- Email: [email protected]
Experimental Investigation into the performance of the Axial-Flow-Type Waterjet according to the Variation of Impeller Tip Clearance
ARTICLE IN PRESS
Ocean Engineering 34 (2007) 275–283 www.elsevier.com/locate/oceaneng
Experimental Investigation into the performance of the Axial-FlowType Waterjet according to the Variation of Impeller Tip Clearance Moon-Chan Kim, Ho-Hwan Chun Department of Naval Architecture and Ocean Engineering, Pusan National University, Pusan 609-735, Korea Received 11 August 2005; accepted 21 December 2005 Available online 11 May 2006
Abstract The tip clearance inside the duct from the tip of the impeller is very important to the performance of waterjet systems, which fact has been proven in the pump field. The tip clearance is especially important on the model scale because it is very difficult in manufacture to keep the tip clearance constant and minimally small along the inside of the duct. In the present study, a flush-type waterjet propulsion unit (duct, impeller, stator, and nozzle) was designed for an amphibious tracked vehicle. Two impellers of different inner diameter were designed and manufactured in order to investigate the gap effect. Resistance and self-propulsion tests with a 1/5-scale model were conducted in PNU towing tank. The flow rate at the nozzle exit, the static pressure at the various sections along the duct and also the nozzle, the revolution of the impeller, and the torque, thrust, and towing forces at various advanced speeds were measured. Based on these measurements, the performance was analyzed according to the ITTC 96 standard analysis method. Based on this analysis method, the full-scale effective and delivered power of the tracked vehicle was estimated according to the variation of tip clearance. r 2006 Elsevier Ltd. All rights reserved. Keywords: Axial-flow-type waterjet; Tip clearance; 1996 ITTC method
1. Introduction As the power and speed of ships have rapidly increased, a propulsion system more effective against severe cavitation has grown in demand. One of the most efficient systems for this purpose is the waterjet, propulsion system which has been used widely for fast ships; furthermore, the application of waterjet to marine vehicles has been increasing. The waterjet system is rather more similar to the pump than the conventional screw propeller; therefore, the tip clearance of the impeller inside the duct is very important to propulsion efficiency (Watterson et al., 1997; see Fig. 1). Studies on the effect of tip clearance on the waterjet system are rare, especially for the axial–flow type waterjet, in spite of their importance. In the present study, the effect of tip clearance was especially focused on to investigate its Corresponding author. Tel.: +82-51-510-2401; fax: +82-51-581-3718.
E-mail address: [email protected] (M.-C. Kim). 0029-8018/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.oceaneng.2005.12.011
relation to efficiency. In order to verify the gap effect, research on the design methodology and analysis of waterjets together with model testing, should be conducted initially. Although waterjet units have been widely used for the propulsion systems of marine vehicles, neither an effective method of model testing nor of performance analysis for the waterjet propulsion system has yet been established, due to highly complex hydrodynamic interactions in the duct-impeller-stator-hull system. The traditional concept of the thrust deduction method (ITTC, 1987) has some drawbacks in the analysis of the waterjet system: for example, sometimes a negative value is found, since water jets to the air and the trim difference between self-propulsion and resistance tests is large. Dyne and Lindell (1994) proposed a new momentum theory by using the frictional wake. Terwisga (1993) refined the definitions presented by ITTC (1987) and separated hull and jet characteristics. Many different approaches to the model testing and performance analysis are still being used by different organizations worldwide. Since the waterjet
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Fig. 1. Variation of efficiency with tip clearance ratio (Watterson et al. 1997).
Table 1 Definition of station numbers for ITTC standard analysis method Station no.
Location
0 1
In undisturbed flow far ahead of the vehicle Far enough in front of the intake ramp tangency point to avoid inlet losses Normal to the internal flow at the aft lip of the intake Just ahead of the pump Between pump and stator or between stages Behind stator At the nozzle outlet plane Behind the nozzle outlet plane where the static pressure coefficient in the jet is zero (vena contracta)
2 3 4 5 6 7
committee of the 21st ITTC (1996) suggested the momentum flux approach, this method, called the ITTC 96 standard method, has been widely adopted worldwide for waterjet performance analysis. For that reason, this method was applied to analyze the model test results in the present study. The details of the momentum flux method have been described in the 21st ITTC Report (1996), so they are omitted from this study. Specifics of the ITTC momentum flux method are shown in Fig. 1 and Table 1. A waterjet propulsion system has many advantages compared with a conventional screw propeller, especially for amphibious military vehicles, because of the good maneuverability at low speeds, the good operating ability in shallow waters, the high thrust at low speeds to aid maneuverability and exits from water, among others. Due to these facts together with other advantages, the demand for waterjet propulsion systems has rapidly increased as applied to ships as well as to various marine and amphibious vehicles. Although there has been much research on waterjet propulsion systems, studies on smallsize waterjet systems, especially for tracked vehicles are very rare. The authors have studied such small-scale
waterjet systems for application to tracked vehicles (Chun, 2001; Chun et al., 2001, 2002; Wang et al., 2001; Jun et al. 2002). To investigate the effect of tip clearance, resistance and self-propulsion for a 1/5-scale model of a tracked vehicle with the designed waterjet, tests were conducted at the PNU towing tank (L ¼ 86 m, B ¼ 5 m, T ¼ 3 m). The static pressures at the various sections along the duct (see Fig. 2) and also the nozzle was measured by a differential-type pressure sensor. Measurements of the revolution of the impeller, torque, and towing forces at various advance speeds using dynamometers and a tachometer were also conducted. Based on these measurements, the performance of the waterjet propulsion system was analyzed according to the ITTC 96 standard analysis method. The full-scale performance measures of the tracked vehicle with the waterjet system were estimated and compared according to the variation of the tip clearance. 2. Design of a Waterjet unit for tracked vehicle For amphibious military vehicles, there are a number of special and unique waterjet requirements, and the following are some that need to be taken into account for the present design:
High thrust at low speeds to aid maneuverability and exiting from waters. Resistance to cavitation when power is applied at low speeds, typically 5 to 12 km/h for amphibious vehicles. Since the vehicle runs over muddy ground, the tip of the impeller is wide and thick enough to resist wearing.
