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Partial-admission impulse turbine is suitable for small-scale power cycles with a high expansion pressure ratio (EPR) and a low mass flow rate. However, the exit velocity of a single-stage turbine is very high, leading to a large exit loss. A new two-stage counter-rotating impulse turbine has been developed to recover the exit loss, which has a novel structure without stator between the two rotors. However, the internal flow characteristic of such a turbine is still unclear. In this study, the aerodynamic performance of the two-stage counter-rotating partial-admission impulse turbine is analyzed. The internal aerodynamic characteristics are estimated via CFD simulation. First, the geometric parameters are determined based on the mean-line model. Then, the characteristic line method is adopted to design the blade profile of the first supersonic stage based on the vortex flow theory. Subsequently, the performances are estimated under the design and off-design conditions via 3D CFD simulation. The results indicate that the modified convergent-divergent supersonic nozzle exhibits a better adaption with the impellers. The load is principally imposed on the upper zone of the blades. The power output of the turbine reaches 1501.3 W with an EPR of 52 and an efficiency of 45.99 % at the design point and the power of the first impeller accounts for 76.5 %. The mass flow rate increases almost linearly with the augmentation of the turbine inlet pressure. The power output proportion of the second impeller may arrive at 27.63 % under off-design conditions, showing the addition of the second stage can improve the turbine performance evidently.

The benefits of turbocharged internal combustion engines are well established in the literature and mass production. Despite the numerous advantages of turbocharging, commercial single-cylinder engines are generally not turbocharged. This is due to intermittent exhaust gas flow to the turbine, typical for a single-cylinder engine. The current work proposes a novel layout wherein a supercharger boosts the engine to reap the benefits of charging the single-cylinder engine. An impulse turbine salvages part of the exhaust energy from the pulsated flow. A one-dimensional simulation study of the novel supercharged and turbo-compounded single-cylinder engine was carried out, and critical parameters of an impulse turbine were designed. CFD simulation study was carried out to validate the performance of the impulse turbine and the novel layout. This study modified a light-duty naturally aspirated (NA) diesel engine currently in mass production into the supercharged version. A positive displacement supercharger was mechanically coupled to the base NA engine with a suitable belt drive, and steady-state experiments were performed with NA and supercharged versions. One-dimensional AVL boost simulation models were validated for the NA and supercharged engines for brake power, air flow rate and BSFC within a 3% deviation. Nozzles of varying exit diameters were employed at the exhaust pipe. One-dimensional simulations were conducted to predict the supercharged engine performance and the kinetic energy rate available at the exhaust nozzle exit. An optimal nozzle exit diameter was chosen. Base calculations for the impulse turbine were performed with the output from the 1D simulation tool to design the critical features of the blade and the power turbine. The designed impulse turbine was further simulated using a commercially available CFD simulation tool, CONVERGE. CFD results showed that the impulse turbine generated about 1.55 kW of power, equivalent to 39% turbine efficiency. Experimental analysis was conducted on the supercharged engine with the optimal nozzle and the simulated impulse turbine. Results showed significantly improved engine performance with the supercharged and impulse turbine compounded version compared to the base NA version. The brake power output and brake thermal efficiency increased by 34% and 3.8%, respectively. Soot and carbon monoxide (CO) emissions were 80% lower, while unburned hydrocarbon (HC) emissions were 45% lower. However, NOx emissions were 60% higher, which can be curtailed by established NOx reduction strategies. Thus, the proposed method of employing a supercharger and compounding an impulse turbine for a single-cylinder diesel engine proved beneficial with improved performance, reduced brake specific fuel consumption and reduced emissions except for NOx. [ABSTRACT FROM AUTHOR]

Fillets on leading edges (LE) of turbine blades alter flow patterns and change the loss profile. A bidirectional flow impulse turbine utilized for harnessing wave energy is introduced and computational fluid dynamics (CFD) was used to perform various analysis. In the present work, fillets of different radii on the leading and trailing edges of both the rotor blade (RB) and guide vane (GV) are modified to study the change in overall performance. After an appropriate gird convergence study, ANSYS-CFX 16.0 solver is used for solving the Reynolds averaged Navier-Stokes (RANS) equations incorporating the k-ω -SST turbulence closure model. The numerical investigations were performed using the high-resolution scheme with the convergence criteria of 10−6 to produce unwavering results. The results show that the boundary layer near the endwall creates flow blockage and losses. Also, the increase in pressure drop due to the thickening of the boundary layers (BLs) across the blade/vane leads to a depletion in the overall performance of the turbine. The rotor and guide vane filleted turbine experience higher losses than the base model due to the radial pressure gradient that is explained with post-processed figures. The concept of fillet shapes has been applied to gas turbines and which improves the performance of the turbine due to a reduction in the secondary flow losses occurring inside the turbine and the same concept has been applied to wave energy air turbine to check its performance based on the losses present across the turbine passage. [ABSTRACT FROM AUTHOR]

