مجله مهندسی مکانیک امیرکبیر
سال پنجاه و دوم شماره 6 (شهریور 1399)

In the present study, with the help of numerical study, the study of the uniform heat flux formation on process tubes as the main parameter in cracking furnaces has been investigated using flat flame burners with different thermal powers. Due to the lack of experimental data for solver validation, two problems of swirl burner and channel with conjugate heat transfer of combustion gases and solid surface have been used. To carry out simulations, the chtMultiRegionReactingFOAM solver in OpenFOAM software has been developed by adding the conjugate heat transfer capability to the ReactingFOAM solver. In simulations, k-ω SST turbulence model has been used for turbulence modeling. The results of the simulations show that the use of a flat flame burner in cracking furnaces allows for the uniform temperature distribution in the furnace with the low maximum combustion temperature. Also, to create the appropriate heat flux around the pipes so that the proper temperature distribution for cracking reactions is provided, the minimum heat flux of the flat flame burners is required, which in less than that, the appropriate temperature distribution does not occur on the pipes.

Condensation occurs on surfaces in two different forms, including fines and dropwise condensation. Investigations have shown that in especial conditions, dropwise condensation could have better heat transfer than filmwise condensation. Dropwise condensation occurs on hydrophobic and superhydrophobic surfaces. The rate of heat transfer depends mostly on the preparing process of hydrophobic surface. Two main features of hydrophobic surfaces are existing micro-nanostructures and their low surface energy. In this paper, a one-step electrodeposition process is used to produce the necessary micro-nano structures on copper surfaces for the first time. A self-assembled mono-layer of 1-Octaecanethiol as surface energy reducing agent. The resulting hydrophobic surfaces are used in dropwise condensation experiments. Effects of different parameters of the electrochemical cell such as electrical current and time of process on the dropwise condensation heat transfer are investigated. The heat transfer experiments are performed on a device fabricated for this purpose. The surface of the specimens are analyzed using Scanning Electron Microscopy images and X-ray diffraction analysis. The results show that some microstructures made from copper grow on the surface. The results show that the current and process time have positive effects on the dropwise condensation heat transfer. It has been seen that surfaces fabricated in low electrodeposition time (15 and 45 sec) have a worse dropwise heat transfer rate than filmwise condensation heat transfer. On the other hand, higher electrodeposition times (135 sec) give 2-4 times higher heat transfer than filmwise heat transfer in subcooling range lower than 10 Kelvin.

A new configuration base on Sabalan geothermal wells is proposed to utilize two wells with different thermodynamic properties in Sabalan region in Iran and generate more power as well as supply of natural gas from Liquefied Natural Gas (LNG). A Kalina cycle and Transcritical CO2 Rankine cycle are using sabalan geothermal wells as a heat source and LNG as a thermal heat sink. A comprehensive parametric study is investigated to understand the characteristics of the system. The effects of the separator 1&2 pressures, pinch point temperature difference in the evaporators, the higher pressure of the Kalina cycle, the ratio of the Transcritical higher pressure to the critical pressure and pressure of LNG pump are studied on the power generation and thermal and exergy efficiencies. The results show that the thermal and exergy efficiencies can be increased by increasing separator 1&2 pressures and LNG pump. Also decreasing the higher pressure of the Kalina cycle and pinch point temperature of evaporators lead to increasing the net output power, thermal and exergy efficiencies. Additionally, exergy analysis results showed that the highest exergy destruction rate belongs to the heat exchanger 1&2. Optimization of the proposed cycle is performed by using genetic algorithm method, and it is observed in the optimal condition that the net output power, thermal efficiency, and exergy efficiency can be obtained as 30610 kW, 29.16%, and 56.92%, respectively. The results of this study indicate that the net output power and thermal efficiency is better performance compared to the previous studies.

A diffusion absorption refrigeration cycle as a car refrigerator, which uses the car exhaust waste heat as a heat source of the cycle has been simulated in this study. An internal combustion engine with a volume of 1.3 liters at different engine speeds and throttle openings was examined experimentally and exhaust conditions such as flow rate and temperature were used as the input of the cycle. Eventually, for engine speeds above 2000 rpm, there was no trouble and the evaporator temperature ranged from -0.4oC to -7.1oC. For 1500 rpm and 1000 rpm, the evaporator temperature did not reach the desired range of variations, which is the case in other reported researches. There is no available solution for the situations where the engine is running at low speed such as in traffic jam or idle condition. Therefore, a new generator was designed and simulated to solve this problem. The simulation results show that by using the modified generator, the heat transfer to the generator improves by 16.8% on average. Consequently, the cooling capacity increases by 4.7%. Therefore, the current diffusion absorption cycle is capable of performing well at the low engine speeds.

