Ashraful Alam

An impulse turbine for bi-directional airflow has been proposed as an air turbine for wave energy conversion using an oscillating water column (OWC) system. In actual sea conditions, the asymmetric cascade is expected to be more advantageous in practice because the exhalation flow rate is smaller than the inhalation flow rate. In this study, the effect of asymmetric cascade on the performance of bi-directional flow impulse turbine for wave power generation was investigated by CFD analysis with steady flow, and the running and starting characteristics in periodic bi-directional flow were clarified by numerical simulation with quasi-steady analysis. It is concluded that the asymmetric cascade improves the performance of the turbine and is effective for wave energy generation in real sea conditions.

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.

Wells and impulse turbines for bi-directional airflow have been proposed as typical turbines for wave energy conversion. Each of them has some advantages and disadvantages. In recent years, the authors have proposed Wells turbine with booster and a counter-rotating impulse turbine for bi-directional airflow in order to overcome their drawbacks. In this study, the starting characteristics of four types of turbines for bi-directional airflow were compared by conducting a CFD analysis under a steady flow condition.

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.

In an oscillating water column (OWC) based wave energy plant, a bi-directional airflow is generated in the air chamber. To harness energy, the bi-directional airflow turbines that rotate in the same direction are used in such wave energy conversion devices. Some turbines for bi-directional airflow have been proposed to date, and their performance were investigated by wind tunnel tests and CFD analyses. Some of the typical turbines have inherent disadvantages, such as severe stall problem and low efficiency. Therefore, authors proposed two unique turbines for bi-directional flow: Wells turbine with booster and counter-rotating impulse turbine. An extensive computational work was conducted to perform a comparative study between the conventional and proposed turbines for bi-directional airflow.

Japan is surrounded on all sides by the sea. Thus, ocean development has been carried out in the midst of environmental protection. In this consequence, a numerous researches have been conducted on various apparatus that can utilize the wave energy. In this study, a pump system based on the wave energy was developed for pumping the seawater, the facility uses (e.g. aquarium, swimming pool with seawater, etc...), the preservation of farming conditions of marine products, and the replacement of seawater by pumping of the deep water. A radial pump that can be operated by an impulse turbine used for wave energy conversion was developed. The performance of this pump system was investigated experimentally. From the experimental results, the pump system could be started approximately in 20 seconds.

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.

The present work reports of the numerical investigation on the passive technique used to enhance the supersonic cavity-induced pressure oscillations. The unsteady flows over a shallow cavity at Mach 1.73 were modified at the leading edge by changing the leading edge geometry. 3 (three) leading edge geometries namely circular-contoured, slanted and circular contour on 30o slant edge were used to intensify the excitation of the shear layer. The density-based algorithm in ANSYS Fluent 13.0 was used to solve the time-dependent Navier-Stokes equations. The SST k-ω model was used to model the turbulence within an unstructured mesh solver. The numerical code was validated comparing the numerical results with the experimental data. The pressure fluctuations with time at different locations in and after the cavity were analyzed. The modification of cavity leading edge shown influencing on the pressure fluctuations.

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.

Wells turbine, widely used to harness the wave energy, has high efficiency at a low flow rate. However, it has severe stall problem. In order to alleviate this problem, a dual-turbine system of Wells turbine with a booster impulse turbine was proposed. In the present study, the effect of impulse turbine geometries on the performance of combined turbine system was investigated by CFD analysis. As a result, the boosting effect of impulse turbine depends on the pressure differences that caused by the setting angles at a high flow rate. Moreover, the decrease of flow rate with the setting angle has a significant influence on the efficiency.

In a wave energy converter with oscillating water column (OWC), an OWC due to the sea wave motion is used to drive an air column in the air chamber. An air turbine for this bidirectional airflow is used to convert the pneumatic energy into the mechanical energy. In this study, in order to achieve further improvement of the performance of a counter-rotating impulse turbine, the effect of turbine geometry on the performance was investigated by using the computation fluid dynamic (CFD) analyses. As a result, it was found that the efficiency of the turbine greatly increased by installing the middle vanes.

