مجله مهندسی مکانیک امیرکبیر
سال پنجاهم شماره 6 (بهمن و اسفند 1397)

Nowadays, a lot of efforts have been spent to develop effective automotive air conditioning systems due to their critical role in passengers comfort. In this study, a transient numerical thermal model has been developed for the first produced vehicle based on the national vehicle platform. At first, each component of the refrigeration cycle is modeled. In the next step, thermal loads for the vehicle’s cabin are calculated along with the solar load at every location of Iran and every hour of the day. The proposed model is used for thermal analysis of the heating, ventilation and air conditioning performance of the first produced vehicle based on the national platform. Also, for thermal analysis of the heat exchangers which are used in the model, ε-NTU method has been used. The proposed model can be an effective computer-aided engineering tool for analyzing the performance of automotive the heating, ventilation and air conditioning systems. Finally, the amount of 5.239 kW thermal load acting on the vehicle cabin is calculated. Also, results indicate that the proposed model can reduce the vehicle cabin temperature from 60 °C to 25 °C within 25 minutes.

In the indoor swimming pools, due to the presence of people with different individual characteristics, it is important to design an air conditioning system which can provide thermal comfort conditions for the occupants. Since the differences in the persons’ individual parameters such as gender and body composition, it is required to study the effect of these parameters on the thermal comfort in swimming pools. On the other hand, the location of the air inlet diffuser is one of the effective factors affecting the thermal comfort conditions. In this study, by using the individual 3-node thermal comfort model, the effect of individual parameters on the thermal sensation of people in an indoor swimming pool has been evaluated. The results show that women are more sensitive than men to the cold conditions. In addition, it can be seen that BMI has a significant impact on people thermal sensation. So, thin people are colder thermal sensation than those with healthy physical fitness, and overweight people have a warmer sensation. Based on the results, with the change in body mass index, the index of the thermal sensation can change up to 0.4 units. This is very important in the evaluation of thermal comfort.

In the present study, a software has been developed using the unsteady Reynolds-averaged Navier–Stokes method to simulate time variation of turbulence coherent structure and reduce computational cost and it is used for numerical simulation of a jet in the cross nozzle flow, accurate determination of the flow structure and nozzle fluidic thrust vectoring. Since this software can capture time dependent physics, in this paper, its capability has been investigated in the simulation of pulse jet in nozzle cross flow and variation of the fluidic thrust vectoring for three frequencies, 50, 100, and 200 Hz. Firstly, software validation has been performed by comparison the results with some experimental data, next, variation of jet in cross flow and its effect on the exhaust flow field and nozzle surface pressure distribution have been studied. Governing equation on the unsteady Reynolds-averaged Navier–Stokes algorithm has been explained and time step and applied boundary conditions have been presented. The Finite volume approach has been used for numerical discretization and the advection upstream splitting method has been utilized for flux computing. Also, to improve the solution accuracy, the multi-block grid and 2nd order monotonic upwind scheme for conservation laws method have been applied and to reduce computational time, the open multi-processing parallel approach has been used.

: In the present research, the problem of mixed convection flow of a non-Newtonian nanofluid in a lid-driven cavity is simulated using a two-phase mixture model. Nanofluid of water-copper in this problem shows a shear-thinning behavior. To study the effects of non-Newtonian fluid with power-law model on the amount of heat transfer, various power-law indices are considered. After applying the governing equations and related models in the computational code, its validation is done by simulating the problem with Newtonian and non-Newtonian fluid behavior and comparing the results with those of other researchers. Afterwards, simulation of the problem is accomplished for the Richardson numbers of 0.001-1 and power-law indices of 0.2-1 while the volume fraction of nanoparticles alters from 0 to 0.09. The obtained results show that the increase in Richardson number decreases the amount of heat transfer. For all Richardson number decrease in the power law index leads to a decrease in the average Nusselt number. Variation of volume fraction from 0 to 0.09 at the power law index of 0.2 leads to an approximate increase of 15.75% and 17.32% in the average Nusselt number for Ri=0.001 and 1, respectively.