This study proposes and evaluates a new cogeneration system for zero-energy buildings. Zero-energy buildings are those in which the net electrical energy exchange between power grid and building equals zero. The proposed system is comprised of a biomass gasifier, an internal combustion engine, a double-effect lithium bromide-water absorption chiller, a backup boiler for hot water production, a gas storage tank, a hot water storage tank, and two heat exchangers. The system is supposed to provide the building with the electricity, hot water, heating and cooling requirements over the year. Besides presenting a functional strategy for the proposed system, this study evaluates the sensitivity of the objectives of the system (i.e., fulfilling the electricity, hot water, and heating and cooling requirements) to some main decision variables, including the capacity of engine, chiller and boiler, the volume of hot water tank, the start-up time of the internal combustion engine. The results indicate that the proposed system is able to support a zero energy building. The results also demonstrate that an increase in the input power of the engine helps to achieve the goal of zero-energy buildings. Moreover, it is observed that the system is most economical when the cooling capacity of the absorption chiller approaches the heating and cooling demands of the building. The results also indicate that the start-up time of the combustion engine would be more influential in the case of high electricity demand conditions.

In the present study, exergo-economic analysis of a combined solid oxide fuel cell (SOFC) with a gas turbine, a generator-absorber heat exchanger (GAX) and heating process heat exchanger for heating, cooling and power production as a tri-generation system is conducted. Also, an external steam reformer is applied to convert dimethyl ether as oxygenated fuel to hydrogen for the electrochemical process of the SOFC. The influence of the primary design parameters (fuel utilization factor and anode inlet temperature) on several variables (energy and exergy efficiencies, exergy destruction and unit costs of the power) are examined. Results show that energy efficiency of proposed system is 38% higher than standalone SOFC. It was found that the maximum exergy destructions occurred in afterburner, SOFC and recuperator. An increase in anode inlet temperature leads to reduction of exergy destruction in afterburner and fuelcell, while it has reverse effect on exergy destruction rate in recuperator. Unit cost of power is equal to 23.51 $⁄GJ at a specific condition and decreases with an increase in fuel utilization factor or increasing of anode inlet temperature. Increasing of utilization factor will increase all exergy efficiencies by 12%. The effect of an increase in anode inlet temperature on exergy efficiencies is positive but compared with the other parameter is lower and will increase them by 8%.

The fuel cell is an electrochemical energy exchanger that directly converts the chemical energy into direct current and heat. Thermal management and water management are two major challenges in designing and efficiency of polymer fuel cells, which are inherent in each other. In this paper, by manufacture PEM fuel cell and test under different conditions, an analysis will be made of how the temperature is distributed in the full cell. By studding this temperature distribution, the relationship between power generation and heat distribution, thermal parameters (temperature distribution, maximum and minimum temperature) are extracted. The innovation aspect of this paper is to achieve an understanding of the distribution of temperature in polymeric fuel cell under different operating conditions. Also, the temperature distribution has been investigated in open-end and dead-end operating modes in different pressures and stoichiometries. When the fuel cell is changed from the open-end to the dead end mode, the maximum temperature changed from the outlet section in to the input section. By increasing pressure, the importance of maldistribution control and design of a suitable cooling system will increase.

Two of the great challenges of today's world are the air pollution of cities and the depleting of fossil fuel resources which forces humankind to replace non-renewable energy sources with renewable energy sources. In both challenges the automotive industry plays a significant role. To overcome the mentioned challenges, during the last decade, the development of electric vehicles has been on the agenda for the automotive industry. At the moment, all types of hybrid electric vehicles (HEVs) are offered at relatively competitive prices by the world's top automakers. Lithium-ion rechargeable batteries play a vital role in these vehicles. The performance, safety and life of these batteries are very much affected by their operating temperature. In this study, we used the experimental data of a Lithium-ion battery cell with using the ANSYS Fluent software in the form of a two-potential model and the NTGK sub-model was simulated. Time evolution of cell voltage and maximum temperature as well as the distribution of temperature during dynamic stress test (special battery test for HEVs) in different intervals are presented. The results of the simulations showed that high temperatures as 45 C are experienced during the dynamic stress test; this can lead to fast exhaustion and safety risk.

