مجله مهندسی مکانیک امیرکبیر
سال پنجاه و دوم شماره 2 (اردیبهشت 1399)

In this work, the effect of using the mixture of a phase change material (PCM) and CuO nanoparticles as a cooling agent on the performance of a photovoltaic module have been investigated experimentally. The phase change material that located in a chamber at the backside of module is cooled with spiral cooper tubes. Results show that using pure PCM significantly causes to decrease in the surface temperature of module from 58.34 ºC to 51.7 ºC. In addition, data depicted that adding CuO nanoparticles to the pure PCM results in increasing the cooling efficiency and the produced power. Increasing the weight of pure PCM and the mixture of PCM-4% CuO from 1 kg to 2.25 kg results in decrease in the surface temperature of PV module from 51.7 ºC to 48.1 ºC and 45 ºC to 42.9 ºC, respectively. In addition, by increasing the nanoparticles in PCM, cooling efficiency and the produced power are increased and the highest values are 22.87% and 3.46 W attributed to the layout of using 2.25 kg PCM and 4% CuO.

Present study is a basin solar still system that a curvy inverted reflector is connected to the basin’s below and some photovoltaic cells are inserted on glass condenser. So, the system produces fresh water and electricity, simultaneously. By writing energy balance for different components of the system such as expressions for PV cells temperature, glass condenser temperature, water temperature, and absorber temperature are obtained. Also, the system thermal and electrical efficiency are introduced. Present study simulation results are in consistent with experimental data of the previous studies. Parametric study results show that increased water depth reduces fresh water productivity and it’s effect on electricity production is negligible. Increasing PV cells reduces fresh water productivity and raises electricity production. Increased wind velocity increases fresh water productivity and electricity production. Increase in Basin area increases fresh water productivity and electricity production. Also, increased PV cells increases electrical efficiency and reduces thermal efficiency, so overall, it decreases system overall efficiency. Water depth effect on electrical efficiency is negligible but, it decreases thermal efficiency and the overall efficiency of the system.

The purpose of the present study is numerical simulation of flameless burner with carbon dioxide injection into the oxidizer stream using open-source openFoam software. Also, the effect of different amounts of oxidizer preheating temperature has been studied. In order to perform simulations from the PaSR combustion model, the standard k-ε turbulence model with modified coefficients and discrete phase radiation model with calculation of adsorption and emission coefficients using WSGGM model with non-gray gas coefficients have been used. The results of the present study indicate that the physical effects of carbon dioxide degradation will reduce the amount of heat release and the amount of carbon monoxide produced, while the chemical effects of this injection result in a significant increase in these amounts. Increasing the mass fraction of injection from 0.25 to 0.75 leads to a change in the maximum mass fraction of carbon monoxide produced from 0.05 to 0.072. Also, the chemical effect of the injection changes the flame structure and increased carbon monoxide emissions by increasing the preheating temperature. The chemical effect of CO2 injection on the production of NOx pollutants is such that, with increasing temperature, the amount of NOx emission is increased.

In this research a two-stage cascade refrigeration cycle with different refrigerants thermodynamically is analyzed and then every component of cycle with the approach to achieve the volume less is studied. Functional variables are including evaporation temperature, pressure ratio and the amount of input work in both high and low temperature cycle and cooling capacity as stated in issue is considered. With the change of refrigerants in high and low temperature circuits, variation the volume of the components and change the coefficient of performance of the system is investigated. The results show that the lowest volume of the system with refrigerant R-134a use in high temperature cycle and R-508B and R- 23 in low temperature cycle archives and the total volume of system reduce. It is observed that at low evaporation temperatures the compressor volume is highly dependent on the amount of cooling capacity. As for the evaporation temperature 173K, with increasing of cooling capacity from 100W to 200 W, the compressor volume from 9100 cm3 to 3.2 times increases. It is also seen with increasing the evaporation temperature, the volume of air cooled condenser volume of condenser reduces and it is observed that in the evaporation temperature 173K, with increasing of cooling capacity from 100 W to 200 W, the volume of air cooled condenser system in two-stage cascade increases from 4500 cm3 to 13000 cm3.

