مجله مهندسی مکانیک امیرکبیر
سال پنجاهم شماره 2 (خرداد و تیر 1397)

The present study aims to address the effect of the presence of a flexible fin on the natural convection heat transfer inside a square cavity. A flexible fin is placed on the left vertical wall by initial tilted angle 30o from the horizontal direction. An Arbitrary Lagrangian-Eulerian method for fluid-structure (fluid-flexible fin) interaction is utilized. Based on this method, the governing system of equations for laminar fluid and heat transfer is formulated into a non- dimensional form and then solved using the finite element method and then results accuracy evaluated against previous valid studies. The results are plotted for an enclosure containing a flexible fin as well as a solid fin in the non-dimensional time interval of 0 to 0.07 and in the Rayleigh number range of 106 to 2×107 and the fin tilted angle of -10° to °. The results show that the presence of a flexible fin deteriorates the heat transfer compared to a solid fin. In other words, using an insulated fin instead of a conductive fin makes different patterns for average Nusselt number curve in a range time and causes a reduction of the rate of heat transfer. Also, the presence of a flexible fin mounted on the hot wall especially affects the average Nusselt number in the areas above the fin location and induces oscillating heat transfer patterns.

In this article, the effect of velocity slip and temperature jump on the flow and heat transfer characteristics of Al2O3 – Water nanofluid in a microchannel with insulated upper wall and constant heat flux on the lower one, is investigated using the lattice Boltzmann method. The problem is solved at Re equal to 5, for base fluid and nanofluid with 0.02 and 0.04 volume fractions, no-slip and slip conditions with 0.04 and 0.1 slip coefficients and also at 5 to 50 nm nanoparticle diameters. The results show that, in general, using the hydrophobic surfaces in addition to making a considerable reduction in wall shear stress, somewhat increases the heat transfer efficacy at uniform wall heat flux condition that can not be seen in the constant wall temperature situations. Also, it is shown that the effect of temperature jump on the average Nusselt number, is more for base fluid than the nanofluid and increases for higher slip coefficients. For nanofluid with 0.04 volume fraction, the average Nusselt number increases continuously with slip coefficient but, for base liquid, firstly it increases and then decreases.

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.

The temperature distribution in the laser-irradiated biological tissue is investigated considering heat convection. The first- and the second-degree burn times are predicted using estimation of thermal damage. A non-Fourier equation of bio-heat transfer based on dual phase lag model is employed. The transport behavior of laser light in the tissue is regarded as highly absorbed and effects of the phase lag times on thermal response and thermal damage are explored considering different heat convection coefficients. The Laplace transform with discretization technique and also using boundary conditions, a set of algebraic equations in Laplace domain is generated which are solved by numerical Laplace inverse transform. The results show that the highly absorbed laser light in the tissue plays an important role in the burned skin time. Also, convective heat transfer boundary condition on the surface provides different results, even by considering the natural convection on the surface, and the first- and the second-degree burns are postponed at least 0.02 second.

In the present paper, the calculation of the optimum characteristics of thin fins attached to the hot wall in closed cavities with different aspect ratios has been investigated. Free convection is predominant in the cavity. The equations of continuity, momentum, and energy are discretized by means of finite volume method and the equations will be solved by a SIMPLER algorithm. The fins are connected to the hot wall and Particle Swarm algorithm is used to optimize the location and the length of fins. In order to model fins with high heat transfer, the dimensionless diffusion coefficients of momentum and energy equations are set equal to infinity and for the modeling of insulator fins, these coefficients are considered infinite and very small, respectively. The aim is to find the optimum characteristics of the array of fins attached to the hot wall in the rectangular cavities in such a way that the heat transfer from the cold wall is minimized. The results of particle swarm optimization algorithm are compared with the reference amounts. Results show that the particle swarm optimization algorithm is capable to find the optimum characteristics of an array of fins that is not calculated by the other methods, until now. The obtained results showed that with the increase of aspect ratio, the increase of the number of fins (increase in the number of variables), the particle swarm optimization algorithm might not have the needed ability to find the general optimum. This issue was studied by some numerical tests. Therefore, it was concluded that by decreasing the number of variables (Fixed location) and finding only the length of each fin, and also by increasing the number of particles in the sample space, the accuracy of the algorithm can be increased.

The main aim of the present study is to investigate the effects of speed and place of ceiling fans on thermal comfort parameters (predicted mean vote and predicted percentage of dissatisfied) and energy expenses in two different office rooms with a certain geometry in winter using district heating system. In all of the models, the regime of a flow is turbulent and governing equations are solved by finite volume method and SIMPLE algorithm. Based on the results, using ceiling fans in winter (heating system) has a considerable influence on the improvement of buildings thermal comfort and reducing consumption of energy. By using ceiling fans the effective temperature of room increases and therefore radiator energy consumption decreases. Furthermore, the results show that the location of ceiling fan does not have any effect on room effective temperature and residents’ thermal comfort. According to the results, predicted mean vote and predicted percentage of dissatisfied parameters improve by turning ceiling fans on and increasing fan normal air velocity. But, the operation of fan leads to cooling system after a certain velocity and it is not acceptable for the heating system. Finally, the CF.A case with p(x)=1.0 m, V=0.2 m/s and Tradiator=51oC, and CF.B.1 case with p(x)=2.5 m, V=0.27 m/s and Tradiator=37oC, by providing thermal comfort conditions and reducing consumption of energy about 58% and 73%, respectively are reported as optimal cases.

