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Wind barriers, according to their sizes and shapes, can effectively control, shift and even modify the airflow field in their downstream. These structures can accelerate the wind flow over the mirror field and move the airborne particles away from the mirrors. For example, in concentrated solar power plants it is highly desirable to engineer the system in a way that fewer particles are deposited within the solar field, in particular, onto mirror surfaces. Therefore, design optimization of dust barriers could significantly impact the mirror soiling and favourably reduce the cleaning water consumption of a solar power plant. This study focuses on the optimization of a solid wind barrier around a parabolic trough collector plant for their protection against dust soiling. The presented simulation results show that an optimum solid wind barrier is able to direct large amount of particles (in this study, more than 86%) to pass over the solar field with very small fraction (around 0.8%) being deposited on the mirrors. In addition, it was found that the barrier wall is more effective in deflecting the larger particles from the solar field. [ABSTRACT FROM AUTHOR] more...

The usage of passive cooling systems such as wind catchers can reduce the energy usage in buildings and provide natural ventilation and comfort to its occupants, particularly in hot and dry regions of Iran and neighboring countries where it was traditionally used. The purpose of this study was to investigate the airflow and thermal comfort in six different designs of wind catchers using computational fluid dynamics (CFD) technique. Simulations of airflow in the wind catcher and the building were done under steady state and turbulent flow regime with boundary conditions based on typical conditions found in Yazd city, Iran. Several commonly used turbulence models were evaluated to assess the accuracy of the simulation. First, the proposed CFD model was validated through comparison of wind tunnel data available in the literature, and then the model was used for design purposes. It was found that the k − ω turbulence model can accurately predict the airflow velocity in the range of parameters studied. The design and performance of wind catcher were evaluated based on the thermal comfort levels using the Center for the Built Environment (CBE) thermal comfort tool and numerical data. Width and height of the wind catcher were varied in the simulations and optimal values were determined. It was found that varying the width of the wind catcher had the greatest impact on the airflow speed and distribution inside the room. Reducing the width from 2.5 m to 2 m showed that airflow velocity in the middle area was increased up to 34%. While reducing the width from 2 m to 1.5 m showed an entirely different flow pattern inside the building and also increase airflow speed in the middle area up to 50%. The addition of curved wall at the bottom of the inlet channel showed that it could increase the airflow speed of the inflow stream, however, it also caused the airflow to be directed towards the lower levels of the room and very large rotating flows in the upper levels. Finally, the results showed that the wind catcher may be optimized for improving comfort for various climates using the tools presented in this work. [ABSTRACT FROM AUTHOR] more...

In this work a series of computational fluid dynamics (CFD) simulations were performed to predict the deposition of particles on a newly designed passive dry deposition (Pas-DD) sampler. The sampler uses a parallel plate design and a conventional polyurethane foam (PUF) disk as the deposition surface. The deposition of particles with sizes between 0.5 and 10 μm was investigated for two different geometries of the Pas-DD sampler for different wind speeds and various angles of attack. To evaluate the mean flow field, the k-ɛ turbulence model was used and turbulent fluctuating velocities were generated using the discrete random walk (DRW) model. The CFD software ANSYS-FLUENT was used for performing the numerical simulations. It was found that the deposition velocity increased with particle size or wind speed. The modeled deposition velocities were in general agreement with the experimental measurements and they increased when flow entered the sampler with a non-zero angle of attack. The particle-size dependent deposition velocity was also dependent on the geometry of the leading edge of the sampler; deposition velocities were more dependent on particle size and wind speeds for the sampler without the bend in the leading edge of the deposition plate, compared to a flat plate design. Foam roughness was also found to have a small impact on particle deposition. [ABSTRACT FROM AUTHOR] more...

