مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 1 (فروردین 1400)

The main part of the input energy to the cylinder of an internal combustion engine is lost by various factors, and only about one third of this energy becomes useful. Therefore, providing solutions that can recover waate heat from engine exhaust gases and coolant is remarkable and useful. In current study the effect of start of injection timing on the RCCI engine on the waste heat recovery capacity has been investigated. A CFD model is used to simulate the engine. At first, the model was verified using experimental data and then the diesel fuel start of injection timing has been changed and their effects on exergy destruction, waste heat recovery capacity, power output and emissions have been investigated. The results showed that advanced start of injection timing, increases engine efficiency and decrwases exhaust CO and HC emissions. In addition to, heat transfer exergy has increased due to the the higher in cylinder temperature, and the higher temperature has led to an increase in irreversibility due to the increased number of reactions. Advanced fuel injection timing has improved the utilization factor and higher value of utilization factor is obsereved when fuel is injected at 20 CAD before TDC

Recently, low-temperature combustion (LTC) methods have become very popular in the field of combustion and IC engines. Reactivity controlled compression ignition (RCCI) is a novel combustion concept with its own advantages. The role of these engines in rectifying disadvantages of other LTC methods is inevitable. This paper studies the influence of using various types of syngas on combustion and emission characteristics of syngas/diesel RCCI engine using CONVERGE CFD. Four types of syngas (ideal syngas composed of solely hydrogen and carbon monoxide, two different types of syngases produced by gasifiers and pure hydrogen) are selected for comparison. Results showed the possibility of using various forms of syngases as low reactivity fuel. Using these kinds of syngases compared with ideal one, results in less NOx in expense of more soot and it gets worse by increasing the fraction of syngas in premixed air. It shouldn’t be ignored, due to the presence of nitrogen in some types, the engine may suffer from weak combustion and sometimes misfire at low loads as well. Using pure hydrogen, despite of its advantages as the main part of syngas, in high quantities, notwithstanding the significant reduction of soot, causes the increase of NOx and pressure rise rate amount which are not desirable.

One of the important characteristics of the flame of liquid and solid fossil fuels, is the flame luminosity and the high radiation heat transfer rate compared to the natural gas flame. The use of gas burners instead of gasoil burners has led to lower thermal efficiency and it poses many problems to industrial manufacturers. In this study, the effect of ion and oil combinations injection with a nebulizer on luminosity, radiative heat transfer and pollutant emission of natural gas diffusion flame is investigated. The experimental results show that the injection of 1% oil combinations, enhance thermal efficiency 2.5% and reduce The temperature of the exhaust gases, production of by 12% and 13% respectively also the injection of 2% oil combinations with 1% water drops, reduce the temperature of the exhaust gases, production of and Carbon monoxide by 6%, 42% and 26% respectively. The experimental results show injection method by the nebulizer can be used as a method in gas burners to increase radiation heat transfer rate and reduce the pollutants of exhaust gases from the chimney

In the present research, combustion species where investigated in perforated natural gas burner based on Flame Emission Spectroscopy (FES). Experimental investigation is performed on a new burner test rig which is one of most popular burners used in condensation boilers through a spectrometer-fiber optic to get the flame emission of a perforated burner. Combustion species H2O*, OH*, CH* ,C2* and CO2* are detected from their chemiluminescence The emission of CO2* which has the main role in heat release rate of burner was investigated for different equivalence ratios (Φ) and burner powers of 11-16 kW showing an intensity peak in the range of Φ between 0.78 to 0.85 that corresponds to the maximum heat release rate. Emission of OH* was also investigated being the main indicator of NO regarding to its producing mechanism. Its maximum was found at the ragne of at Φ=0.78 to 0.85 for different burner powers. The similar experiment showed that OH*/CH* intensity ratio was independent of burner power as is confirmed by previous researchers. One could infer equivalence ratio from the flame emission.

Land-based gas turbines are one of the most practical turbines that use natural gas as the main fuel. Since natural gas is produced with different composition in refinery process, it can lead to change widely the combustion characteristics. Due to avoid from the effect of this undesirable variation on combustion characteristics, different methods are suggested. One of these methods is injection of Hydrogen into the main fuel (i.e. Natural gas). Hydrogen fuel has some desirable properties such as low activation energy, high reaction rate and high burning velocity which makes hydrogen as a suitable additive for the modification of combustion characteristics of hydrocarbon fuels. The main purpose of the present study is to study the effect of two different chemical kinetics, effect of natural gas composition variations, effects of Hydrogen addition into different composition of the natural gas to achieve better combustion characteristics and also, study of pressure effect on combustion properties. It is observed that in working conditions of this study, GRI3.0 kinetic has better results in comparison with AramcoMech1.3 kinetic. Also, addition of Hydrogen in a specific volumetric percentage into main fuel causes to modify combustion properties, for instance increasing flammability limits.