The use of solar greenhouses has increased manifold over the last two decades and this is the main reason for thermal systems development in greenhouses. Most of the energy sources are not reliable because of the environmental problems and continuous climb in energy prices, thus using renewable and pure energies such as solar energy in the thermal systems is very important. In this paper, the use of phase change materials in the heating system of a model solar greenhouse in Dezful is analyzed and discussed experimentally. A model greenhouse with a ground area of 3 m2 has been coupled with a heating system that consists of two flat collectors and a water store that contained 18 kg paraffin wax (latent heat 190 kJ/kg and melting point 55ºC) as phase change material. Temperature variations of soil, greenhouse, and ambient were evaluated to indicate the thermal performance of this energy storage system. Results of this study indicated that the maximum average temperature of a store during energy storage is 67ºC . Over the night in the heat dissipation cycle, the rate of heat transfer and the temperature difference between inlet and outlet of the store is higher in earlier hours in comparison with later hours due to the high-temperature difference between store and greenhouse. The minimum night time temperature of greenhouse rose by 3ºC and nighttime greenhouse average temperature increased by 4ºC. Furthermore, a 6-8ºC increased in temperatures of soil in different depth was achieved.

The effects of magnetic fields on combustion to control and optimize the flame deformation and the flame brightness is a well-known fact. The kinetics and equilibrium properties of chemical reactions of combustion are influenced by the magnetic force exerted on paramagnetic species. In this study, the numerical consideration of the effects of the uniform magnetic fields on one stage methane combustion reaction has been taken. With respect to this fact that NO, OH, and O2 are paramagnetic species and other species and methane have diamagnetic behavior, the effects of the uniform magnetic field at different pressures on 10 methane combustion main product species are studied by minimizing the Gibbs free energy. The results show that the uniform magnetic field at 1 atm pressure has considerable effects on paramagnetic species and their production is influenced dramatically. Also, the role of uniform magnetic fields on product species decreases by increasing the pressure. The results also indicate that uniform magnetic fields decreases the mole fraction of NO simultaneously with the increase in the temperature. Furthermore, applying uniform magnetic field and increasing the pressure reduce NO and CO pollutants and increase the temperature.

In this paper, the forced convective heat transfer of Alumina/water nanofluid is experimentally investigated in uniform-temperature curved tubes in the range of 0

Renewable energy is defined as sort of energy whose producing resources possess the capability to renew through nature during a short period of time. The analytical model of water injection into geothermal reservoirs process which is used to describe more complex matters, can explain the heat transfer processes in the porous media better. The presented corresponding studies so far are based on numerical and semi-analytical methods while here, a fully exact analytical solution is introduced considering phenomena of convection, conduction and dispersion inside fractures, conduction inside matrix blocks, and matrix-fracture heat transfer. In this regard, geothermal fractured reservoir related heat transfer equations are solved, ignoring fracture dispersion and heat conduction phenomena, which appears to be an appropriate assumption in high injection velocity values and then the effects of injection water velocity and distance from injection well parameters on the amount of error percent of these two models are investigated. Moreover, thermal recovery efficiency is employed to investigate cold water flooding into such reservoirs followed by a comparison to a numerical model for the purpose of validation.

Proton exchange membrane fuel cells with a dead-ended anode and cathode achieve high hydrogen and oxygen utilization by a comparatively simple system. In this paper, a new design of proton exchange membrane fuel cell stack is presented. The basic concept of the proposed design is to divide the cells of the stack into several blocks by conducting the outlet gas of each stage to a separator and reentering to the next stage, thereby constructing a multistage anode and cathode. In this design, a higher gaseous flow rate is maintained at the outlet of higher than 85% of 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 the dead-end mode condition has the same performance as an open-end mode. The stack power at the current density of 1200 mA/cm2 is 2.5kW and the voltage of all cells is bigger than 0.6V. This means that the stack can achieve to the power higher than 3kW, although all cells voltage is higher than the restriction voltage of 0.4V. Furthermore, the optimum time for opening and clothing of purge valve are 2 and 4s for anodic cells and 2 and 6s for cathodic cells.

In the present study a two-stage thermoelectric heater and a two-stage thermoelectric cooler are analyzed and compared from the perspectives of the first and second laws of thermodynamics. Based on the first law analysis results, for both two-stage thermoelectric heater and cooler, the coefficient of performance optimizes by current variation. The optimal value of coefficient of performance decreases with the hot and cold junctions temperature difference increasing. Based on the exergy analysis results, the exergy efficiency optimizes by the current variation same as the coefficient of performance. Moreover, for the case of heating, increasing the hot and cold side’s temperature difference increases the exergy efficiency but for the thermoelectric cooler leads to exergy efficiency reduction. Generally, the amount of exergy efficiency for two-stage thermoelectric cooler is lower than that of two-stage thermoelectric heater. Results show that optimum exergy efficiency of two stage thermoelectric heater is 0.181, 0.193 and 0.208 for hot and cold side temperature differences of 15, 30 and 45 K, respectively. Also, the optimum exergy efficiency of two stage thermoelectric cooler is 0.096, 0.073 and 0.04 for the same temperature differences between hot and cold side temperature differences.