In this paper the Lattice Boltzmann method (LBM) in conjunction with the large eddy simulations (LES) and Reynolds Averaged Navier–Stokes (RANS) models was used to study the airflow and particle dispersion and deposition in indoor environments. The turbulent airflow was simulated first using the LES in framework of LBM, in which the sub-grid scale turbulence effects were simulated through a shear-improved Smagorinsky model (SISM). The airflow was also simulated using the k−ε turbulence model within the framework of LBM by the addition of two populations for the turbulence kinetic energy, k, and the dissipation rate, ε. The present simulation results for the airflow showed good agreement with the experimental data and the earlier numerical results. To simulate the particle dispersion and deposition in the room, particles with diameters of 10 nm–10 μm were investigated. For generating the instantaneous turbulence fluctuations for thek−ε model, the discrete random walk (DRW) method was used. Effects of SGS turbulence fluctuation on deposition rate of different size particles were also studied. It was shown that inclusion of the SGS turbulence fluctuations improves the model predictions. The simulated results for particle dispersion and deposition showed that the predictions of the present method are quite similar to the earlier finite volume method and are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR] more...

Full repowering of Be'sat steam power plant has been studied in this work. The methodology is used to simulate the new cycle according to its principal specifications and to optimize it based on the objective functions. Objective functions are electricity cost per kWh and exergy efficiency. These parameters are functions of the pinch and approach point temperature differences at high and low pressure points and at the pre-heater in the heat recovery steam generator (HRSG), steam turbine inlet flow rate, gas turbine (GT) isentropic efficiency, air compressor isentropic efficiency and compressor pressure ratio. Finally, considering the introduced objective functions, it is tried to achieve the most optimized techno-economic characteristics for Be'sat power plant repowering cycle using the genetic algorithm with two scenarios of single and multi-objective optimizations. The results show that the efficiencies of the repowered cycle are 52.59% and 51.3% for two cases of unfired and fired duct burners, respectively. • In the present study, a real steam power plant repowering has been studied (A Case Study). • The five different arrangements of duct burner for proposed scenarios have been considered. • Repowering specifications have been extracted based on domestic technical potentials. • Optimum techno-economic cases for repowering scenarios have been presented. [ABSTRACT FROM AUTHOR] more...

Abstract In this paper, the full repowering of a steam Power Plant with a capacity of 320 MW in Iran was evaluated technically and economically. Implementation of this plan for the target power plant was selected because of its over 30 years of age. Due to the presence of steam flash feed water preheaters and the possibility of preheating feed water entirely or partially prior to entering heat recovery steam generator, the effect of using or not using these low and high pressure equipment was evaluated in different cases. To do this, five different cases were proposed and reviewed. Considering the available gas turbines in Iran, the gas turbines models Siemens V94.2 and V94.3A were selected. In each case, the optimum point is obtained in terms of the lowest cost of electricity generation. Finally, after reviewing the results of all cases, full repowering plan with parallel feed water heating was selected as the best alternative in terms of energy and exergy efficiencies and the price of generating electricity. If two heat recovery steam generators and two V94.3A gas turbines are used, the net exergy and energy efficiencies for the cycle can reach 54.49% and 52.98%, respectively. Moreover, the power capacity of a repowered unit is equal to 700.44 MW with electricity generation price of 0.05986 $/kWh. Graphical abstract Image 1057 Highlights • In the present study, a real power plant has been studied (A Case Study). • The five different arrangements of feed water heaters for proposed scenarios have been considered. • The merging of water-gas heat exchangers with existing preheaters has been investigated. • Optimum techno-economic case for repowering has been presented. [ABSTRACT FROM AUTHOR] more...

One of the main sources of particulate matter (PM) in indoor environments is particle resuspension due to human activities such as walking. In many situations, surfaces are charged, and electric forces become important. As the human gait cycle induced airflow and electric fields and their interaction with particle resuspension, dispersion and deposition are rather complicated, using a simplified model can help to gain a better understanding of process. In this study, the effect of presence of electric field on particle detachment and deposition due to a falling disk was investigated using a computational modeling approach. For this purpose, a charged disk was considered to be falling freely on a rough surface from a specific height. The disk electric potential developed an electric field throughout the domain that intensified as the disk gets closer to the floor. The electric field charged the particles that were resting on the floor by the field charging mechanism. It was found that the presence of electric force decreased the critical shear velocity needed to detach the particles; however, the reduction was negligible for particles smaller than 5 µm. In addition, the simulation results showed that the number of particles deposited on the disk increased with the size of particles in the presence of electric field. The results of this study provide a better understanding of the effect of the electric potential induced due to human activities on detachment and dispersion of particles in indoor environments. [ABSTRACT FROM AUTHOR] more...

