مجله مهندسی مکانیک امیرکبیر
سال پنجاه و دوم شماره 11 (بهمن 1399)

The present study investigates and simulates the status of cold air distribution in a microwave oven hall and proposes a new method to improve the room temperature. Initially, the present hall is simulated by computational fluid dynamics method and validated using empirical data measured by sensors used in rack output. Comparison of the simulation results and the available experimental data shows very good agreement between these data. Temperature measurement with error less than 1 degree indicates the correct choice of numerical solution method in the present study. In the computational fluid dynamics method, the effect of the arrangement of the racks was investigated by changing the arrangement. The results showed that by moving the racks, better cooling conditions can be achieved. In the final step, using the computational fluid dynamics solution and neural network is proposed the best arrangement of racks. Based on the numerical simulation, the lowest and highest supply heat index are 0.456 and 0.631, respectively, and the lower the heat index, the higher the cooling efficiency. The optimal alignment is obtained from the simulation results. The average wall temperature of the racks has been used in optimization. The average temperature of the optimum alignment rack obtained from the neural network is 21.9 ° C which is 0.7 ° C lower than the best simulation.

In present work, aerodynamic performance of a straight-blade Darrieus VAWT is examined in order to use instead of conventional ram air turbines of airplane. These turbines can operate closer to airplane fuselage and it leads them to have shorter torque arm for its drag force; therefore it makes more stability for the whole airplane. In addition VAWTs generally generate their maximum output power in lower TSRs in comparison to HAWTs; this case also can reduce the possibility of shock waves phenomena on the turbine blades. In order to evaluate performance of proposed turbine, the ram air turbine of Airbus a320 is selected and also its dimensions is chosen for proposed turbine. Average of output power and drag force of proposed turbine are computed using 3D simulation and their values are compared with the values of ram air turbine of a320. Results show that proposed turbine which has small and thin endplates, produces almost equal average of output power along with 19.3% less drag force in comparison to ram air turbine of a320. In addition performance of proposed turbine also investigated while it locates in the airplane body between its azimuth angles of -30° to +30°. Although this configuration reduces clearly turbine’s emergence; but it also reduces its output power due to separation in the curved edge. Overllay, performance of proposed turbine indicates its prominent potential to use instead of conventional ram air turbines.

In this study, the effect of steady spanwise blowing on the Aerodynamic Coefficients of a Maneuverable Aircraft Wing Model have been simulated in three dimensions and investigated applying the fluent software. The simulations have been performed at the Mach number of 0.4 and different angles of attack, using unstructured grid and the k-w sst turbulent model. Numerical simulation results showed that the spanwise blowing along the wing leading edge caused a flow along the axis of leading edge vortex and delayed the vortex breakdown until the high angles of attack. As a result, the lift coefficient increases at higher angles of attack, which is directly related to the jet momentum coefficient. By applying blowing, due to the vortex breakdown on the wing surface, drag coefficient is greater than no blowing condition until angle of attack 24 degree and after this angle, the drag coefficient decreases. Therefore, drag coefficient decrease lower at greater jet momentum coefficients. By injecting the flow on the wing, the vortex increases in the different longitudinal sections and causes a greater pressure drop on the upper surface of the wing. Also, the greatest amount of pressure in the inner span of the wing and near the edge of the wing attack is observed.

Usually, ground testing of space engines is performed in high altitude test facility. The facility is equipped with a supersonic diffuser that expels automatically engine gases to the atmospheric pressure and maintains a vacuum pressure around its nozzle and motor. If the motor pressure is lower than a certain amount, the supersonic flow in the diffuser will not be established. In this situation, it is necessary to use auxiliary ejector at the end of the diffuser. In the present study, a new algorithm has been developed in the design of supersonic ejector. Unlike conventional methods, this algorithm can be used for different primary (input from the ejector nozzle) and secondary fluids (outlet from the diffuser end). In the design of the ejector, the main parameters are determined by the algorithm, while the secondary parameters are selected from the empirical test results of the proved references. In this algorithm, a safe margin is considered for the safe operation of the ejector and this margin is predicted by numerical simulation. Also, numerical simulation is used to validate the design method. Finally, using the proposed algorithm, an ejector is designed to reduce the start-up pressure of an engine-diffuser assembly. An integrated simulation of the diffuser-ejector was performed for both cases that rocket motor is off and on, and the appropriateness of the designed ejector was confirmed in both modes of operation

