Abdus Samad

Delaying a stall improves the performance of any turbomachinery system. TC (tip clearance), which is used in a bi-directional flow Wells turbine of an ocean wave energy device, changes the flow pattern on the turbine blade suction surface, while changing or modifying the TC zone can help obtaining a delayed stall. In the present work, a new tip grooving scheme is introduced and the performance is compared for different tip groove depths and TCs of a Wells turbine. The performance is defined in terms of wider operating range or stall delay, power production and efficiency. The problem was solved by a numerical analysis technique. A multi-block meshing scheme was employed to generate structured and hexahedral elements in the computational domain and the flow was solved in ANSYS CFX® v14.5 by solving Reynolds-averaged Navier Stokes equations. It was found that the grooves improve the turbine operating range and power production as compared to those of the turbine without a groove. The groove depth of 3% of the chord length produced highest power and widest operating range. Using the circumferential groove, 26% increase in turbine power output for a particular operating point is achieved.

The present paper investigates the leading edge (LE) undulations of a Wells turbine blade through numerical analysis. The aspiration for this modification came from humpback whales, which have uneven protrusions at the LE of their pectoral flippers. The flippers help whales to maneuver during swimming. The work is performed by using three-dimensional steady, incompressible Reynolds Averaged Navier-Stokes (RANS) equations with turbulent closer model. The LE of the turbine blades is modified with undulations of three different amplitudes: 1mm, 2.5mm, and 4mm. The results show that the undulation changes the turbine performance. The amplitude 2.5mm gives the peak performance. The comparison between blades with different amplitudes and the reference blade has been discussed throughout this study.

The present work focuses multi-objective optimization of blade sweep for a Wells turbine. The blade-sweep parameters at the mid and the tip sections are selected as design variables. The peak-torque coefficient and the corresponding efficiency are the objective functions, which are maximized. The numerical analysis has been carried out by solving 3D RANS equations based on k-w SST turbulence model. Nine design points are selected within a design space and the simulations are run. Based on the computational results, surrogate-based weighted average models are constructed and the population based multi-objective evolutionary algorithm gave Pareto optimal solutions. The peak-torque coefficient and the corresponding efficiency are enhanced, and the results are analysed using CFD simulations. Two extreme designs in the Pareto solutions show that the peak-torque-coefficient is increased by 28.28% and the corresponding efficiency is decreased by 13.5%. A detailed flow analysis shows the separation phenomena change the turbine performance.

Wells turbines are used in oscillating water column wave energy system and the turbine has a stagger angle of 90o. Numerical analysis is performed to analyze the performance of the turbine in the present work. A commercial code ANSYSCFX ® v14.0 was used for the simulations at different flow coefficient, different angles and a constant rotational speed. The turbulence model was k-ω SST. Higher guide vane angle produced higher efficiency of the turbine and the efficiency (enhanced) change was contributed because of the vortex formation in different locations in the flow passage or near the blade surface.

Oscillating water column wave energy converter is having low efficiency because of its principal component, a bidirectional turbine. An analysis of the internal flow of the turbine gives an idea of improving the performance through optimization of geometrical parameters. In this study, an impulse turbine of 0.3 m diameter with fixed guide vanes is numerically simulated by solving three-dimensional incompressible steady Reynolds averaged Navier-stokes equation with two-equation turbulence closure model. This study shows that the numerical results very well match with the experimental results. The detailed flow physics demonstrates that different types of losses occur in this type of turbine and shows that the downstream diverging path of the rotor and guide vane is responsible for low performance. In this study, the effect of guide vane lean, as well as the combined rotor and guide vane lean on the performance of the turbine, has been discussed in detail and found to increase the efficiency of the turbine.

The Wells turbine is a self-rectifying air turbine, used in oscillating water column (OWC) to harvest wave energy. It produces unidirectional torque as the flow oscillates inside the OWC chamber. It has inherent disadvantage of narrow operating range due to stall at high airflow rate. Whereas, a wider operating range is essential to improve the turbine power output. A casing groove modifies the tip leakage flow pattern and improves the operating range. In addition, a radiused tip can alter the tip leakage flow and delay the stall. To enhance the performance further, this paper investigates the combined effect of tip groove and radiused tip (CG&RT) design modification. The flow was simulated by solving steady, incompressible Reynolds averaged Navier–Stokes equations in Ansys CFX 15.0. As expected, the CG&RT blade enhanced the relative operating range and the turbine power output by 44.4% and 23.8%, respectively.

The narrow operating range of a Wells turbine restricts its energy extraction capability from the ocean waves. In this work, the concept of a radiused edge tip blade (RETB) was introduced to overcome such an issue. The RET modifies the tip and changes the tip leakage flow behaviour. The flow through the turbine annulus was simulated numerically and compared with the existing experimental results of the reference turbine. Three-dimensional Reynolds-averaged Navier–Stokes equations with a two-equation turbulence closure model available in ANSYS CFX 14.5 was used for the simulations. The computational domain was discretized with unstructured tetrahedral elements, and the grid independence study gave an optimal grid. The RETB altered the tip leakage flow characteristics and delayed the stall inception. The RETB enhanced the relative operating range by 25% and peak torque by 37%.

The performance of an oscillating water column (OWC) wave energy system depends upon several factors including its turbine performance. OWC turbines have inherently low efficiency and low operating range. Through this work, the shape of a Wells turbine was tried to modify to enhance the efficiency and enlarge the operating range. A numerical investigation with steady state flow condition has been reported to change blade thickness from hub to tip. A new concept to combine NACA0015 and NACA0024 at the hub, the midspan and the tip section were developed and the blade was produced through smooth polynomial so that the blade will have different thickness along the span. A commercial code ANSYS-CFX® v14.5 was used for the simulations. The turbulence k-ω SST model was adopted and the reference turbine performances were compared with existing results reported in the literatures. It was found that a blade thinner at midspan gives lesser flow separation, which helps increasing the turbine performance.

The oscillating water column (OWC), which is a wave energy extracting device, uses a bidirectional flow turbine to generate power. The device performance largely depends on the efficiency, torque and operating range of the turbine. The turbine with a larger operating range produces power during a wider wave height and period, both properties changing throughout the year or during each wave cycle. In this article, a numerical work relying on design optimization is reported to show the dependency of power extraction capability to the operating range of the turbine. Reynolds-averaged Navier-Stokes (RANS) equations were solved and a surrogate approximation model was constructed to find an optimal design. Design variables were blade sweep angles at the tip and mid sections. The objective function, to be maximized, is the torque coefficient. The optimal design delayed the flow separation and the peak efficiency dropped by 3.1%. At the same time, the relative power output and the relative stall point were increased by 29% and 18% compared to the reference case, respectively.

The Wells turbine is used in conjunction with an oscillating water column to harvest wave energy. It is a self-rectifying axial flow reaction turbine which consists of symmetrical blades aligned normal to the incoming flow. A narrow operating range due to flow separation restricts the power extracting capability of the turbine at a higher flow rate. To improve its performance, a turbine blade with combined design modifications such as radiused edge blade (REB) tip, static extended trailing edge (SETE) and variable thickness blade (VTB) was investigated through CFD analysis. Three-dimensional time independent Reynolds-averaged Navier-Stokes equations were solved in a commercial solver to obtain the turbine performance, which was given by non-dimensional torque, pressure drop and efficiency. The combined REB tip, SETE and VTB modifications enhanced relative average turbine power output by 97% and the relative operating range by 22%. However, the average efficiency is decreased by 7.7%, because of the increased pressure drop.