مجید بازارگان

The current study has used three-dimensional simulation of flow around the three-bladed Darrieus ‎turbine to investigate the turbine's self-starting power. Two types of straight bladed and helical bladed ‎turbines, which are equipped with a J-type blades are considered. To fully investigate, the effects of various ‎parameters including wind speed, blade height, and the tips shape of J-type blades which could be open or ‎closed have been studied and the results have been compared with those of the turbine with conventional ‎blades. The analysis of the flow around the J-type blades determined that the vortices formed on the ‎pressure side increase the pressure on this surface and thus rise the torque production. By closing the ends ‎of the J-type blade, these vortices are prevented from exiting at the ends that leads to a substantial ‎improvement on the self-starting and low TSR situations. The results showed that the average torque ‎produced by the straight and helical bladed turbines equipped with closed end J-type blades is respectively ‎‎38.5% and 21% higher than the turbines with full profile blades under self-start-up conditions, and the ‎aforementioned percentages are 49% and 41.9% at low TSRs. It was also concluded that the positive effect ‎of using J-type blades under self-starting conditions is more pronounced at low wind speeds and higher ‎blade heights, which makes these blades a viable option for urban applications.‎

The impinging jet array is capable of enhancing heat transfer rates over the impinging surface. A sever challenge of such cooling method is that it does not transfer the heat across the target plate as uniformly as required in many applications. The main objective of the current numerical research is to study the effects of covering the impingement surface by a porous layer in order to increase the heat uniformity throughout the surface without losing the advantage of high heat transfer rates in a multiple impinging jet flow. The porosity, permeability and thickness of the porous layer are considered. The standard deviation of local Nusselt numbers compared to the average value is used as a measure to evaluate the uniformity of heat transfer along the plate. A performance evaluation factor is proposed to simultaneously account for uniformity as well as total rate of heat transfer. The results show that lowest evaluation factor of 0.137 is obtained once the flow channel is fully covered by the porous medium and the permeability and porosity are at their lowest values of k=1.76*10-12 and ԑ=0.2, respectively. The highest heat transfer performance is obtained for the case when the channel is half filled with the porous layer and the coefficient of permeability and porosity are at their lowest (k=1.76*10-12) and highest (ԑ=0.8) levels, respectively.

The heat transfer and pressure drop of the air flow inside the square channel with perforated ribs are investigated experimentally. The effect of the perforation on the reduction of the low pressure region behind the ribs, and thus reduction in overall pressure drop, is evaluated. The Reynolds number is between 15000 and 50,000. Two values of 0.1 and 0.14 are selected for the ratio of the ribs height to the channel hydraulic diameter and three values of 20, 25 and 30 are considered for the ratio of the ribs pitch to the ribs height. The ratio of the hole diameter to the rib height are 0.3 and 0.5 respectively. It is found that the perforated ribs result in less pressure drop as well as reduced Nusselt number. Moreover, the effect of perforation is found to be unfeasible in the smaller pitch ratios. On the other hand, in long ribs, the perforation improves the performance coefficient up to 17%, the improvement is more evident in low Reynolds. In particular, for applications such as gas turbine or heaters, where the prime concern is to lower the wall temperature or increase the heat transfer and the blower energy consumption is less important, the current results show that using holes in the ribs can effectively enhance the flow thermal performance.

A diffusion absorption refrigeration cycle as a car refrigerator, which uses the car exhaust waste heat as a heat source of the cycle has been simulated in this study. An internal combustion engine with a volume of 1.3 liters at different engine speeds and throttle openings was examined experimentally and exhaust conditions such as flow rate and temperature were used as the input of the cycle. Eventually, for engine speeds above 2000 rpm, there was no trouble and the evaporator temperature ranged from -0.4oC to -7.1oC. For 1500 rpm and 1000 rpm, the evaporator temperature did not reach the desired range of variations, which is the case in other reported researches. There is no available solution for the situations where the engine is running at low speed such as in traffic jam or idle condition. Therefore, a new generator was designed and simulated to solve this problem. The simulation results show that by using the modified generator, the heat transfer to the generator improves by 16.8% on average. Consequently, the cooling capacity increases by 4.7%. Therefore, the current diffusion absorption cycle is capable of performing well at the low engine speeds.

Cooling of blades is a challenge in advancement of gas turbines. Channels are used inside the blades to improve the heat transfer. Repeated ribs have significant impact on the heat transfer and pressure drop characteristics of a channel. In this study the effect of ribs with specified geometry is numerically investigated. In a relatively large range of Reynolds from 10000 to 80000, the fluid flow and heat transfer were simulated. The finite volume method was implemented. To model the turbulence, the standard k– ε model was used. The thermal boundary condition was the constant heat flux at the channel surfaces. The magnitude of the heat flux was assigned in a way to result in temperature difference from 10 K to 15 K. The results of current research were compared to the available experimental data under similar flow conditions. The coefficient of pressure drop was comparable with experimental data both in two dimensional and three dimensional cases. In the two dimensional simulation, the heat transfer coefficient was similar to the experimental data in the range of this study. In the three dimensional simulation, accuracy of the heat transfer coefficient in the case of low Reynolds was not satisfactory. However, it improved progressively as the Reynolds number increased. It was observed that, the maximum improvement in the heat transfer and pressure drop are about more than 200% and 300% respectively. Heat transfer augmentation could be explained by the presence of the secondary flows and disruption of the boundary layer growth due to the ribs.

