1 J. Marine Sci. Appl. (2013) 12: DOI: /s Numerical Study on the Influence of Boss Cap Fins on Efficiency of Controllable-pitch Propeller Ying Xiong 1, Zhanzhi Wang 1* and Wanjiang Qi 2 1. Department of Naval Architecture Engineering, Naval University of Engineering, Wuhan , China 2. The th Unit of People s Liberation Army, Yantai , China Abstract: Numerical simulation is investigated to disclose how propeller boss cap fins (PBCF) operate utilizing Reynolds-averaged Navier Stokes (RANS) method. In addition, exploration of the influencing mechanism of PBCF on the open water efficiency of one controllable-pitch propeller is analyzed through the open water characteristic curves, blade surface pressure distribution and hub streamline distribution. On this basis, the influence of parameters including airfoil profile, diameter, axial position of installation and circumferential installation angle on the open water efficiency of the controllable-pitch propeller is investigated. Numerical results show: for the controllable-pitch propeller, the thrust generated is at the optimum when the radius of boss cap fins is 1.5 times of propeller hub with an optimal installation position in the axial direction, and its optimal circumferential installation position is the midpoint of the extension line of the front and back ends of two adjacent propeller roots in the front of fin root. Under these optimal parameters, the gain of open water efficiency of the controllable-pitch propeller with different advance velocity coefficients is greater than 0.01, which accounts for approximately an increase of 1%-5% of open water efficiency. Keywords: boss cap fins; controllable-pitch propeller; open water efficiency; Reynolds-averaged Navier Stokes (RANS) Article ID: (2013) Introduction 1 When conventional propellers are running, a stream of strong vortex flow is usually generated near the propeller hub. The induced drag generated by hub vortex leads to decrease of efficiency of a propeller near the hub. To address this problem, boss cap fins can be installed additionally on the propeller. Boss cap fins can divert the flow behind bow caps which abates the intensity of hub vortex. In addition, the water flows out of the propeller hub and produces a rotating torque in the same direction of the propeller, thus, reducing the torque on the propeller shafts. Boss cap fins are effective means to improve the efficiency of the propeller. Ouchi and Ogura (1988), Ouchi and Tamashima (1989) provided the earliest research on propellers in Japan at Mitsui OSK Lines of Osaka. The experimentation proved Received date: Foundation item: Supported by the National Natural Science Foundation of China under Grant No *Corresponding author Harbin Engineering University and Springer-Verlag Berlin Heidelberg 2013 that if the boss cap fins attached to the propeller caps it could eliminate hub vortex. Also the generated drag, diverted the flow of water to reduce the rotating speed of wake flow, increased the axial velocity of wake flow to raise the open water efficiency by 3%-7%; and also reduced the vibration and noise generated by the propeller. The researchers further conducted experimental studies on the key parameters of the propeller and provided the research basis for the design of the boss fin caps. Gong and Zhou (1991) investigated the effect of boss cap fins on the performance of propellers and their functions experimentally, and determined the optimal scope of parameter values of boss cap fins. Gong and others found that when the propeller pitch ratio was in the range of , boss cap fins would increase the open water efficiency by 3%-7%; but when the pitch ratio was too small and the hub vortex was weak, it could decrease propeller efficiency. Therefore boss cap fins are more suitable for propellers with large pitch and strong hub vortex. The reverse open water experiment is often used to determine the influence of controllable-pitch propeller on propeller hydrodynamic performance, i.e. to place propeller shaft and open water tank on the upstream of propeller. The front support of the experimental device usually affects the uniformity of incoming flow, leading to certain error of experimental results. Therefore, more attention to the study of propeller hydrodynamic performance by numerical calculation is investigated. Hu and Zhou (1991) provided a theoretical calculation method for boss cap fins and also proposed a set of formulae to determine the performance and shape of boss cap fins based on lifting line theory. The numerical results obtained, were basically consistent with the experimental data. Hsin et al. (2008) presented a design procedure for a PBCF with the use of computational fluid dynamics (CFD). Wang et al. (2009) conducted numerical simulation over hydrodynamic performance of 4119 propellers with boss cap fins using the CFD software. The numerical calculation results showed that when the advance velocity coefficient was not too large, the boss cap fins attached to the propeller causing an increase in the thrust force and decrease in the torque, leading to an increase of the open water efficiency. As the advance velocity coefficient increased, the gain of propeller efficiency
