مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 3 (خرداد 1400)

Nowadays there are a lot of tendencies to use analytical micro-systems in the field of molecular and microbial diagnostics, because of benefits such as less required space, reduced sample and reagent consumption and shorter analysis time. The rotational microfluidic is a sub-branch of the microfluidic systems which by using centrifugal forces, causes the fluid to flow in the networks enclosed in disk-shaped rotating systems. These networks can carry out chemical or biological tests and lead to create more capable devices replaced with current regular devices for medical diagnosis. One of the most commonly used elements in microfluidic systems is the separation element. This element is used to separate particles in the sample by considering mechanical, chemical and electrical specifications. In this study, a novel geometry designed to seperate particles passively in microfluidic systems. The spiral microchannel geometry imports the local centrifugal force caused by the curvature of the channel in addition to the centrifugal and the Coriolis forces, to affect the particles and increase the efficiency of the separation. At first, the particle separation process in the proposed channel studied. Then the effect of the channel length and rotational velocity parameters on the separation efficiency investigated. The results of the experiments showed that the separation efficiency is above 90%, which indicates the high potential of this element in the process of separation. Also, It was observed that the increase in rotational velocity and the length of the spiral channel improves the process of separation and increases the efficiency.

Magneto-Rheological Fluids (MRF) are one of the intelligent fluids which have been extensively used in engineering application including Magneto-Rheological Dampers (MRD). Having yield stress in a magnetic field and ability to control and increase their viscosity is their most important characteristics. . In this study and after three different carbonyl iron powders were subjected to SEM and EDX analysis, five different MRFs were synthesized and were tested for stability. Effect of iron powder weight percentage in the fluid and preservative on level of sedimentation in the MRFs were investigated and the optimized fluid in terms of stability and concertation was selected for rheological characterization in various magnetic fields. The results obtained from the optimized MRF with 85% (weight %) iron powder was similar to that of LORD MRF-140CG oil. Also, a modified non-Newtonian rheological model was developed to predict the behavior of the optimized MR fluid which is more accurate than Bingham and Herschel-Bulkley models and could be implemented in CFD modelling. The modelling of the MR damper was conducted by implementing modified non-Newtonian and Bingham models using analytical quasi-static, unsteady and CFD methods and the results were validated with experimental data. The results show that neglecting factors including fluid shear thinning, wall shear stress and inertia term effects and effect of magnetic field on plastic viscosity in conventional modelling methods results in considerable error that will increase as magnetic field, Reynolds number and gap are increasing. Consequently presented model and methods, could be used for improved MR damper design.

In this paper, the effect of the secondary flow induced by the electrohydrodynamic (EHD) actuator is numerically investigated in the vorticity flux, as a criterion for the secondary flow strength, and the EHD vortices through a smooth channel. In this study, the influence of effectiveness parameters of the EHD as the Reynolds number, applied voltage and the arrangement of the emitting electrode on the vorticity flux, and also relationship between flow and heat transfer characteristics with the vorticity flux are evaluated. The results indicated that in presence of electric field, by increasing the Reynolds number, dimension of the upstream EHD-induced vortices and the vorticity flux are decreased. Also, it is obvious that by increasing the applied voltage, the dimension of the EHD-induced vortices and the vorticity flux are increased. According to numerical results, the heat transfer enhancement is completely depend on the vorticity flux. Also, by changing of the emitter arrangements, the non-dimension average vorticity flux and the average heat transfer enhancement are changed. It is shown that with decrease of the distance between emitter electrode and inlet of channel, from d=50 cm to d=10 cm, the non-dimension average vorticity flux and the average heat transfer enhancement are increased 27.9% and 17.9%, respectively.

In this paper we performed an analytical and 3D numerical investigation for breakup of non-Newtonian droplets in a non-Newtonian carrier fluid. The system geometry is T-junction with unequal length branches that can generates unequal sized droplets. Analytical and 3D numerical results of this research have good agreement with each other. We investigated variation of many quantities during the breakup process such as: flow rate ratio of branches, velocity ratio of branches, droplet length in each branch, whole length of droplet, vorticity, pressure and effective viscosity. The analytical results of this paper reveal the variation of flow rate ratio of branches, velocity ratio of branches, droplet length in each branches and whole length of droplet during the breakup process. The results showed the flow rate ratio of branches and the velocity ratio of branches is constant during the breakup process. Also we observed the droplet length in each branches and whole length of droplet increase linearly during the breakup process. The results showed vorticity near the surface of the droplet is 3 to 7 times the vorticity of the inside of the droplet, therefore the mixing of the materials of the droplet inside increases. Also the maximum vorticity is before reaching the droplet to the center of junction.

