فهرست مطالب فرید دولتی

In this study, the flow field and the impingement heat transfer of fluidic oscillators at narrow spaces are investigated numerically. Simulations are performed in 2-D, incompressible, and unsteady conditions and the aim is to analyze the effects of the jet to wall distance, the external nozzle angle, the Reynolds number, and removing the external nozzle on the heat transfer performance. Also, for a comprehensive review, the results of the fluidic oscillator are compared with the results of the steady jet. To ensure the validity of the numerical simulations, two experimental researches are applied for the fluidic oscillator and the steady jet and a good agreement is observed between the present simulations and the experimental data. The results show that increasing the distance in fluidic oscillators causes a maximum decrease of about 11% at the Nusselt number of the stagnation point, while employing the various distances does not have a significant effect on the steady jet. In addition, the configuration of the different external nozzle angles affects the Nusselt number, but this influence does not have a monotonic behavior. Furthermore, the Nusselt number increases by removing the external nozzle. When the Reynolds number increases for the fluidic oscillator and the steady jet, the Nusselt number of the stagnation point increases by at least about 22 and 28%, respectively.

In this paper, drying of the moist object is numerically investigated in the forced convection with and without the electric field. Finite volume method is used to solve governing equations of electric, flow, temperature, and the concentration fields in flow phase, as well as the temperature and the moisture fields in the moist object. In this study, the effect of applied voltage and the arrangement of the emitting electrode are evaluated. The results indicated that in presence of electric field, the increment of the applied voltage for 18 kV to 24 kV, the mass transfer from porous object 3.78 times and power consumption 7.96 times are increased. It is also found that the drying rate is increased by decreasing the distance between the emitting and collecting electrodes. According to numerical results, the mass transfer enhancement is usually accompanied by penalty of electric energy consumption. Therefore, the specific energy consumption has been evaluated as final criterion. It is shown that the specific energy consumption of the electrohydrodynamic drying process has been remarkably affected by the changing of the emitter arrangements. Finally, an optimum arrangement has been introduced as the affordable arrangement.

Heat transfer and hydraulic performance for flow in a rectangular channel with LVGs (longitudinal vortex generators) are experimentally investigated using the different shape of LVGs. Firstly, a precise and reliable experimental setup was designed and fabricated to generate a constant heat flux boundary condition. In order to analyze the effect of shaped LVGs, three different of the rectangular, trapezoidal and delta winglet pair vortex generators with and without punched holes on the flow field and heat transfer characteristics at the different attack angle of 15º, 30º, 45º and 60º with a small thickness has been studied. Effects of the number of punched holes were evaluated by using dimensionless numbers, friction coefficient ratio (f/f0), Nusselt number ratio (Nu/Nu0) and overall performance ((Nu/Nu0)/(f/f0)). According to the experimental results, the rectangular winglet pair without punched holes vortex generator has the highest values of friction factor ratio and increased with bigger attack angle. The friction factor decreased and heat transfer and also overall performance increased with implementing of perforated rectangular, trapezoidal and delta winglet pair in the channel but in the case of implementing trapezoidal vortex generator with two punched holes heat transfer is a little more than the vortex generator with three punched holes.

In this paper, a two-dimensional numerical approach is used to study the mass transfer in drying process of a moist object affected by electric field in a smooth channel. Finite volume method is used to solve governing equations of electric, flow, temperature, and the concentration fields in flow phase, as well as the temperature and the moisture fields in the moist object. The computational methodology includes the use of a structured, non-uniform quadrilateral grid, and the Standard K-ɛ model was adopted as the turbulence model. The initial temperature of moist object is equal to the air temperature. In this study, firstly, the computed results are compared with the experimental data and the results agree very well. Secondly, the effect of Reynolds number, applied voltage and the position of the emitting electrode on the drying rate of moist object is evaluated. The numerical results show that the drying rate of moist object with increment Reynolds number enhances without the electric field. Also, in presence of electric field, in constant Reynolds the influence of EHD phenomenon on the drying rate increases with increment of applied voltage. In addition, the results show that as the electrode position is established toward the leading edge of moist object, the maximum moisture evaporation reaches.