Future concerns for the limitation of fossil resources and the reduction of environmental pollutants will extend the use of renewable energies such as geothermal energy plus solar energy. The presence of this energy in the country makes it suitable for heating or cooling residential and commercial spaces. In this paper, the thermal load of a home built according to energy efficiency standards is simulated. The air conditioning system based on the geothermal heat pump and the solar collector is modeled on the house using the Transmission software, and the effect of using the heating element of energy recovery from waste water in the system, the technical-economic system performance in different cities of the country in terms of climate and efficiency The solar collector component is examined for the average storage temperature. The results show that the heating used by the recycling component increases the 0.9% increase in the temperature of the inlet to the heat pump in a five-year period and increases by 1.9 degrees Celsius on the average temperature of the hot water tank during one year. The system's performance in warm and dry climates is 25047 $ better than elsewhere in the country.

In this study, the effect of using a wet evaporation layer in the inlet air of the condenser on the performance of split air conditioner has been investigated. Also, based on a new contribution, the drained water generated due to the ambient moisture distillation on the evaporator coil, collected and used in the evaporation layer. The temperature of passed air through the surface of the condenser coil is reduced before entering the condenser due to contact with the evaporation layer and water evaporation, and consequently capturing the latent heat of evaporation from the air. Thereby reducing the temperature of inlet air, the temperature and pressure of the condenser refrigerant decreases. It has caused to reduce the amount of compressor work and also increase the cooling effect and coefficient of performance. Applied experimental studies and results of prototype test showed that by reducing the temperature of condenser inlet air using proposed evaporative cooling system, the average electrical energy consumption decreases more than 12% and the coefficient of performance increases about 15.1%. Also, despite the desired effects of the proposed mechanism on the coefficient of performance and system energy consumption, the results of parametric studies indicated that effectiveness of the evaporative condenser reduced due to the ambient temperature and relative humidity. So that, the system performance changes decreases with increasing ambient temperature (30.5 to 44.5 Celsius) from 18.22% to 15.05%, and also it declines from 18.14% to 6.4%, as well as reduction of the relative humidity (from 35 to 70 percent).

In the present work, a two-dimensional, transient, isothermal, single-phase and two-phase, multicomponent transport model is considered for the anode-side electrode of a PEMFC. The steady-state effects of introducing the CO-contaminated hydrogen on the cell performance were investigated. Then, the dynamic behavior of a PEMFC under CO-poisoning and the effects of air bleeding on the recovery of the output current density were investigated. The numerical results of model were compared and validated with experimental results. The results indicated that even using a low CO concentration, lead to significant degradation of the fuel cell current density (about 70% of the output current was lost within 30 min when the hydrogen is pre-mixed with 10 ppm of CO as the fuel). Injecting a small amount of air into the anode stream, resulted in fast recovery of the lost current density (by injecting about 5% air into the fuel, 80% of the output current was recovered within 2 min at 53 ppm CO). Higher air bleeding ratio only resulted in minor improvement of the cell performance.

In this article, a new power, cooling and heating trigeneration system consisting of a hydrogen-fed SOFC (solid oxide fuel cell) - GT (gas turbine ), a HRSG, a GAX absorption refrigeration cycle and a HR has been studied from a parametric and optimization perspective. In contrast with most of the previous investigations, in which the HRSG is located at the downstream of GAX, in the present research, in order to control the wasted heat, HRSG is located at upstream of GAX. The electrochemical analysis is complete for the fuel cell and the cell temperature is computed for all the working conditions of the cell. Then, the parametric study of the hybrid system, the influences of current density, fuel utilization factor, compressor pressure ratio and air utilization factor on the performance of the system are investigated. The optimization of the system is performed in the method of the genetic algorithm to determine the optimal functional points. After optimization and exergoeconomic analysis, minimum SUCP, the exergy destruction cost rate and exergoeconomic factor for the overall system is equal to 277.2 $/GJ, 40.8 $/h and 27.8%, respectively. Therefore, the increase in the components' capital costs can improve the exergoeconomic performance of the system.

