Performance losses due to ice accretion for a 5 MW wind turbine

A numerical study of power performance losses due to ice accretion on a large horizontal axis wind turbine blade has been carried out using computational fluid dynamics (CFD) and blade element momentum (BEM) calculations for rime ice conditions. The computed aerodynamic coefficients for the normal and iced blades from the CFD calculations were used together with the BEM method to calculate the torque, power and curves of the wind turbine for both normal and icing conditions. The results are compared with the published data. It is shown that icing results in a reduced power production from the turbine and that changing the turbine controller could improve the power production with iced blades. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation method of the energy conversion efficiency of coal gasification and related applications

Traditional gasification parameters, such as cold gas efficiency, hot gas efficiency, or thermal efficiency, only evaluate the heat energy utilisation efficiency of gasifiers and do not take into account the gasification processes expending electricity and other types of energies. Therefore, the energy conversion efficiency cannot be assessed using these parameters. The calculation process on the energy conversion efficiency of underground coal gasification (UCG) is the basis for obtaining quantitative data of carbon emission reduction and establishing the carbon trading methodology of UCG. Moreover, the energy conversion efficiency both for surface coal gasification and UCG is a key research topic because it directly affects the economic and environmental benefits of gasification projects. This study proposed that two parameters, the integrated gasification efficiency (h_com) and the hot gas integrated gasification efficiency ( ), should be included into the coal gasification parameters and used to evaluate the energy conversion efficiency of coal gasification. In addition, the calculation methods of these two parameters for both surface gasification and UCG were established. Using the method, h_com and , of the UCG and Texaco gasification under the same scale was compared and that of various UCG processes was calculated. The results proved the necessity and reasonability of the two parameters and suggested that a certain amount of CO₂ was favourable to improve h_com and of UCG. However, a certain amount of pure O₂ can improve h_com of UCG without direct influences on . Under the condition of each process, to maximise h_com and , there must be an optimal steam (CO₂) to O₂ rate. Copyright © 2015 John Wiley & Sons, Ltd.     

Numerical investigation of the effects of geometric parameters of heaters on mixed covection in a lid‐driven cavity filled with different nanofluids

In this paper, the effects of the thicknesses and locations of two rectangular heaters, located on the bottom and one side of on an enclosure, on mixed convection of nanofluid flows in a lid‐driven cavity are numerically investigated. The enclosure is simultaneously heated partially by these two heaters which have similar or different thicknesses and also filled with different nanofluids containing nanoparticles of Cu, Ag, Al₂O₃, and TiO₂ within the base fluid of water. A finite volume approach by the SIMPLE algorithm is used to solve the governing equations. The effects of different Rayleigh numbers (), Reynolds numbers (), solid volume fractions (), heater lengths (), heater locations () and heater thicknesses () on the streamlines, isotherms and the average Nusselt number along two heaters are studied accurately. Also, variations of average Nusselt number of two heaters are considered whenever one heater is fixed and the other heater moves along on the wall. Moreover, variations of the length of one heater on the average Nusselt number are also studied whenever the length of the other heater is fixed. In addition, variations of the thickness of one heater on the average Nusselt number are studied whenever the thickness of the other heater is fixed.     

Exact,analytic temperature distributions of pin fins with constant thermal conductivity and power‐law type heat transfer coefficient

In this Technical Note, the problem of determining the temperature distribution in a pin fin with power‐law heat transfer coefficients is addressed. It is demonstrated that the governing fin equation, a nonlinear second‐order differential equation, is exactly solvable for the entire range of the exponent n in the power‐law heat transfer coefficients. The exact, closed‐form analytical solutions in implicit form are convenient for physical interpretation and optimization for maximum heat transfer. Furthermore, it is proved that the exact solutions have three different structures: (1) dual in the range of , (2) unique or dual in the range of , and (3) unique in the range of . Additionally, exact analytical expressions for the fin efficiency and the fin effectiveness are provided, both as a function of the dimensionless fin parameter for the gamma of n under study.     

Numerical and experimental study of free convection through a horizontal open‐ended axisymmetric cavity

This paper summarizes a numerical and experimental investigation of free convective heat transfer in an open‐ended cavity between two horizontal parallel circular plates. The upper plate is maintained at an ambient temperature and the lower one is heated. Air is used as the heat transfer medium. The numerical model equations are solved using a control volume‐based finite differences method, and the experimental study was performed using holographic interferometry. Streamlines and isotherm patterns are presented and discussed for different aspect ratios (A) and Rayleigh numbers (Ra). Heat transfer at the surface of the lower plate is thoroughly inspected in the ranges and . Useful correlations of Nusselt numbers in terms of and A are given with their validity ranges. Also, an investigation of both numerical and experimental results is performed. It shows similar temperature field aspect with some differences in the radial boundary layer thickness and a small deviation in the heat transfer.     