In this study, the flow and temperature fields affected by electric field applied to the fine wire are numerically investigated for the incompressible, turbulent, and steady flow over a single slot. The numerical modeling is based on solving electric, flow, and energy equations with the finite volume approach. The computed results are firstly compared with the experimental data in case of flat plate and the results agree very well. Then, the effect of different parameters such as the applied voltage, Reynolds number, and the emitting electrode position on the heat transfer coefficient and pressure drop is evaluated. The numerical results show that the heat transfer coefficient with the presence of electric field increases by incrementing of the Reynolds number but then decreases. And it also increases by the incrementing of the applied voltage. Moreover, the reduction of distance between the emitting electrode and the slot edges can obviously effect on the heat transfer enhancement, power consumption and pressure drop.

In this paper, the flow and temperature fields affected by electric field in the presence of wire collecting electrode are numerically investigated for the two-dimension, incompressible, turbulent, and steady flow conditions with the finite volume approach. The computational methodology includes the use of a structured, non-uniform quadrilateral grid, and the Standard K- model was adopted as the turbulence model. The computed results are compared with the experimental data and the results agree very well. Then, the effect of different parameters such as collecting electrode radius, the applied voltage, Reynolds number, and the distance between emitting and collecting electrodes on the flow pattern and heat transfer coefficient is evaluated. The numerical results show that the influence of electrohydrodynamic phenomenon on the heat transfer enhancement increases with radius of the grounded electrode and the applied voltage but decreases when the Reynolds number and distance between electrodes are augmented. The results indicate that electrohydrodynamic actuator acts as a generator of secondary flow and these vortices were used to enhance the forced convection heat transfer.

Plasma actuator is one of the newest ways in vortex generation and flow control techniques which can enhance heat transfer rate by inducing external momentum to the boundary layer of the flow. In this paper, a 2-D numerical approach was implemented to analyze the presence of plasma actuator on the incompressible, turbulent, steady flow in a flat channel. In this approach, the flow field and heat transfer characteristics such as the stream function and heat transfer coefficient were evaluated through the variety of Reynolds number, at the presence and absence of applied voltages. The present computed results are firstly compared with the numerical data in case of rectangular flat channel and the results agree very well. The numerical results indicate that at a constant Reynolds number with the presence of a plasma actuator, the heat transfer coefficient will be increased but in a constant applied voltage the heat transfer coefficient will increase to the Reynolds of 250 and then will be decreased respectively. In addition, the size of generated vortexes significantly depends on the applied voltage and the upstream flow speed. On the other hand, according to the results, the flow speed affects the size of generated vortex and vanish the actuator effect at high Reynolds. According to the results, there is an optimized point for the applied voltage and flow speed.

Electrohydrodynamic actuator is one of the newest devices in flow control techniques which can delay separation point and reduce the drag coefficient by inducing external momentum to the boundary layer of the flow. In this paper، a 2-D numerical approach was implemented to analyze the presence of electrohydrodynamic actuator on the incompressible، turbulent، steady flow over a NACA 4412 asymmetric airfoil. In this regards، the flow field and aerodynamic characteristics such as the drag and pressure coefficient were evaluated through the variety of attack angles، applied voltages، the location of emitting electrode، and the distance from the upper surface of the airfoil. The numerical results indicate that the drag coefficient with the presence of an electric field decreases with the enhancement of the supplied voltage but increases when the attack angle is augmented. In addition، the location of separation point significantly depends on the position of emitting electrode and the distance between the emitting electrode and the collecting electrode. On the other hand، according to the results، the Electrohydrodynamic effects cause the diminution of the wake region over the airfoil.