In this study, a weakly compressible smoothed particle hydrodynamics (WCSPH) model was used to simulate the forced piston motion of water inside a fixed oscillating water column (OWC) wave energy converter. The considered device was a chamber with a complex entrance geometry attached to the rear wall of the wave flume. The effect of an impulse turbine on the chamber was modeled by an equivalent orifice damping force applied on a thin plate. This way, a balance is established between numerical accuracy and efficiency. The simulations were performed for two different wave steepness values of 0.025 and 0.04. To validate the model results, the experimental work performed in the Technical University of Denmark was used. Fourier Transform analysis was performed on the SPH model results and experimental data to obtain the first-harmonic amplitude of the desired parameters. In general, a good correspondence was found between the SPH calculations and the experimental results for the piston motion of the water inside the chamber. However, the SPH results were slightly larger, especially around the resonance period. A possible reason is the placing of the plate above the free surface which might reduce the small sloshing motions of water inside the chamber. Finally, from the heaving velocity of the plate, the flux and pressure drop through the orifice were evaluated. It is shown that all three parameters reach their maximum around the resonance period of the chamber. • The performance of a fixed, attenuator oscillating water column was analyzed using a 3D SPH model. • The SPH results on piston motion of water and the pressure drop through the orifice were validated using experimental data. • The turbine effect is modeled using the quadratic pressure drop-flux relationship. • Possessing the heaving velocity of the plate, the flux and the pressure drop through the orifice were evaluated. • Critical parameters of the OWC reach their maximum around the resonance period of the chamber. [ABSTRACT FROM AUTHOR]

Oscillating water column (OWC) devices are the technical solution more favourably considered for a significant future exploitation of the energy potential of sea waves. While theoretically quite simple, these systems are indeed rather complex in terms of correct tuning between the caisson and turbine. Recent studies showed that only a holistic approach, also including turbine control, is the way of effectively performing their precise design. To this end, the research presents the application of an analytical wave-to-wire model of an OWC device for the preliminary optimisation of these systems for a small installation in a moderate wave climate in the Mediterranean Sea. More specifically, scatter matrices were determined for imposing the wave states of a selected location positioned in Tuscany (Italy). Regarding previous similar studies, a novel joint optimisation of the caisson and controlled turbine is proposed for the specific site. Either monoplane isolated Wells turbines and axial impulse turbines were analysed. The operating curves, performance parameters, and attended annual energy harvesting were calculated, providing an interesting overview of the functioning of the system. The optimised devices with the Wells and impulse turbomachines convert 36.71 and 42.17 MWh/year operating with a global efficiency of 6.82% and 7.84%, respectively. • Comprehensive wave-to-wire model of oscillating water column systems. • Analysis of the processes of energy conversion from sea wave to the electric wire. • Original modelling of monoplane isolated Wells and axial impulse turbines. • Application to a real site considering annual and seasonal wave conditions. • Integrated optimisation of the oscillating water column chamber and air turbines. [ABSTRACT FROM AUTHOR]

Wave energy presents a promising renewable energy resource capable of significantly mitigating the global energy crisis. Among the various wave energy converters (WECs), the Oscillating Water Column (OWC) stands out due to its simple design, reliability, and robust survivability. A numerical model coupling a bi-directional turbine-generator system with the OWC chamber was developed using the Simulink system design software platform. This integrated model achieves comprehensive coupling between the impulse turbine and a three-phase permanent magnet synchronous generator (PMSG). This is accomplished while concurrently maintaining the dynamic characteristics of the multiphase fluid flows and minimizing the unknown parameters influencing the system performance. The Wave-to-Wire (WtW) time-domain model comprises an energy capture chamber, an impulse turbine, and a PMSG, facilitating accurate output power predictions, short calculation times, and complete process coupling. Validation experiments of an OWC positioned at the energy focused point of a parabolic wall, performed using various wave conditions, including regular and irregular waves, demonstrate strong consistency between the numerical and physical models. This study reveals the damping mechanism of the generator on the OWC system, where the generator generates an electromagnetic torque resisting its movement, when driven by mechanical torque. The model evaluates the output power and conversion efficiency each energy conversion stage. When the incident wave height is 0.075 m, the average electrical power can reach a maximum of 11.2 W, and the overall wave-to-wire conversion efficiency can reach 98 % in the particular experimental configuration investigated. The developed predictive model can be used as a valuable tool for designing effective OWC-WEC-WtW systems. • An OWC-WtW time-domain model was established which achieves a comprehensive coupling between the air turbine and generator. • An experiment was designed and validated against the numerical model under regular waves and irregular waves. • The damping mechanism of the generator on the OWC device was revealed. • The parameters of each energy stage were effectively measured by the proposed numerical model. [ABSTRACT FROM AUTHOR]