Modeling and Optimization of Nanofluid Flow in Flat Tubes Using a Combination of CFD and Response Surface Methodology

In this paper, modeling and optimization of Al₂O₃–water nanofluid flow in horizontal flat tubes is performed using a combination of computational fluid dynamics (CFD) and response surface methodology (RSM). At first, nanofluid flow is solved numerically in various flat tubes using CFD techniques and the heat transfer coefficient () and pressure drop () in tubes are calculated. The numerical simulations are performed using two phase mixture model by FORTRAN programming language. The flow regime and the wall boundary conditions are assumed to be laminar and constant heat flux respectively. In the second step, numerical data of the previous step will be used for a parametric study, modeling and optimization of nanofluid flow in flat tubes using the RSM technique.It is shown that the results include important design information on nanofluid parameters in flat tubes. The important design information about the relationship between design variables and responses will not be achieved without the simultaneous use of CFD and optimization approaches.     

Wind turbine boundary layer arrays for Cartesian and staggered configurations‐Part I,flow field and power measurements

Model wind turbine arrays were developed for the purpose of investigating the wake interaction and turbine canopy layer in a standard cartesian and row‐offset turbine array configurations. Stereographic particle image velocimetry was used to collect flow data upstream and downstream of entrance and exit row turbines in each configuration. Wakes for all cases were analyzed for energy content and recovery behavior including entrainment of high‐momentum flow from above the turbine canopy layer. The row‐offset arrangement of turbines within an array grants an increase in streamwise spacing of devices and allows for greater wake remediation between successive rows. These effects are seen in exit row turbine wakes as changes to statistical quantities including the in‐plane Reynolds stress, , and the production of turbulence. The recovery of wakes also strongly mitigates the perceived underperformance of wind turbines within an array. The flux of kinetic energy is demonstrated to be more localized in the entrance rows and in the offset arrangement. Extreme values for the flux of kinetic energy are about 7.5% less in the exit row of the cartesian arrangement than in the offset arrangement. Measurements of mechanical torque at entrance and exit row turbines lead to curves of power coefficient and demonstrate an increase in efficiency in row‐offset configurations. Copyright © 2014 John Wiley & Sons, Ltd.     

Energy and Exergy Analysis of a Single‐Pass Sequenced Array Baffled Solar Air Heater with Packed Bed Latent Storage Unit for Nocturnal Use

The current study presents an experimental investigation on evaluation of thermal performance of a single‐pass double‐glazed solar air heater with the use of packed bed paraffin wax as a phase change material (PCM). Moreover, the absorber plate is equipped with baffles attached over its top. Galvanized sheets with a thickness of 0.4 mm and total surface areas of 30 cm² are chosen as baffles that are placed in a sequential manner over the absorber plate. The solar energy was stored in the packed bed PCM during the diurnal period (charging process) and was released at night for nocturnal use (discharging process). The tests were performed at three different mass flow rates of 0.009 0.014 and 0.017 resulting in the creation of different Reynolds numbers along the channel. The measured parameters were inlet, outlet, and the PCM temperatures under the meteorological condition of Mashhad, Iran. Energy and exergy efficiencies of the system have been calculated according to the first and second laws of thermodynamics. The experimental results illustrate that the daily energy efficiency varied between 20.7% and 26.8%, whereas the daily exergy efficiency varied between 10.7% and 19.5%.     

Mixed convection inside a fluid‐porous composite cavity with centrally rotating cylinder

The mixed convection in a fluid‐porous composite medium lying inside a square cavity with a centrally rotating cylinder has been investigated in the present work. The bottom half of the cavity is filled with a porous material and the top half is filled with a clear fluid. The bottom wall of the cavity is at a higher temperature, and the top wall is at a lower temperature. The vertical walls are thermally insulated. The convection inside the cavity sets through the combined mechanisms of the thermal buoyancy force and the shearing action of the centrally rotating cylinder. The relative importance of each driving mechanism over the other is featured through the Richardson number. The Darcy–Brinkman–Forchheimer equation is used for the flow modeling in the porous medium, and a single‐domain approach is adopted for the numerical solution in the fluid‐porous composite medium. The simulation is carried out with ANSYS Fluent software, and a parametric analysis involving the Rayleigh number (), Richardson number (), and the Darcy number () is conducted showing their effects on the flow and heat transfer. The phenomena are quite interesting at higher Darcy number and Rayleigh number. The distributions of isotherms, streamlines, and vector plots are plotted, along with the local Nusselt numbers for different parameters, to explore the underlying physics of the phenomenon. The system is found stable at lower Darcy number, and the heat transfer is minimum around Ri = 10. From the numerical study, an empirical correlation for the average Nusselt number is developed as a function of the other dimensionless numbers.     

