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The studies of supercritical carbon dioxide (S-CO2) power generation cycles have attracted wide attention from different energy industries in recent years. At present, there is a lack of studies on the dynamic characteristics of S-CO2 power systems, especially in S-CO2 coal-fired power plants. In order to further provide the theoretical and technical support for the integration of energy internet and the deep peak-load regulation, it is important to establish the dynamic model of the S-CO2 power system. Heat exchanger is a key component of the system. Therefore, the dynamic response characteristics of the heat exchanger need to be studied because the related research will lay the foundation of building the dynamic model of the entire S-CO2 power system. In this paper, the effects of dynamic responses characteristics of thermodynamic parameters (fluid outlet temperature, total surface heat flux and surface heat transfer coefficient of fluid channels) on the printed circuit heat exchanger (PCHE) of 1000 MW S-CO2 coal-fired power plants are studied in detail. Afterward, a neural network is trained to predict the performance of the PCHE. First, the computational fluid dynamics (CFD) method is adopted to establish the dynamic model of the S-CO2/S-CO2 PCHE. Second, when the fluid inlet temperature or the fluid mass flow rate is changed, the dynamic response trends of thermodynamic parameters on the PCHE of 1000 MW S-CO2 coal-fired power plants are analyzed. Third, the comparisons of the equilibration time of the PCHE with different boundary conditions (the fluid mass flow rate and the fluid inlet temperature) are investigated. Finally, a neural network is trained to predict the performance of the PCHE.

The transparent insulation materials (TIM) can effectively improve the performance of flat plate solar collector in cold weather. A three dimension numerical model of flat plate collector with TIM has been developed in this paper. The computational fluid dynamics (CFD) have been used to simulate the model. The Renormalization-group (RNG) k-e model and Discrete Ordinates (DO) radiation model were adopted. The influences of the environment conditions, mass flow rate, tilt angle and transmittance on the performance of the collector with TIM were analyzed. A good agreement was achieved between the CFD prediction and the previous experiment. The result shows that the collector with TIM is more efficient, when ambient temperature is low. For various wind speeds, the new collector's efficiency has a slightly change. The transmittance of TIM is a key parameter to achieve high performance for the collector. When the transmittance is below 80%, the collector with TIM has no the advantage of being good value. The optimum mass flow rate is 0.06 kg/s under corresponding conditions. The tilt angle of the collector with TIM has less effect compared with the conventional one.

The present paper investigates the aerodynamic and aeroacoustic characteristics of H-rotor Darrieus vertical axis wind turbine combined with a promising energy conversion technology, namely wind-lens, employing computational approaches. The Darrieus turbine adequate for low wind speed and urban area conditions. However, its aerodynamic and aeroacoustic characteristics are very complicated. Thus, the main scope of the current work is to enhance the aerodynamic performance of the introduced turbine and then assess the noise production accomplished with such enhancement. The studies are taken on two phases, the first phase is parametric 2D CFD simulations, employing the unsteady Reynolds-averaged Navier-Stokes (URANS) approach, to optimize the design parameters of the wind-lens. The second phase is a 3D CFD simulation of the full turbine using a higher order numerical scheme employing a hybrid RANS/LES method. An estimate of the aeroacoustic noise of the turbine is quantified using the Ffowcs Williams-Hawkings (FW-H) method. As a result, the optimal design parameters of the wind-lens are achieved and the optimized configuration shows a superior augmentation in power generated of about 82 % increase in the power coefficient at a speed ratio of 2.75. However, the turbine equipped wind-lens produces more noisy operation compared to the non-shrouded turbine.

Lost circulation of working fluid into formation fractures is one of the most common and costly problems encountered during the development of petroleum and geothermal resources. Fracture plugging strength and efficiency with loss control material determine the effect of lost circulation control. This paper proposes an integrated method for optimal material selection. The key mechanical parameter of loss control material is determined based on mathematical model and simulation on fracture plugging strength and efficiency. The developed fracture plugging strength model accounts for the shear failure of fracture plugging zone. Simulation with coupled computational fluid dynamics and discrete element method is conducted for fracture plugging efficiency accounting for particles transport and capture in fracture. Model and simulation results show that friction coefficient is the key material mechanical parameter for fracture plugging effect. Laboratory experimental results show that rigid particle with lower roundness, fiber with higher tensile strength and elastic particle with higher deformation rate lead to larger friction coefficient and should be selected as loss control material. Reasonable combination of rigid granule, fiber and elastic particle can create a synergistic effect to achieve the optimal friction coefficient and fracture plugging effect. Material selection strategy is determined and has been successfully applied to field case study in Sichuan Basin, China.

This paper aimed to develop a computational fluid dynamics simulation for an industrial boiler. The industrial boiler is used for the sulfuric acid production. A numerical model is carried out using the commercial code “ANSYS-FLUENT”. The simulations are developed for the case of the industrial boiler and a new proposed design. A clear understanding of the combustion phenomenon and the fluid flow within the boiler is provided from the simulations. The local characteristics of the fluid flow and the heat transfer such as the temperature; the velocity magnitude and the density distributions are presented and analyzed. The comparison between the numerical results and the experimental data is performed and a good agreement is found.

Ejector cooling with hydrocarbon as alternative refrigerant promises great application potential in hot climate regions. In order to unleash the potential of these sustainable cooling systems, it is essential to design high-performance hydrocarbon ejectors. In this paper, a computational fluid dynamic (CFD) simulation approach is proposed for detailed ejector modeling. The NIST real gas model is used to incorporate thermophysical properties of actual refrigerants for accurate ejector performance prediction. A hydrocarbon ejector performance is analyzed according to different operating and geometric conditions, where a linear relationship between the expansion ratio and the compression ratio is obtained for Pentane with an accuracy of 0.5%. A set of Pareto Frontier performance curves are also recommended for designing and operating energy-efficient hydrocarbon ejector cooling systems. Our results show that a small area ratio leads to a high ejector compression ratio, which is desired in hot climate applications. The other geometric design parameters have minor effects on the ejector performance. This work also indicates that ejectors with a converging-area chamber can achieve better overall performance compared to conventional ejectors.