The present study reported of the numerical investigation of a high-speed wet steam flow through an asymmetric nozzle. The spontaneous non-equilibrium homogeneous condensation of wet steam was numerically modeled based on the classical nucleation theory and droplet growth rate equation combined with the field conservations within the computational fluid dynamics (CFD) code of ANSYS Fluent 13.0. The equations describing droplet formations and interphase change were solved sequentially after solving the main flow conservation equations. The calculations were carried out assuming the flow two-dimensional, compressible, turbulent, and viscous. The SST k-! model was used for modeling the turbulence within an unstructured mesh solver. The validation of numerical model was accomplished, and the results showed a good agreement between the numerical simulation and experimental data. The effect of spontaneous non-equilibrium condensation on the jet and shock structures was revealed, and the condensation shown a great influence on the jet structure.

A twin unidirectional impulse turbine for wave energy conversion has been suggested in our previous study, and the performance under unsteady flow has been investigated by quasi-steady analysis. In the present study, the performance of twin impulse turbine under unsteady flow condition has been investigated by unsteady analysis of Computational fluid dynamics. As a result, the mean efficiency of twin unidirectional impulse turbine under unsteady flow is lower than the maximum efficiency of unidirectional impulse turbine. Moreover, it is verified that airflow goes backward in the reverse turbine in low flow rates.

The present study reported of the use of special shaped blade to reduce the difference in pressure across the Wells turbine for wave energy conversion. The blade profile was composed of NACA0020 airfoils and trailing edge was notched like chevron. Experiments were performed investigating the influence of trailing edge shape on the turbine performance. Four notch depths were used to investigate the effect of depth of cut on the turbine performance. As results, by placing a notch-cut at the trailing edge of the blade, it was possible to reduce the pressure difference across the turbine without lowering the efficiency. In addition, the pressure difference substantially reduced at a constant rate with the increase of the cut ratio.

Numerical experiments were carried out on the high speed driven cavity flows in 2D curved channels to investigate mainly the pressure field. A density-based algorithm in ANSYS Fluent 13.0 was used in the present URANS simulations. The SST k-omega model was used for modeling the turbulence within an unstructured mesh solver. Validation of the numerical code was accomplished, and the results showed a good agreement between the numerical simulation and experimental data. Three channels (straight, concave and convex) with a nominal height of H = 4 x 10(-3) m under the transonic flow conditions were considered in the study. The cavity studied is L = 12 x 10(-3) m long with the depth ranging from D = 12 x 10(-3) m to 48 x 10(-3) m to obtain the length-to-depth ratios of L/D=1 to 1/4. The study comprised the analysis of the cavity surface pressures and the associated flow structures. The channel configuration influenced the cavity flowfield, and that influence finally resulted in a change in the surface pressure fluctuations in the cavity. The deep cavity attenuated the flowfield oscillation inside the cavity.

A numerical work is reported of the effect of nonequilibrium homogeneous condensation in the underexpanded supersonic condensing jets issuing from the submillimeter scaled nozzles. Moist air as working gas is used to simulate the condensing jets. The classical nucleation rate and droplet growth equations are used to model the nonequilibrium nucleation phenomena. A TVD numerical method is applied to solve the time dependent Reynolds- and Favre-averaged Navier-Stokes equations that coupled the rate equation of liquid phase production. The influence of size of the nozzle exit diameter on the aerodynamic features of jets is investigated. The shock structure is investigated under different operating conditions—pressure ratios and initial relative humidities. Special attention is given to the effect of homogeneous condensation on the thermo-fluid dynamic features of the jets. The present computational model is validated through comparison of the predicted results with the experimental data.

