A numerical comparison of end-plate effect propellers and conventional propellers

9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China

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2010, 22(5), supplement :495-500 DOI: 10.1016/S1001-6058(09)60242-0

A numerical comparison of end-plate effect propellers and conventional propellers Hsun-jen Cheng 1, Yi-chung Chien 1, Ching-yeh Hsin 1*, Kuan-kai Chang 2 , Po-fan Chen 3 1

Department of Systems Engineering and Naval Architecture, National Taiwan Ocean University, Keelung, China 2 United Ship Design and Development Center, Taipei, China 3 CSBC Corporation, Kaohsiung, China E-mail: [email protected]

ABSTRACT: Unconventional propellers with end-plate effects such as Kappel propellers get designers’ attention due to the environmental concerns and energy saving problems. The computations have been carried out to compare the Kappel propellers and the conventional propellers, and the emphasis is put on the scale effects and the structural performance. The scaleeffect is first investigated, and the computational results show that the Kappel propeller has a larger scale effect than the conventional propeller. The structural analysis is then made, and the comparisons of the Kappel propeller and the conventional propeller show that the Kappel propeller suffers from a stronger stress concentration near the tip. KEY WORDS: end-plate effect, Kappel propeller, BEM, RANS, scale effect

1 INTRODUCTION As a result of fuel resource in high demand and global warning problems, energy saving becomes one of the important issues all around the world. For the purpose of global conservation and fuel cost reduction, the ship building industries have devoted to the development and designs of energy saving devices recently. The unconventional propellers with the end-plate effect have thus brought designers’ attentions[1-2]. The idea of end-plate was inspired by birds in nature. When birds fly in the sky, they will save strength by the control of alula, the feather of wings tip. This idea has been adopted by aircraft designers to the designs of winglets. It is then introduced to the marine propellers, such as Kappel propellers [3-4] and CLT propellers [5-6]. The Kappel propeller will be computed and analyzed in this

paper, and compared to the conventional propellers. Kappel propeller (Fig. 1) is a propeller with the tip smoothly banded toward the suction side, and the blade surface is non-planar which the conventional propeller blade surface is on the helical surface. Kappel and Andersen have conducted a long time research on this kind of propellers, and both the experiments and the full-scale tests have shown that it is more efficient than the conventional propellers [7-8]. The purpose of this paper is to investigate influence of the end-plate at the tip to the propeller performance. The computational results of the Kappel propellers will be compared to the results of a conventional propeller. The emphasis will be put on the scale effects (performances between the model scale and the full scale) and the structural analysis. Two different computational methods are used to analyze the flow field of propellers, and they are the potential flow boundary element method and the viscous flow RANS method. The computational results from both methods are compared to each other, and they are also verified to the experimental data. The computational results from the hydrodynamic analysis are then used for the structural analysis. 2 THE VISCOUS FLOW COMPUTATIONS 2.1 Grid generation For the propeller viscous flow computations, we need first to generate the grids of the propeller, and we use the GRIDGEN software to generate the grids. For a Kappel propeller, we use very dense grids

9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China around the propeller tip in order to accurately compute the flow around the propeller tip. This kind of grid system can also be beneficial to the computational accuracy to estimate the thrusts and torques. The structural grid is originally used around the surface of blades to ensure grids with a better orthogonality. However, it is more difficult to generate structural grids than unstructural grids, particularly to the geometry with a larger degree of curvature. In order to overcome this difficulty, we use the “Semi-Unstructural Grid”. It bases on the structural grid, and divides one structural cell into two triangular cells. Comparing to the structural grid, semi-unstructural grid has a better quality to the computational stability. For the whole computational domain, the unstructural grid is used outside the semi-unstructural grid layer, and the structural grid is used upstream and downstream. That is because the influence of upstream and downstream field to the blade is relatively small, and structural grid can save a lot of computational resources. Fig. 2 shows the grid around the propeller blade.