Taking the above factors into account, the duct-impellerstator unit was designed with an in-house CFD program developed for a waterjet flow analysis as reported by Chun et al. (2002). The impeller diameter was designed to 320 mm for the power absorption of NCR 250 PS. An axial-flow-type waterjet was chosen in consideration of its
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Fig. 2. Definition of station numbers and normalized energy flux for ITTC standard method and configuration of the duct with general arrangement of pump (stator, impeller and shaft) The numbers represent the pressure tab positions for pressure measurements; the numbering is equivalent to ITTC station numbering as given in Fig. 2 and Table 2; and the suffixes -a and -b are the additional points for CFD verification.
Table 2 Design particulars of the model waterjet pump (scale ratio, l ¼ 5) Impeller diameter (mm) Number of blades, Impeller Stator diameter (mm) Number of blades, Stator Nozzle diameter (mm) P/D of Impeller at 0.7 r P/D of Impeller (Mean)
64.0 4 64.0 9 33.58 1.54 1.45
compactness and weight, which are of prime importance for present military tracked vehicles. According to the requirement of resistance to cavitation, the pitch angle at the leading edge was reduced and the area ratio of the impeller blade was designed to be sufficiently large. In particular, the pitch at the tip was reduced to avoid cavitation, since the flow speed at the tip region of the blade is the fastest. According to the same concept, the chord length at the tip region was designed to be larger than those at the other radii. The right pitch angle of the stator leading edge is essential to achieve a good performance of the pump, since this plays an important role in straightening the rotating flow. The number of the stator blade, chosen as 9, also plays a key role in straightening the rotating flow, resulting in good pump efficiency. The thickness of the tip and the leading/trailing edges of the impeller and stator were designed to be thick enough to resist wear. The design particulars of the waterjet pump are shown in Table 2 and Fig. 3. The duct diameters were varied to investigate the gap effect, and the gap ratios nondimensionalized by the diameter of the impeller were chosen as 1.5% and 0.7%, respectively.
3. Model test and analysis 3.1. Model The 1/5 scale model was constructed by FRP, as shown in Fig. 4. The design dimensions of the tracked vehicle model are shown in Table 3. The stator and impeller are made of aluminum with diode ending, and the impeller is shown in Fig. 5. The after-part of the duct including the nozzle is made of aluminum, and the stator is built into this housing, the duct and stator being one body. Since the clearance between the impeller tip and the duct inner surface is so small, the stator and the inner surface of the duct are fabricated with the utmost precision focusing on the right-centering of the shaft. The configuration of the stator and impeller in the shaft assembly is shown in Figs. 6–8. 3.2. Resistance test Although a resistance test is not necessary in applying the ITTC 96 momentum flux method to the analysis of model test results, it is useful to have the resistance curve vs. the speed of the tracked vehicle model to better comprehend the performance characteristics. The test results are shown in Table 4 and Fig. 9. It can be noticed that the resistance increased rapidly with the increase of speed, whereas the trim angle, which was measured to be within 21, was not much changed. 3.3. Self-propulsion test The self-propulsion tests were conducted in the range of impeller rpm that can fully cover the full-scale performance at the design speed. The tests were conducted at 6 model
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Fig. 3. Schematic diagram of the design waterjet system.
Fig. 4. Tracked Vehicle Model.
Table 3 Design dimensions of the model vehicle (scale ratio, l ¼ 5) Length between perpendiculars (m) Length at designed water line (m) Breadth molded (m) Depth molded (m) Draught (m)
1.423 1.428 0.948 0.238 0.284
speeds, 0 (bollard condition), 0.87, 0.99, 1.12, 1.24, and 1.37 m/s, being roughly equivalent to the full-scale speeds of 0, 7, 8, 9, 10, and 11 km/h, respectively. Fig. 10 shows a photo of the model running in a self-propulsion test with the impeller rpm of 4000 at the three full-scale speeds of 9, 10, and 11 km/h. Under the bollard condition, the gross thrust obtained from the momentum change between the inlet and the outlet should be the same or similar (within the experimental error range) to the net thrust (say, the negative resistance) measured by the dynamometer. As shown in Table 5, the two results are almost the same, which shows the high reliability of the present experiments.
