Abdus Samad

The present work describes the design and analysis of a compact and modular wave-powered desalination (WPD) system. The design of the unit is influenced by the United States Department of Energy (DOE) Waves to Water competition rules. The system consists of a flap-type wave surge converter that converts wave energy into hydraulic piston force through a double-acting cylinder working as a power take-off (PTO) device. The dimensions of the flap are determined through frequency domain analysis under different sea states and the required discharge of desalinated water. Further, the whole system, including the reverse osmosis (RO) module, is modeled using WEC-Sim (an open-source code) and Simscape/SimHydraulics toolbox in MATLAB. The analysis of the system under six different sea states shows encouraging results for freshwater production. Considering the sea states studied here, the designed system can generate more than 50 bar pressure of the seawater feed discharge and produce up to 100 L/h of freshwater with TDS less than 500 mg/L. It was also observed that the use of an accumulator reduces the fluctuation in feed flow and feed pressure at the inlet of the RO module, resulting in improved system efficiency.

In this article, a Wells turbine geometry is created with the modification of multiple geometric parameters, i.e., blade sweep/skew, endplate, casing groove, and guide vane. The experiment of a bidirectional turbine is conducted at the wave and fluid engineering laboratory, IIT Madras. The preliminary objective of the study is to measure the starting characteristics and corresponding flow velocity, revolution per minute (rpm), differential pressure of higher and lower pressure sides of the turbine. The output parameters are measured at different cycle times and the stroke length of a piston-cylinder combination, which simulates different wave conditions. After starting the Wells turbine, rotational and axial speeds increase for some time. After that, it will fluctuate between a specific range, and pressure is prepositional to the airflow rate. The wave energy can be converted into pneumatic energy with the help of wave energy converting(WEC) devices, i.e., oscillating water column(OWC) that can be further converted into mechanical energy and then into electrical energy with some appropriate devices. In this article, an experimental analysis of the turbine geometry is reported.

Guide vanes (GVs) improve the performance of a turbine in terms of efficiency, torque, or operating range. In this work, a concept of different orientations of GVs in between a two-row biplane wells turbine (BWT) was introduced and analyzed for the performance improvement. The fluid flow was simulated numerically with a commercial software ANSYS CFX 16.1. The Reynolds-averaged Navier-Stokes equations with the k-Ï ‰ turbulence closure model were solved for different designs and flow conditions. For the base model, the results from simulation and experiments are in close agreement. Among the designs considered, the configuration, where the blades are in one line (zero circumferential angle between blades of two plane) and the midplane guide vane has concave side to the leading edge of the blade, performed relatively better. However, the performance was still less compared to the base model. The reason behind the reduction in performance from the base model is attributed to the blockage of flow and the change of flow path occurring due to the presence of the midplane GVs. The flow analysis of different cases and the comparison with the base model are presented in the current study.

The present article reports optimization of the performance of a Wells turbine, which is an axial turbine utilized for wave energy conversion, through a computational fluid dynamics (CFD) based automated optimization technique. The in-house optimization library OPAL++ was coupled to a commercial CFD solver to get the Pareto front defining the relationships between two objectives. Four different geometric parameters of the turbine with ring-type endplate were used, and the objectives were to maximize the torque coefficient and simultaneously minimize the pressure drop coefficient. From the Pareto front, two designs (G1 and G2) were chosen for further analysis. G1 improved the peak torque-coefficient by more than 120 % and delayed the stall point from φ = 0.225 to φ = 0.3, while the peak efficiency dropped. Whereas, G2 improved the peak efficiency by 9.1 %, but the peak torque coefficient was reduced by about 50 %. The main contribution of the study is to develop an optimum Wells turbine geometry through the coupling of CFD and automated optimization algorithm - first of its kind applied to a Wells turbine with endplate. A detailed flow analysis, the influence of the endplate, and a comparison of the optimized geometries are presented.

Wells turbine is one type of axial turbine exclusively used for wave energy conversion in Oscillating Water Column type wave energy conversion device. It is a bi-directional turbine which can rotate in one direction irrespective of the direction of airflow. One of the main disadvantages of this turbine is the stall phenomenon, where the torque, as well as efficiency, drops drastically at a particular angle of attack. The postponement of stall can be achieved by installing fences along the chord of the blade at a distance from the hub. In the present work, a numerical analysis of Wells turbine with the stall fence is carried out using commercial CFD tool ANSYS. The performance characteristics of the turbine are investigated for different number of stall fences at different distances along the span of the blade. A detail flow analysis is presented to explain the effect of the stall fence on this particular turbine.

