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Air Turbines for Wave Energy Conversion
Manabu Takao,Toshiaki Setoguchi
International Journal of Rotating Machinery , 2012, DOI: 10.1155/2012/717398
Abstract: This paper describes the present status of the art on air turbines, which could be used for wave energy conversion. The air turbines included in the paper are as follows: Wells type turbines, impulse turbines, radial turbines, cross-flow turbine, and Savonius turbine. The overall performances of the turbines under irregular wave conditions, which typically occur in the sea, have been compared by numerical simulation and sea trial. As a result, under irregular wave conditions it is found that the running and starting characteristics of the impulse type turbines could be superior to those of the Wells turbine. Moreover, as the current challenge on turbine technology, the authors explain a twin-impulse turbine topology for wave energy conversion. 1. Introduction Several of the wave energy devices being studied under many wave energy programs in the United Kingdom, Japan, Portugal, India, and other countries make use of the principle of an oscillating water column (OWC). Figure 1 shows the principle of OWC-based wave energy conversion and its energy conversion chain. In such wave energy devices, a water column which oscillates due to wave motion is used to drive an oscillating air column which is converted into mechanical energy. The energy conversion from an oscillating air column can be achieved by using a system of nonreturn valves for rectifying the airflow, together with a conventional turbine. Figure 1: Outline of oscillating water column (OWC) device. On the other hand, the non-return valves can be eliminated by the use of a self-rectifying air turbine, which inherently provides a unidirectional rotation for an alternating airflow. The Wells turbine (Figure 2) [1–13] is of this type and is one of the simplest and probably the most economical turbines for wave energy conversion. However, according to the previous studies, the Wells turbine has inherent disadvantages: lower efficiency, poorer starting characteristics, and higher noise level in comparison with conventional turbines. Consequently, in order to overcome these weak points, a number of self-rectifying air turbines with different configurations have been proposed and improved over decades [14–24]. Figure 2: Outline of Wells turbine. This paper describes the present state of the art of air turbines for wave energy conversion. The types of turbine included in the paper are summarized as follows.Wells type turbines:Wells turbine with guide vanes;turbine with self-pitch-controlled blades;biplane Wells turbine with guide vanes;contrarotating Wells turbine.Impulse turbines:impulse turbine with
A Twin Unidirectional Impulse Turbine for Wave Energy Conversion  [PDF]
Shinya Okuhara, Manabu Takao, Akiyasu Takami, Toshiaki Setoguchi
Open Journal of Fluid Dynamics (OJFD) , 2012, DOI: 10.4236/ojfd.2012.24A043
Abstract: A twin unidirectional impulse turbine has been proposed in order to enhance the performance of wave energy plant. This turbine system uses two unidirectional impulse turbines and their flow direction is different each other. However, the effect of guide vane solidity on the turbine characteristics has not been clarified to date. The performances of a uni- directional impulse turbine under steady flow conditions were investigated experimentally by using a wind tunnel with large piston/cylinder in this study. Then, mean efficiency of the twin impulse turbine in bidirectional airflow has been estimated by a quasi-steady analysis using experimental results in order to investigate the effect of guide vane solidity on the performance.
Computational Study on Micro Shock Tube Flows with Gradual Diaphragm Rupture Process  [PDF]
Arun Kumar Rajagopal, Heuy Dong Kim, Toshiaki Setoguchi
Open Journal of Fluid Dynamics (OJFD) , 2012, DOI: 10.4236/ojfd.2012.24A027
Abstract: Gas flows through micro shock tubes are widely used in many engineering applications such as micro engines, particle delivery devices etc. Recently, few studies have been carried out to explore the shock wave excursions through micro shock tubes at very low Reynolds number and at rarefied gas condition. But these studies assumed centered shock and expansion waves, which are generally produced by instantaneous diaphragm rupture process. But in real scenario, the diaphragm ruptures with a finite rupture time and this phenomenon will significantly alter the shock wave propagation characteristics. In the present research, numerical simulations have been carried out on a two dimensional micro shock tube model to simulate the effect of finite diaphragm rupture process on the wave characteristics. The rarefaction effect was simulated using Maxwell’s slip wall equations. The results show that shock wave attenuates rapidly in micro shock tubes compared to conventional macro shock tubes. Finite diaphragm rupture causes the formation of non-centered shock wave at some distance ahead of the diaphragm. The shock propagation distance is also drastically reduced by the rupture effects.
