فهرست مطالب mehrzad shams

Due to the vast and diverse applications of electrospray in various aspects of human life, this subject has always been of interest to researchers. This article discusses the experimental investigation of the electrospray process for the ethanol-water mixture with three different concentrations of 70%, 96%, and 99.9%. In this article, first, different electrospray modes for 70% ethanol, based on high-speed images, are defined and explained. For three concentrations of 70%, 96%, and 99.9% of the ethanol-water mixture, the Taylor cone angle and the jet diameter at the onset and end of the stable electrospray region have been calculated. For this purpose, high-speed imaging and processing of the resulting images have been utilized. The cone angle and the diameter of the jet exiting from the cone for the three fluids have been calculated for all onset and end points of the stable electrospray region for flow rates ranging from 0.1 to 1 mL/h. The average jet diameter for all points of the stable region for 70%, 96%, and 99.9% ethanol fluids is equal to 34.43, 33.78, and 31.70 microns, respectively. Additionally, the average cone angle for all points of the stable region is 87.26°, 85.80°, and 84.13° for ethanol fluids, 70%, 96%, and 99.9%, respectively. Therefore, the highest cone angle and jet diameter values correspond to 70% ethanol, and the lowest values correspond to 99.9% ethanol.

One of the most common used heat exchangers is the shell & tube heat exchanger. It has many usages in the oil and gas, petrochemical and pharmaceutical industries. The heat exchanger is the best which has the highest heat transfer rate with the lowest energy consumption. In this research, we want to increase heat transfer and reduce pressure drop by changing the cross section of tubes. We used tubes with different cross section; circular , ellipse with 0° attack angle and ellipse with 90°attack angle. We study pressure drop and heat transfer coefficient for Re between 3000 to 15000. Then two combined models are examined. Case 1: Circular tubes in the center of the shell and elliptical tubes with 90° attack angle around the shell, case 2: Elliptical tubes with 90° attack angle in the center of the shell and circular tubes around the shell. Which indicates an increase in heat transfer in elliptical tubes, especially elliptical tubes with 90° attack angle compared to the circular, while circular tubes have the lowest pressure drop. However the heat transfer in the case 1 (STHE-CT&ET90°) and the case 2 (STHE-ET90°&CT) increase by 10% and 3% compared to the STHE-CT respectively, it caused increasing the pressure drop.

Due to increasing environmental pollution, the use of renewable energies is very important. Proton Exchange Membrane Fuel Cell (PEMFC) is for producing renewable energy, which is applied in a wide range of fields due to its many merits, such as low emissions, high power density and high efficiency. Dimension of components of PEMFC plays an important role on its performance. In this article, Numerical solution are performed by assuming two phase, steady and isothermal. The effect of channel dimensions and thickness of gas diffusion layer, catalyst layer and membrane were studied on the performance of the PEMFC. This analysis involves studying two indicators of maximum power density and average current density, which it was investigated in a wide range of voltage operations. The results show that determining proper dimensions for cell components has a great impact on the performance. By comparing the effect of dimensions on two indicators, different rankings are obtained. Channel width and thickness of polymer membrane have the most important role in determining cell performance. Good correspondence between numerical results and experimental data is achieved.

The aim of this study is to investigate the effects of non-Newtonian blood rheology models on the wall shear stress (WSS) distribution in a cohort of patients-specific coronary arteries. Twenty patients with diseased left anterior descending (LAD) coronary arteries (with varying degrees of stenosis severity from mild to severe) who underwent angiography and in-vivo pressure measurements were selected to perform computational fluid dynamics (CFD) simulations. Three-dimensional (3D) patient-specific geometries were reconstructed from 3D quantitative coronary angiography. To compare the effects of rheological properties of blood on WSS along the arteries, each artery was divided into 3 segments; proximal (pre-stenosis), stenosis and distal (post-stenosis). Blood was modelled as a Newtonian and non-Newtonian (Carreau-Yasuda, Casson and Power-law) fluid.Our findings showed that the WSS distributions over proximal and stenosis segments were significantly affected by the non-Newtonian properties of blood whereas the effect was negligible over distal segment. On the other hand, the type of non-Newtonian model is important to achieve accurate results over proximal and stenosis regions, but over distal region, it does not matter what model is used. Therefore, to simplify the simulation, the Newtonian model can be acceptable in finding the wall shear stress distribution over the distal region regardless of severity of stenosis.

For conquering the crisis of the diminution of fossil fuels resources and environmental problems owing to their excessive consumption, some alternative technologies have been introduced. Among these possible choices, the fuel cell technology is a prominent candidate for a large scale and long term energy source. Between different types of fuel cells, the Polymer Electrolyte Membrane Fuel Cell (PEMFC) attracts greatest interest for its noticeable advantages and more over for being a “Green Energy” as it simply generates electricity by combining Oxygen and Hydrogen and producing merely water. However, the most important hindrances in the way of commercializing this technology are those associated with water and heat management in PEM fuel cells. In the present work the dynamic and droplet motion mechanisms is simulated by Volume of Fluid (VOF) two phase technique and implementing of Hoffman function as hysteresis model. In this study, the dynamic contact angle of the droplet motion is considered and the dynamic motion of the droplet is investigated in an application geometry that includes the manifolds of the gas flow distributor in the PEMFC. The effect of the manifold slopes on the discharge of liquid water droplets compare to straight manifold has also been studied. By changing the geometry of the inlet and outlet manifolds, the problem created in conventional geometry, which causes the obstruction of the channel caused by liquid water, is overcome, thus improving geometry improves water management in the channels.

