The objective of the study is to parametrically investigate the performance of Savonius and helical blade turbines in terms of power coefficient, torque coefficient and starting characteristics. Similarly, helical blade turbines were developed and tested by varying its stiffness ratio and number of steps.
Energy Scenario
It plays an important role in ensuring energy supply for the growing needs of the masses and in developing water resources. More recent studies of large water reservoirs created by hydroelectric dams have shown that the processes of decay of aquatic vegetation can cause greenhouse gas emissions (formation and release of methane into the atmosphere) equivalent to emissions from other electricity. resources.
Hydrokinetic Power
Considering this, hydrokinetic turbines can be placed in any water source that has enough speed to move. This type of turbine can be installed in remote areas near rivers that have little or no high flows to harness the energy from the water without using containment structures that cause environmental disruption.
Wind Power
Screens used to protect the machine or reduce impact can themselves injure aquatic animals.
Scope of Research
Further studies can be done to compare the performance of SHT and SWT that have an identical kinetic energy of the incoming fluid (hydro/wind). Separate studies on the effect of different design parameters on the performance of SHT or SWT have been reported in the literature.
Organization of the Thesis
Although this issue has not yet been addressed, such submergence level experiments are essential to provide insight into the performance deviation of the turbines when there is a fluctuation in the water level due to the seasonal changes. However, a comprehensive analysis of a Savonius design for known input kinetic energy but with a difference in the working fluid medium is rarely reported.
Chapter 6 contains the results of field experiments and numerical computations of the helical-bladed hydrokinetic turbines (HHTs). Chapter 7 discusses the results of wind tunnel
Hydrokinetic Turbines and Wind Turbines
- Drag-based Turbines
- Lift-based Turbines
In such a system, the turbine rotates primarily due to the drag force between the concave and convex sides of the blades. The main advantages of the helical blade turbine are its better self-starting properties along with reduced torque fluctuation compared to the straight blade turbine (Scheurich et al.
Performance Parameters
- Power coefficient (C P )
- Torque coefficient (C Q )
- Lift coefficient (C L )
- Drag coefficient (C D )
- Tip-speed ratio (TSR)
- Reynolds number (Re)
The lift is a result of the unequal pressure on the upper and lower surfaces of turbine blades. Drag force (FD) should normally be parallel to the direction of the oncoming air flow.
Design Parameters of Savonius Turbine
- Overlap ratio (β)
- Aspect ratio (AR)
- End plate
- Number of blades (n)
- Blade profiles
This is the ratio of turbine blade height (H) to turbine diameter (D) as shown in the figure. The aspect ratio of the turbine can also be adjusted according to the rotation speed of the electric generator.
Design Parameters of Helical Bladed Turbine
- Solidity ratio (σ)
- Number of blades (n)
- Aspect ratio (AR)
- Helix angle ()
- Blade Profile
- Blade pitch angle (ψ)
Conversely, at constant chord length, as the number of blades increases, so does the strength of the turbine. Blade pitch refers to the position of the mid-thickness of the blade along its length.
Computational Investigations
- Simulation Methods
The standard k–ε model is the basic k–ε turbulence model and has provided better results than the SA model for turbine analysis (Pope et al. 2010; Ghasemian et al. 2017). The shear stress transport (SST) k–ω turbulence model is a two-equation viscosity model that combines the advantages of the k–ε free-stream formulation and the k–ω formulation in the turbine boundary layer.
Objectives of the Present Thesis
In this chapter, details of vertical axis turbine models and their fabrication are discussed along with test sites. In the case of SHTs, three models were developed to study the influence of blade profiles and number of blades. Similarly, three models of HHTs are designed to study the influence of stiffness ratio on turbine performance.
Introduction
Savonius Turbines
Schematic diagrams of two-blade and three-blade turbines with semicircular blades are shown in Fig. In this study, a cutting angle (α) of 47.5° is chosen for the turbine with elliptical blades, as SWT with this value was reported by Alom et al (2016) to perform better than SWT with semicircular blades. The cross-section of an ellipse and an elliptical turbine are shown in the figure. Furthermore, the complete turbine test setup with a mechanical dynamometer is shown in Fig. a) Two-bladed turbine (b) Three-bladed turbine Figure 3.1: Schematic diagram of Savonius turbines with semicircular blades. a) Cross-section of an ellipse (b) Elliptical blades Figure 3.2: 2D view of a Savonius turbine with elliptical blades.