Two phases flows particularly motion droplets into a second fluid have variety of application in different industries including oil and gas industry, medicine and pharmaceutical, extraction of metals, power plants as well as heat exchangers. In this paper, dynamic of a Newtonian drop falling in laminar regime is studied. A fluid which its viscosity is 340 cP is used in order to produce an inertia flow. In experimental section, de-ionized water and glycerin solutions with volume concentrations of 21:79 and 17:83 are used for drop phase. By increasing the volume of drop that leads to rising the inertia force, the drop shape is changed from spherically and a dimple at the rear of drop is appeared. Inertial forces, surface tension and hydrodynamic tension play a significant role on drop shape. Increasing the drop volume causes expanding the dimple consequently drag force is enhanced and terminal velocity of drop is decreased as well. According to the experimental observations, variations of viscosity ratio does not have a profound effect on drop deformation. Moreover, increasing the Reynolds number leads to reduction of pressure coefficient. It is shown that the experimental observations have an appropriate agreement with analytical results.

In this study, the flow physics of two-dimensional liquid jets discharged into still ambient are experimentally studied. Three injectors with thickness of 0.35 mm and aspect ratios of 30, 60 and 90 were manufactured from stainless steel to produce two-dimensional liquid jets. The experiments were conducted for a wide range of liquid volume flow rate varying from 10 to 120 L/h. High speed photography was used to visualize and capture the instantaneous structure of fluid flow at different flow conditions. Based on the visualizations, the behavior of two-dimensional liquid jets was recognized and for the first time categorized into four regimes. These regimes are dripping, column, triangular and perforation. Moreover, different parameters of the liquid sheet including convergence angle, convergence length, retraction velocity and jet length were measured. It was shown that the sheet convergence length increases with aspect ratio. Also, it was observed that the convergence angle and retraction velocity are independent from aspect ratio and manifest similar behavior in all jets. Empirical relations were proposed to describe convergence angle, convergence length and retraction velocity.

Regarding the major determinant of Anastomosis angle in the efficiency of fistula for dialysis, obtuse angle Anastomosises are designed and simulated with angles of90,120,135and145 degree and the obtained results are evaluated from standpoint of flow patterns at the region of the Anastomosis and shear stress in the fistula wall. In this study, in order to compare obtained results in fistula, two Newtonian and carreau non-Newtonian blood models are used at maximum flow rate of blood pulsation curve (flow rate at the time of0.2and0.4seconds respectively). At angle of 90degrees, the formed vortices dimensions, due to the separation of the flow during passing through the region of the Anastomosis, significantly larger than obtuse angles. Consequently, the probability of deposition in the region of the flow increases sharply. from the standpoint of flow pattern, the 90degree angles are inappropriate Anastomosis angle for fistula. At the obtuse angles, the dimensions of these vortexes become much smaller, and then the obtuse angles are considered a better choice. From the standpoint of maximum shear-stress, Anastomosis with obtuse angles in comparison with the Anastomosis angle of 90degrees, has lower maximum shear stress values and the range involved in the maximum shear stress in Anastomosis with 90degree is much wider than the range of Anastomosis with the obtuse. Hence, the probability of manifestation of thrombosis (the main factor of fistula failure) is much higher. In this simulation, the results related to the Newtonian and non-Newtonian models are very close, and the non-Newtonian model predicts shear stress slightly more.

In this study, a multipurpose testing equipment with a sub-scale model body with tandem rotors configuration was used to carry out a number of experiments in order to understand the inflow and down wash behavior of these rotors. The results showed that for single rotor, inflow is descending except in central areas. By using the data obtained from the experiments, an approximate pattern of inflow and downwash of rotor was presented. This pattern was found to be similar to the pattern that was presented for conventional helicopters rotors under settling with power condition. This phenomenon is one of the reasons contributing to the formation of an unstable turbulent flow in the central areas of the rotor. The results showed that interacting of the two tandem rotors has an effect on the behavior of the rear rotor inflow and changes its behavior in comparison with single rotor. It was observed that the results obtained from this study were more realistic due to considering the effects of the body, actual fluctuations of the flow. The experiments in this study showed differences exist in the principles governing the aerodynamic performance of rotors used in UAVs in comparison with conventional helicopters rotors. Therefore, it is necessary to pay enough attention to these differences and behaviors in predicting the aerodynamic performance and design of this type of flying machines.