In this paper, a cascade solar desalination unit with external reflector designed and built and experimented in eigth days. In order to increase the production of fresh water, various techniques were applied such as 1) installing a number of finns on the stairs and on the waterway to create hot spots as a new idea, 2) use of internal reflectors at the base of the stairs and 3) use of an external condenser for increasing condensation rate of produced vapor. Then, the under study system was experimented in eight configuration using combination of above techniques and the experimental results were presented as well as the results of economical analysis. The results showed use of the finns led to the most amount of fresh water although the rest of techniques significantly increased the product. It is worth mentioning that use of the finnes led to fresh water with the lowest cost of product in value of 1343.1 Rials/lit. On the other hand, the system with an external condenser had the maximum efficiency and the system with the finns has the third place among the eight study configuration.

In present study the thermoeconomic analysis of a double slope basin solar still equipped to phase change material (PCM) and photovoltaic thermal (PV/T) collector is carried out. The governing equations of problem are obtained by writing energy balance for different components of the system. The goal of governing equations solution is the calculation of glass cover temperature, absorber temperature, saline water temperature, PCM temperature, freshwater productivity and the useful heat gain of PV/T collector. Also, the output electrical power of PV module is calculated by four parameters current-voltage model. Present study numerical results are in good agreement with experimental data. The system performance evaluation is carried out from the viewpoint of freshwater productivity and energy efficiency for the sample winter and summer day of Zahedan. Results show that the freshwater productivity increases to 10.6% by increasing of mass flow rate from 0.001 to 0.01 kg/s. Increase of saline water mass of basin from 20 to 30 kg decreases the freshwater productivity to 4.8% during the day. On the other hand, it increases the freshwater productivity to 7.43% during the night. The energy efficiency is 37.5% less on winter day than summer day. The freshwater production cost is obtained 0.0314 $/l.m2.

Nowadays, using of renewable sources are popular methods to generating energy. Photovoltaic (PV) technology is one of the most popular ways to producing power. In hot days of year, which the maximum irradiation of sun is available, because of the temperature value is high, the efficiency of PV cells is falls down. In this paper, in order to decrease the temperature of PV cells, using Polyethylene-Glycol 600 (PEG-600) as phase change material (PCM), for cooling the PV panel was studied. The results show the positive effect of fins on controlling the temperature of photovoltaic cells. Moreover, in order to increasing the rate of melting the PCM, integrating some fins was investigated too. The panel with PCM included, in about 80 mins of the end of test, had a same temperature with the conventional panel, as well as panel with both PCM and fins, at the end of experiment, had a temperature difference of about 9°C compared with conventional panel. Furthermore, the maximum efficiency difference between the panel with PCM and panel with PCM + fins, were about 2.4 % and 4.6 %, respectively. This means that fins, due to the increased amount of heat exchange between the panel and PCM, has been able to play an important role to increasing the efficiency and controlling the temperature of the panel. Finally, for economic and industrial feasibility of the proposed prototypes, the economical estimation is also presented.

In this study, the effect of adding of microencapsulated phase change material (MEPCM) to pure water base fluid as a working fluid was investigated. To this end, a laboratory apparatus was prepared and used. The main part of this setup is a tube which is called test section. This tube has a circular cross-section under a constant heat flux and is equipped with 6 thermocouples for measuring wall temperature at 6 different points as well as 2 thermocouples to measure the inlet and outlet flow temperature into the tube. The effect of butterfly tube inserts was also studied and the results were compared with each other. The results showed that adding phase change material to base fluid could improve the heat transfer rate up to 41%. Also, when the butterfly blades were placed in the test section, it was observed that the heat transfer rate increased to 234% for pure water and up to 180% for the slurry of 10 wt% of the phase change material. The blades increase the heat transfer by creating turbulence in the flow and eliminating the thermal boundary layer.