2 14 Ying Xiong, et al. Numerical Study on the Influence of Boss Cap Fins on Efficiency of Controllable-pitch Propeller decreased gradually. Wang et al. (2010) further studied the effect of various parameters of boss cap fins on the open water efficiency. Ma et al. (2011) studied the energy saving effect of propeller boss cap fins using RANS and test methods, proving that boss cap fins led to the sharp decrease of the whole system torque, but it had little effect on the thrust force, and improved the efficiency of the propeller. Numerical calculation results further indicated that the energy saving efficiency of boss cap fins with full scale Reynolds numbers was much higher. Hassan et al. (2012) analyzed the presence of the PBCF in induced downstream effects of the propeller. The performance of PBCF at the end of the hub was evaluated by changing two parameters: the angle of installation of PBCF on the end of the hub and the phase angle between propeller and the PBCF. The research objectives provided by the literature review are mostly on the conventional propellers; however, controllable-pitch propeller has rarely been researched. The controllable-pitch propeller changes the blade pitch by the control mechanism in the hub. Therefore, it has a greater hub than conventional propellers and the intensity and shape of the hub vortex is different from the conventional propellers. In this paper the RANS method is used for numerical simulation of the hydrodynamic performance of a controllable-pitch propeller with and without boss cap fins under the designed pitch condition. Also the research investigated the effect of airfoil profile of boss cap fins, their diameter, axial position of installation and the circumferential angle of installation on the gain of the open water efficiency of the controllable-pitch propeller for the purpose of seeking optimal design parameters of boss cap fins for the controllable-pitch propeller. 2.2 Geometry The controllable-pitch propeller used for calculation is R-R AB propeller, and the main parameters of the propeller are shown in Table 1. Its hub diameter ratio is 0.292; the design pitch ratio at 0.7R is 1.445; and the pitch ratio at propeller root is According to the research results conducted by (Ouchi and Tamashima, 1989), the geometric pitch angle of boss cap fins is the same as the propeller roots. Therefore, the geometric pitch angle of boss cap fins in this paper is set at 49. The coordinate system of the numerical model is the Cartesian coordinate system. X axis points downstream, i.e. the direction of the incoming flow; Y axis is parallel to the reference line on a blade surface; the direction of Z axis is given by the right-hand rule. The models of controllable-pitch propeller without and with boss cap fins are shown in Fig. 1, respectively. (a) Without PBCF 2 Numerical model 2.1 Governing equations The propeller operating in open water satisfies RANS equation: ρ + ( ρu j ) = 0 t x j (1) (b) With PBCF Fig.1 Propeller model without and with PBCF P ( Ui) ( UU i j) t xj xi (2) U U i j 2 U k ' ' 0( ) 0 ij ( uu i j ) Fi xj xj xi 3 xk xj where Ui respectively; and P are the mean value of u i and p, Fi is the mean value of f i ; ' ui is the ' ' fluctuating value of u i, and - ρuu i j the Reynolds stress item associated with fluctuating value of turbulence in the equation, is an unknown variant. SST k-ω turbulence model is used to close the equation; this turbulence model has higher accuracy in the flow field simulation. For details see Zhang et al. (2005). Table 1 Dimensions of controllable pitch propeller model Parameter Propeller PBCF Diameter D/m Blade number 5 5 Hub diameter ratio d/d Pitch ratio P 0.7R /D Skew θ/( ) Rake X m /mm Airfoil profile NACA NACA Grid and boundary conditions Due to the complex geometric shape of the propeller, it is very difficult to divide the hexahedron mesh by establishing a single calculation domain, as the division of the tetrahedral