Double Diffusive Flows over a Stretching Sheet of Variable Thickness with or without Surface Mass Transfer

This paper presents a numerical investigation of the steady two‐dimensional mixed convection flow along a vertical semi‐infinite stretching sheet of variable thickness. The effect of double diffusion on velocity, thermal and concentration fields in presence of power‐law temperature and concentration distributions at wall along with surface mass transfer is considered. The nonlinear coupled partial differential equations governing the flow, thermal and concentration fields are first transformed into a nondimensional set of coupled nonlinear partial differential equations and solved numerically using an implicit finite‐difference scheme in combination with the Newton's linearization technique to obtain nonsimilar solutions at each stream‐wise location. Numerical results are presented to discuss the effects of various physical parameters on the velocity, temperature, and concentration fields. Furthermore, the numerical results for the local skin friction coefficient, local Nusselt number, and local Sherwood number are also reported. For a fixed buoyancy force, the skin friction coefficient and Nusselt number increase with Prandtl number. The increase in the Prandtl number causes about a 30% reduction in the thickness of the thermal boundary layer. The wall thickness parameter enhances the thickness of the momentum boundary layer and the velocity overshoot is observed up to 20% for wall thickness parameter . In contrast, the increase of power‐law index parameter m from to reduces approximately 10% to 25% the momentum and thermal boundary layer thicknesses depending on the values of other parameters     

Peristaltic Transport of a Herschel–Bulkley Fluid in an Elastic Tube

In this paper, we investigate the peristaltic transport of a non‐Newtonian viscous fluid in an elastic tube. The governing equations are solved using the assumptions of long wavelength and low Reynolds number approximations. The constitution of blood has a non‐Newtonian fluid model and it demands the yield stress fluid model: The blood transport in small blood vessels is done under peristalsis. Among the available yield stress fluid models for blood flow, the non‐Newtonian Herschel–Bulkley fluid is preferred (because Bingham, power‐law and Newtonian models can be obtained as its special cases). The Herschel–Bulkley model has two parameters namely the yield stress and the power‐law index. The expressions for velocity, plug flow velocity, wall shear stress, and the flow rate are derived. The flux is determined as a function of inlet, outlet, external pressures, yield stress, amplitude ratio, and the elastic property of the tube. Further when the power‐law index n = 1 and the yield stress and in the absence of peristalsis, our results agree with Rubinow and Keller [J. Theor. Biol. 35 , 299 (1972)]. Furthermore, it is observed that, the yield stress, peristaltic wave, and the elastic parameters have strong effects on the flux of the non‐Newtonian fluid flow. Effects of various wave forms (namely, sinusoidal, trapezoidal and square) on the flow are discussed. The results obtained for the flow characteristics reveal many interesting behaviors that warrant further study on the non‐Newtonian fluid phenomena, especially the shear‐thinning phenomena. Shear thinning reduces the wall shear stress.     

Influence of chemically reactive species and a volumetric heat source or sink on mixed convection over an exponentially decreasing mainstream

In this paper, we investigate mixed convection flow over an exponentially decreasing freestream velocity in presence of nonlinear chemically reactive species and a volumetric heat source or sink. Nonsimilar transformations are used to reduce the boundary layer equations into dimensionless equations and are further solved by the implicit finite difference scheme in combination with the quasi‐linearization technique. The influence of various governing parameters such as the volumetric heat source/sink parameter (Q), the ratio of buoyancy forces (N), the Richardson number (Ri), and the chemical reaction parameter (Δ) on the flow, thermal and species concentration fields are discussed and presented in terms of graphs. The numerical investigation reveals that the increase in volumetric heat source/sink parameter Q increases the temperature profile about 69% in presence of injection and the concentration profile decreases about 56% for and increases around 53% for as n increases from 1 to 2.     

Anti‐windup linear parameter‐varying control of pitch actuators in wind turbines

Operation of wind turbines in the full‐load region mandates that the produced power is kept at a rated value to minimize structural loads and thereby reduce fatigue damage. This is usually achieved by pitching the rotor blades in order to limit the aerodynamic torque in high wind speeds. The pitch actuators usually present a hard constraint in terms of the amplitude and rate of saturation. In this paper, we propose a method to address pitch actuator amplitude and rate saturation by designing anti‐windup controllers in the linear parameter‐varying framework. The proposed design method guarantees the closed‐loop system stability and a prescribed level of performance while it decreases the pitch activity for regulating the generated power to the nominal power during sudden wind gusts. The anti‐windup controller designed to minimize the norm of the closed‐loop system is gain‐scheduled on the basis of the operating condition of the turbine, as well as the states of amplitude and rate saturation of the pitch actuator. The effectiveness of the proposed control design method is demonstrated using high‐fidelity aeroelastic dynamic simulation tools. Copyright © 2013 John Wiley & Sons, Ltd.     