In order to maintain the same condition as for full-scale, the towing force (FD) should be determined in consideration of the difference of the full- and model-scale frictional resistances. Although the shape of a tracked vehicle is somewhat different from that of a conventional ship, the ITTC 78 extrapolation method for the towing force was applied. The analysis data for adjusting the towing force are given in Tables 6a–b for the two different tip clearance cases. In the present study, the full-scale delivered power was estimated from the torque measured in model tests by the scale law, because the full-scale data of the pump were not available. The analyzed overall performance coefficient was comparatively low due to its high loading condition, in spite of the axial-flow-type waterjet design. The difference of the overall performance coefficient according to the tip clearance difference is clearly seen in Tables 6a–6b. In the case of the large tip clearance, 1.5% of diameter, the efficiency was about 75% that for the clearance, 0.7% of diameter. This result shows that the performance is sensitively dependent on the tip clearance, perhaps more so than for the mixed-flow-type waterjet, because the convex curvature of the duct around the pump in the mixed-flow-type waterjet can better prevent the leakage of pressure from the gap than can the axial-flow-type waterjet.
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Fig. 5. Impeller model made of aluminum with diode ending.
Fig. 6. Assembled waterjet system: The inlet part of the duct was made by FRP and the after-part of the duct including the nozzle is made of aluminum. The stator is built into the housing, the duct and stator being a one body.
Fig. 7. Servo-motors, gear box, and waterjet system mounted on the model.
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Fig. 8. Pressure tab fitted with vinyl tubes.
Table 4 Resistance and trim test results Model speed VM (m/s)
Bare hull resistance R (N)
Trim angle t (1)
0.62 0.68 0.74 0.81 0.87 0.93 0.99 1.06 1.12 1.18 1.24 1.30 1.37
20.44 29.18 33.50 39.02 44.44 48.56 55.27 59.84 75.97 85.60 93.86 103.67 116.18
0.348 0.295 0.402 0.509 0.696 0.643 0.750 0.803 0.857 1.231 1.071 1.660 1.713
150 135 120
Resistance (N)
105 90 75 60 45 30 15 0 0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Model speed (m / sec) Fig. 9. Naked model resistance vs. speed.
1.3
1.4
1.5
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Fig. 10. Profile of the self-propulsion model tests at 9, 10, and 11 km/h.
Table 5 Comparison of net thrust and gross thrust under bollard condition (V ¼ 0) Speed of revolution N (rpm)
Flow rate per unit QJ (m3/s)
Net thrust Tnet (N)
Gross thrust Tgross (N)
Difference (%)
3200 3600 4000 4400 4800 5000
0.79 1.00 1.22 1.48 1.74 1.91
136.8 167.1 210.7 256.6 302.8 329.2
136.2 170.0 207.2 250.5 289.6 323.1
0.4 1.7 1.6 2.3 4.3 1.8
In addition, the high loading condition (DP is relatively high), in spite of the axial-flow-type, design, also might have an influence on the large difference. In the case of the large tip clearance (0.313%, 320-2a), the efficiency was increased according to the increase of impeller rotational velocity. This phenomenon is probably due to the tip blockage effect according to the high speed of impeller over the inside surface of the duct. This would be much more important in the prediction of full-scale performance because it is very difficult to maintain the same gap ratio for both the model and the full-scale system. The full-scale speeds estimated at 250 PS (NCR) per unit for the 1.5% and 0.7% tip clearances are to be 10.59 km/h and 11.55 km/h, respectively, as shown in Fig. 11. 4. Conclusions An axial-flow-type waterjet for an amphibious tracked vehicle was designed, and 1/5 scale model tests were conducted. In order to determine the effect of the impeller
tip clearance, two different waterjet configurations were designed and experimented on the towing tank. The ITTC 96 momentum flux method was used in the analysis of the model test results. The experimental accuracy was confirmed by an excellent agreement between the gross thrust obtained from the momentum change and the net thrust measured by resistance dynamometer under the bollard condition. The effect of the tip clearance is clearly seen in the present model test results. The efficiency difference according to the gap variation (1.5% of diameter and 0.7% of diameter) was about 25% in their overall efficiency. This difference is so large that the gap effect should be a very important consideration in enhancing the performance of waterjet systems. This would be still much more important in the prediction of full-scale performance, because it is very difficult to maintain the same gap ratio for both the model and the full-scale system. Experimentation on the scale effect is expected to be conducted in the near future to further establish the performance differences according to the gap effect.