3 Journal of Marine Science and Application (2013) 12: grid may lead to a huge quantity of grids. The research paper, proposes to adapt the hybrid (structured and unstructured) mesh scheme as Liu et al. (2007). Firstly, a cylindrical internal calculation domain enclosing the propeller is established, with the front end of this calculation domain 2D away from propeller disk (D represents propeller diameter), and the tail end 3D away from propeller disk. The diameter of the calculation domain is 1.2D, which is large enough to build the cylindrical external calculation domain within the enclosed area. For mesh generation, the internal calculation domain enclosing the propeller is divided in unstructured mesh with a higher resolution and higher adaptability; propeller tip, leading edge, trailing edge and boss cap fins are locally encrypted, to capture important flow field information; and the external calculation domain is also divided in structured mesh to meet the higher demand for geometric shape. Data transfer is needed at the interface of two calculation domains using the interface model, and the entire calculation domain is thus, divided into about 2.8 million grid cells. The controllable-pitch propeller with boss cap fins and the mesh of calculation domain are shown in Fig. 2 and Fig. 3. Fig. 2 Grid on the controllable pitch propeller propeller is solved in MRF model. The SST k-ω turbulence model is chosen. The governing equation and turbulence model are discretized using the finite volume method, while the convection item and diffusion items are discretized using second-order central difference; a fully implicit numerical calculation method is adopted for the pressure-velocity coupling. 3 Numerical results Firstly, a grid sensitivity study is conducted for the controllable pitch propeller at J=1.1, using three grids: fine, medium and coarse. The fine grid is generated from the medium grid using a refinement factor 2 in each coordinate direction. The coarse grid is geometrically similar to the fine grid. The grid sizes and respective y + values are given in Table 2. The SST k-ω turbulence model is applied in these computations. Table 3 compares the computed thrust and torque coefficients of the three grids with the experimental value (Qian, 1994). Table 2 Grid sizes and y+ values of controllable pitch propeller Grid Number of cells y + Coarse Medium Fine The error in K T and K Q seems to decrease as the grid is refined. The difference of K T is about 0.86% between the fine and the coarse grids and 2.4% for K Q, all the grids predict the K T and K Q of the controllable pitch propeller satisfactorily. The grid study indicates that the K T and K Q values are generally not too sensitive to grid refinement, so the medium grid is used in the following numerical simulations. Table 3 Computed K T and 10K Q of controllable pitch propeller at J=1.1 Grid K T Error of Error of 10K K T /% Q 10K Q /% Coarse Medium Fine Fig. 3 Mesh of domain Boundary conditions: rotational speed of the controllable pitch propeller is r/min; inlet boundary is set as velocity inlet; velocity is determined according to advance velocity coefficient; turbulence intensity at the inlet is 0.01 and the eddy viscosity ratio is 5; outlet boundary is set as pressure outlet, with atmospheric pressure as the pressure value; the walls of the controllable-pitch propeller are set as the boundary conditions for surfaces that are not sliding. The fluid in the internal domain of the controllable-pitch Numerical simulation of the open water performance of the controllable pitch propeller without boss cap fins is first investigated. The numerical results are compared against the experimental results (Qian, 1994), as shown in Fig. 4. As seen in Fig. 4, under different advance velocity coefficients, the numerical results of thrust and torque coefficients are consistent with the experimental values. But as the numerical result of thrust coefficient is slightly lower than the experimental result and that of torque coefficient is slightly higher than the experimental results, the numerical result of efficiency differs significantly from the experimental value, but the numerical results are satisfactory
4 16 Ying Xiong, et al. Numerical Study on the Influence of Boss Cap Fins on Efficiency of Controllable-pitch Propeller on the whole. The numerical model and mesh generation established in this paper have proved to be reasonable and reliable. Fig. 6 Gain curve of controllable pitch propeller with PBCF Fig. 4 Open water performance of controllable pitch propeller According to the research in references (Ouchi and Tamashima, 1989; Gong and Zhou, 1991), when the influence of boss cap fins is taken into account, the parameters setting of boss cap fins has a greater influence on the gain of open water efficiency. In order to investigate the influence of boss cap fins on the open water efficiency of the controllable-pitch propeller, one boss cap fin is first selected for computation according to the suggestions in reference (Ouchi and Tamashima, 1989). The airfoil profile of boss cap fin is NACA16, and the boss cap fins is elliptically shaped, with the