IOP Conference Series: Earth and Environmental Science Performance analysis of a counter-rotating tubular type micro-turbine by experiment and CFD To cite this article: N J Lee et al 2012 IOP Conf. Ser.: Earth Environ. Sci. 15 042025 View the article online for updates and enhancements. Related content - Analysis of a pico tubular-type hydro turbine performance by runner blade shape using CFD J H Park, N J Lee, J V Wata et al. - Study on the leakage flow through a clearance gap between two stationary walls W Zhao, J T Billdal, T K Nielsen et al. - Effect of inner guide on performances of cross flow turbine K Kokubu, K Yamasaki, H Honda et al. This content was downloaded from IP address 148.251.232.83 on 24/08/2018 at 03:08
Performance analysis of a counter-rotating tubular type micro-turbine by experiment and CFD N J Lee 1, J W Choi 1, Y H Hwang 2, Y T Kim 3 and Y H Lee 4 1 Graduate School, Dept. of Mechanical Engineering, Korea Maritime University, Busan, 606-791, Korea 2 Shinhan Precision Co., Gimhae, 621-883, Korea 3 Division of Marine System Engineering, Korea Maritime University, Busan, 606-791, Korea 4 Division of Mechanical and Energy Systems Engineering, Korea Maritime University, Busan, 606-791, Korea Email: lnj@pivlab.net Abstract. Micro hydraulic turbines have a growing interest because of its small and simple structure, as well as a high possibility of using in micro and small hydropower applications. The differential pressure existing in city water pipelines can be used efficiently to generate electricity in a way similar to that of energy being generated through gravitational potential energy in dams. The pressure energy in the city pipelines is often wasted by using pressure reducing valves at the inlet of water cleaning centers. Instead of using the pressure reducing valves, a micro counter-rotating hydraulic turbine can be used to make use of the pressure energy. In the present paper, a counter-rotating tubular type micro-turbine is studied, with the front runner connected to the generator stator and the rear runner connected to the generator rotor. The performance of the turbine is investigated experimentally and numerically. A commercial ANSYS CFD code was used for numerical analysis. 1. Introduction Hydropower has been around for a long time and there are many large hydroelectric facilities around the world. The concept of micro hydro power system is a popular and promising technology in renewable energy. Micro hydro power systems are capable of generating electricity up to a capacity of 100 kw [1] The enormous pressure inside the water pipeline grid that supplies water to homes can be harnessed to generate electricity. Currently, the excess pressure energy contained in the pipelines is wasted by pressure reducing valves. The technology presented here aims to capture this pressure energy to generate electricity. The energy can be captured by using a counter-rotating tubular type micro-turbine. The counter-rotating type machine for water power generation was first proposed by Kanemoto [2]. A lot on the concept followed afterwards [3-5]. The good thing about using this system is that the head and the flow rate are very stable. In addition, the installation is cheap, power generation is environmentally friendly and makes use of the energy which otherwise is wasted. Micro hydro power system using water from the city pipelines does not require large dams or complicated electricity conversion mechanisms. Electricity generated in this manner is often stable and reliable, economical and is a renewable source of electricity. Published under licence by Ltd 1
Some studies on the concept of generating electricity from pressure energy in pipelines have been done previously. Park et al. [6,7] presented results for a small hydro turbine that used the flow rate in sewerage pipes to generate electricity. They studied two plants from Chunrabuk-Do and Chungcheongbuk-Do (in Korea) and the output performance characteristics were studied. They discussed the design specifications such as the design flowrates and the electricity production. Kim et al. [8] investigated the use of a micro hydro turbine in sewerage pipes in Jeju, korea. They studied the flowrates in the pipes at the site and recommended the use of crossflow or tubular turbines for electricity generation. Kim et al. and Cho et al. [9,10] studied electricity generation from sewage water pipes in Sinchun, Korea. Saket [11] presented the technique