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Fig. 1 Computer depiction of a Kappel propeller

2.2 Boundary conditions After generating the computational grids, the boundary conditions are set to simulate the physical problems. We use the commercial CFD software, ANSYS-FLUENT, and the boundary conditions can be classified into the following four types: „ Upstream and downstream: We have to give an initial velocity in the velocity inlet, and set the outlet by pressure outlet. We also have to maintain a distance before the propeller blade to let inflow fully developed. „ Body surface: The solid body boundary condition should be satisfied on the body surface, and the wall boundary condition with no slip condition is set on the body surface. „ Periodic boundary condition: Since we are solving the steady flow problem, the forces and flow field is identical on each blade. Therefore, we will only solve one-blade with the periodic boundary condition such that each blade is axisymmetrical. „ Fluid: We choose fluid to represent the material of flow field, and the density is 998.2 kg/m3, dynamic viscosity coefficient is 1.003×10-3 kg/m-s. We use a propeller-fixed coordinate system, and the fluid rotates along the propeller center. Fig. 3 shows the set-up of the boundary conditions in the viscous flow computations.

Fig.2 Grid system around a Kappel propeller blade

Fig. 3 Set-up of boundary conditions in the viscous flow computations

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9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China

3 COMPUTATIONAL RESULTS For the force and flow field computations, both the potential flow boundary element method and the viscous flow RANS method are used. The boundary element method used here is a perturbation potential based, low-order method developed by Hsin [9], and the wake alignment is included in the computations. The commercial software ANSYS-FLUENT is used for the viscous flow computations [10], and GRIDGEN is used to generate the grid system. 3.1 Numerical results vs. experimental data We will make a comparison between the numerical results and the experimental data for both the Kappel propeller and a conventional propeller. This is to validate the computational accuracy. The energysaving devices usually only gain efficiency by several percents, a consistent grid system has to be established. For the Kappel propellers, a “Jacket” is first established around the propeller surface to build denser grids (Fig. 2). It is found that this kind of arrangement can control the grid density near the propeller surface effectively. Based on this grid system, the Kappel propeller KAP510 is used for the computations. Fig. 4 shows that the computational results from the one-million grids (grid I) and twomillion grids (grid II) are very close, and it indicates that the grid system used here is reliable. In order to make a comparison between the conventional propeller and the end-plate propeller, a conventional propeller referred as “CV1700” is used for the computations. “CV1700” is a well designed containership propeller with the same design requirement as the KAP510 propeller, and its K-J chart is shown in Fig. 5. In Fig. 5, the forces and efficiencies from both the experiments and computations are shown. “BEM” represents the computational results from the potential flow boundary element method, and “fluent” represents the computational results from the viscous flow computations. One can see that the computational results from both methods are within reasonable accuracy. We have made the same computations for the Kappel propeller KAP510, and the computational results and experimental data are shown in Fig. 4 and Fig. 6. From Fig. 4, one can see that the viscous flow method over-predicts the torque, and therefore under-predict the efficiency. On the other hand, Fig. 6 shows that the boundary element method predicts the forces better than the viscous flow method. However, the viscous flow method can compute the flow details, and Fig. 7 shows the vorticity strength at the down-steam of the KAP510 propeller computed by the viscous flow method.

Fig. 4 Computational results of a Kappel propeller KAP510 from viscous flow RANS method

Fig. 5 Computational results of a conventional propeller CV1700

Fig. 6 Computational results of a Kappel propeller KAP510 from the potential flow method

9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China

Fig. 7 Vorticity strength computed at the down-stream of the KAP510 propeller

Both the computational results and the experimental data show that the efficiency of Kappel propeller KAP510 is almost the same as the conventional propeller “CV1700” for the model scale. It means that more effort is needed for the Kappel propeller designs. 3.2 Scale effects The literature claims that the Kappel propeller has a larger scale effect [10], and we will investigate the scale effect by computations in this section. First, we have collected two experimental data carried out by NTOU (National Taiwan Ocean University) and HSVA, and their Reynolds numbers are listed in the Table 1 along with the full scale dimensions. In Table 1, the “extra” scale is an additional scale just for the computations. We then carry out the computations by the viscous flow RANS method, and the computational forces are shown in Fig. 8 and Fig. 9. In Figs. 8 and 9, the x-axis shows the log scale of the relative Reynolds number which based on the Reynolds number from the HSVA experiment. That is, we will set the HSVA experimental data as the unit “Relative Reynolds number”, and the others are then divided by the HSVA Reynolds number. The y-axis shows the computational and experimental thrust coefficients or torque coefficients. The full scale experimental data are estimated from the model scale experiments. From Fig. 8, one can see that the thrust coefficient increases as the Reynolds number increases for both the computational results and the experimental data. However, the computational results have a larger slope. Table 2 shows the slopes of these data. The experimental data show that both the conventional

