Louis Angelo M. Danao

A numerical study of blade thickness and camber effects on vertical axis wind turbines

This article presents the results of a two-dimensional computational study into the effects of rotor blade thickness and camber on the performance of a 5 kW scale vertical axis wind turbine. Validation is provided by reference to experimental data for a pitching aerofoil with dynamic stall phenomenon. The performance of the turbine is mapped out for a variety of different tip speed ratios and detailed investigations are presented to determine how and, most importantly, why the turbine performance varies with thickness and camber as it does. The rotor blades chosen were the NACA0012, NACA0022, NACA5522 and LS0421. The turbine rotor has a diameter of d = 3.1 m with a blade chord length of c = 0.18 m. Over the range of tip speed ratios examined, the NACA0012 profile performed with the highest overall performance of 50% at λ = 3.5. Slightly cambered aerofoils (such as the LS0421) can improve the overall performance of the vertical axis wind turbine, whereas a camber of 5% (NACA5522) results in unfavourable performance. Of the cambered blades tested, the LS0421 performs the best with maximum CP of 0.40 at λ = 3.5. A camber along the blade path causes the blades to generate higher values of torque in both the upwind and the downwind regions. It was also determined that inverted cambered profiles produce power mostly in the upwind region.

Effects on the Performance of Vertical Axis Wind Turbines with Unsteady Wind Inflow: A Numerical Study

Numerical simulations of a micro scale vertical axis wind turbine (VAWT) rotating at constant speed were performed with steady and unsteady wind conditions. The aim of the study was to determine both the performance of a VAWT in unsteady wind conditions and also to determine the flow physics that causes the performance. Frequencies used for the unsteady incoming wind ranged from high values equal to the VAWT rotational frequency to low rates of up to an order of magnitude slower. Mean and standard deviation of wind velocity data from a local wind station were used as mean and amplitude of the variation in wind speed. In general the results show a decrease in performance versus steady wind conditions. The worst cases show a 75% drop in power coefficient (CP) for wind fluctuation rates equal to a quarter of and an order lower to the VAWT rotational frequency. Increasing wind velocities induced earlier stalling and loss of lift on VAWT blades while decreasing wind velocities reduced the apparent angle of attack and subsequently the lift generated by the blades. A hysteresis in the CP was observed when the VAWT was in the low tip speed ratio (λ) region while negative performance dominated the high λ region. For the conditions tested here, any time variation in the wind tends to decrease the energy yield of a VAWT.

Study of CFD simulation of a 3-D wind turbine

A roof-top scaled experimental wind turbine was simulated in FLUENT. Meshing technique for wind turbine blades was studied and a suitable mesh was achieved. The SST k-ω model was set to be the turbulence model in FLUENT. Detailed settings were studied to maintain a convergent and more accurate simulation. The simulation predicted the wind turbine power coefficient in different tip speed ratios. The flow field around the blade, and pressure distributions were also analyzed. The simulation results were compared with its experimental data, showing an agreement. The comparison validated the simulation method, proving that with suitable meshing and setting for FLUENT, simulations could be accurate and reliable.

Visualising Dynamic Stall Around a Vertical Axis Wind Turbine Blade Through Particle Image Velocimetry

The vertical axis wind turbine aerodynamics are highly complex and unsteady. Inherent in the operation of VAWTs is the presence of the dynamic stall phenomenon that has a major influence in the overall performance of the rotor. The acquisition of a reliable experimental flow field data set presents a means to increase the level of understanding of VAWT performance and flow physics through visualisations. The method developed in this study includes the setup of the PIV system in the wind tunnel, surface treatment of the VAWT blades, verification of test settings, and image processing and data analysis. The measurement of the flow fields around a VAWT blade at tip speed ratios of λ = 2.5 and 4 were carried out and the results show significant differences in the stalling characteristics between different λ with increased occurrence of deep and prolonged separation of flow from the blade surface at lower λ. In both cases, however, dynamic stall is observed. The data acquired is an invaluable reference for VAWT flow physics as well as validation of numerical models such as CFD.