The first step in turbine blade design is to select TSR. Therefore, an optimization procedure should be applied to find the best ratio since this directly affects the energy generated from the turbine. In this research the optimum TSR are calculated with regard to the dynamic stall phenomenon. The dynamic stall imposes large amplitude loading on airfoil sections and since it occurs in HAWT operating envelope in unsteady flow. The purpose of this study was to investigate the effect of unsteady flow with periodic oscillation on the performance of HAWT turbines. A DS model is implanted to analyze the static data. Then the Beddoes-Leishman DS model was tailored to the HAWT environment. Then, using this model and BEMT, the optimal TSR is calculated And the impact of the presence of dynamic stall is shown. Also, Cp and CT are plotted in several different TSR with different time steps to express the effects of Unsteady phenomenon. In addition to dynamic results, static and steady results are plotted in power and thrust graphs. This phenomenon affect the efficiency by -3% as compared to the static stall. Also the Optimum TSR increases in dynamic mode. Examination of the time average diagrams of the drag coefficient shows that the delay in separation starts approximately from the midpoints of the blade and reaches the maximum value at the root.

In this work Multi-Relaxation Time Lattice Boltzmann Method (MRT-LBM) is used to investigate the effect of the inlet air position on particle motion in a room. The sub-scale modeled room is one-tenth the scale of a full-size room with dimensions of 0.914 m×0.305 m×0.457 m and two different positions (ceiling and floor) are considered for the inlet air with dimension of 0.101m×0.101m. Large Eddy Simulation (LES) with Standard Smagorinsky model is utilized to simulate the turbulent indoor airflow. Particles with 1 and 10 micrometer sizes are selected for investigation of particle dispersion and deposition on the walls of the room. Number of deposited particles and those exiting the room show that when the inlet air is on the floor, the number of particles leaving through the exhaust register with bigger size (10µm) is more than the case for ceiling position. For smaller particles (1 µm) there is no significant difference between the floor and ceiling position of the inlet air for particles leaving the room thought the exhaust register. Results show that the gravity force significantly affect the particle deposition, as the number of deposited 10 µm particles on the floor are about 100 times that of the deposited 1 µm particles when the inlet air position is at ceiling.

Usually in the test of a solid propellant motor in a high altitude test facility, the deposition of aluminum oxide causes a change in the cross-section area of exhaust diffuser. This may result in some disturbances in the performance of the supersonic exhaust diffuser. This paper is presented to investigate the deposition effect on diffuser performance. In the numerical simulation, the blockage of the diffuser due to the deposition of aluminum oxide is considered by a gradual and time-dependent cross-section constriction. In the initial conditions, the supersonic flow has been established in the diffuser. Diffuser cross-section area is reduced by using a dynamic mesh method during the solution. The numerical method is validated by the available experimental results. Diffuser blockage (n) is equal to the ratio of instantaneous and primary diameters of the second-throat. By changing the value of n from 1 to 0.75, the onset of flow separation is moved to the downstream location in diffuser. This results in a considerable reduction of total pressure loss and then improves the flow pressure recovery. Decreasing parameter n from 0.75 to 0.64, the flow structure is subjected to severe changes and the separation of the flow occurs near the diffuser inlet or inside the nozzle. In this condition, the diffuser state changes from starting to un-starting mode. Therefore, the vacuum condition vanishes in the test chamber.

The present study numerically investigates the feasibility of using multiple dielectric barrier discharge multi-DBD plasma actuators inside the surface of geometry as a novel approach for active flow control over a large vertical axis wind turbine. For this reason, the plasma actuator is modeled based on Suzen Model and the results are validated. Then, a computational study is carried out on a commercial large scale vertical -axis wind turbine to examine the effect of the presence of the plasma actuator. The 530G vertical-Axis wind turbine is used as the baseline case. The plasma actuator was applied inside the surface of the blades of turbine and on all their surfaces in a sequential and simultaneous way. Plasma actuator is one of the newest devices in flow control techniques which can delay separation by inducing external momentum to the boundary layer of the flow. It is revealed that the use of multi-DBD actuators could enhance the induced velocity; this affects the pressure distribution and increases the aerodynamic torque. Consequently, an averaged power increase of 3 % was achieved. Possibility of increase in wind turbine power even in a commercial scale large turbine has been proved by flow separation control using the plasma actuation technology. In addition, the application of the plasma inside the surface of the blades will not effect on its performance.