The hydrodynamic performance of an integrated oscillating water column (OWC) combined with a cylindrical caisson type breakwater was investigated. A three-dimensional numerical wave tank was established to simulate the overall energy conversion performance and hydrodynamics of the integrated system with the software Star-CCM+. The developed Computational Fluid Dynamics (CFD) model was initially verified with the published physical results and numerical results derived from an analytical solution. The effects of the structural geometry and the pneumatic damping of the PTO system on the wave absorption and power conversion efficiency of the proposed device were investigated using this developed model. Based on this proposed numerical model, the velocity of the air flow, the corresponding air pressure characteristics in the air turbine nozzle and chamber zone were discussed. The results indicate that this proposed half-open land based OWC is adapted to absorb shorter nearshore waves, characterized by dimensionless wave number kd approaching 1.49. Furthermore, a larger opening inlet zone for this proposed OWC would shift the resonant corresponding kd to higher wave frequencies, while lowering the opening inlet could help to increase the power extraction efficiency for long waves. The peak hydrodynamic efficiency is observed at a ratio of OWC inlet width to diameter B / D of 0.97, which is approximately six times larger than that at B / D = 0.5. The inclusion of an impulse turbine yields a more uniform pressure distribution within the central chamber and effectively mitigates wave reflection for smaller incident waves, with maximum efficiency achieved using 37 blades. The property of the efficiency mitigation with insufficient certain intake depth of the wave chamber should be avoided for system design. • A novel dual cylindrical OWC incorporated into a fixed caisson type breakwater was numerically investigated. • The effects of the chamber geometry and the PTO damping on the primary-stage power conversion efficiency are studied. • The flow field of the hybrid system in and around the OWC chamber is investigated. • The effect of impulse turbine on the secondary-stage performance as well as efficiency is considered. [ABSTRACT FROM AUTHOR]

In this paper, a fully coupled model for a backward bent duct buoy oscillating water column (BBDB-OWC) wave energy converter (WEC) is established, with the consideration of strong transient nonlinear interactions among ocean waves, BBDB hull motion, mooring system, internal water column oscillation, aerodynamics, turbine rotation and the damping effect of the generator. The model is validated by comparison with experimental results, and then used to investigate the transient characteristics and periodic average performance of the WEC under regular wave conditions. The differences in transient hydrodynamic characteristics and air pressure-flow rate relationship between the fully coupled model and the orifice model are compared. The transient characteristics of an impulse turbine under reciprocating and transient airflows are studied. And the effect of wave period, wave height and load torque coefficient on the average performance of the WEC is investigated. • A fully coupled wave-to-electricity model for a BBDB-OWC WEC is established. • Hydrodynamics between the fully coupled model and orifice model are compared. • Transient performance of impulse turbine using fully coupled model is investigated. • Effect of wave conditions and load coefficient on BBDB performance is studied. [ABSTRACT FROM AUTHOR]

In this work, a bidirectional impulse turbine used in a wave energy device is simulated using a CFD technique and a shape optimization performed with multiple surrogates-assisted multi-objective evolutionary algorithm. The surrogates have been generated using the validated CFD technique. The objectives for the optimization are to maximize the shaft power and to minimize the pressure drop across the turbine. The hub and the tip thickness of the rotor blade profiles are modified and considered as the design variables. The Latin hypercube sampling technique generated design points are evaluated in the CFD solver, the objective responses are used to construct multiple surrogates, and a Pareto optimal front is generated through a multi-objective optimization approach. The turbine efficiency, which is the function of pressure drop and shaft power, is relatively increased by 10.4%. Abbreviations: BPS: best PRESS; GV: Guide vane; KRG: kriging; LE: leading edge; MOO: multi-objective optimization; OWC: oscillating water column; PRESS: predicted error sum of squares; PS: pressure side or pressure surface; RB: rotor blade; RBNN: radial basis neural network; Ref: reference; RMS: root mean square; RSA: response surface approximation; SS: suction side or suction surface; TE: trailing edge; WAS: weighted average surrogates

Wave energy is a renewable source with significant amount in relation to the global demand. A good concept of a device applied to extract this type of energy is the onshore oscillating water column wave energy converter (OWC-WEC). This study shows a numerical analysis of the diameter determination of two types of turbines, Wells and Impulse, installed in an onshore OWC device subjected to a hypothetical sea state. Commercial software FLUENT®, which is based on RANS-VoF (Reynolds-Averaged Navier-Stokes equations and Volume of Fluid technique), is employed. A methodology that imposes air pressure on the chamber, considering the air compressibility effect, is used. The mathematical domain consists of a 10 m deep flume with a 10 m long and 10 m wide OWC chamber at its end (geometry is similar to that of the Pico's plant installed in Azores islands, Portugal). On the top of the chamber, a turbine works with air exhalation and inhalation induced by the water free surface which oscillates due to the incident wave. The hypothetical sea state, represented by a group of regular waves with periods from 6 to 12 s and heights from 1.00 to 2.00 m (each wave with an occurrence frequency), is considered to show the potential of the presented methodology. Maximum efficiency (relation between the average output and incident wave powers) of 46% was obtained by using a Wells turbine with the diameter of 2.25 m, whereas the efficiency was 44% by an Impulse turbine with the diameter of 1.70 m. [ABSTRACT FROM AUTHOR]

The article discusses performance of a power module in a wave-powered navigational buoy developed by National Institute of Ocean Technology. The power module of the buoy consists of an impulse turbine and a generator. Both unidirectional and bidirectional impulse turbine were tested for their performance. The article describes the selection, performance assessment tests and experimentation on these power modules carried out in the laboratory and in open sea trials. It was observed that unidirectional impulse turbine gave better performance than bidirectional impulse turbine for a given range of flow coefficient and pressure drop across the turbine. The performance of the wave powered navigational buoy in the open sea trials has given confidence on its use as a product and has also led to knowledge enhancement for scaling up floating wave energy devices. [ABSTRACT FROM AUTHOR]

