Proceedings Of The Institution Of Mechanical Engineers, Part A- Journal Of Power And Energy-2003-oh-659!64!2

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A practical approach to the hydraulic design and performance analysis of a mixed-flow pump for marine waterjet propulsion H. W. Oh, E. S. Yoon, K. S. Kim and J. W. Ahn Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 2003 217: 659 DOI: 10.1177/095765090321700610 The online version of this article can be found at: http://pia.sagepub.com/content/217/6/659

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659

Technical Note

A practical approach to the hydraulic design and performance analysis of a mixed-flow pump for marine waterjet propulsion H W Oh1*, E S Yoon2, K S Kim3 and J W Ahn3 1 Department of Mechanical Engineering, Chungju National University, Chungju, Chungbuk, South Korea 2 Thermo-Fluid System Department, Korea Institute of Machinery and Materials, Daejeon, South Korea 3 Marine Transport System Research Center, Korea Research Institute of Ships and Ocean Engineering, Daejeon, South Korea

Abstract: The hydraulic design optimization and performance analysis of a mixed-flow pump for marine waterjet propulsion has been carried out using mean streamline analysis and three-dimensional computational fluid dynamics (CFD) code. In the present study, conceptual design optimization has been formulated with a nonlinear objective function to minimize fluid dynamic losses and then a commercial CFD code was incorporated to allow for detailed flow dynamic phenomena in the pump system. The newly designed mixedflow model pump has been tested in the laboratory. Predicted performance curves by the CFD code agree fairly well with experimental data for a newly designed mixed-flow pump over the normal operating conditions. The design and prediction methods presented herein can be used efficiently as a unified hydraulic design process of mixed-flow pumps for marine waterjet propulsion. Keywords: waterjet propulsion, mixed-flow pump, mean streamline analysis, performance characteristics, commercial CFD code

NOTATION b Din Ds D2t Diff Diff g H Imp Imp Ns PS patm Pin ps Q

LE TE

LE TE

width diameter of the inlet duct nondimensional pffiffiffiffi specific diameter ¼ D2t (gH)0:25 = Q impeller diameter at the exit tip diffuser leading edge diffuser trailing edge gravitational acceleration total head (m) impeller leading edge impeller trailing edge nondimensional pffiffiffiffi specific speed ¼ o Q=(gH)0:75 pressure surface atmospheric pressure input shaft power (W) static pressure volume flow (m3/s)

The MS was received on 13 June 2003 and was accepted after revision for publication on 31 July 2003. *Corresponding author: Department of Mechanical Engineering, Chungju National University, 123 Geomdan-ri, Iryu-myeon, Chungju, Chungbuk 380-702, South Korea.

SS U2t V W Z

suction surface impeller speed at the exit tip absolute velocity relative velocity number of blades

Z r t f c o

pump efficiency ¼ rgHQ=Pin fluid density power coefficient ¼ 100
Pin =(ro3 D52t ) flow coefficient ¼ 10
Q=(oD32t ) head coefficient ¼ 10
gH=(o2 D22t ) impeller rotational speed (rad/s)

Subscripts diff imp u 3 4

diffuser impeller tangential component diffuser inlet diffuser exit

1

INTRODUCTION

The propeller as marine propulsion, which propels the marine vessel forward or astern with the aid of the difference

