Ashraful Alam

Natural convection of a low-Prandtl-number conductive fluid driven by a horizontal temperature gradient in an annular enclosure with a square cross section was investigated. The surface temperatures of the inner and outer cylinders were differentially maintained. A static magnetic field was applied in the azimuthal direction. A three-dimensional (3D) numerical simulation was performed for a part of an annulus divided into 20 or 28 equal parts. The natural convection found changes on the order of a two-dimensional (2D) steady, a 3D steady, a 3D non-half-symmetric simply periodic oscillatory, a 3D indefinite oscillatory, a 3D half-symmetric simply periodic oscillatory, and a 3D aperiodic oscillatory flow as the Hartmann number decreases. This transition pattern is identical to that as the Rayleigh number increases in the same system without a magnetic field. In high Rayleigh numbers, the transition is accompanied by an axisymmetric oscillation. A disturbance causing the transition consists of three modes as a 3D steady, a 3D half-symmetric oscillatory, and a 2D axisymmetric oscillatory mode. The Nusselt numbers in most 3D flows are smaller at low Rayleigh numbers and larger at high Rayleigh numbers than that in 2D flows at a same condition, while the kinetic energy of a 3D flow is necessarily smaller than that of a 2D flow.

Abstract In an oscillating water column (OWC) based wave energy device, a water column that oscillates due to the sea wave motion generates a bi-directional airflow in an air chamber, and finally, the bi-directional airflow driven air turbine converts the pneumatic energy into mechanical energy. The counter-rotating impulse turbine for bi-directional airflow has been proposed by M. E. McCormick of the United States Naval Academy in 1978. In a previous study, the authors investigated the effect of the turbine geometry on the performance of a counter-rotating impulse turbine for bi-directional airflow, and it was clarified that the efficiency of the turbine is higher than an impulse turbine with a single rotor for bi-directional airflow in a range of high flow coefficient. Moreover, this impulse turbine has a disadvantage that the efficiency in a range of low flow coefficient is remarkably low due to the deterioration of the flow between the two rotors. In this study, in order to make the counter-rotating impulse turbine practically compatible, the thickness of the middle vanes installed between the two rotors was changed, and the effect of the thickness on the turbine performance was investigated by the computational fluid dynamics (CFD) analysis. As a result, it was found that the efficiency of the counter-rotating impulse turbine with middle vanes increases as the thickness of the middle vanes decreased.

Wave energy can be converted into the electrical energy by using the oscillating water column (OWC) device with air turbine. An impulse turbine for bi-directional flow, as an air turbine in the OWC device, can be obtained a high torque in a wide range of flow coefficient, it has a bit of complicated structure though. Therefore, an impulse turbine with a simple cascade for bi-directional flow was employed in order to overcome the drawback of the complicated cascade, and its performance was investigated by using computational fluid dynamics (CFD) analysis. As the results, the impulse turbine with simple cascade showed peak the efficiency of η_p=0.479 at a setting angle of the guide vane θ =15°, inlet (or outlet) angle of the rotor of γ =80°, solidity of the guide vane of σ_r =2.02, and solidity of the rotor of σ_g =2.27

A twin-impulse turbine for bi-directional flow has been developed for wave energy converter. However, the previous studies elucidated that the mean efficiency of the twin turbine is much lower than that of the impulse turbine for a unidirectional flow because a portion of airflow passes through the reverse flow turbine whose efficiency is very low. Therefore, a fluidic diode was adopted in the twin-impulse turbine in order to reduce the air flow through the reverse flow turbine. In this study, the rectification effect of the fluidic diode was investigated where a bypass is introduced into a blunt body. A computational fluid dynamics (CFD) analysis was conducted to investigate the effect of fluidic diodes on the turbine performance. In this analysis, RANS equations were used as the governing equations and the standard k-ε model was used as the turbulence model. The computational domain is composed of a circular tube and fluidic diode, and the domain meshed with an approximately 1.5 million mesh elements. As a result, it was found that the rectification effect of the fluidic diode is enhanced by installing a blunt body with a bypass hole of diameter 20mm and 16o taper angle.

Wave energy can be converted into the electrical energy by using a wave energy converter. Impulse turbine for bi-directional airflow is a kind of air turbines for wave power generation. This turbine has a high efficiency in a wide range of flow rate thus, it can be converted the wave energy even with a large change in wave height. In addition, this turbine has a good starting characteristic because it can be obtained a high torque in low speed rotation. However, it has problems in maintenance and high production cost because of it complicated cascade. In this study, aiming to resolve this problem, an impulse turbine with a simple cascade was employed, and the turbine performance was investigate using by the computational fluid dynamics (CFD) analysis. As a result, it was found that the efficiency of impulse turbine with simple cascade is very close to the conventional impulse turbine, and the favorable turbine geometry has been clarified. The impulse turbine with a combination of setting angles of guide vane of θ=22.5˚, and setting angle of the rotor of γ=70˚, which obtained a peak efficiency η_P=45.6%, seems to be a suitable simple turbine shape.