In this paper, the conditions of convergence and thermodynamic consistency of the Kupershtokh model for simulating a 2D droplet are investigated. Hence, the coexistence curve of liquid and vapor phases is divided into four levels according to the constant of potential function, k and the weight coefficient of the inter-molecular forces, A. Accordingly, a range is reported for k at each level. This range for level 1 with the lowest density ratio is kmin= 0.05 to kmax = 0.22 and for the fourth level with the highest density ratio is kmin=0.002 to kmax = 0.01. Also, the appropriate weight coefficient for inter-molecular forces, Afit is obtained for yielding the thermodynamic consistency at levels 1 to 4 equal with 0.25, 0.025, -0.082 and -0.125, respectively. The effect of weight coefficient on the amount and direction of interface forces is also investigated. Results show that the choice of A=0.5 produces symmetric and A=0 causes asymmetric forces in the interface. In other words, the combination of two symmetrical and non-symmetric forces provides conditions for reducing the parasitic velocities and increasing the stability conditions. Finally, the problem of mass conservation in four levels of density ratio is investigated. Results show that Kupeshtokh model has a better behavior in controlling the mass of the droplet in the high density ratios. So, the change in the mass of the droplet at level 1 is more than 20% and at the level of 4 is less than 1%.

The aim of this research is the simulation of multi-layer flows of the core-annular type in a 2-dimensional channels, in which the Newtonian fluid in the core is surrounded by a viscoplastic fluid of the regularized Bingham type as a lubricant. This simulation is based on the volume of fluid technique and in the mixing regions of the two fluids, the stress is achieved by harmonic interpolation. Flow and concentration equations are discretized spatially by spectral element method. The velocity correction scheme, as a high order algorithm, is developed for splitting the velocity and pressure variables for the viscoplastically lubricated two phase flow. Applying the developed flow considerations leads to a nonlinear equation in the plastic region of the flow, which is solved numerically and is called semi-analytic solution. This solution, along with previously published works, is used to validate the spectral element results. The effect of the main parameters of the flow, i.e. Bingham number, viscosity ratio and core thickness on the pressure drop and un-yielded region thickness is evaluated. The results show that, for constant Reynolds number, the Bingham number is the most effective parameter on the pressure drop and un-yielded region thickness, respect to the core flow thickness and viscosity ratio. Also the profiles of secondary variables, including apparent viscosity and shear stress, across the channel section is presented and show that in the interface of the fluids, there is a difference between numerical and semi-analytic solutions due to the mixing of the two fluids.

Entropy serves as a key parameter in achieving the theoretical limits of performance and quality in many engineering applications. In this paper, the three-dimensional analysis of entropy generation, local entropy generation and exergy destruction of turbine stator vane by udf code has been done. The current innovation is to measure the exergy destruction rate of the turbine three-dimensional vane with the help of FLUENT. The k-ω (SST) and Spalart-Allmaras models are suitable for prediction of proper effective viscous and thermal conductivity. Due to the sensitivity to the vane tip and the wake flows, k-ω (SST) model obtained the mean value of entropy generation on the reference line by about 85% more than the Spalart-Allmaras model. Local entropy generation has increased with respect to the scale from the root to the tip of the vane. The difference between the values of local entropy generation and the second law of thermodynamic for k-ω (SST) and Spalart-Allmaras models are 7.4% and 10.2%, respectively. Approximate turbulence coefficients have been introduced with the aid of a custom field function that increases the local entropy generation about 130% on the reference line. The k-ω (SST) model calculated the exergy destruction value of a turbine stage (according to Gouy-Stodola theorem) of 1098 kW, which is 4 times the size of the two-dimensional mode due to the scale. The values of local entropy generation calculated in comparison with the stator vane of the turbine of the authentic paper are validated, which has acceptable adaptation.

Low-density lipoprotein (LDL), which is recognized as bad cholesterol, typically has been regarded as a main cause of atherosclerosis. An abnormal accumulation of LDL in the artery wall and, as a result, in the formation of LDL oxide, can lead to atherogenesis. Therefore in present study, the concentration boundary layer of LDL in a straight artery is investigated numerically. The governing equations consist of continuity, momentum conservation and the LDL transport in the blood based on appropriate boundary conditions have been solved using one of the most powerful computational fluid dynamics techniques known as Projection method. Results are obtained and presented as profiles and contours of concentration, blood velocity and wall shear stress, which are in good agreement with numerical and analytical results of previous studies. Effects of factors such as: filtration velocity and wall shear stress on the LDL surface concentration and concentration boundary layer thickness are investigated. The results show that increasing the wall suction (high blood pressure) and reducing the Wall Shear Stress (WSS) results in an increase in the surface concentration of LDL. Increasing the Reynolds number and the Schmidt number decreases the concentration boundary layer thickness, and LDL surface concentration increase about 7% higher than that of the bulk flow.

