. There seems to be almost no bubble at the impeller in the case with the inducer at NPSH=,nom above 0.006; however, it seems that there is a small quantity of bubbles in the case without the inducer even at NPSH=,nom near Therefore, there seems to be a difference in the vibration level between the two cases. At the NPSH region 500 just before the critical cavitation condition, the vibration level experiences a dip for the case without the inducer, while there is no such dip for the case with the inducer. However, it is not clear yet why they show a different vibration behavior near the critical cavitation condition. Inducer performance To investigate the effect of the inducer on the pump performance in more detail, an autonomous test of the inducer, which is taken from the previously tested pump, is conducted. The inducer head is evaluated from the pressure difference between the inlet settling chamber and the collector. In Figure 10, the inducer head is presented together with the pump head. Both the inducer head and the pump head decrease with the flow rate, but the relative decrement of the inducer head is much greater than that of the pump head. In Figure 10, inducer head ratio ranges from 0.03 to 0.05 in the flow range Q=Q nom Typically, the inducer head forms 2 10% of the pump head. 19 A cavitation test of the inducer is carried out and the results are presented together with the results for the pump in Figure 11. The curves are obtained near a design flow coefficient condition. The shape of the cavitation performance curve of the inducer is very different from those of the pump. With a decrease in the NPSH, the inducer head decreases slowly, while the pump head drops abruptly. The slowly decreasing characteristic of the inducer head curve implies that the inducer could work even when the inducer cavitation develops to a certain extent. In turbopumps, cavitation occurs at an inducer first and then propagates downstream to the main impeller. In Figure 11, the pump with the inducer still maintains a normal head while the inducer head with inducer without inducer Casing acceleration RMS [m/s 2 ] Pump H inducer NPSH Inducer Figure 9. Effect of inducer on pump casing vibration at cavitation test. RMS: root mean square. Q/Q nom Figure 10. Head of pump and inducer.

6 316 Proc IMechE Part C: J Mechanical Engineering Science 227(2) Pump Inducer H inducer λ NPSH i [deg] Figure 11. Cavitation performance of pump and inducer. drops significantly, for example, by 40%. By extrapolating the cavitation curve of the inducer, it is found that the pump reaches a critical cavitation point when the inducer head drops by 55%. H inducer =,nom is about at the critical cavitation condition while at the non-cavitating one. For inducers applied to rocket turbopumps, one usually looks for at least a 10% head drop as a critical cavitation point and it is not uncommon to consider a 50% head drop. 6,8 Estimation of reduction of required NPSH of a centrifugal pump with inducer Takamatsu et al. 12 derived a formula to estimate the required NPSH of a centrifugal pump with inducer, and the formula is used in this study. The critical NPSH of a centrifugal pump without inducer is expressed by the following equation ½NPSH re Š m ¼ V2 2g þ W2 ð2þ 2g m where V and W are absolute and relative velocities, respectively, at the inlet of centrifugal impeller and an empirical coefficient, which is called dynamic depression coefficient, representing the local pressure drop on the blade inlet of a centrifugal impeller. The values under the critical cavitation condition of the centrifugal pump with and without inducer are denoted by ½Š mþi and ½Š m, respectively. The critical NPSH of a centrifugal pump with an inducer is expressed by the following equation ½NPSH re Š mþi þ½h i Š mþi ¼ V2 2g þ W2 2g mþi ð3þ Figure 12. Dynamic depression coefficient vs incidence angle on centrifugal impeller blade. 