aspect ratio of 1.5; its geometric pitch angle is 49 and the radial pitch angle remains unchanged. The diameter is 0.385D, and the front end of the root of boss cap fins is D away from the tail end of propeller root; and the circumferential position of boss cap fins is the midpoint of the extension line of the front and back ends of two adjacent propeller roots in the front end of fin root. The open water performance of the controllable-pitch propeller with boss cap fins is compared with that without boss cap fins in Fig. 5, and the gain of efficiency of the propeller with boss cap fins is shown in Fig. 6. From Fig. 5 it can be seen: when the advance velocity coefficient is less than 1.0, the thrust coefficient of the controllable-pitch propeller with boss cap fins increases and the torque coefficient decreases. As a result, the open water efficiency of the controllable-pitch propeller increases, which is consistent with the actual situation observed experimentally. But as the advance coefficient increases, the gain of open water efficiency of the controllable-pitch propeller also reduces subsequently, and finally smaller than that of the parent propeller, which can be seen from Fig. 6. This resulted, because of the higher advance velocity coefficients, and the fact that the boss cap fins not only produced negative thrust, but also increased the torque on the controllable-pitch propeller. Figs. 7 and 8 show the pressure coefficient contours of the blade s suction side and pressure side when J=0.9 for the parent propeller and propeller with boss cap fins respectively. As can be seen from the figure, the low pressure area on the blade s suction side and pressure side of the propeller with boss cap fins is larger than that of the parent propeller, which shows that the thrust of the propeller with boss cap fins is higher than that of the parent propeller. As indicated by the pressure distribution of the pressure sides, the presence of boss cap fins leads to increase of pressure on the leading edge of propeller roots, and the low pressure area is concentrated on the leading edge compared with the parent propeller. On both suction side and pressure side, the pressure at the same point of controllable-pitch propeller with boss cap fins is lower than that of the parent propeller. Fig. 5 Open water performance of controllable pitch propeller with and without PBCF (a) Parent propeller
5 Journal of Marine Science and Application (2013) 12: (b) Propeller with PBCF Fig. 7 Contour of pressure coefficient on controllable pitch propeller suction side (a) Parent propeller, x/d=0.5 (a) Parent propeller (b) Propeller with PBCF, x/d=0.5 (b) Propeller with PBCF Fig. 8 Contours of pressure coefficient on controllable pitch propeller pressure side Fig. 9 shows the vorticity nephogram of the cross section of the controllable-pitch propeller in x direction at x/d=0.5 and x/d=1.0. As the figure displays, the vorticity of the parent propeller in x direction, behind propeller hub is significantly greater than that of the propeller with boss cap fins in both positions. The main reason is the fluid on hub surface flowed backward with the rotating controllable-pitch propeller when boss cap fins were not installed, and the water flow finally converged behind propeller hub to generate strong vortex. However, after the installation of boss cap fins, the water velocity distribution at propeller hub changed. As a result, the water flow did not converge at propeller hub after passing through the propeller hub and vortex intensity decreased, as Fig. 10 shows. If properly designed, the water flow at propeller hub can be effectively diverted by fins. Therefore, no hub vortex will be generated since there is roughly no water flow gathered behind the propeller hub. (c) Parent propeller, x/d=1.0 (d) Propeller with PBCF, x/d=1.0 Fig. 9 Contours of vorticity in x-axis on cross plane after controllable pitch propeller
6 18 Ying Xiong, et al. Numerical Study on the Influence of Boss Cap Fins on Efficiency of Controllable-pitch Propeller water efficiency, three axial installation positions, 0.03D, D and 0.04D are respectively selected for comparative computation utilizing NACA66 mod airfoil profile according to the suggestions in reference (Wang et al., 2009), with other parameters unchanged. The curves of, the difference between open water efficiency of controllable-pitch propellers with and without boss cap fins at the three axial positions under different advance velocity coefficients are shown in Fig. 11. (a) Parent propeller Fig. 11 Gain curves of PBCF with different axis setting positions (b) Propeller with PBCF Fig. 10 Distribution of streamline on controllable pitch propeller hub In order to study the effect of airfoil profile of boss cap fins on open