The purpose of this research was to investigate the hydraulic parameters affecting the performance of the distillation column and the hydrodynamic characteristics of the flow field on the industrial scale-sieve tray using numerical simulation. Computational fluid dynamics method was used for analyzing and predicting flow behavior. The desired geometry including the space between two trays of the distillation column and the down comer region was considered. After plotting geometry, three dimensional grids were generated in Gambit software and the analysis of the flow field was traced in Fluent software. The Eulerian-Eulerian approach was applied to simulate two-phase flow and k-ε RNG model for turbulence modeling. Validation of the results was done successfully using Solari and Bell experiment data and the correlation presented by Colwell. The velocity distribution and volume fraction of liquid and gas in different zones were determined. The influence of inlet volumetric flow rate of liquid and gas, as well as the geometry of the weir, on parameters related to the tray performance such as clear liquid height and froth height were investigated. The results indicated that a better separation would occur in lower gas and liquid loads.

In this research, subcooled flow boiling of water and water-based nanofluid in the different channels cross sections with the same hydraulic diameter is simulated. The subcooled flow boiling of water in the channels is studied by Euler – Euler model. The results of this part was matched with the experimental data very well. To study the effects of nanoparticles in the subcooled boiling flow, copper oxide nanoparticles with 40 nm in diameter were injected at the inlet to the flow. The nanofluid subcooled boiling is simulated by considering three phases, liquid, vapor and nanoparticles. The water and vapor interaction is simulated by Euler-Euler approach; and the motion of nanoparticles in the continuous fluid is modeled by Euler – Lagrange model. Water, vapor and nanoparticles were considered continuous fluid, dispersed fluid and dispersed solid, respectively. After model validation, boiling of nanofluids was modeling in different channels. Volume fraction and temperature variations is obtained along the channels. The results showed that, at low concentrations of nanoparticles (0.001 kg/s) rectangular channels and at higher concentrations (0.005 kg/s) square channels have the greatest changes in vapor volume fraction compared to pure water boiling.

In this study, performance of gas liquid cylindrical cyclone separators and the effect of changing geometrical parameters of cyclone separators entrance are investigated. The cyclone is simulated with computational fluid dynamic methods. After choosing a suitable mesh grid for the cyclone and checking grid independency, the effect of changing entrance geometry on gas carry under and liquid carry over is investigated. Geometrical parameters, especially inlet geometrical parameters, have considerable effect on optimizing cyclone separators performance. RSM model is used for turbulence simulation of the flow and two phase flow is simulated using Eulerian- Eulerian approach. In this simulation, inlet cross section, inlet angle and inlet height relative to the cyclone bottom part are optimized. Results show that GCU decreases with decreasing nozzle’s inlet angle. An optimum point for GCU was given with changing inlet altitude relative to the bottom of the cyclone and inlet nozzle’s width. An optimum point for LCO was obtained with changing inlet altitude and inlet nozzle width. Increasing inlet angle causes a decrease in LCO. In optimum model, gas carry under decreases significantly and liquid carry over is eliminated.

Water is needed to providing proper hydration of membrane and its ionic conductivity in polymer electrolyte membrane fuel cells، but excess water accumulation known as flooding phenomenon decreases the passing way of reactants in the GDL and reaction sites on catalyst layer and increases mass transport loss and leads to performance loss of polymer electrolyte membrane fuel cells. In the present work، the two-phase flow in the cathode channel of transparent polymer electrolyte membrane fuel cell with single serpentine flow field is visualized by direct optical imaging in unsteady and time averaged states. Then the water coverage length ratio and the average of water coverage length ratio are derived as a scale of water content of the cathode channel in the unsteady and time averaged states. In the unsteady and time averaged states، by increase the stoichiometry، decrease the relative humidity and inlet gases temperature in anode and cathode sides، the accumulated liquid water in the channels reduces. The effect of anode stoichiometry on the amount of water in the cathode channel in the unsteady and time averaged states is more than the cathode stoichiometry.

In this study، a two-dimensional، axisymmetric، computational Algorithm has been developed to simulate the plasma flowfield in a MPD thruster for the purpose of determining the flow behavior and electromagnetic characteristics distribution. The solution employs Roe’s flux vector difference method in combination with Powell’s characteristics-splitting scheme. To ensure the stable high-accuracy solution، new modification of MUSCL technique so called OMUSCL2 method is used. According to being supersonic strong gasdynamic expansion near the electrodes tip، HHT entropy correction is employed. Further improvements to the physical model، such as the inclusion of relevant classical transport properties، a real equation of state، multi-level equilibrium ionization models، anomalous transport، and multi-temperature effects، that are essential for the realistic simulation MPD flows، are implemented. Numerical results of a lab-scale thruster are presented، whereby comparison with experimental data shows good agreement between the predicted and measured enclosed current and electric potential.

A Lagrangian-Eulerian numerical scheme for the investigation of bubble motion in turbulent flow is developed. The flow is analyzed in the Eulerian reference frame while the bubble motion is simulated in the Lagrangian one. Finite volume scheme is used, and SIMPLEC algorithm is utilized for the pressure and velocity linkage. The Reynolds stresses are modeled by the RSTM model of Launder. Upwind scheme is used to model convective fluxes. The Guassian Filter White Noise is incorporated to simulate the turbulent fluctuation velocities. The bubble diameter is found by the use of Rayleigh-Plesset equation. Various forces in the equation of motion of the bubble are considered. The Buoyancy, Saffman lift, drag, pressure, and change of volume forces are carefully applied. The effects of all of these forces on bubble path are also examined. The bubbles are created in the low pressure zones, and then traced in the flow field. It is observed that the bubble diameter is highly dependent on the mean stream pressure, and its location. The results are compared with the other published works, and have an acceptable accuracy.