Momentum flux (N)
M1
0.0024 0.0024 0.0024 Energy loss inlet z13
4002 4463 4877
Revolution n (rpm)
Ship speed VS (km/h)
9.02 9.98 11.03
Ship speed VS (km/h)
9.02 9.98 11.03
Ship speed VS (km/h)
9.02 9.98 11.03
1.12 1.24 1.37
Model speed VM (m/s)
(b) 1.12 1.24 1.37
Model speed VM (m/s)
1.12 1.24 1.37
0.0028 0.0028 0.0028
Area A3 (m2)
Revolution n (rpm)
3775 4183 4629
0.564 0.574 0.530
3775 4183 4629
Area A3 (m2)
Revolution n (rpm)
2.512 2.770 3.066
QJ/A3 (m/sec)
0.001 0.001 0.001
Nozzle z57
2.911 3.326 3.615
QJ/A3 (m/sec)
0.002 0.001 0.001
0.606 0.606 0.607
Pump efficiency (E5–E3)/PD
9.69 11.81 15.44
0.442 0.472 0.537
Pump efficiency (E5–E3)/PD
10.66 13.29 14.84
M1
Model speed VM (m/s)
0.385 0.104 0.073
z57
z13
0.153 0.154 0.155
Efficiency overall Zoa
94.02 116.58 143.49
M7
0.107 0.114 0.125
Efficiency overall Zoa
91.38 117.47 139.83
M7
94.34 129.77 175.23
Effective power PE (W)
84.23 104.65 127.90
M7M1
90.30 129.03 171.02
Effective power PE (W)
80.62 104.05 124.83
M7M1
615.85 843.35 1132.24
Shaft power 2pQns (W)
368.64 504.58 679.14
Jet system power (W) E7E1
843.44 1134.26 1363.85
Shaft power 2pQns (W)
370.50 534.59 678.81
Jet system power (W) E7E1
10529 13082 15988
Momentum flux M7M1 (N)
Full scale
373.50 511.27 687.19
Effective pump power (W) PPE ¼ E5E3
38.64 53.45 73.13
Effective power PE (ps)
0.0069 0.0077 0.0085
Flow rate per unit QJ (m3/s)
36.32 52.82 75.54
252.26 347.38 472.55
Shaft power PD (ps)
2.243 2.234 2.238
IVR
339.27 464.33 602.40
PD (ps)
PE (ps) M7M1 (N) 10078 13007 15604
Shaft power
2.600 2.682 2.639
IVR
Effective power
0.007 0.0081 0.0088
Flow rate per unit QJ (m3/s)
Momentum flux
Full scale
372.92 535.80 732.28
Effective pump power (W) PPE ¼ E5E3
282
4002 4463 4877
Nozzle
inlet
9.02 9.98 11.03
Momentum flux (N)
(a) 1.12 1.24 1.37
Energy loss
Ship speed VS (km/h)
Model speed VM (m/s)
Revolution n (rpm)
Table 6 Self-propulsion test analysis results (a) [320-2a] for small gap (0.7% of diameter) and (b) [320-2b] for large gap (1.5% of diameter)
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700 320-2a 320-2b
650 600
NCR
550
DHP (ps)
500 450 400 350 300 250 10.59
200 8
9
10
283
Dyne, G., Lindell, P., 1994. Waterjet testing in the SSPA towing tank. RINA Proceedings of . International Symposium on Waterjet Propulsion, vol. 2, pp. 1–8. ITTC, 1987. Report of the High Speed Marine Vehicle Committee. International Towing Tank Conference, pp. 304–313. ITTC, 1996. Report of the specialist committee on waterjets. 21th ITTC, 1996 International Towing Tank Conference, pp. 189–209. Jun, J.G., Park, W.G., Chun, H.H., Kim, M.C., 2002. Flow analysis on the inside of duct for the waterjet system of tracked vehicle with consideration of interaction of rotor and stator. Proceedings of Spring Meeting of the Society of Naval Architect of Korea, pp. 140–143. Wang, J.W., Chun, H.H., Cha, S.M., 2001. Viscous flow analysis on the inside of duct for the waterjet system of tracked vehicle. Proceedings of Spring Meeting of Society of Naval Architect of Korea, pp. 191–194. Watterson, J.K., Gillan, M.A., Raghunathan, S., Mitchell, R.D., 1997. Applications of computational fluid dynamics to a wave energy conversion device, Energy Conversion Engineering Conference, IELEC-97 Proceeding of the 32nd Intersociety, vol. 3, pp. 1976–1981.
11.55
11
12
VS (km /s) Fig. 11. Estimated delivered power vs. speed.
Acknowledgement This research was partly supported by the MOST (The Ministry of Science and Technology) and partly by the Advanced Ship Engineering Research Center. The support is gratefully acknowledged.
Reference Chun, H.H., 2001. A study on the resistance and propulsion of waterjet propulsion system, ADD Report UD000031(1), p. 125. Chun, H.H., Ahn, B.H., Cha, S.M., 2001. Experiment and analysis on the waterjet propulsion system of the tracked vehicle, Proceedings of Spring Meeting of Society of Naval Architect of Korea, pp. 146–150. Chun H.H., Park, W.K., Jun, J.G., 2002. Experimental and CFD analysis for rotor–stator interaction of waterjet pump, Proceedings of 24th Symposium on Naval Hydrodynamics, Japan, vol. III, pp. 98–111.
Further reading Allison, J.L., 1993. Marine waterjet propulsion, SNAME Transactions, vol. 101, pp. 275–335. Allison, J.L., Jiang, C.B., 1998. Modern tools for waterjet pump design and recent advances in the field, Proceedings of International Conference on Waterjet Propulsion , RINA, Paper No. 2, Amsterdam, pp.1–19. Choi, G.I., Min, K.S., Ann, Y.W., 1996. Waterjet propulsion model experiment for catamaran ship, Transactions of the Society of Naval Architects of Korea, vol. 33(1), pp. 65–76. Coop, H.G., Bowen, A.J., 1993. Hull-waterjet interaction mechanisms: theory and validation. International Conference on Fast Sea Transportation, FAST ’93, Yokohama, pp. 855–866. ITTC, 1978. Report of the Performance Committee. Proceedings of 15th ITTC, Hague. Taylor, T.E., Kimball, R.W., 1999. Experimental validation of a coupled lifting –surface/rans procedure for waterjet pump and design analysis. Proceedings of FAST ’99, Seattle, pp. 893–900. Terwisga, T.V., 1995. Theoretical model for the powering characteristics of waterjet-hull systems. International Conference on Fast Sea Transportation, FAST ’93, Yokohama, pp. 975–991. Terwisga, T.V., Alexander, K.V., 1995. Controversial issues in waterjethull interaction. International Conference on Fast Sea Transportation, FAST ’95, Lubeck-Travemunde, Germany, pp. 1235–1253. Terwisga, T.V., 1996. Waterjet-hull interaction. PhD Thesis, MARIN, The Netherlands, p. 295.