This paper investigates the effects of lateral wind gusts on the aerodynamic performance of vertical axis wind turbines. A synthetic approach coupling computational fluid dynamics (CFD) simulations and the double multiple streamtube method is utilized to calculate vertical axis wind turbine performance. For simulating the wind gust response, the resolved gust approach (RGA) models the gust transport throughout the wind field. The effects of wind gusts on the effective wind velocity, angle of attack, torque and power performance are evaluated. The presence of gusts slightly changes the effective wind velocity and angle of attack. Lateral gusts increase the angles of attack in the upwind half period causing premature flow separations in the flowfield, while suppress flow separation by decreasing the angles of attack in the downwind period. The mean torque and power of the three-bladed turbines are enhanced by the lateral sharp-edge gust of 2 m/s by 5% in comparison to the steady wind. Reducing the number of blades to one and doubling the gust velocity make the performance further enhances by 14.7% and 34%, respectively. Finally, the effects of several gust shapes are investigated and discussed.

In recent years, the world has been experiencing a technological revolution that is transforming the energy sector around the world, and Mexico is no exception. Among these technologies, small wind turbines (SWTs) appear to be promising. This paper presents a methodology according to the specific Task 27 of the International Energy Agency (IEA) in order to improve the wind resource assessment in the urban environment by using computational fluid dynamics (CFD). Many problems that would help boost the use of this technology are still not addressed including wind resource assessment in the urban environment. Among these important issues are the energy yield estimation of SWTs in the urban zones, as well as the effects on the electricity supplied by the turbines in the distribution system. The objective is to present an easy-to-follow methodology, which permits the application of diverse CFD methods and software to assess the installation of SWTs in urban zones. This feasibility study on estimating the wind resource through CFD allows the opportunity to supply local electricity to buildings or public lighting systems. This methodology is based on the insertion of SWTs, in the urban environment; the SWTs are selected based on a market study of these technologies.

Wind forecasting holds the key to the management of wind power. Previous vector or matrix wind forecast methods may not best reflect the intrinsic inter relationship among the wind velocity components of a three-dimensional wind field. Alternatively, a tensor-based model is developed to reconstruct the wind velocity distribution within a short period of time, enabling a new way for wind forecasting. A third-order CFD database is established by CFD simulations and the Tucker decomposition is used to obtain the tensor basis off site. Then in real time, the tensor basis can be employed to rapidly reconstruct wind velocity distributions in any direction, which can also form a new way to reconstruct wind velocity distribution in 3-D spaces. A comparison of the maximum and relative reconstruction errors shows that the newly proposed method performs better than the authors' previously published wind field reconstruction method. The influences of sampling rate, noise level and sensor distributions on the reconstruction error are also discussed in this paper. Finally, a wind tunnel experiment is carried out to evaluate the accuracy of the proposed method, and in most cases, the experimental results show that relative errors drop around 0.03%-0.4% and maximum errors drop around 0.02%-1.7% when using the newly proposed method.

This paper presents a novel 1-D mathematical model to determine complete dimensions of an ejector component of Ejector Refrigeration System (ERS). The concepts of Prandtl's mixing length, Prandtl-Meyer expansion wave, Kelvin-Helmholtz instability and Baroclinic effect are introduced in the model to precisely determine the various diameters, mixing length, nozzle exit position etc. for the given conditions of the primary & secondary fluid, cooling capacity and critical condenser pressure. The area ratios obtained using the mathematical model are compared with the experimental/numerical results available in open literature for the same operating conditions and are found to be in good agreement. Moreover, ejector geometry determined from the proposed model is analyzed using CFD for the same input conditions. Average deviation in the entrainment ratio obtained using CFD and that given to the model is found to be less than 2.48% and thus model is validated again. The experimental test rig of ejector refrigeration system is also fabricated and the performance is evaluated while operating at critical condenser pressure. The deviation in the ejector geometry used in the experiment is found to be less than 7% in comparison to the ejector dimensions calculated by the numerical model for the same input conditions.

This paper presents the results of the application of a shape-optimization technique to the design of the stator and the rotor of a centrifugal turbine conceived for Organic Rankine Cycle (ORC) applications. Centrifugal turbines have the potential to compete with axial or radial-inflow turbines in a relevant range of applications, and are now receiving scientific as well as industrial recognition. However, the non-conventional character of the centrifugal turbine layout, combined with the typical effects induced by the use of organic fluids, leads to challenging design difficulties. For this reason, the design of optimal blades for centrifugal ORC turbines demands the application of high-fidelity computational tools. In this work, the optimal aerodynamic design is achieved by applying a non-intrusive, gradient-free, CFD-based method implemented in the in-house software FORMA (Fluid-dynamic OptimizeR for turboMachinery Aerofoils), specifically developed for the shape optimization of turbomachinery profiles. FORMA was applied to optimize the shape of the stator and the rotor of a transonic centrifugal turbine stage, which exhibits a significant radial effect, high aerodynamic loading, and severe non-ideal gas effects. The optimization of the single blade rows allows improving considerably the stage performance, with respect to a baseline geometric configuration constructed with classical aerodynamic methods. Furthermore, time-resolved simulations of the coupled stator-rotor configuration shows that the optimization allows to reduce considerably the unsteady stator-rotor interaction and, thus, the aerodynamic forcing acting on the blades.

Air manifolds are used in straw-fired batch boilers to provide appropriate excess air as well as homogeneous air distribution in the primary and secondary combustion chambers. The paper presents the results of variant analysis of operation of the air manifold responsible for providing air for the combustion process in a biomass-fired batch boiler. An experimental stand consisting of a set of velocity sensors has been constructed to analyse the operation of the air manifold. The results of the experimental studies were implemented in a computational fluid dynamics model, which allowed to determine the elements of the manifold construction which are crucial from the point of view of the efficient combustion and acceptable low level of the fan’s energy consumption. As a next stage of the studies, a new design of the manifold has been developed and modelled. The results of the studies and the recommendations for designing air manifolds have been presented.