water efficiency of the controllable-pitch propeller, the NACA16 airfoil profile and NACA66 mod airfoil profile are selected for comparative calculation, with other parameters unchanged. The numerical results of the open water efficiency of the controllable-pitch propeller with boss cap fins corresponding to the two airfoil profiles are shown in Table 4. It can be seen from Table 4 that the gain of efficiency of the controllable-pitch propeller using NACA66 mod airfoil profile is slightly higher than that using NACA16 airfoil profile. From Fig. 11 it can be seen that the gain of open water efficiency at the axial position of 0.04D is the highest. As the installation position approached the propeller blades, the gain of open water efficiency of the controllable-pitch propeller decreased gradually, because of the interference to induced velocity of propeller blades generated by fins to form reflux as the distance from boss cap fins to propeller became closer. However, the axial position should not be too distant, because the farther the distance from boss cap fins to propeller, the smaller the induced circumferential velocity at fins and the lower the efficiency would be. To study the effect of the diameter of boss cap fins on open water efficiency, three diameters, 0.35D, 0.385D and 0.42D, are respectively selected for comparative calculation using NACA 66 mod airfoil profile, with other parameters unchanged. The curves of, the difference between the open water efficiency of controllable-pitch propellers of three diameters with and without boss cap fins under different advance velocity coefficients are shown in Fig. 12. Table 4 Efficiencies of PBCF with different airfoil profile J NACA66 NACA The axial position of installation of boss cap fins refers to the distance from the front end of the root of boss cap fins to the tail end of propeller root. In order to study the effect of axial position of installation of boss cap fins on the open Fig. 12 Gain curves of PBCF with different radius From Fig. 12 it can be seen that the gain of efficiency of the controllable-pitch propeller is at its largest when the
7 Journal of Marine Science and Application (2013) 12: diameter of boss cap fins is 0.42D, and the ratio of the diameter of boss cap fins to the propeller hub is 1.438, which is not consistent with the range of 0.18D 0.33D as indicated in the literature by (Ouchi et al., 1989) that studied the conventional propeller as a research objective. The circumferential installation angle of boss cap fins is the position of the front end of PBCF roots on the extension line of the front and back ends of two adjacent propeller roots, 0 is supposed to be at the midpoint of the extension line. In order to study the effect of the circumferential installation angle on the open water efficiency, three circumferential installation angles, 12, 0 and 12 are respectively selected for comparative calculation (as shown in Fig. 13) using the NACA 66 mod airfoil profile, the axial installation position is 0.04D and the diameter is 0.42D, with other parameters unchanged. The curves of, the difference between the open water efficiency of controllable-pitch propellers with and without boss cap fins at the three circumferential installation angles under different advance velocity coefficients are shown in Fig. 14. From Fig. 14 it can be seen that when the front of PBCF root is located at the midpoint of the extension line of the front and back ends of two adjacent propeller roots, the gain of open water efficiency of controllable-pitch propeller is the highest; as the fins move forward or backward circumferentially, the gain decreases to a varying extent. The reason is that when the fins are installed at such position, the boss cap fins have the optimal diversion effect on incoming flow, and the optimal interference on controllable-pitch propellers, which leads to decrease of the overall torque, that improves the efficiency. (a) 12 (b) 0 (c) 12 Fig. 13 Chart of PBCF with different circumferential setting positions Fig. 14 Gain curves of PBCF with different circumferential setting positions 4 Conclusions In this paper the RANS method is used for numerical simulation on the hydrodynamic performance of a controllable-pitch propeller with and without boss cap fins under the designed pitch conditions, and the effect of parameters including airfoil profile of boss cap fins, diameter, axial position of installation and circumferential angle of installation on the gain of the open water efficiency of controllable-pitch propeller is studied. The analysis indicates: 1) When the advance velocity coefficient is small, the thrust of the controllable-pitch propeller with boss cap fins will increase as the torque decrease, resulting in an increase of efficiency. However, the gain of efficiency of the controllable-pitch propeller with boss cap fins will reduce subsequently with the increase of advance velocity coefficient, the efficiency will be even lower than the parent propeller when the advance coefficient is large enough, but this phenomenon could be avoided if suitable parameters of boss cap fins are selected. 