Oscillating water column (OWC) wave energy converters (WECs) have two energy conversion stages. In this study, an axial-flow impulse turbine was installed on an OWC model to replace the traditional substitute, such as the orifice plate. The OWC model was placed in a wave flume for the experimental tests under regular wave conditions. This experimental setup was expected to reflect more actual interactions between two stages at the model-scale level. During the tests, various constant rotation-speeds of the impulse turbine were controlled by a servo motor. The free water-surface elevations at different positions in the OWC model, the air-pressure variations in the chamber, and the torque output of the turbine were recorded. The time histories and phase relationship of typical measured data were analyzed. The primary-stage and secondary-stage efficiencies were calculated under various wave conditions. The overall efficiency of the model peaked at the resonant length ratio of 7.1. All the experimental results can be employed as typical benchmarks for an accurate validation of numerical models. • A model-scale OWC wave energy converter was set up in experimental wave-flume tests. • Interactions between the impulse turbine and the air chamber were incorporated. • The primary-stage and secondary-stage efficiencies of the OWC model were calculated. • A resonant length ratio of the OWC model with the peak performances was found. • Overall performance of the OWC model was presented as experimental benchmarking. [ABSTRACT FROM AUTHOR]

Single-stage axial flow turbines are typically used in underground mining machines and underwater vehicles due to their safety and small size. However, the high-pressure ratio and small mass flow rates force the flow inside the turbine to be supersonic. The blade design of such turbines with high-flow velocity leads to high losses and low turbine efficiencies at conventional design values. In this paper, the 1D design of the turbine is carried out using the meanline design method. The results of the design algorithm are compared with both experimental, CFD (Computational Fluid Dynamics), and a well-known commercial software AxSTREAM. After validating the design, Taguchi method is used to analyze 6 different parameters (Blade Shape, Nozzle Shape, Cord Length, Solidity Ratio, Nozzle Angle and Blade Thickness) affecting the turbine performance and the optimum blade profile is compared with the conventional blade design. The optimum design isentropic efficiency (68.267%) outperformed the conventional design turbine isentropic efficiency (61.759%) by about 11%. This paper provides a different insight into the conventional turbine design method. • Single-stage axial turbine design was discussed in detail. • The optimum admission ratio was determined within manufacturing limits. • 1D calculations and CFD results were compared with experimental data. • Nozzle and Rotor blades were optimized with the Taguchi Method. • The effects of Tip Cr on performance were determined. [ABSTRACT FROM AUTHOR]

Centrifugal pumps can be used as "Pump- as -Turbine (PaT)" by reversing the flow and operating as a Francis turbine. The proposed concept "Impulse Pump- as -Turbine (Impulse PaT)" will use centrifugal pump impeller to be used as hydro turbine by pairing with spear valve injector from impulse hydro turbine. Spear valve injector regulate water inlet flow rate, thus regulate power output of our new concept turbine. Additional benefit from utilized spear valve injector is low loss of turbine efficiency when operate at part-load condition, therefore this new concept turbine can be operated at wide range of flow condition without losing good efficiency while also eliminate risk of turbine damage from cavitation. Flow regulation through spear valve will also simplify control system for this proposed turbine compared to guide vanes. The validation methods from 1-D calculation based on Euler's turbine equation and numerical simulation by commercial CFD package shows that this new proposed concept is feasible with efficiency around 40% and wide operating range from 25% to maximum inlet flow rate. Results from numerical calculation shows that efficiency of this new concept of turbine is limited by number of blades in commercially available pump. • New method of using Pump- as -Turbine (PaT) in impulse turbine mode. • One-dimensional calculation to prove feasibility and performance optimization of PaT in Impulse mode. • Performance validation by using Computational Fluid Dynamics (CFD) method. • Broad turbine operation range with high efficiency compared with conventional PaT. [ABSTRACT FROM AUTHOR]

Abstract Oscillating water column (OWC) is the most widely used wave energy converting technology for onshore structures. The interest in OWC wave energy converters (WECs) has been rising in the last decade because of its capability of long-term operation with high reliability and survivability. The impulse and Wells turbines, acting as axial-flow self-rectifying turbines and rotating unidirectionally in reciprocating airflows generated by the air chamber, play a key role in the pneumatic power conversion of OWC facilities. Although these turbines were developed based on theoretical analysis and steady-state laboratory tests, computational fluid dynamics (CFD) technologies have become a popular and convenient desktop tool for their design and optimization. This review aims to bridge the knowledge gap between engineering applications and CFD achievements for this type of turbine. Recent developments in the CFD modeling, geometry optimization, and performance enhancement have been summarized. Perspectives on the CFD model for OWC axial-flow turbines and its future trends are also proposed. Highlights • A comprehensive review of CFD studies on axial-flow OWC turbines are presented. • Detailed aspects of CFD models for both Wells and impulse turbines are summarized. • CFD achievements of geometry optimization and performance enhancement are reported. • Parameters of CFD models for steady-state analysis are recommended. • Future trends of CFD models for OWC turbines are discussed. [ABSTRACT FROM AUTHOR]