Numerical Study on the Effect of Unsteady Downstream Conditions on Hydrogen Gas Flow through a Critical Nozzle  [PDF]
Junji Nagao, Shigeru Matsuo, Toshiaki Setoguchi, Heuy Dong Kim
Open Journal of Fluid Dynamics (OJFD) , 2012, DOI: 10.4236/ojfd.2012.24014
Abstract: A critical nozzle (sonic nozzle) is used to measure the mass flow rate of gas. It is well known that the coefficient of discharge of the flow in the nozzle is a single function of Reynolds number. The purpose of the present study is to investigate the effect of unsteady downstream condition on hydrogen gas flow through a sonic nozzle, numerically. Navier-Stokes equations were solved numerically using 3rd-order MUSCL type TVD finite-difference scheme with a second-order fractional-step for time integration. A standard k-ε model was used as a turbulence model. The computational results showed that the discharge coefficients in case without pressure fluctuations were in good agreement with experimental results. Further, it was found that the pressure fluctuations tended to propagate upstream of nozzle throat with the decrease of Reynolds number and an increase of amplitude of pressure fluctuations.
Wells Turbine for Wave Energy Conversion —Improvement of the Performance by Means of Impulse Turbine for Bi-Directional Flow  [PDF]
Shinya Okuhara, Manabu Takao, Akiyasu Takami, Toshiaki Setoguchi
Open Journal of Fluid Dynamics (OJFD) , 2013, DOI: 10.4236/ojfd.2013.32A006

Wells turbine has inherent disadvantages in comparison with conventional turbines: relative low efficiency at high flow coefficient and poor starting characteristics. To solve these problems, the authors propose Wells turbine with booster turbine for wave energy conversion, in order to improve the performance in this study. This turbine consists of three parts: a large Wells turbine, a small impulse turbine with fixed guide vanes for oscillating airflow, and a generator. It was conjectured that, by coupling the two axial flow turbines together, pneumatic energy from oscillating airflow is captured by Wells turbine at low flow coefficient and that the impulse turbine gets the energy at high flow coefficient. As the first step of this study on the proposed turbine topology, the performance of turbines under steady flow conditions has been investigated experimentally by model testings. Furthermore, we estimate mean efficiency of the turbine by quasi-steady analysis.

A Twin Unidirectional Impulse Turbine for Wave Energy Conversion—Effect of Fluidic Diode on the Performance  [PDF]
Shinya Okuhara, Manabu Takao, Hideki Sato, Akiyasu Takami, Toshiaki Setoguchi
Open Journal of Fluid Dynamics (OJFD) , 2014, DOI: 10.4236/ojfd.2014.45033
Abstract: As a system using a conventional unidirectional air turbine in oscillating water column (OWC) based on a wave energy plant, a twin unidirectional impulse turbine topology has been suggested in previous studies. However, the average efficiency of the suggested twin turbine is considerably lower than that of a conventional unidirectional turbine in this topology because reciprocating air flow can’t be rectified adequately by a unidirectional turbine. In order to improve the efficiency, using fluidic diode is discussed. In this study, two different fluidic diodes were discussed by computational fluid dynamics (CFD) and a wind tunnel test. Further, its usefulness is discussed from a view point of the turbine efficiency. The fluidic diode was shown to improve rectification of the topology. However, it needs more improvement in regards to its energy loss in order to enhance the turbine efficiency.
A Study on the Half-Ducted Axial Flow Fan Designed by a Diagonal Flow Fan Design Method  [PDF]
Atsushi Kaji, Yoichi Kinoue, Norimasa Shiomi, Toshiaki Setoguchi
Open Journal of Fluid Dynamics (OJFD) , 2017, DOI: 10.4236/ojfd.2017.71002
Abstract: A study on the half-ducted axial flow fan designed by a diagonal flow fan design method was conducted. The rotor which has NACA65 blades was designed, calculated numerically, manufactured and tested experimentally. As a result of the design and CFD, the meridional streamline and three distributions of the meridional, tangential and radial velocity at inlet and outlet go well as designed values of the half ducted fan. On the other hand, the values of the meridional velocity and the tangential velocity are little smaller than the design values at the hub side of the radial distribution. The improvement of the design is prospected for this point, that is, the approach between the design value and the actual flow is prospected if the tangential velocity is assigned small at hub and is assigned large at the tip so as to accord the actual flow in the vortex design of the rotor blade. Then the designed half-ducted rotor with four NACA65 blades was fabricated by a three-dimensional printer and tested in the wind tunnel in order to validate the half-ducted design method. For the comparison between the design values and the experimental values at the design flow rate coefficient of φ = 0.264, the experimental values of the pressure rise coefficient ψ and the efficiency η are rather small than the design values, while the experimental value of the torque coefficient τ is almost the same as the design value. However, the experimental value of approximately 0.45 of the maximum efficiency is comparably large value considering for the limitation of the situation of half-ducted. For the comparison between the experimental values and the CFD values at φ = 0.264, the CFD values are almost the same values as the experimental values for all the values of ψ, τ and η. In addition, the tendencies of the CFD values when the flow rate coefficient changes are almost similar as the experimental tendencies, though the flow rate coefficient for the CFD values when ψ or η takes the peak value shifts toward larger flow rate. For the case at rotor outlet at φ = 0.264, two values of the meridional velocity and the tangential velocity are larger than the design values at the tip side of the radial distribution.