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in pressure on the two sides of the blade, has predominated over the past sail-dominated centuries. However, such a conventional propulsion system is not efficient for very large and high-speed vessels because of the degradation of propulsion performance, for example, flow-induced erosion, hydro-acoustic noise, overload, and so on. On the other hand, the waterjet propulsion system, which pressurizes the water and utilizes the reaction force created by the water leaving the pump as the thrust on the ship, has become the most favorable means of propulsion for increasingly larger and faster vessels. In addition, the waterjet absorbs approximately the same power, regardless of the ship speed at constant rotational speed, that is, the engine cannot be overloaded for maximum acceleration and stopping performance, and no reversible gear box is needed to propel the vessel backward. Koushan [1] suggested a preliminary design and offdesign prediction method for the early design stage of waterjet propulsion systems that is based on laws of conservation of mass and conservation of energy coupled with empirical parameters available in the open literature. Huntsman and Hothersall [2] developed the hydrodynamic design methodology for the waterjet impeller and diffuser components using their own quasi-three-dimensional method and fully three-dimensional flow solver and applied it to the design and performance analysis of a tubular-type waterjet pump of which the meridional shroud line is parallel to the rotational axis. In this paper, taking into account the marine waterjet propulsion system (as shown in Fig. 1) and the required operating conditions, the type of pump available for waterjet propulsion falls into the regime of mixed-flow pumps, 1.9 4 Ns 4 2.5, with the inclined hub and shroud lines in the meridional plane. The present study is aimed at carrying out hydraulic design optimization and performance analysis of a mixed-flow pump satisfying the required operating conditions for a marine waterjet propulsion system. First, in order to determine the overall meridional configurations as a conceptual design phase, this study adopts the design optimization and performance prediction codes developed in previous works [3–7], of which methods are based on mean streamline analysis using fluid dynamic loss correlations. Commercially available CFD software [8], one of the engineering software products whose predictive capabilities have already been verified in the open literature [9–11], has been employed to determine the detailed three-

dimensional configurations and predict the performance characteristic curves as well as the internal flow phenomena. The following procedure of design and performance analysis for a waterjet mixed-flow pump presented herein can be used efficiently as a practical design guide for industrial naval waterjet architects.

2

ANALYSIS VALIDATION FOR CFD CODE

The prediction performance of the commercial CFD code adopted in this paper is first validated in comparison with the experimental data. A schematic diagram and photograph of the test set-up is presented in Fig. 2. The waterjet experimental apparatus consists of a long duct-like section mounted inside the test section of the water tunnel. Basic experimental measurements including pump static pressure, discharge flow rate, shaft torque, and impeller rotational speed (rpm) are taken in the present study. The pump flow rate is calculated from the pressure difference across a downstream nozzle. Wall static pressures at the inlet and outlet planes are averaged in a manifold with four duct taps. The bulk total pressure, delivered head, is derived from the wall static pressure measurements and the assumption of uniform velocity at the inflow and outflow stations. Power absorbed is obtained from the torque transducer and rpm readings. Uncertainties are 1 per cent for head, flow rate, and shaft torque and 1 rpm for shaft speed. Unfortunately, no experimental data for specific flow field within the pump system were available in this study. The present study introduces nondimensional parameters to describe effectively the performance characteristics of a mixed-flow pump for a waterjet propulsion system. Figure 3a shows model pump I, a scale model based on an existing pump in the industry. The authors measured the threedimensional configuration data of the impeller and diffuser blades in order to obtain the specific geometric data for each blade because the blade data have been classified as competition sensitive by the industry. Figure 3b shows the comparison of predictions by the CFD analysis code and test results for model pump I. Although the computation overestimates the measured performance for the total head rise below the best efficiency point flow rate, the predicted overall characteristic curves for the total head, the shaft torque, and the efficiency are qualitatively accurate over the normal operating conditions. The employment of this CFD

Fig. 1 Overview of a marine waterjet propulsion system Downloaded from pia.sagepub.com at National Univ of Sciences and on November 21, 2011

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HYDRAULIC DESIGN AND PERFORMANCE ANALYSIS OF A MIXED-FLOW PUMP

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code for performance analysis required to design a new mixed-flow pump model in the present work has, therefore, been validated effectively.