Highlights • Self-starting performance of an OWC impulse turbine is studied comprehensively. • A fully passive fully-driven numerical model is used for unsteady simulations. • The peak efficiency of 0.44 is achieved under the constant flow and load conditions. • The self-starting speed of impulse turbine is mainly determined by the incident air-flow velocity. • The moment of inertia has limited effects on the turbine performance at the stable stage. Abstract In recent times, self-rectifying axial-flow air turbines are being widely employed in oscillating water column (OWC) wave energy converters (WEC). The steady performance of air turbines has been systematically investigated in previous studies. However, there still exists a lack of information on their unsteady performance, such as in the self-starting characteristics and subsequent running behavior. In this study, the unsteady behavior of impulse turbine under various constant-flow conditions is investigated. Experimental studies were conducted to investigate the effects of constant-load on the variations in the rotation speed, the pressure drop and the torque output of the turbine starting from rest. A fully passive flow-driving numerical model is employed for further detailed analysis of the flow and pressure fields. Followed by a well-agreed validation using the corresponding experimental data, the three dimensional (3D) transient model is used to study the effects of the air-flow velocity magnitude and the rotors' moment of inertia on the self-starting performance of the turbine. Except for the variations in the rotation speed, the pressure drop and the pneumatic torque, the distributions of the flow-field and the pressure over the blades at specific time-points are analyzed. [ABSTRACT FROM AUTHOR]

Oscillating water column (OWC) systems are among the most credited solutions for an effective conversion of the notable energy potential conveyed by sea waves. Despite a renewed interest, however, they are often still at a demonstration phase and additional research is required to reach industrial maturity. Within this framework, this study provides a wave-to-wire model for OWC systems based on an impulse air turbine. The model performs a comprehensive simulation of the system to estimate the attendant electric energy production for a specific sea state, based on analytical models of the primary (fixed chamber) and secondary (air turbine) converters coupled with the tertiary converter (electric generator). A rigid piston model is proposed to solve the hydrodynamics, thermodynamics, and hydrodynamics of the chamber, in a coupled fashion with the impulse turbine aerodynamics. This is solved with a novel method by considering the cascades as sets of blades, each one consisting of a finite number of airfoils stacked in the radial direction. The model was applied for two Mediterranean sites located in Tuscany and Sardinia (Italy), which were selected to define the optimal geometry of the turbine for a specified chamber. For each system, the developed analytical wave-to-wire model was applied to calculate the performance parameters and the annual energy production in environmental conditions typical of the Mediterranean Sea. The selected impulse turbines are able to convert 13.69 and 39.36 MWh/year, with an efficiency of 4.95% and 4.76%, respectively, thus proving the interesting prospects of the technology.

In this work, a bidirectional impulse turbine used in a wave energy device is simulated using a CFD technique and a shape optimization performed with multiple surrogates-assisted multi-objective evolutionary algorithm. The surrogates have been generated using the validated CFD technique. The objectives for the optimization are to maximize the shaft power and to minimize the pressure drop across the turbine. The hub and the tip thickness of the rotor blade profiles are modified and considered as the design variables. The Latin hypercube sampling technique generated design points are evaluated in the CFD solver, the objective responses are used to construct multiple surrogates, and a Pareto optimal front is generated through a multi-objective optimization approach. The turbine efficiency, which is the function of pressure drop and shaft power, is relatively increased by 10.4%. Abbreviations: BPS: best PRESS; GV: Guide vane; KRG: kriging; LE: leading edge; MOO: multi-objective optimization; OWC: oscillating water column; PRESS: predicted error sum of squares; PS: pressure side or pressure surface; RB: rotor blade; RBNN: radial basis neural network; Ref: reference; RMS: root mean square; RSA: response surface approximation; SS: suction side or suction surface; TE: trailing edge; WAS: weighted average surrogates [ABSTRACT FROM AUTHOR]

Wave energy harvesting systems mostly have low power production capability because of unoptimized design of the system components. A bidirectional flow impulse-turbine used in such a system has efficiency less than 40%, and it is required to design the turbine for a higher efficiency. Present work finds an optimal design and shows design-variable sensitivity to the turbine efficiency. The problem is solved using numerical analysis technique. The flow through the turbine was analyzed by solving the Reynolds-averaged Navier-Stokes equations (RANSE). The design variables; namely number of rotor blades and number of guide vane, guide vane angle and guide vane profile were modified to maximize the turbine efficiency. Using the Latin hypercube sampling technique, sample points were selected from a design space defined by lower and upper limits of the variables. Then, several surrogates were constructed using the RANSE calculated results, and the turbine performance was optimized. The results show that guide vane angle is the most sensitive parameter, while the guide vane profile has negligible effect on efficiency. A hybrid genetic algorithm searched the optimal design point. The relative mean efficiency enhancement over a wide range of flow coefficient was approximately 24%, while it was 28% at maximum efficiency point. [ABSTRACT FROM AUTHOR]

The rapid development of air turbines for oscillating-water-column wave energy conversion proposes a crucial demand for deep understanding of its unsteady performance in the reciprocating air flow condition. A 3D unsteady numerical model is presented in this study, which is established on the platform of commercial computational fluid dynamics (CFD) software ANSYS-Fluent 12.0. The passive rotation of turbine rotor is fully driven by the bi-directional air-flows following the Newton's second law. The numerical model is validated by experimental results within the grid independency and time-step sensitivity studies. The 3D overall pneumatic characteristics in a stable wave cycle are presented to demonstrate the unsteady performance of the turbine rotor. Effects of air-flow velocity amplitude and rotor's moment of inertia on the flow field and pressure distribution on the blades, pneumatic torque and rotation speed, and the transient input and torque coefficients are studied. [ABSTRACT FROM AUTHOR]