Computational Analysis of Mixing Guide Vane Effects on Performance of the Supersonic Ejector-Diffuser System  [PDF]
Fan Shi Kong, Heuy Dong Kim, Ying Zi Jin, Toshiaki Setoguchi
Open Journal of Fluid Dynamics (OJFD) , 2012, DOI: 10.4236/ojfd.2012.23005
Abstract: The flow field in the ejector-diffuser system and its optimal operation condition are hardly complicated due to the complicated turbulent mixing, compressibility effects and even flow unsteadiness which are generated inside the ejector- diffuser system. This paper aims at the improvement in ejector-diffuser system by focusing attention on entrainment ratio and pressure recovery. Several mixing guide vanes were installed at the inlet of the secondary stream for the purpose of the performance improvement of the ejector system. A Computational Fluid Dynamics (CFD) method based on Fluent has been applied to simulate the supersonic flows and shock waves inside the ejector. A finite volume scheme and density-based solver with coupled scheme were applied in the computational process. Standard k-ω turbulent model, implicit formulations were used considering the accuracy and stability. Previous experimental results showed that more flow vortexes were generated and more vertical flow was introduced into the stream under a mixing guide vane influence. Besides these effects on the secondary stream, the mixing guide vane effects on the shock system of the primary stream were also investigated in this paper. Optimal analysis results of the mixing guide vane effects were also carried out in detail in terms of the positions, lengths and numbers to achieve the best operation condition. The comparison of ejector performance with and without the mixing guide vane was obtained. The ejector-diffuser system performance is discussed in terms of the entrainment ratio, pressure recovery as well as total pressure loss.
Effects of Supersonic Nozzle Geometry on Characteristics of Shock Wave Structure  [PDF]
Shigeru Matsuo, Kousuke Kanesaki, Junji Nagao, Md. Tawhidul Islam Khan, Toshiaki Setoguchi, Heuy Dong Kim
Open Journal of Fluid Dynamics (OJFD) , 2012, DOI: 10.4236/ojfd.2012.24A019
Abstract: Interaction between the normal shock wave and the turbulent boundary layer in a supersonic nozzle becomes complex with an increase of a Mach number just before the shock wave. When the shock wave is strong enough to separate the boundary layer, the shock wave is bifurcated, and the 2nd and 3rd shock waves are formed downstream of the shock wave. The effect of a series of shock waves thus formed, called shock train, is considered to be similar to the effect of one normal shock wave, and the shock train is called pseudo-shock wave. There are many researches on the configuration of the shock wave. However, so far, very few researches have been done on the asymmetric characteristics of the leading shock wave in supersonic nozzles. In the present study, the effect of nozzle geometry on asymmetric shock wave in supersonic nozzles has been investigated experimentally.
Numerical Study on Transonic Flow with Local Occurrence of Non-Equilibrium Condensation  [PDF]
Shigeru Matsuo, Kazuyuki Yokoo, Junji Nagao, Yushiro Nishiyama, Toshiaki Setoguchi, Heuy Dong Kim, Shen Yu
Open Journal of Fluid Dynamics (OJFD) , 2013, DOI: 10.4236/ojfd.2013.32A007

Characteristics of transonic flow over an airfoil are determined by a shock wave standing on the suction surface. In this case, the shock wave/boundary layer interaction becomes complex because an adverse pressure gradient is imposed by the shock wave on the boundary layer. Several types of passive control techniques have been applied to shock wave/boundary layer interaction in the transonic flow. Furthermore, possibilities for the control of flow fields due to non-equilibrium condensation have been shown so far and in this flow field, non-equilibrium condensation occurs across the passage of the nozzle and it causes the total pressure loss in the flow field. However, local occurrence of non-equilibrium condensation in the flow field may change the characteristics of total pressure loss compared with that by non-equilibrium condensation across the passage of flow field and there are few for researches of locally occurred non-equilibrium condensation in a transonic flow field. The purpose of this study is to clarify the effect of locally occurred non-equilibrium condensation on the shock strength and total pressure loss on a transonic internal flow field with circular bump. As a result, it was found that shock strength in case with local occurrence of non-equilibrium condensation is reduced compared with that of no condensation. Further, the amount of increase in the total pressure loss in case with local occurrence of non-equilibrium condensation was also reduced compared with that by non-equilibrium condensation across the passage of flow field.

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