The present paper describes a novel CFD approach for the flashing of CO2 through nozzles and ejectors. The novelty of the method is represented by the possibility of defining both the liquid and vapor phases as compressible materials. The properties of each phase are obtained via look-up tables calibrated against standard fluid libraries and are valid in the whole domain of interest, including the supercritical, subcritical and metastable regions. The model has been implemented within a commercial CFD solver and is completely general, i.e., it can be applied to any type of compressible multiphase flow. In the present study, the proposed approach has been validated against an experimental test-case available in literature.

Wind turbine airfoil operates in the atmosphere with uncertain turbulence and relatively low Reynolds number all year round. Meanwhile, due to the complexity of blade airfoil fabrication, there are inevitable geometric deviations to the theoretical airfoil shape. These uncertainties from manufacturing and operating environment couple together and lead to performance degradation. In the traditional wind turbine airfoil design process, the uncertainties are not the design variables, objectives, and constraints are deterministic. This paper presents a novel approach for uncertain analysis and aerodynamic robustness optimization of wind turbine airfoil considering turbulence and geometric error uncertainties. An interval method coupled with the Kriging model is applied to quantify the uncertain influence, and is integrated in the optimization. The target of optimization is to find an optimal airfoil with low sensitivity to uncertainties, as well as maintaining lift to drag ratio. After optimization the min std best airfoil shows 17.96% reduction of fluctuation range and no decreased average of lift to drag ratio compared to the baseline airfoil. The optimization was validated through flow field analysis by non-deterministic CFD approach. The proposed methodology can be further applied to other engineering designs making product less sensitivity to uncertainties thus more reliable.

This research paper presents a CFD analysis of small scale ORC scroll expanders using variable and constant wall thicknesses by providing back-to-back aerodynamic performance comparisons. The evaluation of the three-dimensional and transient CFD results shows that the shorter scroll profile length of the variable wall thickness design (VWD) generated lower average radial and axial gas forces. In addition, higher pressure gradients in between individual working chambers contributed to a higher peak of the tangential gas moment despite higher transient gas force and tangential gas moment variations. Moreover, the pressure trace analysis reveals that the expansion process was finished at a crank angle of 816° in VWD, compared to 996° in the constant wall thickness design (CWD). The studies of the static pressure distributions along the surface of the fixed scroll of the two geometries indicate that static pressure drops through local radial clearances were higher in VWD. However, a higher number of static pressure drops occurred in CWD. The expansion process of CWD was driven by lower pressure gradients resulting in a complete dissipation of the large-scale vortices in the expansion chambers of CWD at the crank angle of 672°, in contrast to 600° in the expansion chambers of VWD.

To capture tidal current energy to the greatest extent possible, the turbines must to be large in-scale. When the turbine is close to the free surface, with high energy-flow density, unsteady cavitation on the blade surface has a large negative impact on the efficiency and life of the turbine. This paper presents a revised theoretical analysis of hydrodynamics optimization of horizontal-axis tidal turbines, including cavitation effects, based on improved inverse blade element momentum (BEM) theory. The cavitation performance is reflected by the minimum pressure coefficient peak value on the blade surface, based on which the cavitation prediction model is established. The cavitation prediction model and improved inverse BEM theory are combined; additionally, the mathematical model of multi-objective optimization is established. To verify the effectiveness of the proposed methodology, a 10-kW current turbine was designed, and computational fluid dynamics (CFD) was used to validate the hydrodynamics characteristics of the resulting turbine blades. The experimental model was designed according to the similarity theory, and a cavitation mechanism visualization experiment and a performance parameter test experiment were carried out in a cavitation water tunnel. The simulation and experimental results revealed that the proposed design method achieves the optimization goal.

Compared to other traditional engines, the Stirling engine is an externally closed heat engine loop with a high theoretical efficiency and low emissions. Due to its multi-fuel capacity, including solar, biogas and geothermal energy, this property is now becoming very advantageous. However, the performance of the Stirling engine is penalized when being integrated with a low heat source. Thus, research on enhancing the engine's performance through heat transfer is highlighted in this paper. A numerical investigation of the effect of circular, pin, and rectangular fins on the efficiency of the Stirling engine is presented in this report. The engine's model was analyzed using computational fluid dynamics (CFD) method and validated with previous study without any additional heat enhancement material. The result recorded an average of difference around 2.15% with other CFD models and 3.13% with experimental results. The engine's model is then added with fins. An increment in the heat transfer rate, efficiency and power output of the engine are obtained and the highest efficiency is achieved by rectangular fins with 19.03%.

This paper presents characteristics of heat accumulation system operation in one of the Polish combined heat and power (CHP) plants. The heat source cooperating with this installation is cogeneration units consisting of gas engines with a total heat output of 8 MWt and total electrical power of 8 MWe. The total thermal capacity of the non-pressure hot water accumulator is 75 MWh. An important issue in the case of hot water tanks is thermal stratification caused by the difference in density between higher-temperature water, located at the top of the hot water storage tank and colder water at its bottom. The article presents the results of numerical analysis based on the axial-symmetric model of a hot water tank using Ansys Fluent R19.2 software. Simulations with the Computational Fluid Dynamics (CFD) method were carried out for the process of charging the heat accumulator lasting 10 h. The effect of mesh density on the results obtained was analysed. The results were compared with the measurement data from the CHP plant. Heat losses from the hot water tank were taken into account and the density characteristics of the heat flux lost to the environment were presented.

This paper introduces a bi-level reduced-order models (ROMs) approach for quickly deciphering the optimal exergy fields in closed wet cooling towers (CWCTs) with consideration of weather variations. First, an efficient sampling method based on stochastic reduced-order model is performed for the approximation of the multivariate probability distributions by generating a finite set of samples. The uncertainty associated with input variables is propagated via multi-sample CFD simulations of the CWCT model for each of the samples. The results of the state and output variables stored in the CFD solutions are used to construct the data-driven and physics-based ROMs by combining principal component analysis and artificial neural network methods. The constructed data-driven ROM is embedded in a sampling-based stochastic optimization model that seeks the maximization of the expected exergy efficiency ratio. The physics-based ROM is used to visualize the optimal field profiles of the thermal-, mechanical-, and chemical-exergy fluxes. Finally, the results of a case study demonstrate that the main strengths of the proposed approach is to simultaneously obtain the optimal exergy efficiency ratios and the exergy field profiles of the CWCT system in a computationally efficient manner.