Ocean Engineering 34 (2007) 275–283 www.elsevier.com/locate/oceaneng
Experimental Investigation into the performance of the Axial-FlowType Waterjet according to the Variation of Impeller Tip Clearance Moon-Chan Kim, Ho-Hwan Chun Department of Naval Architecture and Ocean Engineering, Pusan National University, Pusan 609-735, Korea Received 11 August 2005; accepted 21 December 2005 Available online 11 May 2006
Abstract The tip clearance inside the duct from the tip of the impeller is very important to the performance of waterjet systems, which fact has been proven in the pump field. The tip clearance is especially important on the model scale because it is very difficult in manufacture to keep the tip clearance constant and minimally small along the inside of the duct. In the present study, a flush-type waterjet propulsion unit (duct, impeller, stator, and nozzle) was designed for an amphibious tracked vehicle. Two impellers of different inner diameter were designed and manufactured in order to investigate the gap effect. Resistance and self-propulsion tests with a 1/5-scale model were conducted in PNU towing tank. The flow rate at the nozzle exit, the static pressure at the various sections along the duct and also the nozzle, the revolution of the impeller, and the torque, thrust, and towing forces at various advanced speeds were measured. Based on these measurements, the performance was analyzed according to the ITTC 96 standard analysis method. Based on this analysis method, the full-scale effective and delivered power of the tracked vehicle was estimated according to the variation of tip clearance. r 2006 Elsevier Ltd. All rights reserved. Keywords: Axial-flow-type waterjet; Tip clearance; 1996 ITTC method
1. Introduction As the power and speed of ships have rapidly increased, a propulsion system more effective against severe cavitation has grown in demand. One of the most efficient systems for this purpose is the waterjet, propulsion system which has been used widely for fast ships; furthermore, the application of waterjet to marine vehicles has been increasing. The waterjet system is rather more similar to the pump than the conventional screw propeller; therefore, the tip clearance of the impeller inside the duct is very important to propulsion efficiency (Watterson et al., 1997; see Fig. 1). Studies on the effect of tip clearance on the waterjet system are rare, especially for the axial–flow type waterjet, in spite of their importance. In the present study, the effect of tip clearance was especially focused on to investigate its Corresponding author. Tel.: +82-51-510-2401; fax: +82-51-581-3718.
E-mail address: [email protected] (M.-C. Kim). 0029-8018/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.oceaneng.2005.12.011
relation to efficiency. In order to verify the gap effect, research on the design methodology and analysis of waterjets together with model testing, should be conducted initially. Although waterjet units have been widely used for the propulsion systems of marine vehicles, neither an effective method of model testing nor of performance analysis for the waterjet propulsion system has yet been established, due to highly complex hydrodynamic interactions in the duct-impeller-stator-hull system. The traditional concept of the thrust deduction method (ITTC, 1987) has some drawbacks in the analysis of the waterjet system: for example, sometimes a negative value is found, since water jets to the air and the trim difference between self-propulsion and resistance tests is large. Dyne and Lindell (1994) proposed a new momentum theory by using the frictional wake. Terwisga (1993) refined the definitions presented by ITTC (1987) and separated hull and jet characteristics. Many different approaches to the model testing and performance analysis are still being used by different organizations worldwide. Since the waterjet
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Fig. 1. Variation of efficiency with tip clearance ratio (Watterson et al. 1997).
Table 1 Definition of station numbers for ITTC standard analysis method Station no.
Location
0 1
In undisturbed flow far ahead of the vehicle Far enough in front of the intake ramp tangency point to avoid inlet losses Normal to the internal flow at the aft lip of the intake Just ahead of the pump Between pump and stator or between stages Behind stator At the nozzle outlet plane Behind the nozzle outlet plane where the static pressure coefficient in the jet is zero (vena contracta)
2 3 4 5 6 7
committee of the 21st ITTC (1996) suggested the momentum flux approach, this method, called the ITTC 96 standard method, has been widely adopted worldwide for waterjet performance analysis. For that reason, this method was applied to analyze the model test results in the present study. The details of the momentum flux method have been described in the 21st ITTC Report (1996), so they are omitted from this study. Specifics of the ITTC momentum flux method are shown in Fig. 1 and Table 1. A waterjet propulsion system has many advantages compared with a conventional screw propeller, especially for amphibious military vehicles, because of the good maneuverability at low speeds, the good operating ability in shallow waters, the high thrust at low speeds to aid maneuverability and exits from water, among others. Due to these facts together with other advantages, the demand for waterjet propulsion systems has rapidly increased as applied to ships as well as to various marine and amphibious vehicles. Although there has been much research on waterjet propulsion systems, studies on smallsize waterjet systems, especially for tracked vehicles are very rare. The authors have studied such small-scale
waterjet systems for application to tracked vehicles (Chun, 2001; Chun et al., 2001, 2002; Wang et al., 2001; Jun et al. 2002). To investigate the effect of tip clearance, resistance and self-propulsion for a 1/5-scale model of a tracked vehicle with the designed waterjet, tests were conducted at the PNU towing tank (L ¼ 86 m, B ¼ 5 m, T ¼ 3 m). The static pressures at the various sections along the duct (see Fig. 2) and also the nozzle was measured by a differential-type pressure sensor. Measurements of the revolution of the impeller, torque, and towing forces at various advance speeds using dynamometers and a tachometer were also conducted. Based on these measurements, the performance of the waterjet propulsion system was analyzed according to the ITTC 96 standard analysis method. The full-scale performance measures of the tracked vehicle with the waterjet system were estimated and compared according to the variation of the tip clearance. 2. Design of a Waterjet unit for tracked vehicle For amphibious military vehicles, there are a number of special and unique waterjet requirements, and the following are some that need to be taken into account for the present design:
High thrust at low speeds to aid maneuverability and exiting from waters. Resistance to cavitation when power is applied at low speeds, typically 5 to 12 km/h for amphibious vehicles. Since the vehicle runs over muddy ground, the tip of the impeller is wide and thick enough to resist wearing.