Helical-bladed Turbines
- Design of HHTs to analyze the effect of solidity ratio
- Design of HHTs to analyze the effect of multi-staging
A mold is then produced from the pattern from which the spiral blades are made. The experimental setup consisting of a turbine with a central shaft, bearings, brake pulley mounted on the central shaft and two digital spring balances shown in Fig. The turbine blades are attached to the 16 mm diameter turbine shaft with the help of spoke arms on either side of the blade.
Testing Facility for Hydrokinetic Turbines
- Open channel water flume
- Irrigation sluice
- The Brahmaputra river
The water current velocity at the site is measured using a water velocity meter (Make: Global Water, model: FP111) with operating range 0.1–6.1 m/s (smallest count 0.1). Water inlet velocities were measured at the beginning of the straight section (upstream of the turbine) of the flume. The entire structure is then held between the two boats and the flume inlet is held facing the incoming water flow.
Wind Tunnel Test Facility
Torque and Power Measurements
Starting Torque Experiments
Summary
This chapter examines the computational methodology used to simulate Savonius and helical blade turbines. In the present study, symmetric vanes along the vertical axis are used and therefore, 2D planar simulations are performed to investigate the performance of the Savonius turbine, which significantly reduces the computational time without compromising accuracy remarkably compared to 3D simulations. In the case of SHT, the inlet boundary condition is set according to the actual experimental water velocity condition, V = 0.8 m/s.
Introduction
4.2 2D unsteady Simulation for Savonius Turbine
- Description of computational domain
- Details of problem set-up
- Selection of turbulence model
- Details of flow solver
- Calculation of power coefficient
The upper and lower edges of the computational domain are assigned as symmetry boundary condition. The known rpm value is imposed on the inner zone of the computational domain for rotation of the turbine blades. For a realistic modeling of the flow field around a turbine, it is essential to account for turbulence.
4.3 3D Unsteady Simulation for Helical-bladed Turbine
Description of computational domain
Details of problem set-up
Selection of turbulence model
Details of the solver
Calculation of power coefficient
Summary
This chapter describes both the experimental and computational results of the drag-based Savonius hydrokinetic turbines (SHT). The study shows that the SHT with two hemispherical blades exhibits superior performance compared to the SHT with two elliptical blades and the SHT with three hemispherical blades under full and partial immersion conditions. In addition, the SHT with three semicircular blades performs better than the SHT with two elliptical blades.
Introduction
Effect of Number of Blades
- Load characteristics experiments
- Grid refinement and comparison of computational model
- Flow physics analysis
Additionally, the two- and three-blade turbines stall at maximum brake loads of 1.18 N.m and 0.91 N.m respectively. Therefore, there is reduced maximum CP and CQ for the three-bladed Savonius turbine compared to the two-bladed turbine. Some of the reported results for the three-bladed Savonius wind turbine/hydro turbine are shown in Table 5.1.
Effect of Blade Profile
- Load characteristics experiments
- Flow physics analysis
In order to obtain the flow physics and reasoning behind the improved performance of the semicircular blade turbine over elliptical blade turbine, CFD simulations were performed for the experimental conditions. At θ = 90°, on the concave halves of both advancing and receding blades are subjected to a higher magnitude velocity vector for elliptical blade turbine. This reduces the performance of the elliptical blade SHT compared to semicircular blade SHT.
Immersion Experiments
As observed, the value of CP drops abruptly and the point of optimal TSR also shifts towards the lower value as the submergence level decreases (Figs. 5.9 to 5.11). When the submergence level drops from 100% to 60%, the CPmax values of three-bladed semicircular, two-bladed semicircular, and two-bladed elliptical turbines decrease by 70%, 75%, and 90%, respectively. In addition, when the submergence level is changed from 100 to 80%, the CPmax value drops by and 80% for three-bladed semicircular, two-bladed semicircular, and two-bladed elliptical turbines, respectively.
Summary
In all cases, mechanical power measurement was made using a rope brake dynamometer and power coefficients were obtained for a variety of mechanical load conditions. In the initial set of experiments, the effect of the stiffness ratio on the performance of the HHT is evaluated together with the initial torque characteristics. The experimental results showed a superior performance of HHT that has higher durability under full and partial immersion.