This paper has attempted to investigate the aerodynamic characteristics of the straight-bladed vertical axis wind turbine (VAWT) with bionic blades for better application of bionic blades to VAWTs. The study was based on the computational fluid dynamics numerical simulation (CFD numerical simulation) technique to investigate the differences of the power and tangential force coefficients of the wind turbine between the bionic blades and standard blades at three different tip speed ratios (TSRs = 1.38, 2.19, and 2.58) for the mainstream wind velocity of 8.0 m/s. The results showed that the average power coefficient enhancing effect of the bionic blade increases as the TSR increases, and the average power coefficient of the bionic blade is higher than that of the standard blade mainly because of the high wind energy utilization in the wave trough section. Moreover, the power coefficients of the wave crest sections for bionic blades are better than that of the standard blades at the downstream region of 200°

In this paper, an auto-tuning AR (area ratio) ejector and an auto-tuning NXP (nozzle exit position) ejector are designed to solve the contradictions between the fixed structure and the dynamic primary pressure driven by multi-condition low-grade heat energy. A CFD (Computational Fluid Dynamics) model is used to calculate the performance of the ejector under different operational conditions. The effects of both AR and NXP on the performance of the ejector are systematically studied. The optimal area ratio increases linearly along with the primary pressure, and the optimum NXP decreases as the primary flow pressure increases. Following this, auto-tuning AR and NXP ejectors are designed to enhance the performance of the ejector by self-adjusting the AR and NXP with a variation in primary flow pressure. Simulation results show that the auto-tuning ejector has a strong ability to improve the ejector performance compared with the constant structure ejector.

The upcoming environmental requirements outlined at the 2017 European Commission Integrated Pollution Prevention and Control (IPPC) are becoming more and more strict in comparison to the existing standards – IED 2010/75/EU. Questions arise about changes in boiler operating conditions after the adoption of the regulations to fulfill specified emission limits. Of initial concern is the question of which technology to pursue in order to reach the specified level of NOx emissions at 150 mg/Nm3@6%O2. By introducing an air staging technique the NOx limit can be partially achieved; however these changes affect the boiler operation in a detrimental way. In order to investigate impact of de-NOx installation on the temperature and heat flux distribution, numerical techniques can be used. In order to determine the impact of the changes in the boiler operation, this paper presents the application of an advanced, multiphase open-source code Arches, developed at the University of Utah for modeling pulverized-coal combustion using Large-Eddy Simulation (LES). Numerical simulations showed the effect on the boiler's working parameters e.g., heat-transfer rate, temperature distribution, species concentration, and NOx emission before and after proposed modifications.

In this paper a new mechanistic model for the diabatic annular two-phase flow is presented and applied to prediction of dryout and post-dryout heat transfer in various channels. The model employs a computational fluid dynamics code – OpenFOAM® – to solve the governing equations of two-phase mixture flowing in a heated channel. Additional closure laws have been implemented to calculate the location of the dryout and to predict wall temperature in the post-dryout region. Calculated results have been compared with experimental data obtained in pipes and good agreement between predictions and measurements has been achieved. The presented model is applicable to complex geometries and thus can be used for prediction of post-dryout heat transfer in a wide variety of energy conversion systems.

While the limitation of the entrainment ratio in supersonic ejectors is a well-known phenomenon, there is still a need to gain insight on the choking phenomena at play in on-design operation. In state-of-the-art simplified models of supersonic ejectors, the secondary stream is assumed to reach sonic velocity in a hypothetical throat (Fabri-choking). However, an alternative explanation of the entrainment limitation known as the compound-choking theory states that a nozzle flow with two streams at different stagnation pressures may be choked with a subsonic stream if the other one is supersonic. In this paper, the compound-choking is highlighted in a supersonic ejector through a thorough analysis of numerical simulations validated against experimental data. In addition, comprehensive experimental data of supersonic ejectors are used to assess the performance of the compound-choking theory to predict the entrainment ratio in the on-design regime in various configurations. Most predictions are in the ± 10 % range when compared to the experimental data. Compared to state-of-the-art 1D models relying on the Fabri-choking assumption, the compound-choking theory is shown to generally perform better regarding the prediction of the on-design entrainment ratio. This study suggests that the compound-choking theory is well suited to model the choking process in supersonic ejectors.

Thermal energy storage is a key issue in efficient energy systems applications. In this regard, phase-changing material (PCM) has been of interest as a passive solution for efficient energy management, especially in buildings. This paper presents a thermal analysis of a brick, which includes a particular type of PCM and ten air cavities, so as to be utilized in building envelop system. The objective of current study is to reduce heat transfer from outdoor to indoor space and vice versa by investigating latent and sensible heat storage through the PCM. Eventually, the fluctuations of indoor and outdoor air are analyzed in both the coldest and hottest days of Tehran in 2016. Computational fluid dynamics (CFD) was performed to estimate the behavior of the confined PCM while exposed to the external time-dependent conditions. The results indicated that in the summer heat storage was conducted mostly via latent heat, while in winter the thermal storage was mainly made up of sensible heat. However, in both the summer and winter, PCM could be incorporated in buildings envelop systems in order to operate as a passive indoor thermo-regulator.

There have been continuous efforts by researchers and power producing companies to reduce various costs involved in solar power tower (SPT) plants. Economically deploying conventional SPT plants for industries using thermal or thermo-chemical processes which need temperatures >1100 K could be challenging. Here, the overall economics of deployment of conventional SPT could go unfair as such design needs larger heliostat field, therefore costlier stiffer heliostats to reach high concentration ratio (CR). These challenges can be solved by beam-down SPT which uses secondary reflector mounted on tower top and receiver cum secondary concentrator on the ground could achieve desired CR, is one potential candidate to save on tower construction and pumping costs. Using beam-down SPT heat can be made available at the ground which opens an option of extracting the heat using natural circulation loop (NCL). The current paper explores the new proposed configuration of NCL in terms of understanding thermal hydraulics using 3-D CFD simulations. Further, it also incorporates optimization of the proposed design configuration using formulated heat transfer model. The optimized geometry is simulated using 3-D CFD simulation which gave the desired rating.