Reducing of air pollutions which are due to combustion of Mazut fuel is one of the essential requirements of the power plants to prevent the environmental damages. On the other hand, toxic contaminations in the air combine with atmospheric precipitation and cause acid rain, and thus their damaging effects increase and their effects are greenhouse gas and global warming. This study investigates the mechanical and metallurgical properties of sediments ash due to combustion of Mazut fuel on torch and superheater pipes of Iranshahr power plant as well as its pollutants. Then, the factors that cause environmental pollution involve sulfur and nitrogen oxides using Mazut fuel are studied. Also, new methods of reducing these contaminations such as nanotechnology and the best solution for reducing these pollutants and preserving the environment in the Iranshahr power plant are analyzed. By determining the ten main criteria, two methods of Mazut nano-emulsion and wet FGD method have been selected and are suggested to reduce the contamination of Mazut combustion in Iranshahr power plant. In addition, with the feasibility of using new technologies, an ideal method for reducing the pollution caused by the combustion of Mazut fuel at Iranshahr Power Plant has been carried out.

Seawater greenhouse using humidification-dehumidification method can desalinate saline water and utilize fresh water for the greenhouse and drinking. Many parameters affect the performance of the seawater greenhouse. In this study, the effect of the width and length of the greenhouse, the height of the first evaporator and the roof transparency parameters on the energy consumption in the seawater greenhouse were investigated with the artificial neural network method. Artificial neural networks of the multi-layer perceptron (MLP) have been used for modeling. An appropriate structure for this method was obtained and the mathematical statistics of %AARE, RMSE and R2 were used to evaluate the network performance. The existing method is in good agreement with experimental data. Using this optimized network, the effect of each parameter on the energy consumption was evaluated. Finally, a greenhouse with a width of 125 meters, a length of 200 meters, an evaporator height of 4 meters, and a roof transparency of 0.6, which produces 161.6 m3/day of fresh water and 1.558 kWh /m3 of energy consumption, was introduced as an optimal seawater greenhouse.

In this investigation experimental measurements have been done over a wide range of wave parameters, water temperatures and air velocities for . The measurements were performed over a wide range of on a large heated wave flume equipped with a wind tunnel. The effects of forced, free and mixed convection regimes on the water evaporation rate in surface gravity Waves have been investigated. The experimental results shows that wave motion on the water surface increase the rate of evaporation for all air flow regimes. Also Results reveal the evaporation rate increases with air velocity but increasing of , , has different effects on evaporation rate. For free convection regime evaporation rate increases with, , . For forced and mixed convection regime at medium values of , the leeward air flow structures which form barrier for the vertical transport of vapor, decrease water evaporation rate. Results reveal that the effect of induced turbulence at the wavy interfacial surfaces on the water evaporation increment is more than the effect of interfacial area increment percentage.

In this research, a new analytical solution of one dimensional coupled equations of moisture and heat transfer in capillary-porous body is presented. These equations are known as the Luikov system of equations. These partial differential equations are coupled and nonhomogenous, which is assumed linearly with the assumption that the coefficients of the equations are independent of space, time, and every dependent variables.In innovative method of this survey, considering the governing equations are coupled, at first , assuming that the independent equations system from each other (removal coupling), it has been resulted a public answer for equations by using the method of separation of variables. Next, the private answers will be gotten by considering coupled equations and using laplace transform method, in this survey it has been searched the effect of dimensionless coefficients such as Lu, Fo, ε concluding on the rate of heat and moisture transfer. The outcome shows the effect of Luikov number on the rate of heat and moisture transfer of capillary-porous body equations coupling. It has also been resulted the depending of phase change coefficient and this outcome has been planned in the surveys by Luikov