Experimental validation of model‐based blade pitch controller design for floating wind turbines: system identification approach

This paper discusses the model‐based design of a blade pitch controller for a floating offshore wind turbine (FOWT) scale model. A mathematical model of the FOWT is constructed from an input–output measurement in an experiment using system identification. The blade pitch controller is designed by an control method, and the effectiveness of the controller is evaluated by means of a basin experiment using the FOWT scale model. The results show that the blade pitch controller is effective in reducing platform pitch motion and rotor speed fluctuation. Copyright © 2017 John Wiley & Sons, Ltd.     

Analysis and design of Coleman transform‐based individual pitch controllers for wind‐turbine load reduction

As the size of wind turbines increases, the effects of dynamic loading on the turbine structures become increasingly significant. There is therefore a growing demand for turbine control systems to alleviate these unsteady structural loads in addition to maintaining basic requirements such as power and speed regulation. This has motivated the development of blade individual pitch control (IPC) methodologies, many of which employ the Coleman transformation to simplify the controller design process. However, and as is shown in this paper, the Coleman transformation significantly alters the rotational system dynamics when these are referred to the non‐rotating frame of reference, introducing tilt–yaw coupling in the process. Unless this transformation is explicitly included in the model employed for IPC design, then the resulting controllers can yield poor performance. Therefore, in this paper, we show how to model the Coleman transformation in a form that is amenable to IPC analysis and synthesis. This enables us to explain why traditional design parameters of gain and phase margin are poor indicators of robust stability and hence motivate the need for a multivariable design approach. The robust multivariable IPC approach advocated in this paper is based upon loop shaping and has numerous desirable properties, including reliable stability margins, improved tilt–yaw decoupling and simultaneous rejection of disturbance loads over a range of frequencies. The design of a robust multivariable IPC is discussed, and simulation results are presented that demonstrate the efficacy of this controller, in terms of load reduction on both rotating and non‐rotating turbine parts. Copyright © 2014 John Wiley & Sons, Ltd.     

Forced Convection Heat Transfer of Newtonian Fluids from a Backward‐Facing Step: Effects of Expansion Ratio,Reynolds, and Prandtl Numbers

This study involves the numerical solution of the laminar heat transfer in a separating and reattaching flow by simulating the flow and heat transfer downstream of a backward‐facing step. The in‐house finite volume code has been implemented employing a hybrid differencing scheme and the SIMPLE algorithm for the pressure–velocity coupling. Three principal parameters governing heat transfer in this geometry, that is channel expansion ratio (ER), Reynolds number (Re), and Prandtl number (Pr), are systematically varied in the range ER = 1.111 to 2, Re = 1 to 200, and Pr = 0.71 to 100, and the simple correlations between these parameters have been elucidated. A series of important findings have been established by analyzing the results some of which are: (1) there is an associated shifting of the point of maximum heat transfer with respect to the flow‐reattachment point with gradually decreasing the values of ER and (2) the heat transfer enhancement increases with the increase in Pr number as a result of the compression of the thermal boundary layer and the maximum Nusselt number varies as .     

An experimental investigation on the free convection heat transfer in a horizontal cavity consisting of flow diverters

The free convection heat transfer in a horizontal cavity with adiabatic vertical and isothermally horizontal walls and adiabatic diverters has been studied experimentally in this paper. The experiments have been carried out using a Mach–Zehnder interferometer. The effects of the diverter angle ranging from 0 to 90$\circ$ and Rayleigh number based on the cavity side wall length, from 6 × 10³ to 1.2 × 10⁴ on the heat transfer from the heated wall, were studied. The results indicate that, at each Rayleigh number, the maximum and the minimum heat transfer occur at a diverter angle of 30 and 90$\circ$ , respectively. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21014     

CFD Investigation of the Impacts of Variation in Geometry of Twisted Tape on Heat Transfer and Flow Characteristics of Water in Tubes

In this research paper, the influence of variations in geometry of tape insert on thermal performance and flow characteristics of water inside tubes is investigated by means of computational fluid dynamics (CFD). The tape considered is alternate‐axis triangular cut twisted tape. The perimeter of the cuts on the tape, the pitch of the tape and the width of the tape were varied. Turbulent flow is considered and uniform heat flux is imposed on the walls of the tubes. The RNG turbulence model is selected for the simulations and RANS equations are applied to render the Navier‐Stokes equations tractable. The findings of the investigations indicated that the thermal performance of all the tubes fitted with twisted tape is better than that of the tube without twisted tape, and also that the performance is influenced by the geometry of the twisted tape. In particular, the thermal performance diminishes as the tape pitch increases but it is augmented by an increase in the size of the cuts on the tape and an increase in the width of the tape.