Pool boiling has many applications in industrial processes. Use of nanoparticle, heater surface expansion and magnetic field applying are important and effective elements on boiling heat transfer. In this article, pool boiling of dionized water and γ-Fe2O3/water magnetic nanofluid have been analyzed on smooth and copper grooved surface at one atmosphere pressure in the presence and absence of magnetic field, experimentally. For comfirmation of results accuracy, the test of dionized water was done in three different days that had good agreement with reference relations. Nanofluid has been built in one stage and has high stability. The result showed that the boiling heat transfer coefficient of dionized water has increased in circular and rectangular grooved surfaces and has decreased in triangular grooved suface toward the smooth surface. The boiling heat transfer coefficient of nanofluid has increased 24% in circular grooved surface and has decreased 8% and 37% in rectangular and triangular grooved sufaces, respectively, toward the smooth surface. The corners existence and wettability reduction in vertical wall of rectangular and triangular grooves and the bubbles slipping cause thermal resistanse increasing toward circle groove. Two flat constant magnets have been used in two sides of boiling reservoir for magnetic field creation. By applying magnetic field with negative gradient, the boiling heat transfer cofficient of nanofluid has enhanced in circular and rectangular grooved surfaces and has reduced in triangular grooved surface. The upward magnetic force causes the formed bubbles diameter decreasing, but groove type is effective on result, too.

In this study, flow boiling heat transfer in a vertical copper tube with internal diameter of 16 mm under constant heat flux conditions and at atmospheric pressure is experimentally investigated by using water as working fluid. To validate the results and to ensure the accuracy of the experimental loop, several experiments are performed in the single-phase mode, and the results are compared with the existing relationships for single-phase heat transfer, which indicates the validity of the test section. The experiments are carried out in both empty vertical tube and the vertical tube containing a metal foam. The two-phase tests are within the range of the slug flow regime which is visualized by a glass tube placed at the end of the test section. Heat transfer parameters such as Nusselt number and Convection heat transfer coefficient are measured in different mass flow rates and heat fluxes. The results of the experiments are also compared with the experimental data available for two-phase flow in the literature. The effects of porous material, heat flux and mass flux on heat transfer parameters are investigated. It was found that in low vapor qualities and in the range of mass fluxes (i.e. 38-53 kg/m2s), and heat fluxes (i.e. 23-36 kW/m2), the inserted metal foam leads to an improvement in the heat transfer coefficient from 1.5 to 1.82 times (compared to the empty tube). It was also found that despite the use of metal foam, the slug flow regime is remained in the pipe.

In this study, the effects of the porous burners in comparison with the conventional burner on the performance of Gas-tank water heater have been investigated experimentally. A partially premixed burner with natural gas fuel is used in the experiments. Four different types of porous burners and a conventional burner have been applied. In the Results, water temperature, chimney temperature, flame temperature, emissions and combustion efficiency for different burners at various flow rates of fuel have been reported. Observation shows that the use of porous burner with 10 cm porous metallic solid reduces the time that water temperature reaches 60 oC. Also, it is observed that the conventional burner has the lowest flame temperature and the highest chimney temperature in all of the tests. Moreover, it is showed that the burner with 10 and 5 cm porous metallic solid has the highest flame temperature and the lowest chimney temperature, respectively. The burner with 5 cm porous metallic solid and the conventional burner has the highest and lowest amount of carbon dioxide, respectively. The combustion efficiency of Gas-tank water heater increases about 15 percent by using the burner with 10 cm porous metallic solid.

One of the effective approaches of heat transfer enhancement is use of fins. In this investigation, effect of five different geometries of fin on turbulent forced convection heat transfer have been studied. The governing equatios for fluid flow and heat transfer are discretized with Finite Volume Method (FVM) and solved. Also for linking of velocity and pressure of fluid, SIMPLE algorithm is employed. For modeling the turbulent flow the is applied. At first, independency of numerical method from computational domain and validation of numerical results in compare of other experimental and numerical investigation are provided. Then, geometries are investigated for fins include of triangle with different angles, trapezoidal, square and rectangular. For these geometries, numerical results for velocity vector, pressure and temperature of fluid have been presented. Finally, heat transfer results are studied with regards to nusselt number. Triangle fin with angle of 60o respect to 90o, has much nusselt number and trapezoidal fin has much nusselt number among five fins.Also it is seen that fin with 60o has maximum pressure coefficient and pressure loss for square and trapozide fins is the most.