According to the development of technology of automated highways, the platooning can be benefited for improvement in fuel consumption. Platooning has already been exercised in the nature and industry as an effective tool to reduce the drag coefficient. The goal of this article is to investigate the effect of longitudinal distance between two passenger cars on drag coefficient and hence the fuel consumption numerically and experimentally. It has already been shown by various investigators that a passenger car travelling in the wake of another car will experience a reduction in drag coefficient. The optimized distance and speed to attain the least drag coefficient and fuel consumption are reported. The car’s speeds of 70, 90 and 110 kilometers per hour have been examined in this study. To evaluate the numerical results, some field tests are done. A device which was particularly designed and fabricated to measure the fuel consumption was installed on a sedan midsize car. The results show that the car platooning with an optimized longitudinal distance between two cars can reduce the drag coefficient by 7 percent.

In this research, a lattice Boltzmann model is proposed to simulate free convection flow through pressure filtration. High compressibility effect needs to be considered near the critical point. A Poiseuille flow has been used as the first example to examine the effects of boundary condition model used in this study. It has been shown that the encountered error is of second order, which is considered to be desirable. The effect of the present boundary condition on the stability of solution to a Rayleigh-Benard problem has also been demonstrated. Finally, the filtered pressure equations have been implemented to model flow of a supercritical fluid in a cavity. The results are in good agreements with available data in the literature.

Piston effect is an important mechanism of heat transfer in a supercritical fluid flow under microgravity condition. In this study, a Lattice Boltzmann Model (LBM) has been introduced to simulate the piston effect. Variations of diffusion coefficient has been accounted for by adding a corresponding term to equilibrium distribution function. To calculate the intermolecular forces and compressibility in the LBM, a van der Waals equation of estate has been employed. Boundary conditions corresponding to compressible LBM at the presence of van der Waals forces have been set to eliminate the speed jump at the wall. It has been shown that such boundary conditions provide high accuracy in problems involving forces with an error of second order of magnitude in terms of space. The developed thermal LBM together with compressible LBM have been applied to simulate the heat transfer to supercritical fluid flows. The piston effect has been modeled by considering van der Waals inter molecular forces. The errors associated with each of the schemes used have been evaluated. A comparison between a pure conduction case and heat transfer due to piston effect has been made. It has been shown that the heat transfer occurs faster once the piston effect is in effect.

Repeated ribs are used to enhance heat transfer in turbine blades and solar air heaters. The ribs have significant effect on the characteristics of heat transfer and fluid flow. Application of ribs enhances the heat transfer significantly, however simultaneously the pressure drop increases. As a result a balance must be made between the improvement in heat transfer and additional power to overcome the elevated pressure drop. In this study, the effect of ribs on the thermal performance of a flat plate solar air heater is investigated to find the optimum set of parameters in a flat plate solar air heater with ribbed surfaces. Genetic algorithm used to perform the optimization. The optimization carried out to meet two objectives, to attain higher thermal efficiency and to guarantee a practical temperature difference in the inlet and outlet of air flow. In a relatively large range of Reynolds numbers from 2300 to 25000, the fluid flow and heat transfer were evaluated. In order to validate the results, comparison has been made with available data in literature. It was found that the application of ribs improve the thermal efficiency about 10% in low air mass flows, however at higher air flow the additional power due to the pressure drop diminish the effect and even might decrease the efficiency.

In this study¡ a Lattice Boltzmann Method (LBM) has been developed to calculate the distribution of a scalar quantity¡ like temperature¡ in a natural convection flow field under the condition of varying fluid thermal conductivity. The standard form of an LBM usually considers the fluid properties to be constant without any source term in conservation equations. The model developed is to account for variation of thermal conductivity with temperature in the presence of an external heat source. The proposed model has been examined against various case studies. It is shown that it is capable of modeling the extremely nonlinear problems. To magnify the nonlinear term in the natural convection case of under study¡ the radiation and other thermal sources have been used. The multiple relaxation time scheme has been applied to assure the solution stability. Using Chapman-Enskog analysis¡ the error associated with the proposed model has been estimated. The part of error which was not due to variations in the fluid properties¡ may be eliminated by introducing a correction term in higher order terms in Chapman-Enskog analysis. In addition¡ it has been shown that the correction term associated with the fluid conductivity variations¡ create an error of second order in terms of Knudsen number and is negligible. The present LBM model has an error of the second order of magnitude with respect to time.