Repowering as a formerly-experienced method can be considered as a suitable solution in raising the quantity and quality of generated electricity. The full repowering can provide appropriate solutions for converting the status of existing steam power plants (SPP) to assemblies with acceptable generation quality and reach current world standard levels. In this study, due to the importance of Bandar Abbas steam power plant, in terms of positioning and the amount of generated electricity, full repowering is considered. After modeling and verifying the model, the results of modeling the desired states in performing repowering are presented. Optimization is performed using functions of exergy efficiency and cost of electricity generation, using genetic algorithm method. Finally, the obtained diagram (Pareto Curve) indicates the cost changes based on the exergy efficiency that are effective in deciding on and selecting the desired and appropriate investment state. The results of the multi-objective optimization of repowering with two single-pressure and double-pressure level Heat Recovery Steam Generators (HRSGs) indicate an increase in the exergy efficiency of the plant up to a level above 46%. [ABSTRACT FROM AUTHOR] more...

In the present study, turbulent airflows and particle deposition around two inline buildings were investigated numerically. A computational modeling approach including the RNG k-ε and Realizable k-ε turbulence models were used for these simulations. The Lagrangian particle tracking approach was implemented for evaluating dispersion and deposition of spherical dust particles. For simulation of turbulence fluctuations, an improved discrete random walk (DRW) model that includes the near wall correction was implemented into several user-defined functions (UDFs) that were linked to the ANSYS-Fluent code. It was shown that the new improved DRW stochastic model led to results that are more accurate compared to the standard model. The improved DRW model was then used for simulating turbulent deposition and dispersion of dust around building models. The presented results showed that putting buildings on elevated supports reduces dusts deposition from downstream of single and inline buildings at short distances, particularly for small particles of about 1  μm. It is also shown that particle deposition fractions around buildings on rough ground are higher than those for smooth ground. Deposition fractions were also predicted by the improved model for faces of single and inline buildings. [ABSTRACT FROM AUTHOR] more...

In many situations, inhaled droplets experience phase change in the human nasal passages during the breathing cycle. The associated size change of droplets can significantly influence the transport and deposition patterns in the nasal cavity. In this study, the transport, evaporation, and deposition of inhaled droplets under unsteady cyclic breathing conditions were simulated. A computational model of the nasal cavity was developed from magnetic resonance imaging (MRI) scans of the nose of a healthy male and a computational fluid dynamics (CFD) method was used to solve the governing equations of airflow and droplet motion. The Lagrangian trajectory approach was then used to investigate the transport and deposition of droplets. The size change of droplets was simulated using a convection–diffusion controlled model. In order to evaluate the effects of evaporation on transport and deposition of droplets, two inhaled compounds (water and linalool) with different saturation vapor pressures were simulated. The results showed substantial differences between the evaporation behavior of water and linalool droplets. Water droplets, which is more volatile than linalool drops, with the initial diameter ≤ 8 µm completely evaporated while complete evaporation of linalool droplets was observed for the initial diameter < 2 µm. During the time-varying inspiratory airflow, the minimum evaporation rates were observed when the airflow rate was near its maximum value. Comparing the deposition fractions of water droplets in the presence and absence of evaporation showed that droplet evaporation decreases the deposition of droplets by about 10–20% mainly due to the reduced inertial impaction effects. Furthermore, it was shown that a steady airflow simulation does not accurately estimate the deposition and evaporation of droplets in the human nasal cavity. [ABSTRACT FROM AUTHOR] more...