In the present study, the effects of plasma actuator on the flow and thermal fields of the film cooling through a single row of inclined fan-shaped hole over a flat plate model have been investigated. Numerical simulations have been conducted for analyzing incompressible, turbulent and steady fluid flow. Simulations were implemented using non uniform structured grid and Low- Re k-ε turbulence model. The present computed results have been compared with the numerical and experimental results and it can be seen that the gained film cooling effectiveness distributions on the flat plate agree very well with them. Then, at constant degree injection angle, length-to-diameter ratio and density ratio, investigation has been implemented, with different blowing ratios of 0.25, 0.5 and 1, applied voltages of 0, 16 and 24kV and different velocities of 4.5, 9 and 45m/s. Based on the results, fan-shaped injection hole creates better film cooling performance. And also, the effect of plasma actuator on film cooling effectiveness is better at lower blowing ratios and this improvement becomes more obvious at higher applied voltages. Also, its performance is better in low velocities. Finally, the best situation for improvement of effectiveness is in higher voltages, lower blowing ratios and low velocities.

This study is an experimental effort to investigate the heat transfer from the asymmetry concave surface. For this purpose, an asymmetric cylindrical surface with curvature radiuses of 8 and 12 cm has been considered. Constant heat flux of 2000 W/m2 is applied on the concave surface using a silicon rubber heater mat. In the steady-state condition, the temperature distribution of the concave surface is measured with an infrared camera. The asymmetric distribution of Nusselt number is compared with two symmetrical concave surfaces with curvature radiuses of the 8 cm and 12 cm. In this study, the effects of jet Reynolds number and jet to surface distance on Nusselt number distribution are investigated. The study of asymmetric flow and heat transfer in symmetric and asymmetry surfaces have been carried out for three Reynolds numbers of 23000, 35000 and 50000 and three jet-to-plate distance of 2, 4 and 6. Results show that the concave surface with the curvature radius of 8 cm has more values of Nusselt number distributions in comparison to the surface with the curvature radius of 12 cm. The present results confirm that the Nu distribution is asymmetry along the S axis. In the axial direction, symmetry distribution is observed for the Nusselt number. Also, by reducing the distance of jet from the surface the Nusselt number increases across the asymmetric concave surface.

Natural convection heat transfer of Al2O3-water nanofluid with variable properties in the turbulent flow inside a cavity with a heat source and heat sink on the vertical walls is studied numerically. Base fluid viscosity, thermal conductivity, and viscosity of nanofluids, are a function of temperature and volume fraction. The governing equations in the two-dimensional space are discretized using the control volume method. Turbulence computations are performed using the k-w-SST model. The results show that change in the placement of heat source and heat sink and Raleigh number have the effect on streamlines and isotherms. For Rayleigh numbers 107 and 108, the Nusselt number increases with increasing volume fraction of nanoparticles to 1%, and then decreases with increasing volume fraction of nanoparticles. Also, for some cases it is observed that the Nusselt number of nanofluids is less than the base fluid and therefore in these cases using nanofluids for enhanced heat transfer is not proposed. For Rayleigh numbers 107 and 108, the least Nusselt number occurs in top-bottom case, and the most Nusselt number occurs in bottom – bottom case.

Developing new energy storage systems using Phase Change Materials (PCMs) have been recently attracted considerable interest, since these materials during phase transition, could absorb and release energy at a constant temperature. Due to this feature, they are widely used in energy systems. In this investigation, the effects of applying nonuniform magnetic field with negative and positive gradients on the heat transfer and also on the development of solidification and melting processes of a non-electrical conductive magnetic nano-fluid as a PCM in an enclosure in the presence of different magnetic fields have been studied numerically using single phase homogenous model and control volume technique. In the present study, the enthalpy-porosity method has been used for analyzing the solidification and melting process of phase change materials enhanced with nanoparticles (in this study Fe3O4 nanoparticles have been used). The obtained results show using nanoparticles and applying magnetic fields increase the development of solidification and melting processes. Due to heat transfer through vertical walls, the effect of the magnetic field with positive and negative gradients only in the y-axis direction has been investigated. Since the magnetic field is applied only to the mushy zone, a magnetic field with a negative gradient in the vertical axis direction will have the greatest effect on the progression of the solidification and melting process so that the time of these processes will be decreased.