7 Therefore, the reduction of the required NPSH of a centrifugal pump by the attachment of an inducer is derived from equations (2) and (3) NPSH re ¼½NPSH re Š m ½NPSH re Š mþi ¼½H i Š mþi V2 2g þ W2 V2 2g mþi 2g þ W2 2g m ð4þ Now, we just have to calculate the right-hand side of equation (4) to obtain the reduction of the required NPSH, NPSH re : Note that the curly bracket on the right-hand side of equation (4) is h in equation (1). Calculation of h or the curly bracket on the righthand side of equation (4) is carried out step by step. First, the velocities at the impeller inlet without inducer are calculated from a simple velocity triangle on the condition of zero inlet prewhirl. Second, the velocities at the impeller inlet with inducer are calculated by a commercial computational fluid dynamics (CFD) computation. The velocities at the inlet of the centrifugal impeller are calculated here on non-cavitating conditions because the measured velocities at cavitating condition are very similar to those of non-cavitating one. 12 Third, the coefficient in equation (4) is obtained using a chart for centrifugal impeller (Figure 12) which Furukawa and Ishizaka 7 presented. The coefficient values for centrifugal, diagonal, and axial impellers are presented in Furukawa and Ishizaka. 7 The CFD used here is the three-dimensional Reynolds-averaged Navier Stokes method. 20 The authors used the method in previous studies to calculate the flow in inducers, and the method shows a good agreement with the experiments. 16,18 The method uses

7 Hong et al. 317 Pump w/ inducer w/o inducer Inducer ΔNPSH re by test ΔNPSH re by estimation H inducer Δh NPSH Figure 13. Grids for computation of inducer and impeller. an explicit Runge Kutta scheme and second-orderaccurate central-difference scheme with artificial dissipation for integration in time and space. The k-" turbulence model with an extended wall function is used to simulate the turbulence effects. A uniform flow condition is imposed at the inlet of the inducer and static pressure is assigned at the impeller outlet. Periodic boundary conditions are set at corresponding positions, because only one flow passage is solved for the inducer and impeller. Grids for the computation of inducer and impeller are presented in Figure 13. The cell number for the computation is 314,249. The value of y þ, the dimensionless quantity of the distance of the first grid point from the wall, is kept between 10 and 50 because the wall function is used. Mean value of velocity components is obtained by mass average of the computational flow results at the impeller inlet. Now, only ½H i Š mþi is unknown on the right-hand side of equation (4). To solve equation (4), a graphical method proposed by Takamatsu et al., 12 which is illustrated Figure 1, is applied to the present case, as shown in Figure 14. It is noted that the abscissa and the righthand ordinate in Figure 14 are drawn with the same scale. Equation (4) can be written as follows by rearranging it and using h ½H i Š mþi þ½npsh re Š mþi ¼½NPSH re Š m þ h where h is calculated by the previous procedure. Then, the 45 line intersects with the extrapolated cavitation curve of the inducer at NPSH=,nom Therefore, NPSH re =,nom is from the estimation, while it is from the test. ð5þ Figure 14. Comparison of reduction of the required NPSH by estimation and test. NPSH: net positive suction head. A few assumptions are used for the estimation, for example: (a) the velocities at the impeller inlet at the cavitating condition are similar to those of the noncavitating condition; (b) the cavitation curve is still valid at the extrapolated region; and (c) the effect of the rotational speed on the cavitation performance is negligible. In spite of the assumptions, the estimation shows a reasonable agreement with the test case in Figure 14. Conclusions Hydraulic performance tests for a fuel pump of a rocket engine turbopump are conducted in water environment in order to see the effect of an inducer on pump performance. The main results are summarized as follows: 1. By performing pump tests with and without an inducer, it is experimentally shown that the inducer has a negligible effect on the head and efficiency of the pump, but a significant effect on the cavitation performance. The critical NPSH of the pump increases more than three times when the pump is tested without the inducer. The above result might be unsurprising since this is the reason that inducers are used. However, the function of the inducer is illustrated by an experiment in this study. 2. By performing an autonomous inducer test, it is found that the pump reaches a critical cavitation point of 3% head drop when the inducer head drops by 55%. A 3% head drop is the acceptable operating regime for industrial pumps. However, for inducers applied to rocket turbopumps, one usually looks for at least a 10% head drop as a critical cavitation point.