A twin-impulse turbine for bi-directional flow has been developed for wave energy converter. However, the previous studies described that the mean efficiency of the twin turbine would be much lower than that of impulse turbine for a unidirectional flow, since a portion of airflow gets through the turbine under reverse flow whose efficiency is very low. For that reason, a fluidic diode was adopted in the twin impulse turbine in order to reduce the air flow through the turbine under reverse flow. In this study, we investigated the rectification effect of the fluidic diode when a blunt body was provided with a bypass. Computational fluid dynamics (CFD) analysis were conducted to investigate the effect of fluidic diodes on the turbine performance. In the CFD analysis, RANS equations are used as governing equations and the standard k-ε model is used as the turbulence model. The computational domain is composed of a circular tube and fluidic diode and meshed with an approximately 1.5 million mesh elements. As a result, it was found that the rectification effect of the fluidic diode was enhanced by installing a conical bypass with a bypass width ratio of d/D=0.088 and an angle of 44 degrees of a blunt body.

A twin-impulse turbine system has been developed for wave energy converter. However, an impulse turbine, which is used for a unidirectional flow, cannot obtain a high efficiency in reverse flow. A fluidic diode can be used to alleviate this problem. In this study, in order to improve the rectification performance, the performance characteristics of fluidic diode has been investigated by computational fluid dynamic (CFD) analysis. The commercial CFD tool, SCRYU/Tetra of Cradle Co. Ltd, is used to perform the CFD analysis. RANS equations are used as governing equations, and the standard k-ε model is used as the turbulence model. The computational domain is composed of a circular tube and fluidic diode and meshed with an approximately 1.5 million mesh elements. The pressure and velocity fields inside the fluidic diode are visualized and analyzed to obtain a deep insight about the flow rectification effect. As a result, it is clarified that the rectification characteristics of the fluidic diode do not depend on the length of the blunt body. The flow passage area through which the air flows inside the fluidic diode greatly affects the rectification characteristics.

An alternating flow generates in the wave energy plant with an oscillating water column and in the thermo-acoustic engines. Turbines used in such devices generally rotates in a same direction. The performance of some turbines for alternating flow have been investigated by wind tunnel test and computational fluid dynamics (CFD) analysis in order to clarify their usefulness. Conventional turbines for alternating flows have some inherent disadvantages such as severe stall and low efficiency. Therefore, the authors proposed two unique turbines for bi-directional flow in recent years: Wells turbine with booster and counter-rotating impulse turbine for alternating flows. An extensive numerical works was conducted to perform a comparative study between the conventional and proposed turbines.

In a wave energy plant with oscillating water column (OWC), an air turbine is adopted as an energy conversion device. The counter-rotating impulse turbine for wave energy conversion has been proposed by M. E. McCormick in 1978. In a previous study, authors investigated the effect of turbine geometry on the performance, and clarified that the efficiency of this turbine is higher than single rotor impulse turbine in the range of high flow coefficients. In this study, the pitch-controlled guide vanes were installed in order to achieve a further improvement in the performance of a counter-rotating impulse turbine, and the effect of guide vanes on turbine characteristics was investigated by using the computation fluid dynamic (CFD) analyses. As a result, it was found that the effect of the setting angle of the guide vanes has significant influence on the turbine performance, and inlet and outlet setting angle of θi=22.5° and θo=45°, respectively, obtained the better efficiency.

A twin impulse turbine that rectifies the reciprocating bi-directional airflow by two impulse type turbines without using a rectifier valve has been proposed as an air turbine for the oscillating water column (OWC) based wave energy conversion. However, a numerical analysis of the turbine performance in bi-directional airflow conducted in previous studies revealed that a portion of airflow passes through the turbine for reverse flow with a very low efficiency, and resulting the average efficiency of the turbine decreases markedly. Subsequently, the use of fluidic diodes has been proposed in some previous studies. In this study, the performance of the fluidic diode for the twin impulse turbine has been investigated by wind tunnel test. In addition, the flow in the fluid diode has been clarified by computational fluid dynamic (CFD) analysis.

The Wells turbine has long been used as a secondary converter for wave energy conversion. In order to alleviate the stall problem of this turbine, the authors have proposed a Wells turbine with booster. In the present study, the counter-rotating impulse turbine for wave energy conversion, developed by M. E. McCormick of the United States Naval Academy, was proposed as a booster turbine. This turbine shown a better efficiency than an impulse turbine with single rotor in a previous numerical analysis. In this study, the Wells turbine with a counter-rotating impulse turbine was simulated to investigate the fluid dynamic performance by using CFD analysis. As a result, the stall characteristics of Wells turbine shown a little improvement by installing the counter-rotating impulse turbine. Further, it was found that, as a booster, the impulse turbine with single rotor is more efficiency than the counter-rotating impulse turbine.