Spontaneous condensation of moist air in supersonic jets is of considerable interest in a variety of natural and industrial processes. During impingement of supersonic moist air jets, the nonequilibrium homogeneous condensation can be experienced at the region between downstream of nozzle exit and an obstacle. The subsequent release of latent heat thus results in a deceleration of the flow and a rise in pressure, known traditionally as the condensation shock likely have strong effect on the flow features. The present paper reported of the effect of spontaneous nonequilibrium homogeneous condensation of moist air on the aerodynamic and oscillatory flow features of supersonic jets impinging on cavity. A total variation diminishing (TVD) scheme was used to solve the time dependent Favre averaged Navier-Stokes equations, and the droplet growth equation of liquid phase production for simulating the condensing jets. Both qualitative and quantitative validations of the numerical model were accomplished, and the results showed a good agreement between the computed results and experimental data. Predicted flow and oscillatory features of jets were presented.

Steam or moist air is used as working gas in a wide range of engineering applications of supersonic jets. In these cases, nonequilibrium homogeneous condensation may occur at the downstream of nozzle throat. The surrounding gas will be heated by the release of latent heat of condensation, and may results a change in the flowfield. The present report will describe numerical investigations predicting the effect of nonequilibrium condensation on the flow characteristics of ideally-expanded supersonic free jets. A TVD numerical method is applied to solve RANS and droplet growth equations. The predicted results are compared with the experimental data.

This paper presents computational fluid dynamics (CFD) studies on turbulent confined jets of H&lt sub&gt 2&lt /sub&gt gas during fast filling of a cylindrical tank at high pressure. The CFD model is validated by comparison with experimental results. An attempt is made to study the thermo-fluid dynamics of hydrogen gas alongside the characteristics of confined jets in the tank during filling. A pressure-velocity coupling algorithm is applied to solve the Favre averaged Navier-Stokes equations with an SST k-ω turbulence model. The real gas model using Redlich-Kwong equation of state is considered for density computation to model compressibility effects at high pressure. Convective heat transfer from the compressed gas to tank wall is estimated through the coupling between the energy equations of gas and solids. A standard type III, 74L hydrogen cylinder composed of aluminum and CFRP for liner and laminate is used in the present study.

Over half a century researchers have engaged themselves in doing research on supersonic impinging jets for their fundamental fluid dynamic perspectives as well as their huge practical applications. In a variety of engineering applications steam or moist air is used as working gas. Rapid expansion of steam or moist air leads to an occurrence of non-equilibrium homogeneous condensation at the region between downstream of nozzle exit and an obstacle. Surrounding gas is heated due to the release of latent heat of condensation, and may affect the jet flow features. The present numerical work deals with the effect of condensation on the aerodynamic and oscillatory flow features of supersonic impinging jets onto a. cylindrical cavity. A TVD scheme is used to solve the RANS equations and droplet growth equations for simulating condensation in the jet flowfield. The numerical code is validated through comparison with experimental results. Predicted flow and oscillatory properties of jets are presented.

In this study, a transonic flow past NACA0012 profile at angle of attack alpha=0(0) whose aspect ratio AR is 1.0 with non-equilibrium condensation is analyzed by numerical analysis using a TVD scheme and is investigated using an intermittent indraft type supersonic wind tunnel. Transonic flows of 0.78-0.90 in free stream Mach number with the variations of the stagnation relative humidity(I broken vertical bar(0)) are tested. For the same free stream Mach number, the increase in I broken vertical bar(0) causes decrease in the drag coefficient of profile which is composed of the drag components of form, viscous and wave. In the case of the same M-a and T-0, for more than I broken vertical bar(0)=30%, despite the irreversibility of process in non-equilibrium condensation, the drag by shock wave decreases considerably with the increase of I broken vertical bar(0). On the other hand, it shows that the effect of condensation on the drag coefficients of form and viscous is negligible. As an example, the decreasing rate in the drag coefficient of profile caused by the influence of non-equilibrium condensation for the case of M-a=0.9 and I broken vertical bar(0) =50% amounts to 34%. Also, it were turned out that the size of supersonic bubble (that is, the maximum height of supersonic zone) and the deviation of pressure coefficient from the value for M=1 decrease with the increase of I broken vertical bar(0) for the same M-a.