Nowadays، solid tumor modeling and simulation results are used to predict how therapeutic drugs are transported to tumor cells by blood flow through capillaries and fluid flow in tissues. This model involves processes such as fluid diffusion، convective transport in extracellular matrix، and extravasation from blood vessels. In this paper، a complete model of interstitial fluid flow in tumor and normal tissue is presented with considering multi scale of solution such as blood flow through a capillary (as the smallest scale) to interstitial flow (as the biggest scale). The advanced mathematical model is used to generate a capillary network induce by tumor with two parent vessel around the tumor for the first time. In the following، the blood flow is modeled through the network with considering the non-continuous behavior of blood rheology and adaptability of capillary diameter to hemodynamics and metabolic stimuli. This flow is simultaneously simulated with interstitial flow which is coupled to blood flow through capillary with extravascular flow. The results predict elevated interstitial pressure in tumor region and heterogeneous capillary network which are introduced as barriers to drug delivery.

Tumor induced angiogenesis is the bridge between benign and malignant tumor growth stages. In this process، growth and migration of endothelial cells build capillaries to supply the tumor with blood for its further growth. Regarding the importance of capillary formation and blood flow in angiogenesis، simulation of this phenomenon plays important role in tumor growth and cancer development studies. In this work، considering intracellular، cellular، and extracellular scales a mathematical model of tumor-induced angiogenesis is used to consider mechanical effects of extracellular matrix on growth and migration of endothelial cells. These effects are matrix density and its fiber length. In this study، to model cellular dynamics، a discrete lattice based Monte Carlo method is used. Results show that migration of endothelial cells and development of capillaries are possible in a specified range of matrix density and matrix fiber length. Based on the results، medium matrix densities and low fiber length provide a suitable environment for capillaries growth and development. The model is a promising tool for modeling tumor induced angiogenesis and is a base for development of models for loop formation and blood flow in capillaries around tumor.

Due to growth of energy consumption and depletion of fossil fuels sources, power generation of renewable energy sources such as wind energy has become one of the main interests of researchers. Among different types of wind turbines used for extracting electric power from wind flow, vertical axis wind turbines can be implemented in urban areas and in proximity of energy consumers because of their independence of wind direction, low sensitivity to wind turbulence and lower noise production. In this paper a straight-bladed vertical axis wind turbine has been simulated 3 dimensionally by use of a commercial CFD code. The numerical results have been validated against available experimental data. To improve the performance of the turbine, the effects of blade mount point offset and preset pitch have been investigated. The results show that appropriate blade offset and preset pitch for this case study leads to a 60 and 65 percent increase in the maximum performance coefficient respectively.

In this paper, pollutants flows coming out of stacks or cooling towers with different outlet shapes have been numerically studied. The effects of exhaust outlet on plume rise, and the pollutant dissipation are investigated. To simulate the flow turbulence, realizable k – ε model is employed for the case of a stack flow with more than one exhaust outlet on the influence of wind condition. The plume rise and dissipation of pollutant are depending on the direction of the wind and the shape of exhaust outlet. Depending on wind direction and shape of exhaust outlet, higher or lower levels of plume rise can be obtained with various kind of pollutant dissipation. These changes are due to the influence of high pressure upstream and low-pressure downstream flow fields of outlet from stacks on the counter vertex rotating pair. The maximum concentration and dissipation of pollutant for various wind directions and output configurations are examined.

Heat Chndjryany Mbdlhayy that they are simultaneously heat transfer between the cold stream and multi stream multi-gram takes place. Heat Chndjryany of cases where high thermal efficiency and low weight and volume are considered, is used. Cases, their application can be used in the above processes as gas cooling before cooling fluid input, condenser, chiller Rybvylr and, in order to aerospace industries and fuel oil as well Khnkkrdn Khnkkrdn whole collection, and ventilation systems in the condenser and Tbkhyrknndhha Air exchangers and pointed. Converters due to the great economic benefits and high thermal efficiency, are an excellent alternative to heat exchangers network, which mainly consists of shell and tube exchangers are. Of this application as the new converters is named. This rticle originally designed in the manner expressed Chndjryany converters and thermal and economic performance of these converters in comparison with the shell and tube heat exchangers for heat recovery networks will be discussed.

The propellants in a liquid fuel rocket engine are guided into the combustion chamber by means of a back pressure provided by a pressurizing system. There are two common ways to optimize the mass of a pressurizing system, namely minimizing the pressure of the system and cutting off the inlet gas flow to the propellant tank at the right moment. In the former, the minimum pressure required by the propellant pumps depends on accurate calculation of the inlet pressure of the pumps. In the later however, the cut-off time of the gas flow to the propellant tank is a function of the rocket acceleration. The above parameters have direct impact on the total mass of the pressurizing system. In this work, the system pressure minimization and the inlet gas flow cut-off have been analytically studied. Our results have been compared with the existing experimental data, showing relatively close agreements.