This study, an enhanced computational coupling method is proposed for the transient problems of incompressible fluid-elastic structure interaction based on the smoothed particle hydrodynamics (SPH) method. The coupling process is conducted between an incompressible SPH (ISPH) fluid model and a totally Lagrangian SPH (TLSPH) structural model. In the ISPH method, due to the importance of smoothing particle distribution for the accurate and stable simulations with noise-free pressure field, a new scheme for particle shifting has been proposed to regulate particle distribution. In contrast to numerical errors at the free surface in traditional particle shifting algorithm, this proposed algorithm as a suitable treatment for discontinuous boundaries such as the free surface presents an optimized particle shifting scheme without need to adjust the new parameter. The performance of the totally Lagrangian structural model is evaluated using simulation of the dynamic problem. The proposed numerical coupling method was examined by simulating several benchmarks in fluid-structure interaction and the results were compared with experimental and numerical results. The considered problems of fluid-structure interaction in this paper include the dam-breaking with an elastic gate and the deflection of an elastic obstacle due to fluid sloshing. The agreement between presented results with literature data shows the ability of the proposed model to simulate the phenomenon of fluid-structure interaction.

The intake is part of the aircraft, which provides air to the engine uniformly and with a minimum total pressure loss. Today, due to the abundant application of S-shaped inlet, optimization of these inlets has been considered by the researchers. The uniform distribution of the flow at the inlet of the compressor has a direct effect on engine performance. On the other hand, the separation of the flow through the duct reduces the pressure recovery and the engine thrust. In this article, an S-duct intake has been optimized to reduce the total pressure loss and flow distortion. The neural network, coupled with the genetic algorithm, is used to optimize the objectives in the shortest possible time. Two optimizations have been done. In the first case, new geometries have been generated by changing the center line coordinate and the cross-sectional area ratios. The first optimization results in an enhancement of 32.5% for pressure recovery coefficient and a reduction of 35.8% for flow distortion. In the second optimization, the length of the duct has also been decreased. By decreasing the length of the duct, the weight of the aerial vehicle is reduced, and on the other hand, the useful space inside the body is increased. This optimization gave an enhancement of 35.96% for pressure recovery coefficient and a reduction of 39.4% for flow distortion and a 25% reduction in the duct length.

In the present study, mixing of two fluids of different viscosities in a micro channel with an oscillating curved stirrer was simulated by MRT-LBM and the effect of geometric shape, oscillating speed and amplitude and viscosity logarithmic ratio on mixing efficiency was analyzed. Most researches in this field have studied the mixing of two same fluids but the mixing makes sense when two miscible fluids have different properties. Also in these researches, stirrers in micro channel are considered cylinder or rectangle shape. In this study, a curved stirrer was used for mixing two fluids of different viscosities in micro channel for the first time. The simulation was performed at Re=80 and Sc=10 and NASA/LANGLEY LS(1)-0417 (GA(W)-1) airfoil shape was used for curved stirrer. At the study of geometric shape, results showed that mixing efficiency of two fluids of same and different viscosities in micro channel with oscillating curved stirrer was higher than micro channel with oscillating rectangle stirrer. The results of stirrer oscillating amplitude and velocity survey revealed that increase in Strouhal number causes increase in mixing efficiency on studied oscillating amplitude and Optimum efficiency is on oscillating amplitude 0.5 on all studied Strouhal numbers. Also mixing efficiency decreases with increase of viscosity logarithmic ratio on studied Strouhal numbers and this decrease is noticeable in high Strouhal numbers.

Todays, natural circulation loops (NCLs) are used in various industrial applications such as nuclear reactors, geothermal systems, electronic device cooling and so on. In a natural circulation loop, working fluid receives heat from a heat source (heater) and rejects it to a heat sink (cooler). The main objective of this paper is to investigate the effects of orientation of heater and cooler on the mass flow rate and temperature distribution in the natural circulation loop since the mass flow rate of the natural circulation loop is directly related to the heat transfer rate .The governing equations of the natural circulation loop are written as dimensionless form Cu-Water nanofluid considered as the working fluid and the effect of nanoparticles percentage on mass flow rate is investigated. Also, the effects of other parameters such as pipe diameter, height of the loop, loop inclination angle and the heater power on the mass flow rate of the loop and temperature distribution are investigated. The results show that increasing the diameter of the pipes, the percentage of nanoparticles, and the heater power increase the mass flow rate. However, the mass flow rate decreases with increasing loop inclination angle. The mass flow rate may be decreased or increased with increasing height depending on the orientation of the heater and cooler. In a constant heater power, the increase in the temperature of the nanofluid is much higher than other orientations when both the heater and the cooler are in the vertical position.