2) Boss cap fins could change the distribution of flow velocity at the propeller hub, and cause partial change to the flow direction rotating with the propeller. The majority of the water flow will be blocked or abated by fins, and will not gather at propeller hub after flowing through the fins, leading to a decrease of the intensity of hub vortex. 3) With regard to similar controllable-pitch propellers, the
8 20 Ying Xiong, et al. Numerical Study on the Influence of Boss Cap Fins on Efficiency of Controllable-pitch Propeller boss cap fins of the NACA 66 mod airfoil profile have a good effect of increasing the gain of efficiency of controllable-pitch propellers. 4) There is an optimal axial installation position for boss cap fins. The optimal installation position for similar controllable-pitch propellers is about 0.04D, which is more distant away from the controllable-pitch propeller than for conventional propellers. 5) With regard to similar controllable-pitch propellers, the thrust effect generated is the optimal when the radius of boss cap fins is 1.5 times of propeller hub. This optimal radius is larger than that for conventional propellers. 6) For similar controllable-pitch propellers, the optimal circumferential installation position for boss cap fins is the midpoint of the extension line of the front and back ends of two adjacent propeller roots in the front of fin roots. The numerical simulation in this paper has been carried out by means of a fixed pitch. What requires to be further research is further recommended is the effect of boss cap fins on the gain of open water efficiency of controllable-pitch propellers under a varying pitch and how to select optimal design parameters. References Gong Qifu, Zhou Heyu (1991). A new type marine energy-saving device-pbcf. Journal of Wuhan University of Water Transportation Engineering, 15(2), Hassan G, Amin M, Abdollah A (2012). Numerical analysis of hub effect on hydrodynamic performance of propellers with inclusion of PBCF to equalize the induced velocity. Polish Maritime Research, 19(2), Hsin CY, Lin BH, Lin CC (2008). The optimum design of a propeller energy-saving device by computational fluid dynamics. The Proceedings of the Fifth Int. Conf. of Computational Fluid Dynamics (ICCFD), Seoul, Hu Zhian, Zhou Hanren (1991). A method of the characteristic calculation and shape decision for propeller boss cap fins. Journal of Hydrodynamics (Ser.A), 6(3), Liu Zhihua, Xiong Ying, Ye Jinming (2007). Study on the prediction of propeller open-water performance using RANS formula and multi-block hybrid meshes. Journal of Hydrodynamic (Ser. A), 22(4), Ma Yan, Xin Gongzheng, Shi Xiaoyong (2011). Numerical evaluation and experimental study of energy saving device propeller boss cap fins. Shipbuilding of China, 52(Special), Ouchi K, Ogura M (1988). A research and development of PBCF, improvement of flow from propeller boss. Journal of the Society of Naval Architects of Japan, 163(6), Ouchi K, Tamashima M (1989). Research and development of PBCF, new and practical device to enhance propeller efficiency. The 4 th International Symposium on Practical Design of Ships and Mobile Units (PRADS), Varna, Qian Xiaonan (1994). Open water test of R-R AB propeller. SJTU towing tank report. Shanghai Jiao Tong University, Shanghai. Wang Chao, Huang Sheng, Chang Xin (2009). The prediction of hydrodynamic performance of propeller boss cap fins. Ship and Ocean Engineering, 38(6), Wang Chao, Chang Xin, Huang Sheng (2010). Discussion of main parameters influencing assistant thrust efficiency of propeller boss cap fins. Journal of Naval University of Engineering, 22(4), Zhang Zhaoshun, Cui Guixiang, Xu Chunxiao (2005). Theory and modeling of turbulence, Tsinghua University Press, Beijing, Author biographies Ying Xiong was born in He received his PhD degree in Fluid Mechanics from Wuhan University of Technology. He is a professor at Naval University of Engineering. His current research interests include hydrodynamics, design and optimization of marine propulsors. Zhanzhi Wang was born in He is a PhD candidate at Naval University of Engineering. His current research interests include ship hydrodynamics and computational fluid dynamics. Wanjiang Qi was born in He received his master degree in Naval Architecture and Ocean Engineering from Naval University of Engineering. His current research interests include hull optimization and CFD.