In this work, the behavior of inhaled water droplets with initial sizes of 1–30 μm was studied under a sinusoidal cyclic breathing pattern. The maximum flow rate was 15 L/min, and the effect of inhaled air humidity and a humid mucus layer was considered. The geometry of the nasal airway model was created using magnetic resonance imaging (MRI) scans of a healthy male's nose. The airflow and droplet motion governing equations were solved using an Eulerian-Lagrangian computational fluid dynamics (CFD) approach. A convection-diffusion-controlled model was used to analyze the size change of droplets. The main focus of this study was on the behavior and fate of the droplets, including their changes in size, as well as their evaporation, deposition, and penetration patterns. However, other parameters, including airflow velocity, temperature, and vapor concentration, were also evaluated. Based on the results, the mucus layer can significantly reduce the droplets evaporation to the extent that evaporation of initially 12 μm droplets reduces by about 90% due to the presence of the mucus layer. However, the inhaled air humidity and the mucus layer have little effect on the deposition fraction. The deposition rate mainly linearly depends on the initial droplet size. In the presence of the mucus layer, droplets with an initial size of 10 μm are mostly susceptible to penetration; around 80% can exit the nasal cavity. The critical range of droplets initial size is 5–10 μm. As the relative humidity of the inhaled air changes, the evaporation and penetration behavior of droplets with the critical range of size alters dramatically. Besides, in all cases, the deposition fraction of droplets smaller than 5 μm is almost zero. • The effect of mucus layer and air humidity on droplets fate is studied for 15 L/min. • The mucus layer heat flux decreases from the nostril toward the back of the turbinate region. • The evaporation fraction is influenced by the mucus layer and inhaled air humidity. • The mucus layer has a more significant effect on the size of droplets initially smaller than 5 μm. • The mucus layer and air humidity have no major impact on the deposition fraction. [ABSTRACT FROM AUTHOR] more...

In this study, the influence of a novel converging-diverging (CD) vortex finder on the performance of a gas cyclone was studied using the computational fluid dynamics (CFD) method. In addition, the influence of the CD outlet pipe on the erosion rate of the cyclone was investigated. The cutoff diameter, pressure drop, and erosion rates of the new vortex finder design were compared with those of the conventional straight outlet pipe cyclone. The throat of the CD pipe was at the same level as the cyclone's roof. The simulation showed that the CD outlet pipe significantly reduced the cyclone pressure drop. The height and diameter of the converging part of the vortex finder affected the cyclone pressure drop and separation efficiency. Increasing the converging part's inlet diameter reduced the outer vortex tangential velocity and the cyclone separation efficiency. Increasing the converging part inlet diameter reduced turbulent intensity at different cyclone levels. The CD vortex finder with the shortest height and inlet diameter of the converging part (model C1) reduced the cutoff diameter from 6.2 µm to 5.9 µm. The CD outlet pipe with the largest height and inlet diameter of the converging part (model C6) reduced the pressure drop by 42.2% compared to the conventional cylindrical vortex finder. The erosion rate also decreased significantly with an increase in the converging part inlet diameter. However, the converging part height only slightly affected cyclone performance. • Effect of a novel design of CD vortex finder on the performance of a gas cyclone. • CD outlet pipe causes a significant reduction in the cyclone pressure drop. • Model C1 reduces the cutoff diameter from 6.2 µm to 5.9 µm compared to the conventional model. • model C6 reduces the pressure drop by 42.2% compared to the conventional model. • the cyclone with the new vortex finder of model C6 showed the lowest erosion rate. [ABSTRACT FROM AUTHOR] more...

With people spending most of their time indoor, the air quality in indoor environment has become a subject of serious concern. Of particular interest is the mitigation of transmission of airborne infectious diseases in indoor facilities. The airflow pattern in indoor environment is affected by the ventilation system, thermal plume around human bodies, human respiration, human walking and other activities. In the present study, the effects of thermal plume, respiration and motion of the body on the indoor air quality in a ventilated cubicle were investigated using a computational modeling approach. Attention was given to the transport and fate of particulate pollutant in a ventilated room in the presence of a heated, breathing and rotating manikin. The cases of displacement and mixing ventilations were studied. In particular, the concentration of pollutant in the breathing zone of the manikin under various conditions was evaluated. The simulation results indicated that the rotation of the manikin significantly distorted the thermal plume of the body and the associated transport of particulates. Furthermore, the rotation decreased the concentration of particles in the breathing zone of the manikin. It was also found that the thermal plume generated by the body was distorted by the airflow of the mixing ventilation system, while there is little if any distortion by the displacement ventilation system. [ABSTRACT FROM AUTHOR] more...