Oscillating water column (OWC) axial impulse turbines permit the conversion of wave energy into electrical power. Unlike other hydropower units with a mature and well established technology, such turbines have been recently developed, there are still few prototypes operating and therefore there is a large space for optimizing its design. Many recent studies focus on the improvement of the efficiency and transient characteristics by means of experimentation and also simulation techniques. In the present paper we use a 3D numerical simulation model (computational fluid dynamics model with ANSYS-Fluent 18) to analyze the influence of different geometrical parameters on the efficiency of the turbine, which have been less discussed yet. A reference configuration case has been used to validate our simulation model by comparing it with previous experimental results. Then, parametric variations in the guide vane number and type, gaps between the rotating and stationary part and hub to tip ratio have been introduced in the model to discuss the influence of these effects. It is found that some of these parameters have an important influence on the efficiency of the turbine and therefore, the results presented in this paper can help to optimize future designs of OWC impulse turbines.

In recent years, the oscillating water column (OWC) wave energy converter, which can capture wave energy from the ocean, has been widely applied all over the world. As the essential part of the OWC system, the impulse and Wells turbines are capable of converting the low pressure pneumatic energy into the mechanical shaft power. As an enhanced technique, the design of endplate or ring attached to the blade tip is investigated numerically in this paper. 3D numerical models based on a CFD-software FLUENT 12.0 are established and validated by the corresponding experimental results from the reports of Setoguchi et al. (2004) and Takao et al. (2001). Then the flow fields and non-dimensional evaluating coefficients are calculated and analyzed under steady conditions. Results show that the efficiency of impulse turbine with ring can reach up to 0.49 when ϕ=1, which is 4% higher than that in the cases for the endplate-type and the original one. And the ring-type Wells turbine with fixed guide vanes shows the best performance with the maximal efficiency of 0.55, which is 22% higher than that of the original one. In addition, the quasi-steady analysis is used to calculate the mean efficiency and output-work of a wave cycle under sinusoidal flow condition. Taking all together, this study provides support for structural optimization of impulse turbine and Wells turbine in the future. [ABSTRACT FROM AUTHOR]

Turbines are widely used as the expanders in low-grade power systems. Increasing the expansion pressure ratio can improve the system efficiency whereas supersonic flow may occur inside the expander, increasing the turbine design difficulty. For small-scale power system, impulse turbine is a suitable option when the mass flow is small and the expansion pressure ratio is high. In this study, a 1.5 kW single-stage partial-admission impulse turbine is designed for waste heat recovery of internal combustion engines. The aerodynamic characteristics of the designed supersonic impulse turbine are analyzed. First, a preliminary design of the nozzle and the impeller is conducted based on the mean-line model and the main geometry parameters are obtained. Then, the blade profile is designed using the characteristic line method. A 3D CFD simulation is performed based on the designed impulse turbine and the flow field is analyzed under the design and off-design conditions. The results indicate that the mean velocity and the Mach number at the nozzle outlet arrive at 1082.87 m/s and 2.645, respectively. A high load is generated on the blades in the partial-admission region due to the pressure difference between the pressure and suction surfaces. The two blades in the middle output the largest torques, taking up nearly 48% of the overall torque. The output power of the turbine achieves 1.385 kW with a total-to-static efficiency of 42.52%. However, some special flow phenomena are found in the edges of the partial admission region such as the inverse flow and the endwall cross flow at the blade hub, the transverse forward flow at the blade tip. The shock wave at the leading edge of the flow channel and the expansion acceleration of the working fluid at the trailing edge are also observed. The results provide a reference for small-scale impulse turbine design with a small mass flow and a high pressure ratio. • A 1.5 kW single-stage partial admission impulse turbine is designed. • The blade profile is determined based on vortex flow theory. • Two shock waves occur in the inlet region of the rotor. • Torque is generated mainly by the upper part of the blades. [ABSTRACT FROM AUTHOR]

To obtain a high specific work output, the large pressure ratios across the turbine are required. This can be achieved using a supersonic turbine. When the fluid mass flow is low, the impulse kind of one or two stages supersonic turbine is employed. To prevent losses due to low blade aspect ratio and issues related to manufacturing and industrial problems, the turbine is used in partial admission conditions. Studies show that the turbine efficiency is highly dependent on the amount of partial admission coefficient. The turbine efficiency in full admission is high, but the use of partial admission lowers the additional losses. Therefore, there will be a degree of partial admission in which the turbine will have the highest efficiency. The aim of this work is to achieve the optimum partial admission for a special impulse turbine as a case study. Therefore, in the beginning, an appropriate model of losses is presented. Then, using a nonlinear design optimization code, the partial admission of an impulse supersonic turbine is optimized. This code is written using a genetic algorithm. Then, using three-dimensional numerical analysis, the optimal model will be selected. In the optimization problem, the turbine efficiency is the objective function. The amount of design parameters and constraints used in this process are ten and eight, respectively. After the optimization process, prototypes of designed and modified turbines are made and tested. Test results were compared and analyzed. The results showed that the turbine efficiency is improved between 2.5% and 3% depending on various operation conditions.