In recent years, homogeneous charge compression ignition engines are a promising idea to reduce emissions and fuel consumption of internal combustion engines. Thus many researchers and industries have focused on this topic. Combustion phasing control is the main problem in the mentioned engines’ commercialization. There are some key parameters affecting the combustion characteristics. A single zone thermodynamic model can investigate these effects numerically. In this paper, a single zone thermodynamic model is developed and validated with experimental data. The developed model includes detailed chemical kinetics. This model is used to study the effects of inlet temperature and pressure, equivalence ratio, exhaust gas recirculation rate, inlet air humidity and engine speed on the start of combustion and its duration. The fuel was pure methane and the start of combustion is defined at a crank angle where 5% of the fuel is consumed. As a result, an interpolated relation was introduced to be used in control-oriented models. The third derivation of pressure due to crank angle is either calculated as the most important start of the combustion indicator in control utilization. The results show the proper accuracy of the model and the introduced relations. The effect of inlet air humidity on the start of combustion is negligible.

The previous researches show the great effect of soot on natural gas flame radiation. The addition of soot into methane flame is widely proposed by the researchers as a good method to enhance radiation from non-luminous flames. But the effect of adding soot on the chain reactions of methane combustion and NO & CO emission have been not reviewed. The present study investigates the effects of adding gaseous soot on the progress of natural gas combustion and NO & CO emission in the zero-dimensional zone by means of GRI 3.0 mechanism in the Cantera python package. Also, methane incomplete combustion products are considered as an additive to control the pollutants. The results indicate that the addition of gaseous soot into natural-gas flame leads to increase in adiabatic temperature, the rate of decomposition reactions of fuel and brings more CO and NO production. In the other hand, the addition of methane-air incomplete combustion products into natural gas reaction cause to reduce CO production and decrease in the adiabatic flame temperature. So, in order to enhance radiation of methane flame and pollutants reduction the additive that composed of of gaseous soot and methane-air incomplete combustion can be used.

In most cases, the effect of radiation is not taken into account in enclosures in which convection is taking place. However, this needs to be addressed given the many applications of radiation, including nuclear reactor design, furnaces, electrical coolers, and solar collectors. The combined convection-radiation heat transfer on the moving surface of a square enclosure with laminar flow and gray surfaces, absorber, emitter, and radiation isotropic scattering was solved by the finite volume method. The effects of scattering and emissivity of the cold wall on heat transfer, streamlines, and isothermal lines were investigated. All walls were assumed to be black surfaces except for the one on right. The radiation problem was solved by the discrete ordinates method and the absorption coefficient was assumed to be fixed at 0.1. The SIMPLER method was employed in the convection problem given the correlation of velocity and pressure fields, while the power law method was used to investigate the significance of convection and diffusion terms. The results showed although scattering does not significantly affect streamlines and ambient temperature, it reduces the heat flux through the wall thus lowering the radiation flux. The major difference in surface reflection and scattering is in the temperature distribution at the center of the enclosure away from the surface which is evident despite the change in reflection. Moreover, the greater influence of scattering on local radiation and convection Nusselt number on the lower insulation surface was shown, suggesting the cooling of the insulation surface with changing of scattering at a fixed absorption coefficient.