of micro hydro power generation system based on sewage system that has been designed, developed, and practically implemented to provide reliable electric energy to a suitable load in the campus of the Institute of Technology, Banaras Hindu University, Varanasi, (Uttar Pradesh), India. He discussed several factors relating to the pipe, such as the design pressure, the roughness of the pipe's interior surface, method of joining, weight, ease of installation, accessibility to the sewage system, design life, maintenance, weather conditions, availability of material, related cost and likelihood of structural damage have been considered for design of a particular penstock for reliable operation of the sewage system. Lee [12] did a performance analysis of a small hydro tubular through model tests. Kim et al. [13] presented CFD studies on a tubular-type hydrodynamic performance for variable guide vane openings. Kim et al. [14] performed studies on the internal flow characteristics of a tubular-type hydro turbine for variable runner openings. Lee et al. [15] performed CFD performance studies on a counter rotating tubular type micro hydro turbine. In the present paper, a counter-rotating tubular type micro-turbine is studied, with the front runner connected to the generator stator and the rear runner connected to the generator rotor. The performance of the system is investigated experimentally and numerically. A commercial ANSYS CFD code was used for numerical analysis. The performance is studied by varying the water flowrates. Model tests, both experimentally and numerically, are normally performed to study the flow as well as performance prediction. To improve the performance, it is first necessary to understand the flow through all the components and then to determine the performances against various parameters in the design stage. Numerical analysis of models has made it possible to predict their efficiency and other characteristics early in the design stage. 2. Numerical and experimental procedure 2.1. Numerical Procedure Figure 1 shows the model simulated using ANSYS CFD. The total length of the turbine from inlet to outlet is 1.6m. The output power is transferred to a generator from the passageway for output power transfer using a belt and pulley system. There are five blades on the front runner, and four blades on the rear runner. The outer diameters of the blades are 280 mm. The pipe diameter at inlet and outlet (after diffuser) is 350 mm. For numerical calculations, the commercial ANSYS-CFX Version 11 code was used to solve the incompressible turbulent flow using SST turbulence model. The inlet of the calculation domain was assigned total head and at the outlet the flow rate was specified as boundary conditions. Unstructured grids, i.e. Tetra-prism grids, were used to generate mesh for the entire model using ICEM CFD as shown in Figure 2. The total number of nodes was approximately 3 million. All the calculations were conducted under steady state condition. 2
Figure 1. Schematic of the counter-rotating turbine Figure 2. Computational domain of the turbine model 2.2. Experimental Procedure The experimental equipment were manufactured and tested at the Shinhan Precision Company, Gimhae, Korea. The dimensions of the experimental equipment and the CFD model were the same. Figure 3 shows the experimental equipment. Figure 3. A) Whole view of experimental set-up. B) Front runner C) Rear runner Figure 4 shows a schematic diagram of the experimental set-up. A pump, model DSN-3530, with a rated flowrate of 13m 3 /min, circulated water from a 50 tonne (about 50000 L) water tank. An inverter controlled the pump flowrate. The flowrates were varied from 0.133 0.165 m 3 /s. A butterfly valve 3
was each installed before and after the turbine, to be shut down when setting up the turbine. A 6-pole motor was used as a generator. The maximum rated rpm was 1200 rpm. The generator was connected to the control panel. The generator torque and rpm, and the pressure drop across the turbine were recorded by the control panel. Figure 4. Schematic diagram of the experimental set-up The shaft power, effective head, and turbine efficiency for a gear ratio of 1:1 and 1:1.16 are presented for varying water flowrates. 