In this paper, effect of the angle of injection on the film cooling effectiveness with square wave pulsation is investigated at various frequencies. Four angles of injection are selected at 20, 25, 30 and 35 degrees. Film cooling is used to cool turbine blades and extend life of blade. The pulsed flow is investigated at three frequencies of 2, 50 and 500 Hz. Finite volume method was used to solve governing flow equations. The SST k-ω model was used for modeling turbulence. results showed that injection angle between 20 and 25 degrees in three frequencies studied had the most film cooling effectiveness of the central and lateral line, especially in the areas far from the edge of the hole. Higher frequencies increase the effectiveness of film cooling at the lower initial distances of the hole. At far distances, the lower frequency is the most effectiveness. The largest difference in centerline effectiveness was achieved at frequency of 500 Hz , and between angles of 20 and 35 degrees, and this value was 64.3%. This value was 98.9% for laterally effectiveness . As the frequency increases, interruptions of flow-off and flow-on are reduced, and as a result, the instantaneous effectiveness also has a slower variation than the lower frequencies. The blowing ratio of 0.5 had the most value in comparison with blowing ratio of 0.75 and 1 in all angles and frequencies. The maximum difference in effectiveness, at blowing ratio of 0.5 , in comparison with other two blowing ratios, was 187.4%.

Due to an increasing need for refrigeration systems and their growing electrical demand and greenhouse gases production, using ejector refrigeration systems would be a suitable substitution for conventional cooling systems. Main drawback of ejector refrigeration systems is their low coefficient of performance. The key component to improve the cycle performance is the ejector. Prerequisite of improving ejector performance is an accurate CFD simulation for predicting its entrainment ratio. In this study, a two dimensional, axisymmetric, steady state, compressible flow CFD simulation of a supersonic ejector is performed. In the second part of this study, geometrical optimization of the simulated ejector for two different objective functions is performed. The first objective function considered was the ejector entrainment ratio. The optimization with this objective function led to 53% relative improvement in the entrainment ratio with a negligible decrease in critical pressure. The second, objective function considered was the exergy efficiency in which the optimization showed 39.6% relative improvement. The exergy efficiency is used for the first time in the literature as the objective function for optimization of ejector geometry.

Nowadays,most people spend their time in the interior spaces.Therefore,particulate pollutants in these spaces are serious threat to the health of human.Hence,the investigation on distribution and deposition of particle pollutants in indoor spaces is important for assessing indoor air quality.Due to the wide use of fan coils in interior spaces for cooling and heating,in this article the effect of fancoil airflow velocity on the concentration of micron particles of1,10and100microns in the breathing zone of seating(0.9 to 1/1meter) and standing(1.5to1.7meters)people has been studied.For this purpose,by using the computational fluid dynamics and the solver package developed by the authors in OpenFoam,the particle concentration is investigated in a room with a fancoil airflow velocity at1.5and3 m/s.The results show that large particles(100 microns)have a small effect on air quality and settle down after10seconds,in the other hand the smaller particles(10and1micron)have a major impact on ventilation conditions and they settle down after more time(about800seconds).The results indicate that the fan coil velocity increases,also the results show that more particles are filtered by the filter in the fancoil by increasing fancoil velocity and the percentage of particles deposited on the floor reduced.For example,for10microns particles,by increasing fan coil velocity from1.5to3m/s,the percentage of particles deposited on the floor is reduced from26%to17%.

In this research, a transient two-component radiation-penetration problem is solved numerically. This study aims at investigating the effect of the radiated absorbing gas density in low pressures on the distribution of the ambient temperature in terms of the time, position, and the amount of heat transferred from the radiation environment in terms of the time in two modes of radiation equilibrium and constant temperature of the environment. MDOM was used to solve the radiation problem; while, the finite volume method was used to solve the transient time penetration problem. The problem was analyzed in two transient and steady states and it was observed that the time effect of density led to the time effects on the ambient temperature distribution with a radiated balance and the effects cannot be neglected. Furthermore, by taking into account the influence of the penetration coefficient as a function of temperature, the correlation of the penetration problem with the radiation results in the dependency of the adsorbent component density on the temperature, so that the effects of the density in terms of temperature cannot be ignored. The problem analysis shows that although the radiation equation can be solved in a steady manner for the time interval of interest, one cannot neglect its time effects due to changes in the density of adsorbing gas. This effect is higher in high-mass absorbing coefficients, emphasizing the importance of considering the penetration coefficient in terms of temperature.