3

Fig. 2

Experimental test rig: (a) schematic diagram of performance test set-up; (b) view of test section

CONCEPTUAL DESIGN OPTIMIZATION

In order to accomplish the specific geometric modeling of a three-dimensional configuration of turbomachinery, the overall configuration data have to be first specified, that is, the inlet/outlet blade angles and other design variables for each component in the meridional plane have to be determined from the preliminary design process. The present study adopted the design optimization code, which has been developed for the preliminary design works of mixed-flow pumps in a previous work [7], in order to carry out the conceptual design optimization of a mixedflow pump for marine waterjet propulsion. This paper states the loss of waterjet pump efficiency, 1 7 Z, as an objective function and then deals with a nonlinear constrained optimization problem. Most of the geometric/fluid dynamic design constraints herein are based on the field data suggested in past studies [6, 7] related to the design of industrial pumps. Table 1 summarizes several geometric constraints additionally required to design mixed-flow pumps for a marine waterjet propulsion system. Figure 4a depicts the overall meridional configuration of a mixed-flow pump system with geometric parameters. The waterjet propulsion system should satisfy the performance of the head coefficient 0.548 at the flow coefficient 0.756 as a mixed-flow pump with nondimensional specific speed 2.43, considering the design target based on the marine performance specifications in the present study. The specific speed and the specific diameter of the mixed-flow pump preliminarily optimized by using the mean streamline analysis are 2.43 and 1.74, respectively, which lie on the collected field data [12] of efficient turbomachines (Fig. 4b). This confirms that the present conceptual design optimization for the meridional configuration has been accomplished within the reliable design regime. As a reference, the Cordier line Table 1 Additional design constraints for a waterjet pump system

Fig. 3 Model pump I for marine waterjet propulsion: (a) side view of pump system; (b) performance characteristic curves

1 2 3 4 5 6 7 8 9 10 11 12 13

0.25 4 D1h/D1t 4 0.50 13 4 d1 4 30 15 4 d2 4 60 30 4 yimp 4 55 4 4 Zimp 4 9 1.45 4 Dc/Din 4 1.48 1.73 4 Lz,diff/Lz,imp 4 1.80 0.6 4 D4t/D3t 4 0.9 1.0 4 b4/b3 4 1.5 15 4 d3 4 60 25 4 d4 4 40 40 4 ydiff 4 55 4 4 Zdiff 4 10

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Fig. 4

Conceptual design optimization of model pump II: (a) meridional cross-section of mixed-flow pump with geometric parameters; (b) the specific speed—specific diameter diagram

[12], which is a fair curve through the field data in the statistical sense, is represented in Fig. 4b.

4

DETAILED DESIGN OPTIMIZATION OF A MIXED-FLOW PUMP FOR WATERJET PROPULSION

The detailed design work on the impeller and diffuser blades proceeds in consideration of the overall meridional geometric data determined from conceptual design optimization. The present study decides the three-dimensional configuration by using the commercial CFD code that enables the user to create a new blade geometry and analyse three-dimensional viscous flow phenomena within the blade passage in a graphical environment. Investigating the internal flow dynamics and the performance characteristics of a pump system, the iterative design process is continued until geometric feasibility is reached. Some representative design factors to be considered first in accomplishing detailed design optimization can be briefly stated as follows. The blade angles along the streamline should be distributed such that the blade loadings (velocity distributions) are optimized along the blade surface and the incidence, defined as flow angle minus metal angle, has to be designed to improve the pump efficiency and the pump performance at the off-design points. In the case of the diffuser, in order to improve the propulsive performance and the maneuverability of a waterjet pump, the blading has to