Two-phase flow instabilities are observed in many areas of industrial applications such as turbomachinery, refrigeration systems, water boiling reactors and similar systems. Predicting fluid flow parameters such as pressure drop, stability region during boiling and oscillation characteristics are the determining factors in the design and operating conditions of two-phase flow equipment. In this paper, density wave oscillations type instability in boiling process is analyzed.By introducing appropriate dimensionless variables, an integrated model for the process is presented. The model is solved for steady state response of the system by using numerical analysis of a developed numerical method based on weighted residual method (WRM). Stability region is determined in reaction frequency versus ratio of reaction frequency to inlet mass flow plane. In addition, friction number effect on stability threshold is assessed. The effect of mass flow rate, inlet subcooling, system pressure and other important process parameters on the oscillation characteristics as well as the instability boundary are investigated.The results show that with increasing mass flow, the system becomes more stable for DWO occurrence. The critical quality of the exhaust vapor also decreases with increasing mass flow. On the other hand, the period of DWO oscillation and its amplitude increases with increasing mass flow.

In the present article, the effects of Squealer on aerodynamic performance and thermal load distribution on the blade tip region have been investigated. Experimental results are presented at Blowing Ratios of BR = 0.5, 0.75, 1.0, and 1.5. The Film-Cooling Effectiveness is measured via the steady-state Heat Transfer Measurement Technique using an IR camera. The RANS approach has been applied to compare the Film Cooling performance and Aerodynamic Losses in the plane and recessed blade tip. The experimental results indicate that, as the blowing ratio increases, the coolant jets provide better cooling coverage on the cavity surface. The numerical results show that the Plane Tip Averaged Film-Cooling Effectiveness is lower than that for the Squealer Tip. It can be observed that, for the Plane Tip and Squealer Tip configurations, as the blowing ratio increased, the area-averaged Heat Transfer Coefficient decreased by about 43% and 44%, respectively. Moreover, the area-averaged Film-Cooling Effectiveness on Squealer tip surface and Rim walls increased by 15% and 23%, respectively. Furthermore, the lower Heat Transfer Coefficient was observed at a higher Blowing Ratio on the surfaces mentioned above. The Squealer Tip geometry showed better aerodynamic performance, which results in weaker tip leakage vortex and lower tip leakage flow rate with respect to the plane tip geometry.

In recent years, the applications of under-floor air distribution (UFAD) systems have commonly increased for ventilation of office buildings. In the present study, the effects of inlet cooling air temperature from the diffusers of under-floor air distribution system have been experimentally investigated on the occupants’ local thermal sensation. In the experiments, the inlet air temperature is controlled at 12°C, 16°C and 20°C, and the inlet velocity is kept constant. Also, the room thermal conditions have been controlled at the mean temperature of 24±0.5°C and mean relative humidity of 25±2%. During the experiment, 8 healthy male subjects with common office clothing and metabolic rate were exposed to an under-floor air distribution system for 30 min and their thermal sensation and satisfaction were assessed on the basis of thermal comfort standards. Based on the results, the head and chest thermal sensations are not significantly depended on inlet temperature. But, by decreasing the inlet temperature, the thermal sensation and satisfaction of hands and feet are decreased. Moreover, the results indicated that the overall body thermal sensation is significantly depended on the sensation of the body parts with extreme thermal conditions. Also, the results show that the feeling of air movement can be increased during the time; so, the subjects reported about 75% draught discomfort after 30 min exposure to UFAD system.

In this paper, the dynamic operation of a desiccant cooling system combined with solar and ground source renewable energies has been presented. Solar energy has been used for providing the required energy for regenerating the desiccant wheel (DW), and the ground source heat exchanger (GSHE) has been exploited as an air pre-cooling component. Transient operation of the desiccant system has been performed using TRNSYS software. The potential of the system in providing thermal comfort has been assessed in hot-humid regions. Results reveal that the studied desiccant system is capable of providing thermal comfort in these regions with low-grade regeneration temperatures (lower than 75 ℃). As a new perspective, the maximum value of the DW regeneration temperature has been controlled. The effect of the DW performance and its maximum regeneration temperature has been evaluated on the behavior of the system. Regarding the results, high performance DW can increase the provided thermal comfort up to 40% and contribution of solar energy up to 14% compared with its low performance. With reduction in maximum value of regeneration temperature to 50 ℃, the achieved thermal comfort will be decreased to lower than 30%. Using the GSHE in the system enhances the thermal comfort and a specified level of that can be provided with lower regeneration temperatures. The conducted economical assessment show that in the situation in which the system entirely provides thermal comfort, the payback period is calculated to be 8.2 years.