The propagation and shape of methane-air premix flame in an enclosure with dimensions of 50 × 11 × 8 cm and the effect of porous obstacle with a porosity percentage of 95 with 20 cavities per square inch in the flow path has been studied in a laboratory. The study of flame behavior has been done with high speed camera photography. One side of the enclosure is made of transparent plexiglas with dimensions of 8 x 50 cm for photographing. The pressure variations inside the enclosure are recorded with the help of a pressure sensor and converter located on top of the enclosure. The results for the unobstructed enclosure correctly represent the process of forming the classical tulip flame inside the enclosure. The range of pressure variations has been adapted to the reference samples and has been validated. The location of the porous obstacle has been tested for four different spacings of 5 cm, 10 cm, 15 cm and 20 cm from the spark location. In the chamber without obstacle, the tulip flame at a location of 25 cm from the spark and 50 ms from the time of sparking was created. The results for the four porous obstacle states indicate that the turbulence created in the flow field can change the location of formation of the tulip flame, as well as its formation time. For a distance of 20 cm the porous obstacle from the spark location, the turbulence of the flow field is at its maximum.

Designing of an efficient emergency ventilation system is one of the main approaches to prevent the perilous fire in tunnel phenomenon.One of the most considerable factors in smoke control is critical velocity.In the present work, a parameter is called a critical volume flux, which indicates at least a volumetric flow that prevents smoke from flowing upstream of the fire. In this study, fire in tunnel is simulated using Fire Dynamics Simulator code (FDS) and The effect of blower location on maximum temperature, spread of smoke and critical volume flux of fire in the tunnel has been investigated. The results show that the blower location has a significant effect on critical velocity and volumetric flux and it can reduce critical volumetric flux by at least 11 percent . Also, for a closer look by creating a variety of conditions in the blower system, the effect of fire source height has been investigated on the critical volume flux. The results also show that increasing the fire source's height does not have a significant effect on volumetric flux and critical velocity. In the study of longitudinal distance of the blower system from the fire source, in the case of back-layering smoke, the results show that the approach of the blower to the fire would reduce the smoke back-layering length and increase the maximum temperature.

In this research, the effect of anode stoichiometry, cathode stoichiometry and temperature of inlet gases on water management and performance of a PEM fuel cell is studied by means of DOE (Design of Experiments) and direct visualization. In order to visualize the liquid water accumulation in cathode flow channels, a transparent PEM fuel cell is designed and manufactured in fuel cell research laboratory of Amirkabir University of Technology. Design of experiments is based on response surface method and central composite design. Cell’s performance is recorded over the test time and a video is simultaneously captured from its transparent cathode flow channels. Then, a digital image processing technique is used to detect and quantify channel areas that are occupied by liquid water. Area of regions containing liquid water is divided by total area of flow channels to calculate a parameter called water coverage ratio (WCR) which is then used to study flooding phenomenon. Results show that increase in cathode stoichiometry, anode stoichiometry and gas inlet temperature leads to a decrease in WCR. Also, WCR lies between 1.8 and 4.3 when an optimized produced power is reached. It is also proved that anode and cathode stoichiometry has to be minimized to reach the maximum produced power at a high inlet gas temperature.

The contact resistance between polymer exchange membrane fuel cell (PEMFC) components has a crucial effect on cell performance. The geometry of the endplate, on the other hand, plays an essential role in the contact pressure distribution on the membrane electrode assembly (MEA) and the amount of contact resistance between plates. In this paper, the effect of endplate geometry on the contact pressure distribution over the MEA is simulated using ABAQUS software. In addition, a new geometry endplate is provided and the obtained results are compared with flat endplates. Geometrical parameters of an endplate with curvature (bomb-shaped endplate) are considered, and the effects of these parameters on the contact pressure distribution over the MEA are investigated. In this simulation, a 3D model of the fuel cell is developed. Our simulation results show good performances for the designed endplate and uniform contact pressure distribution on the fuel cell active area. Finally, a single fuel cell was manufactured and assembled using the simulation parameters, and experimental tests are conducted using pressure measurement films to validate the design.