8 318 Proc IMechE Part C: J Mechanical Engineering Science 227(2) 3. The shapes of cavitation performance curves (chart of head vs NPSH) with the inducer is different from the case without the inducer: with a further decrease in the NPSH after the critical condition, the head decreases slowly for the case without the inducer, while the head drop is very sharp for the pump with the inducer. Once the cavitation at the inducer reaches the threshold, then even a slight decrease in the NPSH could result in a serious cavitation at the main impeller. 4. The vibration level increases when the head decreases due to the pump cavitation, and it increases sharply at the region of head breakdown. The vibration level in the case without the inducer is higher than that of the case with the inducer at both the head breakdown and constant head regimes. 5. A reduction of the required NPSH of the centrifugal pump by attachment of an inducer is estimated considering the flow interference between the inducer and the centrifugal impeller. A formula and a graphical method which were introduced in Takamatsu et al. 12 are applied in this study. The estimating procedure is: (a) the velocities at the impeller inlet without inducer are calculated from a simple velocity triangle; (b) the velocities at the impeller inlet with inducer are calculated by a commercial CFD; (c) the dynamic depression coefficient is obtained using a chart; 7 and (d) a graphical method is used. The estimation shows a reasonable agreement with the test case in spite of a few assumptions. Funding This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. References 1. Jakobsen JK. Liquid rocket engine turbopump inducers. NASA SP-8052, Furst RB. Liquid rocket engine centrifugal flow turbopumps. NASA SP-8109, Sobin AJ and Bissell WR. Turbopump systems for liquid rocket engines. NASA SP-8107, Huzel DK and Huang DH. Modern engineering for design of liquid-propellant rocket engines. Washington: DC: AIAA, Brennen CE. Hydrodynamics of pumps. White River Junction, VT: Concepts ETI, Japikse D. Overview of commercial pump inducer design. In: Proceedings of the 9th international symposium on transport phenomena and dynamics of rotating machinery, Honolulu, Hawaii, February Furukawa A and Ishizaka K. Experimental data for basic design of pump inducer. In: Proceedings of the 9th international symposium on transport phenomena and dynamics of rotating machinery, Honolulu, Hawaii, February Bonhomme C, Rebattet C and Wegner M. Inducer design criteria. In: Proceedings of the 9th international symposium on transport phenomena and dynamics of rotating machinery, Honolulu, Hawaii, February Kamijo K, Yoshida M and Tsujimoto Y. Hydraulic and mechanical performance of LE-7 LOX pump inducer. AIAA J Propul Power 1993; 9(6): Tsujimoto Y, Yoshida Y, Maekawa Y, et al. Observations of oscillating cavitation of an inducer. ASME J Fluids Eng 1997; 119(4): Cervone A, Bramanti C, Rapposelli E, et al. Experimental characterization of cavitation instabilities in a two-bladed axial inducer. AIAA J Propul Power 2006; 22(6): Takamatsu Y, Furukawa A and Ishizaka K. Method of estimation of required NPSH of centrifugal pump with inducer. In: Proceedings of 1st China Japan joint conference on hydraulic machinery and equipment, Hangzhou, China, October 1984, pp Kim J, Hong SS, Jeong EH, et al. Development of a turbopump for a 30 ton class engine. AIAA paper , Balje OE. Turbomachines. New York: John Wiley & Sons, Kim DJ, Hong SS, Choi CH, et al. Performance tests of a fuel pump for a turbopump unit. In: Proceedings of the 6th KSME-JSME thermal and fluids engineering conference, EA05, Jeju, Korea, March Choi CH, Noh JG, Kim DJ, et al. Effects of floating-ring seal clearance on the pump performance for turbopumps. AIAA J Propul Power 2009; 25(1): Hong SS, Kim JS, Choi CH, et al. Effect of tip clearance on the cavitation performance of a turbopump inducer. AIAA J Propul Power 2006; 22(1): Choi CH, Noh JG, Kim JS, et al. Effects of bearing strut on the performance of a turbopump inducer. AIAA J Propul Power 2006; 22(6): Sutton GP and Biblarz O. Rocket propulsion elements, 8th edn. New York: John Wiley & Sons, NUMECA Fine/Turbo. Software Package, ver , NUMECA International, Brussels, Appendix Notation H NPSH re Q V W h NPSH re head required NPSH volumetric flow rate absolute velocity at the inlet of the centrifugal impeller relative velocity at the inlet of the centrifugal impeller head reduction due to flow interference between an inducer and the centrifugal impeller reduction of the required NPSH of a centrifugal pump by attachment of an inducer

9 Hong et al. 319 Subscripts dynamic depression coefficient efficiency m m þ i critical cavitation condition of the centrifugal pump without an inducer critical cavitation condition of the centrifugal pump with an inducer nom nominal flow coefficient condition at tested rotational speed