Improving VAWT performance through parametric studies of rotor design configurations using computational fluid dynamics

Vertical axis wind turbines present several advantages over the horizontal axis machines that make them suitable to a variety of wind conditions. However, due to the complexity of vertical axis wind turbine (VAWT) aerodynamics, available literature on VAWT performance in steady and turbulent wind conditions is limited. This paper aims to numerically predict the performance of a 5 kW VAWT under steady wind conditions through computational fluid dynamics modeling by varying turbine configuration parameters. Two-dimensional VAWT models using a cambered blade (1.5%) were created with open field boundary extents. Turbine configuration parameters studied include blade mounting position, blade fixing angle, and rotor solidity. Baseline case with peak Cp of 0.31 at tip-speed ratio of 4 has the following parameters: mounting position at 0.5c, zero fixing angle, and three blades (solidity = 0.3). Independent parametric studies were carried out and results show that a blade mounting position of 0.7c from the leading edge produces the best performance with maximum Cp = 0.315 while the worst case is a mounting position of 0.15c with peak Cp = 0.273. Fixing angle study reveals a toe-out setting of −1° producing the best performance with peak Cp of 0.315 and the worst setting at toe-in of 1.5° with peak Cp of 0.287. The solidity study resulted in the best case of four blades (solidity = 0.4) with peak Cp = 0.316 and the worst case of two blades (solidity = 0.2) with peak Cp = 0.283.

Design of a Screw Heat Exchanger as the Liquid-Suction Heat Exchanger in a Vapor Compression Refrigeration System

The aim of this study is to optimize the design of a screw heat exchanger (SHX) in terms of varying coil aspect ratio (CAR) as a liquid-suction heat exchanger (LSHE) in a vapor compression refrigeration system through Computational Fluid Dynamics (CFD) simulation. The novelty of this study is that it focuses on the fluid-to-fluid heat transfer of SHX to measure its heat transfer effectiveness as compared to many research studies that focus only on helical duct heat exchanger under constant wall temperature and constant heat flux. Optimization of the SHX was done through conjugate heat transfer in a CFD model. High confidence on the computational package was determined as the results of the package on heat transfer between water flows through a shell-and-coil heat exchanger match well with available experimental data of the same setup in related literature.The performance of SHX with square coil section (SHXSCS) was compared to that of the SHX with circular coil section (SHXCCS). The optimum SHXSCS was then compared to a commercially available LSHE in the form of tube-in-tube heat exchanger (TTHE) which is the most commonly used LSHE in vapor compression refrigeration systems.The SHXSCS has nominal degrees of superheat of 22.17 °C and nominal degrees of subcool of 17.63 °C. The ratio of the heat of superheating to that of subcooling is seen to increase with increasing CAR and NTU. The RE of SHXSCS is larger by an average of 3.87%, 18.25% and 26.46% compared to those of the SHXCCS, TTHE and the standard vapor compression cycle (VCC), respectively. The COP of the SHXSCS is 2.55%, 10.74% and 5.94% higher than those of the SHXCCS, TTHE and the standard VCC, respectively. The SHXSCS is more capable than SHXCCS in splitting the heat due to the less complicated square cross sections of flows and even less interface materials for heat transfer. Moreover, the SHXSCS is more effective in splitting the heat from subcooling into moderated superheating and largely to the surounding compared to the TTHE mainly due to longer length of interaction of the flows.Copyright

A Numerical Study on the Effects of Unsteady Wind on Vertical Axis Wind Turbine Performance

Numerical simulations using RANS–based CFD have been utilised to carry out investigations on the effects of unsteady wind in the performance of a wind tunnel vertical axis wind turbine. Using a validated CFD model, unsteady wind simulations revealed a fundamental relationship between instantaneous VAWT CP and wind speed. CFD data shows a CP variation in unsteady wind that cuts across the steady CP curve as wind speed fluctuates. A reference case with mean wind speed of 7m/s, wind speed amplitude of ±12%, fluctuating frequency of 0.5Hz and mean tip speed ratio of 4.4 has shown a wind cycle mean power coefficient of 0.33 that equals the steady wind maximum. Increasing wind speed causes the instantaneous tip speed ratio to fall which leads to higher effective angle of attack and deeper stalling on the blades. Stalled flow and rapid changes in angle of attack of the blade induce hysteresis loops in both lift and drag. Decreasing wind speeds limit the perceived angle of attack seen by the blades to near static stall thus reducing the positive effect of dynamic stall on lift generation. Three mean tip speed ratio cases were tested to study the effects of varying conditions of VAWT operation on the overall performance. As the mean tip speed ratio increases, the peak performance also increases.Copyright