Introduction
Effect of Solidity Ratio
- Experiments at open channel flume
- Immersion experiments
It should be noted that the current CPmax lies in the range as reported by most of the researchers. The main reason for this shift is the increase in diameter of the turbines with a decrease. Therefore estimation of static torque is extremely essential as it is the characteristics of the turbine.
Load Characteristics Experiments at Irrigation Sluice
- Validation of computational model
Also, the total pressure in the circular region is much lower in the case of HHT at 64.9 rpm than at 38 rpm. Therefore, there is a significant pressure drop for HHT at 64.9 rpm compared to HHT running at 38 rpm. This indicates a higher energy extraction from the incoming fluid in the case of HHT at 64.9 rpm.
Effect of Multi-staging
At no load, the single-stage turbine configuration rotates at 486 rpm while the two-stage turbine configuration rotates at 376 rpm. Further, the maximum braking torque obtained for the single-stage and two-stage turbine configurations is 0.52 N.m and 0.41 N.m, respectively.
Summary
Similarly, the experimental investigation of single-stage and two-stage HHT presents the largest CP of the same order of magnitude. However, in the presence of multiple turbines, the drag due to the spoke arms increases and thus reduces the performance of the overall system. The lift and drag characteristics of SWT and SHT are evaluated and found to be complementary.
Introduction
Load Characteristics Experiments
Similarly, two-elliptical blades SWT and SHT run at TSR of 1.31 and 1.10 respectively under the same condition. Again, the maximum braking load results in the braking torque of 0.13 N.m and 1.17 N.m for two semicircular blades SWT and SHT, respectively. Similarly, for three-semi-circular blades SWT and SHT, the maximum braking loads lead to the braking torque of 0.085 N.m and 0.91 N.m, respectively.
Computational Analysis
Thus, it is quite clear that the two-semi-circular blade SWT demonstrates better tensile and lifting properties compared to two-elliptical blade SWT and three-semi-circular blade SWT. Along similar lines, comparisons of lift-mod characteristics for SWT and SHT are presented in Figs. Therefore, it can be concluded that regardless of the fluid (wind/water) medium, the vertical axis Savonius turbine generates approximately the same lift-drag characteristics for a given inlet power.
Summary
The performance of the vertical axis drag-based and lift-based turbines is investigated through onsite tests and expressed in terms of torque and power coefficients. In another set of experiments, a performance comparison of SWT and SHT with an identical kinetic energy of the oncoming fluid (wind/hydro) is made. In addition, the lift-drag characteristics of the SWTs and SHTs are estimated to further verify the experimental observations.
Contribution of the Present Work
- Savonius Hydrokinetic Turbine (SHT)
- Helical-bladed Hydrokinetic Turbine (HHT)
- Savonius Wind Turbine (SWT)
At various partial immersion levels, the SHT with two hemispherical blades shows superior performance compared to the SHT with three hemispherical blades and the SHT with two elliptical blades. At 60% and 80% immersion levels, the SHT with two hemispherical blades indicates the highest CPmax, followed by the SHTs with three hemispherical blades and two elliptical blades. The two hemispherical vane SHT operates at higher optimum TSR values at all immersion levels compared to two other Savonius turbine designs.
Application Potential
The performance of SHTs is compared with SWTs for identical input power condition of 15.9 W through wind tunnel experiments and 2D unsteady simulations. SWTs are found to operate over a slightly wider range of TSRs than SHTs especially towards increasing TSR value beyond the optimal TSR. At identical power input, SWTs and SHTs are found to demonstrate similar lift-drag characteristics, which justify the fact that a turbine produces approximately the same power output for a given power input, regardless of the type of fluid (hydro /young).
Scope of Future Work
Chen L, Chen J and Zhang Z, (2018), Overview of the Savonius rotor blade profile and its performance, Journal of Renewable and Sustainable Energy, Vol. Golecha K, Eldho TL, and Prabhu SV, (2011), Influence of the deflector plate on the performance of modified Savonius water turbine, Applied Energy, Vol. Marsh P, Ranmuthugala D, Penesis I, and Thomas G, (2015), Numerical investigation of the influence of blade helicity on the performance characteristics of vertical axis tidal turbines, Renewable Energy, Vol.