In recent years, fluidized bed reactors have been widely used due to the uniform temperature distribution, good mixing of phases and high heat transfer rate. High heat transfer rate in fluidized beds is attributed to hydrodynamic mechanism. Therefore, in this research the important hydrodynamic parameters such as the minimum fluidization velocity, pressure drop, bed height, bubble formation and flow regime are investigated experimentally and numerically by using a two-fluid model coupled with the kinetic theory of granular flow and two different drag models of Gidaspow and Syamlal-O'Brien. The results showed that the numerical results with Gidaspow drag model are in better agreement with experimental results in comparison with the Syamlal-O'Brien drag model. So, the minimum fluidization velocity with an approximate error of 13.8% and the bed height with an average error of 9% are predictable in comparison with the experiments. Furthermore, in order to investigate the effects of particles density and thermal diffusivity on temperature distribution of a bubbling fluidized bed, particles with the same density and different thermal diffusivity coefficients and also solid particles with equal thermal diffusivity and different densities were investigated. Finally, to demonstrate the advantages of fluidized beds to receive the required hot air in industrial units, the temperature distribution in a bubbling fluidized bed was compared with the air temperature in a similar constant surface temperature simple channel. The results showed that the outlet air temperature of a bubbling fluidized bed is about 28 degrees higher than the air temperature in a similar simple channel.

Since the nuclear energy has been recognized as a useful energy, the subject of structure, operation and safety and environmental protection have also been important. in the nuclear reactors, one of the most dangerous accidents that can occur is the loss of coolant accident, that the most important of these events is the guillotine breaking in cold or hot leg coolant, which, This will melt the reactor core if it is not stopped. This paper presents one of the most dangerous accidents in reactor containments known as Loss of Coolant Accident (LOCA) in its worst condition which is called large LOCA.The specific type of large LOCA is DECL (Double Ended Cold Leg) break which means totally guillotine type of break in cold leg pipe. This modeling is performed in single volume method in AP1000 reactor which is one of the most sophisticated safe reactors that has ever been built. The conservation mass and energy equations have been used in this modeling and the modeling software applied in our analysis is MATLAB, and the results are compared with the AP1000 safety, security and environmental reports.

In the current paper, heat transfer in conduction pumping of n-hexane and n-decane dielectrics (as working fluids) using flush electrodes is investigated by conducting experimental tests. The Study has been carried out for different fluid film thicknesses and variable applied electric voltage, and the effects of various parameters such as physical properties (ion mobility difference, density and viscosity), as well as fluid working temperature on heat transfer performance of the conduction pump have been investigated. The results show that higher ion mobility difference, as well as lower density and viscosity, increases the flow rate and heat transfer in the conduction pump, due to the improvement of the vortices formation in the vicinity of the electrodes. Moreover, it significantly increases the heat transfer in the pump by creating turbulent flow around the electrodes. On the other hand, higher operating temperatures enhance the flow rate and heat transfer due to decreasing density and viscosity and also increasing the temperature gradient between the source and the destination of heat transfer. The intense heat transfer enhancement by using the conduction phenomenon compared to the ordinary fluid flow pumping through a simple duct (having no electrodes) is seen for all film thicknesses and working temperatures. Maximum observed enhancement of Nusselt number for n-hexane and n-decane are equal to 10401% and 568%, respectively.

In the present research, methods of using natural ventilation in the architecture of the buildings have been studied. Architectural elements such as: the atrium, the ceiling light aperture, the courtyard, the staircase, dome roof, duct of the instalations, and vertical channels of the building such as channel of the elevator are widely used in Iranian buildings. In the present research, the feasibility of using these elements for natural ventilation has been studied taking into account the relevant standards.Natural ventilations based on the ceiling light aperture, vertical paths such as atrium and duct, combination of staircase and duct, and pass through ventilation have been studied. For the ventilation based on vertical paths such as the installation duct or the atrium, the ratio of the path area to the opening to it should be 0.78 times of the hydraulic diameter of the path. The area of the opening of the room to outdoor should 1.5 times of the opening of the room to the path.For ventilation using the light ceiling aperture the area of the opening to outdoor is independent of buiding area and should be 1 m2. In ventilation using duct and staircase the should be an opening on the doom roof with area equal to the area of the duct.The ban on the use of the elevator canal for ventilation, the limitation of the use the staircase for exhaust air according to the relevant regulations, and the suggestion have given in the present study.

