Computed effects of tip clearance on performance of impulse turbine for wave energy conversion
Introduction
For the last two decades, scientists have been investigating and defining different methods for power extraction from wave motion. These devices utilize the principle of an oscillating water column (OWC). OWC-based wave energy power plants convert wave energy into low-pressure pneumatic power in the form of bidirectional airflow. Self-rectifying air turbines (which are capable of operating unidirectionally in bidirectional airflow) are used to extract mechanical shaft power, which is further converted into electrical power by a generator. Two different turbines are currently in use around the world for wave energy power generation, Wells turbine, introduced by Dr. A. A. Wells in 1976 and Impulse turbine with self-pitch controlled guide vanes by Kim et al. [1]. Both these turbines are currently in operation in different power plants in Europe, Canada, Australia and Asia on an experimental, as well as a commercial basis. The present work deals with the Im pulse turbine. A 1.0-m diameter Impulse turbine with self-pitch controlled guide vanes was designed, fabricated and is being operated by National Institute of Technology at Vizhinjam, a site near Thiruvanandapuram, which is a city on the west coast of India [2]. The guide vanes pitch at the wave frequency. Such moving parts lead to maintenance and operating life problems and increased cost and hence the performance of the turbine with fixed turbine has been investigated by Maeda et al. [3].
There are few reports presented on the numerical analysis on Impulse turbine and Wells turbine. An optimal installation angle of the Impulse turbine has been investigated by numerical and experimental analysis (Kim et al. [4]). The performance of the Impulse turbine with unstructured grids and various turbulence models has been studied by Thakker et al. [5]. CFD analysis on CA9 Wells turbine has been made to validate the performance of the turbine and to analysis aerodynamics characteristics [6]. In all the earlier studies, tip clearance has not been incorporated in the numerical model. The tip leakage flow is one of the most prevalent and influential features of the flow through turbomachine rotors. In ad dition, the tip leakage flow is a phenomenon that is difficult to measure in most turbomachines. Computed effects of solidity on Wells turbine performance with tip clearance have been investigated by Watterson and Raghunathan [7]. The predicted effect of solidity on the turbine pressure drop, torque and efficiency agreed qualitatively and quantitatively with the experimental data. Few authors [8], [9], [10], [11] have been investigated the effect of tip clearance on the performance of Wells turbine experimentally and numerically with CFD codes and found that the turbine is very sensitive to tip clearance when compared to a conventional turbine. They have concluded that the decrease in tip clearance advances the stall but increases the cyclic efficiency as a result of reduced leakage losses. Also it has been proved that the turbine with a relatively large tip clearance could operate over a much wider range of flow rate range of flow rate without stalling. To investigate the ef fect of blade sweep on the performance of the Wells turbine, numerical investigation was carried out under steady flow condition with a fully 3-D Navier–Stokes code for two kinds of blades, NACA 0020 and CA9 by Kim et al. [12]. Extensive work has been performed in the realm of tip clearance studies on conventional turbine [13], [14], [15].
This paper describes the use of CFD method to investigate the effect of tip clearance on performance of Impulse turbine, which is working under bidirectional airflow for wave energy conversion. The method employs structured grids, which allow inclusion of such features as the blade tip and casing treatments. The 3-D CFD model has been generated with tip clearance to validate the computed results with experimental data. The study has shown that the numerical method is able to predict with reasonable accuracy; the variations of pressure drop across the turbine rotor, torque and efficiency with flow coefficient, and the effect of tip clearance. An optimum tip clearance has been suggested where the effect of tip clearance is almost negligible. Furthermore, the design tip clearance (1 mm) has been validated numerically. In addition, the performance of Impulse turbine with various tip clearances has been computed under irregular wave condition by using numerical simulation technique.
Section snippets
Review of experimental apparatus
A schematic layout of the experimental rig of Wave Energy Research Team at University of Limerick is shown in Fig. 1. It consists of a bell mouth entry, 0.6 m test section with a hub-tip ratio of 0.6, drive and transmission section, a plenum chamber with honeycomb section, a calibrated nozzle and a centrifugal fan. Air is drawn into the bell mouth shaped open end, it passes through the turbine and then enters the plenum chamber. In the chamber, the flow is conditioned and all swirls/vortices
Governing equations
Gambit 2.0 and FLUENT V6 were used for meshing and analyzing the problems, respectively. FLUENT V6 solves the Navier–Stokes equations for conversion of mass and momentum (, , , ). Additional conservations of k and ε equations are solved for turbulence closure. Governing Navier–Stokes transport equations are:
MASSMOMENTUM
Solver parameter
The solver treats each cell in the domain as a finite
Validation of numerical procedure
The present numerical model has been validated with the experimental data with 1% tip clearance. Fig. 5a–c show the comparison between computed and measured values for input coefficient, torque coefficient and efficiency against flow coefficient, respectively. From Fig. 5a, it can be observed that the computed values overpredict the measured values at high flow coefficients. But good agreement has been reached between computed and measured CT values, Fig. 5b. Computed efficiency of turbine
Conclusion
The present computational model has been validated with experimental results with reasonable accuracy and found to be suitable for further design analysis. It is found that k–ε turbulence model can predict the performance of turbine in the low rotational speed of turbine. The performance curves of the Impulse turbine with various tip clearances have been arrived at numerically. The flow physics of the blade passage flow interacting with tip leakage flow has been analyzed with the computed
Acknowledgements
The authors would like to acknowledge the financial support given by ESBI, Ireland and also by the Wave Energy Research Team, Department of Mechanical and Aeronautical Engineering, University of Limerick.
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