be achieved to minimize the tangential (or swirl) velocity component, Vu, at the exit of the diffuser. In accordance with the above detailed design procedure, a new designed scale model, model pump II, of a mixed-flow pump for a waterjet propulsion system is specifically represented in Fig. 5a. In the present study, the incidence angles are established at 5.6 to 6.4 from hub to tip in order to properly distribute the blade loading within the impeller, which leads to an increase in the momentum of incoming fluid onto the pressure surface of the impeller blade. As can be seen in Fig. 5b, the computed relative velocities normalized by the impeller exit tip speed are ideally distributed around the blade surface for the design flow rate (f ¼ 0.756), which means that the impeller blading has been well optimized in the present work. As for the diffuser blading, this paper in particular focuses on the fact that the tangential velocity component at the diffuser exit should be diminished and the total head performance must fall in the feasible margin of the required condition (c ¼ 0.548) at the design flow rate and rotational speed (f ¼ 0.756). Under these circumstances, it is observed that the component matching between the impeller exit flow angle and the inlet blade angle of diffuser becomes unattainable. Consequently, an excessively negative incidence (15.5 ) near the inlet tip of the diffuser blade appears and thus the flow separation on the inlet suction surface inevitably occurs, which deteriorates the structure of the flow field at the diffuser inlet. Figure 5c shows such reversed flow phenomena between the pressure and suction surfaces near the tip of the diffuser inlet. Figure 5d represents the calculated results for the massaveraged tangential velocity distribution through the diffuser passage at the design flow rate. As can be seen, the normalized tangential velocity at the diffuser exit remains satisfactorily constant at about 0.025. Figure 5e shows that the predicted static pressure of a pump system, model pump II, gradually increases without any abrupt change in the performance curve. Finally, a comparison between the predicted and required total head characteristics of model pump II at the design flow rate (f ¼ 0.756) is illustrated in Fig. 5f. It is noted that the agreement between the calculation (c ¼ 0.530) and the required design condition (c ¼ 0.548, denoted by w) is nearly accurate in the engineering consideration. The present study made a scale model, model pump II, of a mixed-flow pump for waterjet propulsion and carried out an experiment on the performance characteristics of this model pump II, whose configuration was obtained from the previously mentioned conceptual and detailed design optimization procedures. As can be seen in Fig. 5f, the experimental data are in fairly good agreement with the computed performance curves, which were prepared prior to the experimental work, over the normal operating conditions. Although the predicted curve for the total head seriously overestimates the measured data at the low flow region, the overall performance characteristics between them yield the best agreement

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Fig. 5

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Detailed design of model pump II for marine waterjet propulsion: (a) three-dimensional configurations for impeller (left), diffuser (right) respectively; (b) relative velocity distributions around the impeller at the design flow rate; (c) absolute velocity distributions around the diffuser at the design flow rate; (d) tangential velocity distribution through the diffuser at the design flow rate; (e) overall static pressure distribution of pump system at the design flow rate; (f) performance characteristic curves Downloaded from pia.sagepub.com at National Univ of Sciences and on November 21, 2011

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over the operating flow regime. The torque curve estimated by the CFD code slightly overpredicts the test data; however, the overall performance trends are in good agreement. While such differences between the predicted and measured performances for the torque have an effect on the efficiency curve, it is found that the calculated efficiency curve underpredicts the experimental data near the design flow rate, and the experimental result shows that the model pump II yields hydraulically excellent performance, that is, the best efficiency of model pump II amounts to more than 90 per cent.

5

CONCLUSIONS

A practical design and performance analysis procedure of a mixed-flow pump for marine waterjet propulsion, in which the conceptual approach to turbomachinery design using mean streamline analysis is followed by detailed design and analysis based on validated CFD code, has been presented in this paper. The additional design constraints for the waterjet mixed-flow pump system selected in the present study have been described in Table 1. Performance curves predicted by a coupled CFD code were compared with the test data of a newly designed, hydrodynamically efficient, mixed-flow pump. The results agree very satisfactorily with the measured performance curves over the normal operating conditions. Future work is definitely needed to improve the blade loadings of the diffuser blade by investigating the incidence distributions along the diffuser leading edge and to eliminate the swirl component of the diffuser exit flow. It would also be of great interest and value if research work could be carried out to obtain an optimal configuration of a mixedflow pump for a waterjet propulsion system under the simultaneous consideration of pump efficiency and net positive suction head required to suppress blade cavitation.

ACKNOWLEDGEMENTS This work was supported by the Dual-Use Technology Program of the Ministry of Science and Technology. The authors express their sincere gratitude and would also like to thank all of the people who have helped them in carrying out this research project.

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