Taking the above factors into account, the duct-impellerstator unit was designed with an in-house CFD program developed for a waterjet flow analysis as reported by Chun et al. (2002). The impeller diameter was designed to 320 mm for the power absorption of NCR 250 PS. An axial-flow-type waterjet was chosen in consideration of its
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277
Fig. 2. Definition of station numbers and normalized energy flux for ITTC standard method and configuration of the duct with general arrangement of pump (stator, impeller and shaft) The numbers represent the pressure tab positions for pressure measurements; the numbering is equivalent to ITTC station numbering as given in Fig. 2 and Table 2; and the suffixes -a and -b are the additional points for CFD verification.
Table 2 Design particulars of the model waterjet pump (scale ratio, l ¼ 5) Impeller diameter (mm) Number of blades, Impeller Stator diameter (mm) Number of blades, Stator Nozzle diameter (mm) P/D of Impeller at 0.7 r P/D of Impeller (Mean)
64.0 4 64.0 9 33.58 1.54 1.45
compactness and weight, which are of prime importance for present military tracked vehicles. According to the requirement of resistance to cavitation, the pitch angle at the leading edge was reduced and the area ratio of the impeller blade was designed to be sufficiently large. In particular, the pitch at the tip was reduced to avoid cavitation, since the flow speed at the tip region of the blade is the fastest. According to the same concept, the chord length at the tip region was designed to be larger than those at the other radii. The right pitch angle of the stator leading edge is essential to achieve a good performance of the pump, since this plays an important role in straightening the rotating flow. The number of the stator blade, chosen as 9, also plays a key role in straightening the rotating flow, resulting in good pump efficiency. The thickness of the tip and the leading/trailing edges of the impeller and stator were designed to be thick enough to resist wear. The design particulars of the waterjet pump are shown in Table 2 and Fig. 3. The duct diameters were varied to investigate the gap effect, and the gap ratios nondimensionalized by the diameter of the impeller were chosen as 1.5% and 0.7%, respectively.
3. Model test and analysis 3.1. Model The 1/5 scale model was constructed by FRP, as shown in Fig. 4. The design dimensions of the tracked vehicle model are shown in Table 3. The stator and impeller are made of aluminum with diode ending, and the impeller is shown in Fig. 5. The after-part of the duct including the nozzle is made of aluminum, and the stator is built into this housing, the duct and stator being one body. Since the clearance between the impeller tip and the duct inner surface is so small, the stator and the inner surface of the duct are fabricated with the utmost precision focusing on the right-centering of the shaft. The configuration of the stator and impeller in the shaft assembly is shown in Figs. 6–8. 3.2. Resistance test Although a resistance test is not necessary in applying the ITTC 96 momentum flux method to the analysis of model test results, it is useful to have the resistance curve vs. the speed of the tracked vehicle model to better comprehend the performance characteristics. The test results are shown in Table 4 and Fig. 9. It can be noticed that the resistance increased rapidly with the increase of speed, whereas the trim angle, which was measured to be within 21, was not much changed. 3.3. Self-propulsion test The self-propulsion tests were conducted in the range of impeller rpm that can fully cover the full-scale performance at the design speed. The tests were conducted at 6 model
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Fig. 3. Schematic diagram of the design waterjet system.
Fig. 4. Tracked Vehicle Model.
Table 3 Design dimensions of the model vehicle (scale ratio, l ¼ 5) Length between perpendiculars (m) Length at designed water line (m) Breadth molded (m) Depth molded (m) Draught (m)
1.423 1.428 0.948 0.238 0.284
speeds, 0 (bollard condition), 0.87, 0.99, 1.12, 1.24, and 1.37 m/s, being roughly equivalent to the full-scale speeds of 0, 7, 8, 9, 10, and 11 km/h, respectively. Fig. 10 shows a photo of the model running in a self-propulsion test with the impeller rpm of 4000 at the three full-scale speeds of 9, 10, and 11 km/h. Under the bollard condition, the gross thrust obtained from the momentum change between the inlet and the outlet should be the same or similar (within the experimental error range) to the net thrust (say, the negative resistance) measured by the dynamometer. As shown in Table 5, the two results are almost the same, which shows the high reliability of the present experiments.
In order to maintain the same condition as for full-scale, the towing force (FD) should be determined in consideration of the difference of the full- and model-scale frictional resistances. Although the shape of a tracked vehicle is somewhat different from that of a conventional ship, the ITTC 78 extrapolation method for the towing force was applied. The analysis data for adjusting the towing force are given in Tables 6a–b for the two different tip clearance cases. In the present study, the full-scale delivered power was estimated from the torque measured in model tests by the scale law, because the full-scale data of the pump were not available. The analyzed overall performance coefficient was comparatively low due to its high loading condition, in spite of the axial-flow-type waterjet design. The difference of the overall performance coefficient according to the tip clearance difference is clearly seen in Tables 6a–6b. In the case of the large tip clearance, 1.5% of diameter, the efficiency was about 75% that for the clearance, 0.7% of diameter. This result shows that the performance is sensitively dependent on the tip clearance, perhaps more so than for the mixed-flow-type waterjet, because the convex curvature of the duct around the pump in the mixed-flow-type waterjet can better prevent the leakage of pressure from the gap than can the axial-flow-type waterjet.