Numerical ascertainment of improving performance of modified Savonius rotor as decreasing the minimum wind speed required for initiating the rotation is investigated using 3D flow predictions executed in ANSYS-FLUENT. For making the comparison feasible, the design represented in a similar paper is simulated and performance is compared with empirical data. Different types of nozzles are added after observing acceptable agreement between empirical and simulation results. Nozzle acts like an air intake on the advancing blade and fortifies the acting flow; it also eliminates the reverse moment caused by contacting the flow with returning blade. Therefore, increasing in power coefficient is anticipated. Systematic analysis of obtained torque and power coefficients with and without nozzle are presented. In the next step, this work is dedicated to bucket shape optimization. Buckets with multi-curvature are introduced and have been investigated in three stages. By installing a simple nozzle with tail in front of a Savonius wind turbine with double-curved two stage buckets, the Savonius turbine maximum power coefficient is improved from 0.13 to 0.39.

Stirling engines have recently been studied theoretically via two different methods, thermodynamic analysis and CFD simulation. Evidently, second order thermodynamic analysis is simpler and less time consuming than CFD. However CFD considers more details as well as the engine geometry and consequently reflects more details about the flow field and losses phenomena. In this paper, numerical simulation of a typical Stirling engine was conducted by OpenFoam open source software. Dynamic mesh was used to simulate moving pistons and the working fluid was considered compressible. Volume averaging technique was applied to simulate flow in the regenerator as a porous media. The well-established PIMPLE algorithm was used to handle pressure coupling in momentum equation. To investigate the effect of turbulence, a simulation was conducted employing k-ω SST turbulence model and no significant change in the results was observed.

Coal seams are converted to syngas by advanced thermo-chemical processes through Underground Coal Gasification (UCG) method. Inability to predict the shape and volume of the underground cavity is an important scientific gap in UCG method which is the main subject of this paper. For this purpose, firstly, a series of equations are introduced to predict the cavity growth dimensions over time. Subsequently, these equations are extended in numerical simulation of the Computational Fluid Dynamics (CFD), incorporating the commercial COMSOL software. According to the simulation, the amount of oxidant necessary to convert a certain amount of coal (in the heterogeneous phase) is calculated. The model results indicated that the shape and volume of cavity could be predicted at the onset of the gasification process. The numerical results agreed well with the field data.

This paper optimizes global reaction mechanisms under the MILD (Moderate and Intensive Low-oxygen Dilution) oxy-combustion combustion. Seven global mechanisms are compared and validated with two detailed mechanisms under the oxy-fuel combustion, MILD air-combustion, and MILD oxy-combustion conditions. The ability of these global models to capture the combustion process under different conditions is compared first using computational fluid dynamics (CFD) simulations of both the MILD oxy-combustion in a laboratory-scale furnace and non-premixed turbulent jet open flames, and later using a plug flow reactor (PFR) approach. Experiments of the MILD oxy-combustion are also carried out for the mechanism validation. The detailed comparison shows that the present optimized global mechanism significantly improves the prediction of temperatures, equilibrium concentrations of major species, and the peak CO concentration, relative to other global mechanisms, for the MILD oxy-combustion. The present refined mechanism is an appropriate global reaction mechanism for methane MILD oxy-combustion, if the computational cost of detailed reaction mechanism is unaffordable.

The developed reduced-order model (ROM) of the R744 two-phase ejector was presented in this paper. The proper orthogonal decomposition (POD) model was employed together with the radial basis function (RBF) to evaluate the ejector performance at the motive nozzle operating regime from 70 bar to 100 bar. The proposed model was built based on the full CFD model of the R744 two-phase ejector with homogeneous equilibrium flow assumption. The validation procedure was performed to evaluate the ejector nozzles mass flow rate discrepancies of ROM compared to the CFD results and experimental data. In addition, the accuracy analysis of the ROM flow field results compared to the CFD results was performed. The validation process based on the CFD results and experimental data indicated the high accuracy of ROM for both nozzles mass flow rate within ±±10% for most of the investigated operating points. Hence, the high accuracy of the computed mass flow rates allows ROM implementation into the dynamic simulations of the refrigeration system to evaluate the ejector performance at given operating points with negligible time effort. © 2017 Elsevier Ltd. All rights reserved.

Tesla turbine is a bladeless, radial turbine which might be suitable for the applications in various energy systems, e.g. in Organic Rankine Cycle or combined heat and power systems if its efficiency improves. The investigations presented in the paper concern efficiency based numerical optimization of a Tesla turbine. Parameters subjected to the optimization process are inlet nozzle height, inter-disc gap, nozzle angle, pressure and rotational velocity. The optimization was conducted using response surface methodology. The first stage of the optimization excluded pressure ratio as neutral to efficiency. The second stage of the optimization allowed determination of the most important parameters: tangential velocity ratio, partial admission coefficient, reaction degree and aspect ratio required for maximization of the turbine's efficiency. Efficiency was improved almost twice with the respect to the nominal model (from 9% to 17%) and validated experimentally. Performance characteristics of the optimized turbine model were calculated using computational fluid dynamics software.

The purpose of the present investigation is to explore the feasibility of applying the micromix combustion principle (MCP) to design low NOx burners using 100% H2 as fuel for domestic and industrial boilers. Previous investigations studying the MCP on stationary and aero gas turbine applications, showed low NOx emissions without flashback risk, which represent the two main issues when burning 100% H2. Since boiler burner operating conditions differ from gas turbine combustors, the present paper studies the MCP through CFD calculations under re-defined conditions for domestic and industrial burners, adapting the energy densities, air-fuel equivalence ratios and pre-heated air temperatures. A reference geometry was built to validate the selected numerical models with experimental results from literature. Afterwards, burners were re-dimensioned following an existing geometry-scaling methodology. The obtained results evidenced that MCP characteristics were still maintained for the defined cases, keeping low NOx values between 4 and 14 ppm. The numerical results were also validated through experimental NOx measurements in a laboratory scale micromix burner prototype. In order to assess the benefits of using hydrogen micromix burners in domestic and industrial boilers, the present work includes a final discussion with practical design considerations. The present study lays the groundwork for complementary experimental research work, which is being carried out in laboratory-scale prototypes.