The aim of this work is to provide an impulse type hydraulic turbine to utilize unexploited water resources as nano hydropower in the mountainous area. The turbine is simplified to make cheap energy and uses inexpensive components for widespread utilization. The turbine model is tested experimentally to reveal the power characteristics. The flow visualization and numerical simulation are conducted to clarify the behavior of free surface flow in the runner with different nozzle positions. The experimental results show that the maximum runner efficiency of the prototype turbine is 0.56. Numerical simulation shows that the output power depends on the nozzle positions with the impingement of the tail of the jet portion to the backside of the blade. This study gives the fundamental information of the turbine performance to acquire a guideline for future practical applications.

The objective of this paper is to present the effect of end plate on the performances of the impulse turbine for wave energy conversion by experimental investigation. The experiments have been performed by model testing under steady flow conditions in the study. And then, the performances of the impulse turbine with end plates have been compared with those of the original impulse turbine, i.e., the impulse turbine without end plate. As a result, it is found that the characteristics of the impulse turbine with end plates are superior to those of the original impulse turbine. Furthermore, the effects of end plate size and penetration on the turbine characteristics have been clarified in the study.

This paper investigates a small size wave power device with an impulse turbine installed in the breakwater near Niigata Port, Japan. The device consists of an air chamber, a turbine, a generator and pressure-relief valves. This study reveals the characteristics of each component in this system with impulse turbine and a direct current dynamo the power of which is consumed by a constant resistor. In this paper special features of the impulse turbine are found, and the system characteristics are briefly represented. The overall plant performance was analyzed using mathematical model of an oscillating water column (OWC) based on linear water wave theory and the special features of the impulse turbine.

In the case of impinging water jets or droplets, air entrainment processes are crucial to the casing design of hydraulic impulse turbines in the micro-hydro sector. To initiate first steps towards a precise prediction of the complex, multi-phase casing flow of impulse turbines, single aspects such as the penetration depth of impinging liquid jets have to be separated and fully understood. Existing investigations determining penetration depths are related to a very small range of flow rates and therefore show an underestimation of the penetration depth being applied to the casing flow of impulse turbines, which are generally operated at higher flow rates. For a more general description of the air entrainment process, investigations of plunging water jets within an extended flow rate range are conducted and the penetration depth is modelled using a data driven artificial neural network (ANN) approach and a non-linear regression model. At low flow rates, experiments results are in accordance with existing studies, whereas penetration depths up to 170 cm are measured at higher flow rates. For the mathematical models to achieve a wide range applicability, a large data base is used, including published and measured data. The modelled penetration depths can be precisely verified by the performed measurements and show correct physical behaviour, even in areas without underlying data. Calculation rules, weight matrices and biases of the trained ANN are published to achieve high transparency and scientific improvement in neural modelling of penetration depths of impinging liquid jets. [ABSTRACT FROM AUTHOR]

For the practical application of a fixed oscillating water column (OWC)-type wave energy converter, it is necessary to develop a design method which can consider the characteristics of incident wave motion, the motion of the internal free surface affected in the structure such as a partly submerged wall, the fluctuation of air pressure in an air chamber, and the rotation of the air turbine. On the other hand, the impulse turbine as the secondary conversion device in the OWC unit is expected to achieve high efficiency. In this paper, firstly, the steady air flow tests for a single-impulse turbine were conducted to grasp the characteristics of the turbine without the effect of water waves. Secondly, the 2-dimensional wave tank tests in regular waves for the performance evaluation of the fixed OWC-type wave energy converter with the same impulse turbine were conducted to obtain the data needed to make this design method. As the results, the effects of the air chamber length and the guide vane's setting angle on the primary and secondary conversion efficiencies are clarified experimentally. [ABSTRACT FROM AUTHOR]

The increased demand for clean renewable sources of electricity has fostered strong growth in wave energy development in recent years. This led to greater understanding of the various systems involved in the conversion of wave energy to electrical energy, which in turn led to more accurate and sophisticated models of each system. Mathematical models have been developed independently to represent the various interactions that take place within wave energy converters (WEC). In this paper, models used to represent the various stages of energy conversion that occur within an oscillating water column (OWC) have been combined to create a single wave-to-wire model of an OWC. The model was then compared to the experimental results from the FP-7 CORES OWC deployment project. The results show good agreement between modeled and experimental data. The model can be used to estimate the power output of an OWC, as well as to test control strategies and algorithms allowing for development in the control of OWCs before physical deployment. [ABSTRACT FROM PUBLISHER]

A number of self-rectifying air turbines for wave power conversion have been proposed so far. This paper shows the comparison of the performances of all these turbines proposed for use in the near future. As a result, the impulse turbine with self-pitch-controlled guide vanes is found to have the best performance.

A classical one-dimensional analysis in turbomachinery was presented to estimate aerodynamic characteristics of an impulse turbine with self-pitch-controlled guide vanes which is proposed by the authors for ocean wave power conversion. With some simplified assumptions, the efficiency vs/flow-rate coefficient curves were calculated and compared with the experimental results both in a unidirectional steady flow condition and a sinusoidally oscillating flow condition. The estimated results reveal a behavior of the actual characteristics curve of the turbine. Possibility of further improvement in efficiency was discussed from a viewpoint of specific speed and specific diameter.