Scheffler fixed focus concentrator is used for domestic and industrial applications in various parts of the world. This concentrator has an elliptical frame and designed in such a way that can provide fixed focus during a year with proper rotation. In this research, design and manufacturing principles of a Scheffler standing concentrator with an area of 2.7 m2 is presented. To this end, concentrator and its receivers were designed using Solidworks software.. Thermal efficiency of a manufactured concentrator was obtained from its operation for many days with different conditions. The results show that a concentrator with an area of 2.7 m2 can provide incident radiation up to 9.82 times that on the horizontal surface at the focal point. In addition, results of the experiment show that this concentrator can provide enough heat for boiling of 12 liters of water in less than 2 hours. A thermal efficiency of 37% was obtained for a receiver with surface of black color and glass cover. Finally, the standing concentrator was used to heat 100 lit of water resulting in an increase of water temperature to 60 °C.

Film boiling has various industrial applications, especially in heat exchangers. Studying this phenomenon on complex geometries and investigating heat transfer coefficient is desired by many industries. The numerical method used here is a finite difference/ front tracking method which is developed independently for film boiling in complex geometries. The film boiling over two or more cylinders is simulated using this method. The effects of spacing, angle, and diameter are investigated for two cylinders. For the case with many cylinders, the effects of different geometrical configurations (regular and staggered) and the number of rows are investigated by calculating the average Nusselt number on each cylinder. It is observed that the cylinder spacing does not have any significant effect on the Nusselt number for the upper cylinder. However, the angle and cylinder diameter significantly affect the Nusselt number for the upper cylinder. In the regular configuration, the Nusselt numbers for the upper cylinders are relatively uniform and higher than lower cylinders. In the staggered configuration, however, the Nusselt numbers of the upper cylinders are different, non-uniform, and higher than those of the simple geometry.

The present study aimed to recognize the optimized configuration of hybrid multiple effect evaporation and reverse osmosis desalination and gas turbine cycle. To achieve this goal, first, a thermodynamic and thermoeconomic model was developed for different parts of the cycle. Six configuration for hybrid desalination plant were. In fact, one of the important goals of the present study was to investigate whether the integration of hybrid desalination plants is useful from thermodynamic and economical points of view. Two approaches were considered in the optimization study. In the first approach, the water production of multiple effect evaporation desalination plant was fixed at 70000 m3/ day and the capacity of reverse osmosis desalination was considered as 50%, 75% and 100% of thermal desalination capacity. In the second approach, the water production of multiple effect evaporation desalination plant was not fixed but the total production rate of hybrid desalination plant were given at 105000, 122500 and 140000 m3/day. The final conclusion showed that the first configuration could be chosen as the best one because it had the maximum value of exergy efficiency and minimum value of cost of water in both first and second optimization approaches.

Drying process demands high power consumption due to the low efficiency of this process in the industries. Therefore, modeling and simulation of dryers in order to optimize them and to design more efficient dryers is necessary. In this research, a cross-flow fluidized bed dryer has been simulated in order to evaluate a new design of the dryer. A mathematical model has been used to predict outlet solid moisture and temperature as well as outlet air humidity and temperature of a single story fluidized bed. The simulation results were evaluated using the experimental data of a pilot cross flow fluidize bed dryer. The comparison showed that the model can reasonably predict the process. Therefore, the simulation was used to investigate a new design of three story dryer (three sequential single story dryers). The simulation results showed while inlet solid moisture was 0.32 and outlet solid moisture for the single-story dryer was 0.19, in the same conditions, outlet solid moisture for the three-story dryer was 0.13. This means the new design can reduce solid moisture by 50 percent more, indicating the better use of hot air and subsequently more energy saving.