The main purpose of current study is investigation on the effect of water addition on n-heptane homogeneous combustion. Reaching the goal a homogeneous charge compression ignition engine is simulated using a multi zone model. N-heptane is used as engine fuel and its combustion process is modeled utilizing a semi detailed chemical kinetics mechanism. At first the developed model is validated against the experimental data and its accuracy in prediction of engine performance and emissions is calculated. At the next step different values of water are added to the in-cylinder mixture and their effects are studies. Thermal, chemical and dilution effects of water are studied separately in current study. The results show that water addition decreases the in-cylinder peak pressure and temperature and retards the start of combustion. When exhaust gas is not recirculated to the combustion chamber, low contents of water increase the first law efficiency and decrease the exhaust carbon monoxide and unburned hydrocarbons. In the presence of recirculated exhaust gases, low contents of water has no significant effect on engine performance and emissions. Higher contents of water cause to misfire and increase the exhaust emissions and decrease the first law efficiency sharply. The main important effect of water on engine performance and emissions is its thermal effect. Chemical effect of water on heptane combustion is negligible.

PEM fuel cells requires humidification the reactive gases before entering the fuel cell for good performance. Using a flat membrane humidifier with important advantages such as simple building, no need for electrical energy and no moving parts, is one of the best methods to humidification the reactive gases discussed in this paper. In this study, it is proposed to insert porous layers (gas diffusion layers) on both sides of the membrane, to increase the residence time gases. Therefore, by using three-dimensional and numerical modeling of the humidifier, the effect of porous layers and the effect of their properties on the humidifier performance are investigated. For this purpose, a non-porous humidifier is first modeled, and then the porous layer is inserting on the wet side, on the dry channel side, and on two sides of the membrane, and the performance of these models is compared. The results show that the highest dew point temperature of dry side outlet as well as the highest water flow rate of dry side outlet were related to the use of gas diffusion layers on both dry and wet sides, on the dry side, on the wet side and humidifier without gas diffusion layers respectively. In all cases of laying gas, with increasing porosity coefficient and permeability, dew point and water flow rate of dry side outlet increases and improve humidifier performance. The pressure drop on both dry and wet sides does not change significantly with the insert of the gas diffusion layer.

Proton exchange membrane fuel cells with a dead-ended anode and cathode can obtain high hydrogen and oxygen utilization by a comparatively simple system. Accumulation of the water in the anode and cathode channels can lead to a local fuel starvation, which degrades the performance and durability of PEMFCs. In this paper for the first time, a new design for a PEM fuel-cell stack is presented that can achieve higher fuel utilization without using hydrogen and oxygen recirculation devices such as hydrogen pumps or ejectors, which consume parasitic power and require additional control schemes. Unified humidifier is another novelty that is applied for the first time. All part of systems is compact in a module. The basic concept of the design is to divide the anodic cells of a stack into two blocks by conducting the outlet gas of each stage to a separator and reentering to next stage, thereby constructing a multistage anode and cathode. In this design, a higher gaseous flow rate is maintained at the outlet of the cells, even under dead-end conditions, and this results in a reduction of purge-gas emissions by hindering the accumulation of liquid water in the cells. The result shows that with this new design the dead-end mode condition has the same performance as open-end mode. The all performance tests were carried out at an integrated power system.

In this study, modeling of a semi-solar greenhouse has been done using Matlab software from the viewpoint of energy, exergy and economic, for the first time to the best of the author’s knowledge. This simulation predicts the four different point’s temperatures of the semi-solar greenhouse, considering mass and heat transfer between greenhouse components and the crop evapotranspiration. The results of the proposed modeling have evaluated by data recording from the constructed typical semi-solar greenhouse experimentally. Noticeable value of 19.5 ℃ has obtained for the temperature difference of the inside air and the outside air during tests. For statistical error functions of for R2, EF, MAPE, TSSE and RMSE, average values of 97.5%, 87%, 6.08 %, 〖213.4℃〗^2 and 2.1 ℃ have been calculated which shows the acceptable accuracy of the thermodynamic analysis. Moreover, exergy destruction of heat and mass transfer processes in the greenhouse system has been inspected. Considering the aim of this study as providing suitable thermal conditions for the inside air, the greenhouse air unit cost for each time step of one minute was analyzed. The unit cost of the air inside the greenhouse increases considerably by raising the interest rate from 10% to 20%. Using the technique of double layer glass separated with air filled space as the greenhouse cover, total exergy destruction of the semi-solar greenhouse decreases about 45.36%.