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propeller and the Kappel propeller have the same slope, and the numerical results show that the Kappel propeller has a larger slope. That means the numerical results show that the Kappel propellers have a larger scale effect for thrusts. Fig. 9 shows the same comparison of the torque coefficients, and the slopes are also shown in Table 2. The torque coefficient decreases as the Reynolds number increases, and both the experimental data and the numerical results show that the Kappel propeller has a larger slope. That is, both agree that the Kappel propellers have a larger scale effect for torques. Note that the full scale data are estimated from the model scale experiments based on the method developed for the conventional propellers; therefore, an estimated method should be derived especially for the end-plate effect propellers. From this investigation, we find that the computational results agree with the literature, that the end-plate propellers have a stronger scale effect than the conventional propellers. Table 1 Dimensions of different scales for the investigations Va Rn D (m) /(m·s-1) 2.4×106 2.7191 0.2778 HSVA 4.0×106 5.0000 0.2500 NTOU 8.7×107 8.3413 3.3000 extra 8.4557 6.6000 full scale 1.8×108

sclae effect N /(r/min) 13.4700 27.5217 3.4783 1.7630

Table 2 Slopes from the Computational results of different scales for the sclae effect investigations Slope*104 (KQ) Slope*104 (KT) KAP510(Exp.) 0.3704 -3.6083 CV1700(Exp.) 0.3704 -2.6891 KAP510(Cal.) 1.3994 -1.2897 CV1700(Cal.) 0.5193 -0.8815

Fig. 8 Thrust coefficients computed from different Reynolds numbers

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9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China a larger deformation. 4 CONCLUSIONS

Fig. 9 Torque coefficients computed from different Reynolds numbers

3.3 Structural analysis We then move to the structural analysis of both propellers. The commercial software NASTRAN based on the finite element method is used for the structural computations. In the hydrodynamic analysis, the forces are computed on the surface of the propellers; therefore, the surface grids in the hydrodynamics analysis first have to be transferred to solid elements in the structural analysis. The surface pressure forces from the hydrodynamic analysis are then mapped to the finite element grids. The four-node tetrahedral mesh is first adopted for the computations, and the ten-node tetrahedral mesh is then used for the computations. The computational results show that the four-node tetrahedral mesh gives poor results due to its low order approximation. We then compare the conventional propeller “CV1700” and the Kappel propeller KAP510 using the ten-node tetrahedral meshes. Fig. 10 shows the computed stresses and geometry deformations of the conventional propeller “CV1700”, and Fig. 11 shows the computed stresses and geometry deformations of the Kappel propeller “KAP510”. It is obvious that the KAP510 has a stress concentration around the area that the Kappel rake begins. For full-scale propellers (propeller diameter = 6.6 meters), the deformation at the tip is 1.65cm for the CV1700 propeller, and 4.74cm for the KAP510 propeller, and the Kappel propeller has

The purpose of the presented paper is to investigate the end-plate effect on propellers. We have compared the performances of the end-plate effect Kappel propeller and a conventional propeller with the same design requirement by computational methods and experimental data. The hydrodynamic analysis first performed by the potential flow boundary element method and the viscous flow RANS method. The scale effect is then investigated. Finally, the stress and deformations are computed by the finite element method. The conclusions are as follows: z The potential flow boundary element method has better force predictions than the RANS computations; however, the RANS method can provide more details in flow field. z Both computational results and experimental data predict that trust will increase as the Reynolds number increases, and the torque will decrease as the Reynolds number increases. z Computational results have shown that the Kappel propellers have a larger scale effects for both the thrust and the torque. However, the experimental data show that the Kappel propellers have a larger scale effect only for the torque. In this paper, an integrated analysis by both the hydrodynamic analysis and the structural analysis has been adopted for the comparisons of the endplate effect propellers and the conventional propellers. The same procedure is expected to assist to the designs of newly developed unconventional propellers. ACKNOWLEDGEMENTS The study presented here is conducted in support of CSBC Corporation, Taiwan under ES-10 project and USDDC. The authors gratefully acknowledge CSBC and USDDC, for their continuous support research at National Taiwan Ocean University.

9th International Conference on Hydrodynamics October 11-15, 2010 Shanghai, China

Fig. 10 Computed stresses and geometry deformation of the conventional propeller “CV1700. The upper figure shows the stress and deformation on the suction side, and the lower figure shows the stress on the pressure side

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Fig. 11 Computed stresses and geometry deformations of the Kappel propeller “KAP510”. The upper figure shows the stress and deformation on the suction side, and the lower figure shows the stress on the pressure side

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