Oscillating Water Column (OWC) is one of the most popular wave energy converters being used for the last two decades. The pneumatic energy from water waves inside the air chamber of OWC is converted into mechanical energy with the help of Wells turbine. Biplane Wells turbine has inherent advantage over the monoplane turbine in terms of starting characteristics and operating range. The main parameters affecting the performance of biplane Wells turbine are the gap between the planes and the offset angle between blades in two planes. Surrogate-based optimization represents the optimization methodologies that use surrogate modelling techniques to find out maxima or minima. Surrogate modelling techniques are very useful for design analysis that uses computationally expensive codes such as Computational Fluid Dynamics (CFD). In the present work, flow over a biplane Wells turbine is simulated using CFD and optimized using surrogate approach. Radial Basis Neural Network (RBNN) method is used to create the surrogate. Blade thickness and the offset angle defining the circumferential position of blades in two planes are considered as the two variables and the objective function is taken as efficiency of the turbine rotor. The comparison of performance between the reference blade and the optimized blade is presented in this article.

The oscillating water column type wave energy converters equipped with Wells turbine are one of the popular wave energy conversion devices. In most of the numerical and experimental studies, the Wells turbine characteristics are examined in no-load condition or with a fixed loading to achieve a fixed rotational speed. In the present work, a biplane Wells turbine is designed and tested in an experimental test facility. The test facility consists of a piston-chamber assembly that can generate sinusoidal airflow inside a duct. The turbine is placed inside the duct and tested for different stroke lengths and time periods of the piston, which produces a sinusoidal inlet airflow of different amplitude and time period. The turbine characteristics are studied at the no-load condition and for different values of resistive loading connected with the generator. The hysteresis behavior of the turbine is studied for two different flow coefficients based on experimentally observed and numerically calculated volume flow rates. Based on the experimental results, a detailed analysis of the turbine performance is presented for different operating conditions.

Wells turbine is an axial flow air turbine extensively used in the oscillating water column (OWC) of ocean energy harvesting device. The turbine has low aerodynamic efficiency at higher flow rate and poor starting characteristics. In this paper, the characteristics of the hysteresis behavior of a Wells turbine for a wave energy conversion device under alternative axial flow conditions are reported. The numerical work is carried out by solving the three-dimensional unsteady Reynolds Average Navier–Stokes equations (URANS) with two-equation eddy viscosity model. It is noticed that the unsteady numerical results are associated with two hysteresis loop. In the clockwise hysteresis loop, larger flow separation can be noticed on the blade suction side due to stronger vortex while flow separation decreases due to weaker vortex during counterclockwise hysteresis loop. Also, the effect of the blade sweep and blade profile thickness on the hysteresis behavior of the wave energy conversion device are reported.

Remarkable advancement in wave energy conversion technology has taken place in recent years. Due to its simplicity, the Wells turbine has been one of the most widely used power take-off mechanisms in an oscillating water column type wave-energy conversion device. However, the turbine suffers from several challenges due to its narrow operating range, which hinders the commercial feasibility of the system. Several aerodynamic applications have successfully used passive control methods to modify the flow conditions. This work applied a combination of stall fences and casing grooves for passive flow control of a Wells turbine. The computational fluid dynamics (CFD) technique is used to analyze the modified turbine numerically. The casing groove modified the tip-leakage vortices, interacted with local vortices created by the stall fences, and helped reattach the flow at higher flow coefficients. As a result, the modified turbine increases the operating range up to 33.3%. In addition, the peak-to-average (PTA) power ratio decreased by up to 27.7%.

A pair of turbines can harness power from a wave energy Converter. Their performance is poor than individual turbines due to flow reversal. A fluidic diode (FD) which offers variable resistance to the flow, can be used to prevent flow reversal and improve the performance of these units. Its performance is given by diodicity (ratio of reverse to forward flow pressure drop). A higher diodicity enables it to prevent flow reversal better and improve the turbine unit’s overall efficiency. In this work, the geometrical shape of the FD is optimized to obtain higher diodicity. Six geometrical variables of the FD are varied to obtain sample points using the sampling technique, which is numerically investigated by solving steady-state Reynolds averaged Navier–Stokes (RANS) equations. These numerical results were fed into a neural network code that produced an optimal FD design. The optimum model showed a 36.5% improvement in diodicity at 0.35 m3/s. The fluid flowing through the optimized model experience higher resistance in the reverse direction because of the increased vortex strength than the base model. Among all the design variable considered, nozzle angle is a highly sensitive parameter in the optimization process. The optimum FD model enhanced the overall efficiency of the turbine unit by 13.3.