In a mixed-flow pump, the energy performance curve always appears as an unsteady region under rotating stall condition. In this paper, computational fluid dynamics (CFD) technology is employed to study the effect of variation in blade thickness of the impeller for the purpose of improving the stall characteristics of the mixed-flow pump. The local loss caused by the turbulent flow is investigated using the entropy generation method that considers the wall effects. The results show that the stall region of mixed-flow pump is closely related to the blade thickness, and the total entropy generation (TEG) in impeller and guide vane are affected. At design flow rate, the tip leakage vortex (TLV) accounts for most of the hydraulic loss in the impeller and the change in the blade thickness does not influence the pump head or efficiency significantly. Whereas, within stall region, the stall vortex and TLV determine the high TEG region in the impeller, the backflow and secondary flow are the source of major hydraulic loss in the guide vane. A small increase in the blade thickness reduces the loss caused by the stall vortex which delays the stall point. However, once the blade thickness increases significantly, the stall vortex becomes bigger which deteriorates the flow fields in the impeller and guide vane resulting in rotating stall. Thus, the blade thickness can be considered as a key parameter to minimize the occurrence of rotating stall in a mixed-flow pump for improving its efficiency and safety.

Thermoacoustic phenomena can be modeled by means of analytical closed-forms and numerical models. The analytical solution can be obtained with a minimum computational cost, whereas the numerical approach is computationally expensive. The former, however, provides an average solution along the engine, while within the numerical simulations non-stationary phenomena can be observed. This paper includes results related to the modeling of the operation of a thermoacoustic engine both by means of the analytical approach and numerical models. Some computational details and assumptions adopted in both models are discussed. The main focus is placed on the numerical approach, where the model size (number of pores analyzed) is taken into account. The results show differences between a single-pore symmetrical model and multiple-pore ones. The object of the comparisons is to determine the reliability of the modeling of the operation of a thermoacoustic engine and to evaluate sources of discrepancy between models, both numerical ones, as well as the numerical and analytical approaches.

This paper presents the set-up, the validation and the application of a two-phase incompressible three-dimensional Computational Fluid Dynamic (CFD) model for an extensive parameter study on the performance of a fixed, detached Oscillating Water Column (OWC) wave energy converter. The numerical study aims to assess the combined effect of relevant design parameters (chamber length, front wall draught, damping applied by the turbine) and of the wave conditions (wave height, wave period and water depth) on the device performance, with reference to its hypothetical installation in moderate wave climates (e.g. in the Central Mediterranean Sea). The OWC device examined here is fixed, detached from the seabed, and with asymmetric front and back walls as a characteristic feature. The results show that, by appropriately tuning the design parameters, a maximum value of the capture width ratio of approximately 0.87 can be achieved.

Vertical axis wind turbines (VAWTs) have distinctive attributes, for example, exceptional performance in built environments and high packing density in wind farms. This study reviews recent research in the wake aerodynamics of H-rotor VAWTs. The general mathematical expressions for the analysis of the rotor aerodynamics of VAWTs are presented, followed by a classic aerodynamic model. Velocity deficit and turbulence increase are common features of wind turbine wakes, and the deep dynamic stall of operational VAWTs further complicates their wakes. This study systematically compiles the investigations of the wake flow fields and vortical structures of VAWTs. In these investigations, a wide range of research methods have been used, for instance, wind tunnel testing, particle image velocimetry (PIV) tests, field tests, computational fluid dynamics (CFD) simulations, and theoretical development. The results of these studies suggest that the wakes of VAWTs are characterized by distinctive features, such as strong asymmetry and counter-rotating vortical motion. Based on these characteristics, researchers have developed various wake models. The proposed wake models can be used to design standalone VAWTs or layouts for multiple VAWTs. Finally, this paper provides discussions and suggestions pertaining to the investigations of the wakes of VAWTs.

This paper demonstrates the capability of CFD to accurately simulate the effect of the flow turbulence intensity on the performance of vertical-axis wind turbines (VAWT). This effect is quite important as it increases the performance of small VAWTs. For this study, two and three dimensional CFD analysis has been performed on different straight-bladed Darrieus-type rotors. Both a small and a large H-Darrieus VAWT with diameters of 0.5 m and 35 m respectively are investigated using NACA0018 blades. Computational results based on a commercial CFD code (Star CCM+) are compared with experimental measurements. Several simulations based on full URANS calculations are proposed. Firstly, the sensitivity of time step, the number of iterations by time step, and discretization schemes are studied. Secondly, the effect of turbulence intensity on the VAWT performance is simulated and compared with experimental data. Finally, results reveal that the power coefficient of a small turbine is increasing for higher turbulence intensity, up to 20%, but stops increasing afterward. For a small turbine H-Darrieus turbine, the power coefficient is increased by 22% when the turbulence intensity is changed from 0.7% to 20%, however, there is no increase detected in the case of a large H-Darrieus wind turbine. The impact of the turbulence intensity was assessed and a range of behaviors was identified.

The present paper is intended to assess the ability of the state-of-the-art computational fluid dynamics (CFD) and blade element momentum (BEM) approaches for accurate load predictions on a 2.3 MW wind turbine rotor. Three different cases are considered, a steady uniform inflow condition, a turbulent uniform inflow condition and a turbulent inflow case in combination with shear and yaw. The CFD computations employ a delayed-detached eddy simulation (DDES) approach in combination with a high order (5th) WENO method for flux discretization. The BEM calculations apply several correction factors including recently developed dynamic stall and yaw models. Furthermore, a well established procedure at IAG to set-up BEM calculations consistent to CFD will be presented and verified. The results are compared with the field experimental data of the turbine for these three different flow conditions. The studies show that both CFD and BEM results are in a very good agreement with the experimental data not only on the mean load levels but also with regards to the load fluctuations. The differences between BEM, CFD and experimental data for most radial stations are less than 10%.