In this paper computerized simulation of water-cooled photovoltaic/thermal system has been investigated. The configuration of the selected system was a plate and spiral tube type. Energy simulation was done through code development in MATLAB software. The proposed model has been validated with practical data. Thereafter, overall performance of the system has been evaluated. Afterwards, parametric analysis was done in order to compare the effect of the variation of operational parameters on thermal and electrical energy efficiencies. To cover this aim, variation of overall efficiency and pressure drop have been investigated versus variation of water flow rate and the distance between tubes as well as the diameter of tubes. According to the results of simulation and analytical studies, the expected output can be specified logically with evaluation of operational and performance parameters. This procedure is a key step which should be considered for the optimal design of the system in different weather conditions considering energy efficiency improvements. According to the results of this research, for the simulated photovoltaic/thermal system following values shows the optimum amounts: water flow rate of 0.016 kg/s, the outer diameter of the water side pipe 1 cm and the distance between pipes in a range of 7 to 11 cm.

In present research, the performance evaluation of a single effect (LiBr-H2O) absorption cooling system connected to photovoltaic thermal collectors is carried out. The main components of the system include generator, evaporator, condenser, absorber, heat exchanger, pump, expansion valves and photovoltaic thermal collectors. The governing equations of the problem are obtained by writing the mass balance, concentration balance and the first law of thermodynamics for the components of the system and it is solved numerically. The validation of simulation results has been carried out with the experimental data of the previous studies. The results show that there is a desired number for photovoltaic thermal collectors which is 50 number with total area 38.5 m2. It can supply a 5 kW cooling load. Finally, the effect of various operating parameters on the daily coefficient of performance of the system has been investigated. The increase of temperature of generator, condenser and absorber increases the daily coefficient of performance of the system, while the increase of evaporator temperature decreases the daily coefficient of performance of the system. The usage of PV/T collectors besides the supply of inlet heat of generator can provide the consumed pumping power through the system and additional electrical power for other applications.

In this study by experimental measurements and mathematical methods, the energy and exergy efficiencies, wasting energies and fuel consumptions of different traditional flatbreads bakeries are obtained. The knowledge about these parameters can pave the way to find useful solutions for reducing energy consumptions in bakeries. Based on obtained results, the energy efficiencies of Sangak, Barbari and Taftun bakeries are about 21%, 12% and 5%, respectively. On the other side, the exergy analyzes for these bakeries illustrate that thermo-dynamical quality of wasted energy of flue gases is low for Sangak and Barbari bakeries. According to results, usage of insulator for side-walls and roof of oven and reducing excess air in combustion reaction, are useful solutions to decease wasting fuel in bakeries. By using insulator for walls and roof of oven the wasted energy of walls can reduce about 65%. Also by controlling the combustion reaction to perform with 5% excess air the wasted energy of excess air decreases about 90%. Finally, it was cleared that above solutions can increase the energy and exergy efficiencies of bakeries and also their can reduce the annual energy consumption of Sangak, Barbari and Taftun bakeries about 58%, 66% and 82%, respectively.

Use of thermal energy storage using latent heat fusion of phase change materials is an effective and efficient way for energy storage in solar water heaters. The present paper is an experimental study carried out at last week of August 2014, in Jundi-Shapur Industrial University of Dezful City. First, in this study, two eccentric double tube heat exchangers with different eccentricities were built and each was separately placed in the circuit of a forced solar water heater including a flat plate solar collector, a storage tank, and a hot water circulating pump. Through the internal tube of heat exchanger passes the water heated by the solar collector, while the shell contains paraffin wax as the phase change material to which copper oxide nanoparticles are added to increase thermal conductivity. This study experimentally investigates the effect of copper oxide nanoparticles and also the effect of eccentricity on the charge of paraffin wax as the phase change material and energy storage due to the change in both of these parameters. Experimental results indicate that paraffin wax containing 3%, 1% and without nanoparticles, have reached their melting points at 14:20, 15:20 and 16:20, respectively, in the heat exchanger with 1 inch eccentricity , Paraffin wax with 3% nanoparticles has reached its melting point at 14:20 and 15:40 in the heat exchangers with 1-inch eccentricity and 0.5-inch eccentricity, respectively. . It is worth noting that lowering the heat exchanger internal tube and adding nanoparticles significantly improve the fusion (charge) of paraffin wax and also thermal energy storage.