In present work, for the first time, gas flow with considering slip velocity and temperature jump boundary condition is studied in a heat sink consisting rectangular fins and microchannels with calculating conjugated heat transfer.In this paper, helium gas flow with Knudsen number between 0.048 to 0.06 has been studied. Heat flux applied to the bottom of aluminum heat sink is 500W/m2. The governing equation for fluid flow has been discretized using second order upwind method and solved with using Coupled algorithm in Ansys-Fluent commercial software.Results show that inlet and local Knudsen numbers decrease with increasing pressure ratio and also local Poiseuille number decreases with increasing inlet Knudsen number. Also with increasing inlet Knudsen number (reduction of pressure ratio), first the average Nusselt number decreases and then increases.In this case, the average Nuselt number decreases about 54.4% with increasing Knudsen number from 0.006 to 0.024 and the average Nusselt number increases with increasing Knudsen number from 0.024 to 0.048.With increasing Knudsen number, thermal resistance increases continuously. The results show that with increasing inlet Knudsen number, slip effects are pronounced and slip and jump temperature coefficients increase.

Use of a magnetic field, wavy wall heat exchanger and dispersion of nanoscale particles are the new methods for improving the thermal systems performance. Flow vortex, which is formed by means of chips, or appendages such as blades or fins, is very effective in improving the heat transfer rate. In this study, the effect of magnetic field and vortex generator in compulsory displacement is investigated separately and simultaneously. In the present work, a wavy wall heat exchanger is simulated in various geometries of the Vertex generator under magnetic fields in various Hartmann and Reynolds numbers filled with nanofluid. For this propose fluid flow and heat transfer were simulated using Navier Stokes, conservatism of mass, momentum and energy with incompressible flow assumption. The system of nonlinear governing equations is solved explicitly using a fluent software based on the basic pressure solver and finite volume method. Turbulent equations are discriminated using the first order upstream method, as well as momentum, continuity and energy equations with upstream second order method. The results show that with increasing Reynolds number, the Nusselt number and friction coefficient increase and decrease respectively. As the Hartman number increases, the Nusselt number increases and the friction coefficient decreases.

Inactive methods of enhancing heat transfer in the shell and tube heat exchangers, such as using micro-fins and helically coiled tubes and employing nanofluids as the working fluid, are investigated experimentally. Experiments are carried out to measure the pressure drop and the heat transfer coefficient of the Al2O3-water nanofluid flowing in the shell and helically coiled micro-finned as well as smooth tube heat exchangers. The performed experiments are for the Dean number ranging from 500 to 4000, the fin helix angle between 18 and 25º, and the nanofluid volume fractions of 0, 0.5 1%. The overall heat transfer coefficients of the heat exchangers are calculated using the Wilson plot method. Empirical correlations are proposed for the heat transfer coefficient of nanofluid through the tube-side in terms of the Dean number, the fin helix angle, the fin height and the volume fraction of nanofluid. Based on the experimental results, the microfining of the coiled tube and increasing the microfin helix angle and using nanofluid enhance the heat transfer and increase the pressure drop.

This study focuses mainly on employing trombe wall systems to provide the heat required for restoring the desiccant wheel and investigating the optimal surface area of the wall for attaining air conditioning comfort. In this study, a solar desiccant wheel was modeled that receives the thermal energy required for regeneration from a trombe wall. In this system, first, the components of the desiccant wheel, the trombe wall, and the insolation were separately modeled in MATLAB and then assembled. The integrated system may be examined in all humid weather conditions around the globe. The results of the model are compared with the experimental results and have acceptable agreement with each other. The model had been developed for cooling the building in July in Rasht city by using the ground heat exchanger. A ground coil was incorporated in this system to pre-cool the process air. The optimal surface area of the trombe wall was extracted as a function of the parameters of the desiccant wheel. For a wall output temperature of 66 °C, the comfort temperature was found to be 24 °C, the humidity ratio to be 12 gw/kgra, and the optimal wall surface area to be around 52 m2.