The effect of intermediate electrode on the performance of the square tubular PEMFC was studied in the present paper. Four different configurations were developed and evaluated for the first time, namely: simple square tubular (without intermediate electrode i.e. Simple), double parallel intermediate electrode (DPIE), double bisectors intermediate electrode (DBIE) and triple parallel intermediate electrode (TPIE). Calculations were performed maintaining the same active area and boundary conditions for all four configurations. The results of polarization curves showed that the insertion of the intermediate electrode increases current density, and leads to higher consumption of reactants and better performance of PEMFC. Employing the intermediate electrode not only does not cause any additional cost compared to simple square tubular configuration, but also the cost is reduced in comparison to the base model due to the reduction of fuel cell length. The results also revealed that the intermediate electrode arrangement also has a significant influence on the performance of PEM fuel cell; For instance, DPIE compared to DBIE delivers higher power and has better performance. Moreover, it was proved that the addition of more than one intermediate electrode layer increases the pressure drop dramatically which fades the advantage of increasing power output.

This paper presents a study on the effect of solidity and airfoil profile on the performance of Vertical Axis Wind Turbines (VAWTs). A 1.1 kW commercially viable Darrieus VAWT was studied using ANSYS Fluent. Four different airfoils – NACA 0012, NACA 0015, NACA 0030 and AIR 001 – were considered in the analysis. The tip speed ratios (λ) were varied from 1 to 2.5 with an incoming wind velocity of 10 m/s. It was observed that NACA 0030 performed better at lower values of λ due to long duration of attached flow; while NACA 0012 performed better at λ > 1.8 with a wider range of λ. The shed vortex dissipates much faster for thinner airfoils than for thicker airfoils at higher values of λ. Two bladed VAWTs generated more power than the three bladed turbines. This indicated that turbines with lower solidity perform better at high λ.

Highlights - A computationally-efficient layout optimization scheme is proposed. - Wind turbine speed on complex terrain is calculated through wind multiplier method. - Non-flat terrain can yield 1% higher of wind farm efficiency than flat terrain. - Real wind farm topography must be considered for better optimization results. Abstract In this paper, a novel computationally-efficient optimization scheme is proposed to account for the effect of wind speed variations over non-flat terrain and irregular land plot boundary. In particular, typical analytical wake models are augmented with a wind multiplier (obtained from CFD simulations) to account for the effect of altitude variations, while irregular boundaries are addressed through novel constraint handling method in an unrestricted coordinate optimization formulation. In contrast to the optimization via CFD simulation alone, the augmented wake model provides acceptably accurate results with a reasonable computational burden. The new optimization approach is subsequently applied to two real wind farms with significant altitude variations: the Gokceada Island wind farm and the Grasmere and Albany wind farm. The optimized Gokceada wind farm layout exhibits a significant cost of energy (CoE) reduction when topography is considered (over conventional, flat-terrain optimization). However, the Grasmere and Albany wind farm shows a relatively small decrease in CoE over conventional optimization. This discrepancy can be attributed to the wind characteristics in each location: the Gokceada wind farm has a relatively consistent wind direction and the optimization can use topography to carefully avoid wake interactions. In contrast, the Grasmere and Albany wind farm with less-consistent wind directions leaves no clear method to avoid wakes using topography. These results indicate that the consideration of wind farm topography can yield significant benefits and the wake interactions can be greatly reduced through careful placement of wind turbines in the case where a narrow wind directions is dominant.

When offshore floating vertical axis wind turbines (OF-VAWTs) face the ocean waves and wind loads under normal operation conditions, they have six-degrees of freedom (6-DOF) movement. Each of the 6-DOF movements will influence the aerodynamic performance of the OF-VAWTs in turn. In view of this, the present paper uses the computational fluid dynamics (CFD) method and the Improved Delayed Detached Eddy Simulation (IDDES) to investigate the aerodynamics of an OF-VAWT in periodic surge motion. The overset mesh technique is employed to simulate the rotor's surge motion. In order to verify the present CFD model, the power coefficients of a bottom-fixed VAWT at different tip speed ratios are compared between the experiments and the simulations. By contrast with the non-surge motion, the aerodynamic forces (torque, tangential force, normal force and pressure) and vortex structures of an OF-VAWT are analyzed. Subsequently, the unsteady aerodynamic performance of an OF-VAWT in different amplitudes and periods of surge motion is investigated. It is shown that the surge motion can widen the variation ranges of the aerodynamics forces, and change the flow field around the rotor. The smaller surging amplitude and larger surging period are proposed as they can reduce the variation ranges of the aerodynamics forces, and then keep the floating wind turbines more steady. In addition, the durability and power output of the wind turbines will be improved in surge motion with smaller amplitude and larger period.

Unsteady wind is characterized by low energy content and large fluctuations. A Computational Fluid Dynamics (CFD)-based method for capturing wind energy in a fluctuating free-stream, supported by analytical formulations, is investigated in this paper. We implemented unsteady Reynolds-Averaged Navier-Stokes (RANS) solver to control the dynamic mesh motion. Using an urban wind resource, characteristic fluctuation frequencies at 0.5 Hz, 1.0 Hz, and 2.0 Hz have been selected to demonstrate the enhanced wind energy capture. The numerical energy coefficient marginally changed from 0.36 at 0.5 Hz to 0.37 at both 1 Hz and 2 Hz cases. The results reveal that the highest frequency of fluctuation with meaningful energy content in unsteady wind condition is ≈ 1 Hz. The study findings promote our understanding about the energy associated with short-period fluctuations reflecting realistic unsteady wind environment. Additionally, the present study approach to analyze wind energy capture on a H-Darrieus wind rotor in a fluctuating free-stream can be extrapolated to other slightly complex VAWT configurations.

Divergent chimney is proposed to be an alternative for Solar Chimney Power Plants (SCPPs) because of their reported remarkable improvement in power output over cylindrical chimneys. However, the power output of divergent SCPPs in those studies changed from several percentage to >100 times higher than that of cylindrical ones. In our hypothesis, this large deviation was related to the various configurations of the SCPPs examined. Therefore, this paper examined comprehensively the effect of geometry of divergent chimneys on system performance of SCPPs to further reveal their hydrodynamic features. The geometric parameters under investigation included the area ratio (AR) of chimney exit over entrance, the divergent angle (DA) of chimney wall and the size of system. Our numerical simulations indicated a parabolic tendency in the performance of the divergent SCPPs when increasing the ARs (or DAs). Reasons for this tendency were proposed based on its hydro- and thermo-interaction. Furthermore, the normalized power output showed good consistency among the SCPPs with different sizes when geometric similarity was adopted to the entire system geometry. The similar normalized outputs found were almost insensitive to the variations in the solar insolation. These outcomes would be a valuable reference for designing SCPPs with divergent chimneys.