An impulse turbine with self-pitch-controlled guide vanes has been developed specially for wave energy conversion by the authors. Based on that work, a large turbine of 1 m diameter has been fabricated and operated on the west coast of India. In a practical turbine system, the guide vanes need substantial supports in order to heighten the strength. Such supports affect the performance of the turbine. In this paper, the etfect of guide vane support geometry on starting and running characteristics of the turbine was investigated experimentally. Two types of cross sections of support were studied, that is, circular type and airfoil type. The effect of support length was also studied.

Impulse turbine, working as a typical self-rectifying turbine, is recently utilized for the oscillating water column (OWC) wave energy converters, which can rotate in the same direction under the bi-directional air flows. A numerical model established in Fluent is validated by the corresponding experimental results. The flow fields, pressure distribution and dimensionless evaluating coefficients can be calculated and analyzed. Effects of the rotor solidity varying with the change of blade number are investigated and the suitable solidity value is recommended for different flow coefficients. [ABSTRACT FROM AUTHOR]

Real-life engineering problems have multiple objectives, which mostly are conflicting in nature, and these problems can be solved through multi-objective optimization (MOO) procedure. In the present problem, a high-fidelity computational fluid dynamics model coupled with multiple-surrogate assisted genetic algorithm based MOO has been solved for performance enhancement of a wave energy extracting axial impulse turbine. Response surface approximation, Kriging, neural network and a weighted-average surrogate (WAS) were used to generate population for the MOO procedure and Pareto optimal fronts (PoF) of the objectives were produced. The design variables were number of rotor blade and guide vanes and the objectives were minimization of pressure drop and maximization of shaft power of the turbine. It was found that a cross-validation error analysis is inevitable to find the degree of fitness of a surrogate. The WAS-produced PoF shows better performance as compared to that of the other surrogates. The surrogates based on minimum cross-validation errors produce slightly lesser performance than the WAS. The efficiency, which is a function of both the objectives, was relatively increased by ∼11% through the current investigation. [ABSTRACT FROM AUTHOR]

OWC (Oscillating water column) wave power plants are widely used in ocean wave energy conversion. It is universally accepted that for a good efficiency of the plant, the turbine and the chamber should match each other according to the wave climate site. In this work an alternative methodology is presented for coupling the chamber and the turbine efficiently, carrying out a whole dimensional analysis of the OWC plant. Basically, the wave data and the chamber geometry are the input data. The behaviour of the chamber is numerically simulated. From the numerical results, the optimum damping could be identified for every wave condition. Once the optimum chamber damping is found, the objective is to determine which turbine matches this damping. Numerical results, based on the quasi-steady assumption, were used to simulate turbine performance under periodic conditions. The calculation of the diameter is based on the damping caused by the turbine on the OWC, whereas the rotational speed is fixed to maximize the turbine mean efficiency. Furthermore, this methodology is worked out to compare the size and the rotational speed of different kinds of turbines. Finally, the influence of sea state changes on the parameters of the optimum turbine is also studied. [ABSTRACT FROM AUTHOR]

Both numerical and experimental studies were conducted to evaluate the performance of an omni-flow wind turbine designed to provide renewable electricity on the top of urban buildings like skyscrapers. The numerical approach was based on Finite Volume Method (FVM) and the turbulence flow was studied with several commonly used Reynolds-averaged Navier–Stokes turbulence models. The results of the study were evaluated with the wind tunnel test results over a range of tip speed ratios. The numerical results showed the effect of blade number on both power output and starting capability. Although both the power and torque coefficient were improved significantly by the optimisation of the blade number, there was only a slight change when the blade number was greater than twenty. The results from wind tunnel testing also showed excellent starting capability with a starting wind velocity as low as 1.6 m/s. A numerical simulation was also conducted for the wind turbine working under non-uniform flow conditions. The numerical results have shown that the peak power coefficient of such a wind turbine under non-uniform flow, was lower than that under the uniform flow. Additionally, the applied thrust on a blade was subject to frequent and periodical changes. However, the effect of the change of thrust in magnitude and frequency was not significant. Therefore the omni-flow wind turbine has the potential to meet the challenge of unpredictable wind velocity and direction as a consequence of the urban environment. [ABSTRACT FROM AUTHOR]

Oscillating water column based wave energy extracting system has a low efficiency due to the poor performance of its principal power extracting component, the bidirectional turbine. In the present work, flow over a bidirectional impulse turbine was simulated using CFD technique and optimized using multiple surrogates approach. The surrogates being problem dependent may produce unreliable results, if a wrong surrogate is selected. Hence, multiple surrogates such as response surface approximation, radial basis function, Kriging and weighted average surrogates were incorporated in this problem. Same design points were used to generate multiple optima via multiple surrogates to enhance the robustness of the optimization process. Numbers of guide vanes and rotor blades were chosen as the design variables, and the objective was to maximize the blade efficiency. Reynolds-averaged Navier–Stokes equations were solved for analyzing the flow physics. The computed results were used to train the surrogates and find the optimal points via hybrid genetic algorithm. The surrogates were further applied to find the optimal flow parameters by changing flow velocity and turbine speed. The relative efficiency enhancement through our present approach was about 16%. Detailed methodologies, analysis of the results and surrogate applicability have been presented in this paper. [ABSTRACT FROM AUTHOR]