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Fig. 5. Impeller model made of aluminum with diode ending.
Fig. 6. Assembled waterjet system: The inlet part of the duct was made by FRP and the after-part of the duct including the nozzle is made of aluminum. The stator is built into the housing, the duct and stator being a one body.
Fig. 7. Servo-motors, gear box, and waterjet system mounted on the model.
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Fig. 8. Pressure tab fitted with vinyl tubes.
Table 4 Resistance and trim test results Model speed VM (m/s)
Bare hull resistance R (N)
Trim angle t (1)
0.62 0.68 0.74 0.81 0.87 0.93 0.99 1.06 1.12 1.18 1.24 1.30 1.37
20.44 29.18 33.50 39.02 44.44 48.56 55.27 59.84 75.97 85.60 93.86 103.67 116.18
0.348 0.295 0.402 0.509 0.696 0.643 0.750 0.803 0.857 1.231 1.071 1.660 1.713
150 135 120
Resistance (N)
105 90 75 60 45 30 15 0 0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Model speed (m / sec) Fig. 9. Naked model resistance vs. speed.
1.3
1.4
1.5
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Fig. 10. Profile of the self-propulsion model tests at 9, 10, and 11 km/h.
Table 5 Comparison of net thrust and gross thrust under bollard condition (V ¼ 0) Speed of revolution N (rpm)
Flow rate per unit QJ (m3/s)
Net thrust Tnet (N)
Gross thrust Tgross (N)
Difference (%)
3200 3600 4000 4400 4800 5000
0.79 1.00 1.22 1.48 1.74 1.91
136.8 167.1 210.7 256.6 302.8 329.2
136.2 170.0 207.2 250.5 289.6 323.1
0.4 1.7 1.6 2.3 4.3 1.8
In addition, the high loading condition (DP is relatively high), in spite of the axial-flow-type, design, also might have an influence on the large difference. In the case of the large tip clearance (0.313%, 320-2a), the efficiency was increased according to the increase of impeller rotational velocity. This phenomenon is probably due to the tip blockage effect according to the high speed of impeller over the inside surface of the duct. This would be much more important in the prediction of full-scale performance because it is very difficult to maintain the same gap ratio for both the model and the full-scale system. The full-scale speeds estimated at 250 PS (NCR) per unit for the 1.5% and 0.7% tip clearances are to be 10.59 km/h and 11.55 km/h, respectively, as shown in Fig. 11. 4. Conclusions An axial-flow-type waterjet for an amphibious tracked vehicle was designed, and 1/5 scale model tests were conducted. In order to determine the effect of the impeller
tip clearance, two different waterjet configurations were designed and experimented on the towing tank. The ITTC 96 momentum flux method was used in the analysis of the model test results. The experimental accuracy was confirmed by an excellent agreement between the gross thrust obtained from the momentum change and the net thrust measured by resistance dynamometer under the bollard condition. The effect of the tip clearance is clearly seen in the present model test results. The efficiency difference according to the gap variation (1.5% of diameter and 0.7% of diameter) was about 25% in their overall efficiency. This difference is so large that the gap effect should be a very important consideration in enhancing the performance of waterjet systems. This would be still much more important in the prediction of full-scale performance, because it is very difficult to maintain the same gap ratio for both the model and the full-scale system. Experimentation on the scale effect is expected to be conducted in the near future to further establish the performance differences according to the gap effect.