3. Results Figure 5 shows the experimental characteristic curves of the turbine, for runner model RA54, at different flowrates and a gear ratio of 1:1. For this gear ratio, an almost constant turbine rotational speed of 1200 rpm was obtained. This was not desirable because it gave a low efficiency. As can be seen from Figure 5, the efficiency decreased continuously from 76.95% to 57.75% with increasing flowrate. The almost constant and limited rpm of the turbine limits the efficiency by limiting the water flow through the turbine. When the turbine rpm is high, more water will pass through it, therefore there will be less effective head, thus more efficiency. But in the current case, the speed limit of the turbine allows the turbine to act as a blockage to most of the water that would try to pass through the turbine. This reduces the exit flow, thus increasing the effective head and reducing the efficiency. Figure 6 shows the experimental and numerical results for a gear ratio of 1:1.6. The turbine rotational speed obtained in this case was close to 1800 rpm. It is seen that the efficiency increases with increasing water flowrate. A maximum efficiency of 73.94% is achieved in the system, for a flowrate of 0.155 m 3 /s. The high rotational speed of the turbine increases the efficiency by allowing a continuous water flow through the turbine. The turbine does not act as a blockage to the water that would try to pass through it, thus the effective head reduces and the efficiency increases. The CFD results predict the efficiency a little bit higher than the experimental results. The error is not so high, therefore it can be stated that there is good correlation with CFD and experiment results. Numerical analysis was done to validate the experimental results, as well as to observe the flow phenomena. 4
Figure 5. Experimental results for a gear ratio of 1:1 Figure 6. Experimental and numerical results for a gear ratio of 1:1.16 Figure 7 shows the velocity and pressure distribution for the front and rear blades for a flowrate of 0.133 m 3 /s and 0.155 m 3 /s. The rotation of the front runner causes the flow in the wake region of the front runner to be guided to the rear runner at an angle, which increases the rotational speed of the rear runner. This swirling flow gives the counter rotating torque to the rear runner. Thus the front runner also acts as a guide vane for the rear runner. When the flowrate increases, the velocities before the two runners also increase. The pressure distribution for the larger flowrate is higher compared to the smaller flowrate. The pressure loss across the front blade is larger compared to rear blade. Most of the pressure energy in the water is used by the front blade. 5
Figure 7. Velocity and pressure distribution for the front and rear runner blade A) 0.133 m 3 /s B) 0.155 m 3 /s Figure 8 shows the pressure distribution on the pressure and suction sides of the front and rear blades, for a flowrate of 0.155 m3/s. The pressure difference for the front runner is higher compared to the rear runner. The pressure distribution on the pressure side of the blades has some variations across the face of the blades. However, the pressure distribution on the suction side of the blades is almost constant. This is also shown in the Cp graphs in Figure 9. Figure 8. Pressure distribution on the faces of the front and rear blades, for a flowrate of 0.155 m 3 /s Figure 9 shows the Cp values at 10%, 50%, and 90% of the blades, for both the front and rear runners, for a flowrate of 0.155 m3/s. It can be seen that the front blade has higher pressure difference compared to the rear blade. The suction side for both the blades becomes almost constant after 0.2 6
R/RLT. For the front blade, for 10% and 50%, the constant suction pressure is due to the effects of the rear blade on the flow. For 90%, the flow effects on the front runner due to the presence of the rear runner is not as high as compared to 10% and 50%. However, for the suction side of the rear runner, the suction gets constant for all three positions. This could be due to the higher pressure region in the diffuser after the rear runner, which may lead to backflow. In future analysis, the diffuser can be moved further downstream to minimize the effects of backflow on the rear blade. Figure 9. Cp distribution on the front and rear runner blade for flowrate of 0.155 m3/s 4. Conclusions A counter-rotating tubular type micro-turbine is studied, with the front runner connected to the generator stator and the rear runner connected to the generator rotor. The studies are carried through experiments and ANSYS CFD. The shaft power, effective head, turbine efficiency