This paper aims to assess the reliability of turbulence models in predicting the flow fields around the horizontal axis wind turbine (HAWT) rotor blades and also to improve our understanding of the aerodynamics of the flow field around the blades. The simulations are validated against data from the NREL/NASA Phase VI wind turbine experiments. The simulations encompass the use of twelve turbulence models. The numerical procedure is based on the finite-volume discretization of the 3D unsteady Reynolds-Averaged Navier-Stokes equations. The resulting simulations are compared with the full range of experimental data available for this case. The main contributions of this study are in establishing which RANS models can produce quantitatively reliable simulations of wind turbine flows and in presenting the flow evolution over a range of operating conditions. At low (relative to the blade tip speed) wind speeds the flow over the blade surfaces remains attached and all RANS models tested return the correct values of key performance coefficients. At higher wind speeds there is circumferential flow separation over the downwind surface of the blade, Moreover, within the separation bubble the centrifugal force pumps the flow outwards, which at the higher wind speeds suppresses the formation of the classical tip vortices. RANS models which do not rely on the linear effective viscosity approximation generally lead to more reliable predictions at higher wind speeds. By contrast some popular linear effective viscosity models perform the poorest over this complex flow range. Finally all RANS models are also able to predict the dominant (lowest) frequency of the pressure fluctuations.

This paper introduces an aero-acoustic computational study that investigates the noise caused by one of the most promising wind energy conversion concepts, namely the "Wind-Lens" technology. The hybrid method - where the flow field and acoustic field are solved separately, was deemed to be an appropriate tool to compute this study. The need to investigate this phenomenon increased gradually, since the feasibility of utilizing Wind-Lens turbine within densely populated cities and urban areas depends largely on their noise generation. Ffowcs Williams-Hawkings (FW-H) equation and its integral solution are used to predict the noise radiating to the farfield. CFD Simulations of transient three-dimensional flow field using (URANS) unsteady Reynolds-averaged Navier-Stokes equations are computed to acquire the acoustic sources location and sound intensity. Then, the noise propagates from the before-mentioned sources to pre-defined virtual microphones positioned in different locations. ANSYS-FLUENT is used to calculate the flow field on and around such turbines which is required for the FW-H code. Some effective parameters are investigated such as Wind-Lens shape, brim height and tip speed ratio. Comparison of the noise emitted from the bare wind turbine and different types of Wind-Lens turbine reveals that, the Wind-Lens generates higher noise intensity.

The flow-field around the offshore floating vertical axis wind turbines (OF-VAWTs) is affected by the six-degrees of freedom (6-DOF) movement of the platforms. Understanding the impact of a certain DOF motion on the aerodynamics is beneficial to the design of wind turbines. In this paper, the aerodynamics and performance of a scale OF-VAWT in pitch motion are investigated. The computational fluid dynamics (CFD) method with the turbulence model of improved delayed detached eddy simulation (IDDES) and the overset mesh technique are employed to analyze the characteristics of pitch motion of wind turbines. The CFD model is verified by the experimental data from available literature when the turbine has no-pitch motion. Then, the aerodynamic forces, power, and wake of an OF-VAWT in periodical pitch motion are analyzed. Because of the varying wave loads, an unsteady aerodynamic analysis considering the pitch motion with different periods and amplitudes is performed. The results show that the pitch motion can improve the power output of the OF-VAWTs, and enlarge the variation ranges of aerodynamic force coefficients. Meanwhile, the graphs of the vortex structure show that the complex flow interaction emerges around the rotor blades. Additionally, the power coefficient and the instantaneous aerodynamic forces coefficients sensitively change with different pitching periods and pitching amplitudes.

Thermal stress is one of major causes of cracking in high-temperature components of gas turbines. This paper presents a thermo-structural analysis of cracks on the vane of a gas turbine for power generation. The vane components include three airfoils with hub and shroud sections. The airfoils have serpentine-type internal passages and film cooling holes on the pressure-side surfaces for cooling. The conjugate heat transfer problem was solved to accurately evaluate heat transfer on the vane using computational fluid dynamics software, CFX. Based on the conjugate heat transfer result, thermal expansion and thermal stress were evaluated using structural analysis software, ANSYS. The results showed that an irregular temperature distribution induced anisotropic thermal expansion in the vane segments, including the shroud and hub sections, and that the anisotropic thermal expansion caused serious stress concentrations. Among the three airfoils, the middle one was the most stressed because the thermal expansion was constrained by deformed hub and shroud sections. The predicted locations of stress concentration coincided with the locations of cracks on the actual vane after an operating period. The prediction provides general information on the initiation of cracks on a vane segment having multiple airfoils.

Small scale open solar thermal Brayton cycle with Concentrated Solar Power (CSP) system can provide an efficient, clean, sustainable and low cost energy conversion system to produce different forms of energy such as electricity. The current paper investigates various configurations of open cavity solar receivers including cylindrical, conical and spherical with an aperture of 0.02835 m2 and average irradiance values of 2.5 kW/m2. Advanced ray tracing (OptisWorks®12), as this is the latest available version) and computational fluid dynamic (CFD, ANSYS® 15) simulations are used to reduce optical and thermal losses and maximize the exit temperature of the working fluid. The received irradiance on the external surface of the helical coils inside these receiver geometries was used to predict the heat transfer fluid temperature through CFD analysis. Investigations were carried out to discover the effects of coil pitch and tube diameter on the working fluid's exit temperature. The results showed that for a pitch value of 3 mm and a tube diameter of 10 mm, the exit temperature is 401.3 K, 405.7 K and 409.4 K for each of the spherical, cylindrical and conical geometries respectively; indicating that the conical shaped receiver is more efficient than the other geometries. Moreover, the best pitch value, when the higher outlet temperature was achieved, depends on both the tube diameter and the cavity configuration. The effect of some other factors such as the ambient temperature and the pressure losses on the receiver's performance have also been investigated in this study.