Momentum flux (N)
M1
0.0024 0.0024 0.0024 Energy loss inlet z13
4002 4463 4877
Revolution n (rpm)
Ship speed VS (km/h)
9.02 9.98 11.03
Ship speed VS (km/h)
9.02 9.98 11.03
Ship speed VS (km/h)
9.02 9.98 11.03
1.12 1.24 1.37
Model speed VM (m/s)
(b) 1.12 1.24 1.37
Model speed VM (m/s)
1.12 1.24 1.37
0.0028 0.0028 0.0028
Area A3 (m2)
Revolution n (rpm)
3775 4183 4629
0.564 0.574 0.530
3775 4183 4629
Area A3 (m2)
Revolution n (rpm)
2.512 2.770 3.066
QJ/A3 (m/sec)
0.001 0.001 0.001
Nozzle z57
2.911 3.326 3.615
QJ/A3 (m/sec)
0.002 0.001 0.001
0.606 0.606 0.607
Pump efficiency (E5–E3)/PD
9.69 11.81 15.44
0.442 0.472 0.537
Pump efficiency (E5–E3)/PD
10.66 13.29 14.84
M1
Model speed VM (m/s)
0.385 0.104 0.073
z57
z13
0.153 0.154 0.155
Efficiency overall Zoa
94.02 116.58 143.49
M7
0.107 0.114 0.125
Efficiency overall Zoa
91.38 117.47 139.83
M7
94.34 129.77 175.23
Effective power PE (W)
84.23 104.65 127.90
M7M1
90.30 129.03 171.02
Effective power PE (W)
80.62 104.05 124.83
M7M1
615.85 843.35 1132.24
Shaft power 2pQns (W)
368.64 504.58 679.14
Jet system power (W) E7E1
843.44 1134.26 1363.85
Shaft power 2pQns (W)
370.50 534.59 678.81
Jet system power (W) E7E1
10529 13082 15988
Momentum flux M7M1 (N)
Full scale
373.50 511.27 687.19
Effective pump power (W) PPE ¼ E5E3
38.64 53.45 73.13
Effective power PE (ps)
0.0069 0.0077 0.0085
Flow rate per unit QJ (m3/s)
36.32 52.82 75.54
252.26 347.38 472.55
Shaft power PD (ps)
2.243 2.234 2.238
IVR
339.27 464.33 602.40
PD (ps)
PE (ps) M7M1 (N) 10078 13007 15604
Shaft power
2.600 2.682 2.639
IVR
Effective power
0.007 0.0081 0.0088
Flow rate per unit QJ (m3/s)
Momentum flux
Full scale
372.92 535.80 732.28
Effective pump power (W) PPE ¼ E5E3
282
4002 4463 4877
Nozzle
inlet
9.02 9.98 11.03
Momentum flux (N)
(a) 1.12 1.24 1.37
Energy loss
Ship speed VS (km/h)
Model speed VM (m/s)
Revolution n (rpm)
Table 6 Self-propulsion test analysis results (a) [320-2a] for small gap (0.7% of diameter) and (b) [320-2b] for large gap (1.5% of diameter)
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700 320-2a 320-2b
650 600
NCR
550
DHP (ps)
500 450 400 350 300 250 10.59
200 8
9
10
283
Dyne, G., Lindell, P., 1994. Waterjet testing in the SSPA towing tank. RINA Proceedings of . International Symposium on Waterjet Propulsion, vol. 2, pp. 1–8. ITTC, 1987. Report of the High Speed Marine Vehicle Committee. International Towing Tank Conference, pp. 304–313. ITTC, 1996. Report of the specialist committee on waterjets. 21th ITTC, 1996 International Towing Tank Conference, pp. 189–209. Jun, J.G., Park, W.G., Chun, H.H., Kim, M.C., 2002. Flow analysis on the inside of duct for the waterjet system of tracked vehicle with consideration of interaction of rotor and stator. Proceedings of Spring Meeting of the Society of Naval Architect of Korea, pp. 140–143. Wang, J.W., Chun, H.H., Cha, S.M., 2001. Viscous flow analysis on the inside of duct for the waterjet system of tracked vehicle. Proceedings of Spring Meeting of Society of Naval Architect of Korea, pp. 191–194. Watterson, J.K., Gillan, M.A., Raghunathan, S., Mitchell, R.D., 1997. Applications of computational fluid dynamics to a wave energy conversion device, Energy Conversion Engineering Conference, IELEC-97 Proceeding of the 32nd Intersociety, vol. 3, pp. 1976–1981.
11.55
11
12
VS (km /s) Fig. 11. Estimated delivered power vs. speed.
Acknowledgement This research was partly supported by the MOST (The Ministry of Science and Technology) and partly by the Advanced Ship Engineering Research Center. The support is gratefully acknowledged.
Reference Chun, H.H., 2001. A study on the resistance and propulsion of waterjet propulsion system, ADD Report UD000031(1), p. 125. Chun, H.H., Ahn, B.H., Cha, S.M., 2001. Experiment and analysis on the waterjet propulsion system of the tracked vehicle, Proceedings of Spring Meeting of Society of Naval Architect of Korea, pp. 146–150. Chun H.H., Park, W.K., Jun, J.G., 2002. Experimental and CFD analysis for rotor–stator interaction of waterjet pump, Proceedings of 24th Symposium on Naval Hydrodynamics, Japan, vol. III, pp. 98–111.
Further reading Allison, J.L., 1993. Marine waterjet propulsion, SNAME Transactions, vol. 101, pp. 275–335. Allison, J.L., Jiang, C.B., 1998. Modern tools for waterjet pump design and recent advances in the field, Proceedings of International Conference on Waterjet Propulsion , RINA, Paper No. 2, Amsterdam, pp.1–19. Choi, G.I., Min, K.S., Ann, Y.W., 1996. Waterjet propulsion model experiment for catamaran ship, Transactions of the Society of Naval Architects of Korea, vol. 33(1), pp. 65–76. Coop, H.G., Bowen, A.J., 1993. Hull-waterjet interaction mechanisms: theory and validation. International Conference on Fast Sea Transportation, FAST ’93, Yokohama, pp. 855–866. ITTC, 1978. Report of the Performance Committee. Proceedings of 15th ITTC, Hague. Taylor, T.E., Kimball, R.W., 1999. Experimental validation of a coupled lifting –surface/rans procedure for waterjet pump and design analysis. Proceedings of FAST ’99, Seattle, pp. 893–900. Terwisga, T.V., 1995. Theoretical model for the powering characteristics of waterjet-hull systems. International Conference on Fast Sea Transportation, FAST ’93, Yokohama, pp. 975–991. Terwisga, T.V., Alexander, K.V., 1995. Controversial issues in waterjethull interaction. International Conference on Fast Sea Transportation, FAST ’95, Lubeck-Travemunde, Germany, pp. 1235–1253. Terwisga, T.V., 1996. Waterjet-hull interaction. PhD Thesis, MARIN, The Netherlands, p. 295.