In this study, the optimal performance of an irreversible regenerative Brayton cycle is sought through power maximization using the finite-time thermodynamic concept in finite-size components. Optimization is performed on the maximum power as the objective function using a genetic algorithm. In order to take into account the time and the size constraints in the current problem, the dimensionless mass-flow parameter is used. The behavior of the system parameters, such as maximum output power, exergy, exergy destruction, first and second law efficiencies, and effectiveness of the heat exchangers are investigated using the dimensionless mass-flow rate parameter. The influence of the unavoidable exergy destruction due to finite-time constraint is taken into account by developing the definition of thermal exergy. According to the results, the external exergy destruction increases and consequently the second law efficiency and heat exchangers effectiveness decrease with an increment of the dimensionless mass-flow rate parameter. However, as the dimensionless mass-flow rate parameter tends to zero, the efficiency and the power of the system approaches Carnot efficiency and zero value, respectively. Finally, the improved definitions are proposed for the heat exergy and the second law efficiency which will be compared with the conventional definitions and then their cumulative effects on cycle’s performance will be discussed.

In the present study, a MATLAB computer code has been prepared for the simulation of a multi-effect desalination unit with thermal vapor compression. The first step is to obtain the parameters’ effect on the performance, including motive steam flow rate, temperatures, and dimensions of the system. Comparison of the present simulation results with the data reported for an actual desalination system shows a good consistency. System performance in two different cases of extracted secondary vapor from the last effect and all effects is investigated. It is observed that a higher performance ratio and specific heat transfer area are obtained by receiving secondary vapor from the last effect. Finally, the genetic algorithm is used to maximize the performance ratio of the system which is considered as the fitness function. Optimization results show that one can achieve a performance ratio higher than 17 and specific heat consumption less than 107 kJ/kg for a system with 10 effects.

This paper presents a new rotary flow control valve with cam-nozzle structure that consists of a pressure compensator valve with an electronic actuator. This configuration installed in a turbojet engine fuel control system. This valve actuation is accomplished directly with a rotary servomotor. The purpose of this new design is to modify and optimize of a single speed turbojet engine performance for new missions. The rotary actuator selection, rotary metering valve design with high travel, direct drive rotary metering valve design and the special metering flow area are the innovations in the presented design. Because of rotary direct drive metering section, versus usual methods, number of parts(15-20%) and cost and hence weight of system are decreased. The fuel metering area is the lateral area of a cylinder with variable height. The mathematical model and simulation of the system is performed to obtain optimized design parameters. After manufacturing the prototypes, they are tested on a special stand for evaluation of system performance and adjusting the fuel system. The test results compared with the simulation results. Maximum (5%) deviation between model and test results shows that the model is accurate for prediction of system function.

One of the major parameters which affects fuel cell performance is the ohmic loss due to electrical resistance among fuel cell components. Assembly and design parameters affect the pressure distribution on gas diffusion layer. In this study, the influence of effective parameters such as the amount of clamping force, sealant groove depth and the thickness of end plate on the uniform pressure distribution over gas diffusion layer were investigated. By decreasing clamping force, the amount of end plate deformation decreases and uniform pressure distribution on gas diffusion layer increases. By reducing pressure on the gas diffusion layer, the possibility of leakage increases. By using an experimental sealing test, the minimum compression stress over washer for no leakage condition was achieved to be 2 MPa. According to the gas diffusion layer manufacturer, the most efficiency was achieved in 1 MPa compressive stress. Furthermore, the influence of effective parameters on the uniform pressure distribution over gas diffusion layer was examined and discussed. Finally, optimum parameters were obtained using radial basis function neural network and Bee algorithm.

Multi-phase fluid flow through porous media is strongly dependent on interfacial tension of immiscible fluids. Fluid mechanics will be affected by changing the interfacial tension. This paper describes a new approach to predict the interfacial tension at the oil-water system in the presence of an ionic surfactant. This study equation is based on Butler equation, often used for obtaining surface tension equations at different interfaces. The Debye–Hückel theory is used to determine activity coefficients of surfactant in the bulk phase. Cationic surfactants, including decyl trimethylammonium bromide (C10TAB) and dodecyl trimethylammonium bromide (C12TAB), are used to validate the equation. The new final equation can properly describe the alkane-water interfacial tension in the presence of single surfactant solutions. In this study, alkanes, including hexane, heptane, octane, decane, dodecane, and tetradecane are considered as the oil phase. The following parameters are obtained by curve-fitting: 1- molar surface area, and 2- bulk-surface distribution coefficient of surfactant. The alteration of equation parameters at different alkane-water systems is discussed. The newly developed equation is in a good agreement with the literature experimental data. This approach can be particularly important in the practical use of surfactants for the reduction of oil-water interfacial tension when experimental data are rare.