for a gear ratio of 1:1 and 1:1.16 are presented for varying water flowrates. Numerical analysis is only done for a gear ratio of 1:1.16. It has been found through experiments that for a gear ratio of 1:1, the efficiency decreased continuously from 76.95% to 57.75% with increasing flowrate. This was due to the limitations to the water flow through the turbine due to the limiting rotational speed of the generator. For a gear ratio of 1:1.16, the efficiency increased with increasing flowrate, since the high rotational speed of the generator did not limit the flow of water through the turbine. A maximum efficiency of 73.94% is achieved in the system. Numerical results show that most of the pressure energy is used by the front runner. The rotation of the front runner causes a swirling flow in the wake, which gives a counter rotating torque to the rear blade. The front runner acts like a guide vane for the rear runner. The pressure distribution on the pressure side of the blades has variations across the face of the blades. However, the pressure distribution on the suction side of the blades is almost constant. The suction side for both the blades becomes almost constant after 0.2 R/RLT. This could be due to the higher pressure region in the diffuser after the rear runner, which may lead to backflow. In future analysis, the 7
diffuser can be moved further downstream to minimize the effects of backflow on the rear blade. Other future plans are to study the turbine performance using a Powder Brake rather than a generator, since the generator limited the rotational speed in the present studies. Also, cavitation as well different blade types and blade angles will be studied. Acknowledgments This work is the outcome of a Manpower Development Program for Marine Energy by the Ministry of Land, Transport and Maritime Affairs(MLTM) and New & Renewable Energy R&D program(2009t100100265) under the Ministry of Knowledge Economy, Republic of Korea. Nomenclature D Diameter of runner[m] g Gravitational acceleration [m/s 2 ] H Effective head [m] N Rotational speed [ rpm] P w Water power [kw] P s Q T η ρ Shaft power[kw] Volume flow rate[m 3 /s] Torque[N m] Turbine efficiency[-] Fluid density [kg/m 3 ] References [1] U.S. Department of Energy 2001 Energy Efficiency and Renewable Energy Small hydropower systems [2] Kanemoto T, Kaneko M, Tanaka D and Yagi T 2000 Trans. of the Japan Soc. of Mech. Eng. 99 1158 [3] Kanemoto T, Tominaga K, Tanaka D, Sato T, Kashiwabara T, and Uno M 2002 Trans. of the Japan Soc. of Mech. Eng. 2 205 [4] Kasai T, Usui M, Nakamura Y, Kanemoto T and Tanaka D 2010 Current Applied Phys. 10(2) S133-S136 [5] Kanemoto T 2010 Current Applied Phys. 107(2) S4-S8 [6] Park W S and Lee C H 2005 Capacity computation and application techniques of small hydro power Korea Institute of Energy Research 134-139 [7] Park W S and Lee C H 2005 Performance prediction of small hydro-power using sewage Korea Institute of Energy Research 70-75 [8] Kim G S, Han W, Mun I S and Park J G 2005 Micro hydropower generation by discharge water of sewage treatment plant in Jejudo The Korean Soc. for Power Sys. Eng. 22-27 [9] Kim D S and Hong W H 2005 A study on feasibility analysis for small hydro power plants in the sewage treatment plants Architectural Institute of Korea 223-230 [10] Cho E J and Hong W H 2006 A study on analyzing efficiency of small hydro power in the sewage treatment plant Architectural Institute of Korea 757-760 [11] Saket R K 2008 Design, development and reliability evaluation of micro hydro power generation system based on municipal waste water IEEE Elec. Power & Energy Conf. (Vancouver, Canada, October 6-7, 2008) [12] Lee W Y 1985 Performance analysis of the small hydro tubular turbine through the model test Korea Advanced Institute of Science and Technology [13] Kim Y T, Nam S H, Choi Y D, Hwang Y H, Nam C D and Lee Y H 2007 Tubular-type hydroturbine performance for variable guide vane opening by CFD 5th Int. Conf. on Fluid Mech.( Shanghai, China, August 15-19, 2007) 424-427 [14] Kim Y T, Nam S H, Cho Y J, Choi Y D, Nam C D and Lee Y H 2007 Internal flow characteristics of tubular-type hydroturbine for variable runner vane opening 9th Asian Int. Conf. on Fluid Machinery (Jeju, Korea, October 16-19, 2007) 198 [15] Lee N J, Park J H, Hwang Y H, Kim Y T and Lee Y H 2010 CFD performance analysis of a counter-rotating tubular type micro-turbine Renewable Energy (Yokohama, Japan, June 27- July 2, 2010) O-Sh-3-1 8