In this paper, the simultaneous study of the effects of the excess air ratio and the combustion mechanisms on the temperature and distribution of species in the porous medium burners with continuous porosity variation has been investigated. For this purpose, multi-step chemical kinetics have been used and their effects on the temperature profile, mass fraction of the main species and emission of pollutants for different values of the excess air ratio have been investigated. Problem-solving equations include continuity equation, momentum equations, gas, and solid phase energy equations, and chemical equilibrium equation are solved using the finite volume method and SIMPLE algorithm is used for the relationship between velocity and pressure. The results showed that for excess air ratio of 1.5, the results of combustion mechanisms have the same accuracy in predicting the temperature profile and mass fraction of the main species, and then, for additional values of the excess air ratio, the results of the combustion mechanisms Show a slight difference. This is while the greatest difference in the results is observed for the stoichiometric condition. Also in stoichiometric conditions, the NO emission rate using the GRI-3.0 combustion mechanism is predicted to be zero, and for the rest of the coefficients of the excess air ratio, its value will be of the order of magnitude 10-6.

In the last decade, renewable energy sources have been proposed as an appropriate alternative to meet the energy needs of remote rural areas. The present work focuses on the feasibility of locally available renewable energy production systems development (including solar systems and biomass resulted from manure dung). The studied rural area, Eastern Alamut, located in Qazvin province and consists of three cold and impassable villages of Pichebon, Narmelat and Dinheroud. In order to fulfill energy supply of the rural region, three scenarios have been presented and related techno-economic analyses have been done. Firstly, all heating (cooking and hot water are included) and electricity demands have been met solely by photovoltaic system. In the second scenario, heating and electricity have been supplied by biogas as well as CHP system. Moreover, the heat loss of CHP has been used to warm up the digester. In the last survey, the electricity need has been supplied by photovoltaic system along with covering heating demand through biogas. In this scenario, the required heat to operate digester is provided using the photovoltaic system. In each scenario, volume of digester and equipment of solar system have been calculated. The techno-economic analysis shows that the second scenario that has a positive Net Present Value in each discount rate (5%, 10% and 15%) and Internal Rate of Return is 24.24% 23.26% and 38.6% for Pichebon, Narmelat and Dinehroud villages, respectively, is as a practical design and economic option.

This paper dealing with a novel algorithm based on features of Fuzzy-PID controllers to estimate heat flux on the inverse heat conduction problems. The main structure of Fuzzy-PID is a PID controller in which the proportionalو integrator and the derivative gains are obtained online by fuzzy system. The input of the algorithm is the measured temperatures within the model. In each time-step, the smart controller calculates the proper heat flux in order to adjust the measured temperature with desired input temperature. The model studied a flat plate with an insulated surface and an active level that affects the variable heat flux at the time. The variation of heat flux with time can be considered to be constant, step, and triangular. The measured temperatures are obtained at the active and inactive surface with numerical simulation. The effect of noise level at the measurement temperatures on the accuracy of the proposed method is investigated. The estimations and error analysis indicate that this algorithm is very successful in estimating the different forms of heat flux with different amounts of noise and the different thermocouple positions in the wall.

With the increasing utilization of polymer electrolyte membrane (PEM) fuel cells in cars, ships and airplanes, the study of vibrational behavior of fuel cells has gained particular importance.In this paper, modal analysis of 400-watt 4-cells fuel cell with an active surface area of 225 〖cm〗^2 has been performed numerically and experimentally. Time domain method has been used to extract the global fuel cell frequencies. This method has found widespread applications for models that do not have finite element simulation before the modal test.The considered fuel cell model has been excited from several directions and the output data of the sensors were recorded at several points. Then, by interpreting the results in the time domain and using the phase response angle, two natural frequencies of the model were extracted. The results of the test showed that the first transverse and longitudinal frequency of model is 500 Hz and about 2500 Hz, respectively.Then, simulation of finite elemental fuel cell components was studied in detail.Comparison of the frequencies obtained from the test and numerical analysis showed that maximum difference is about 8%. Therefore, numerical analysis of the model with sufficient detail can adequately cover the vibrational properties of the real model. Also, the results showed that by changing the geometrical and mechanical properties of the membrane by 45%, the natural frequency of fuel cell changes through 4%. Furthermore, Removing the membrane plates, in addition to reducing the number of model elements, reduces the contact constraints.

Liquefied natural gas (LNG) is obtained by cooling the natural gas to −162℃ at the atmospheric pressure. Methane is the major chemical component of LNG which varies between 87.0–99.8% for different sources. The cryogenic power generation cycle using LNG as its heat sink is known to be one of the considerable ways for the LNG exergy recovery. A double-stage Rankine power generation cycle using the single component working fluid in each stage for liquefied natural gas (LNG) cold exergy recovery is used as a base case in the present study. To improve the recovery of LNG cold exergy, a three-stage Rankine power generation cycle has been proposed using mixture working fluid. Optimization is done using the particle swarm algorithm. The performance of three-stage Rankine power generation cycle is studied regrading to the effects of thermal efficiencies, exergy efficiencies, overall heat transfer coefficient of condensers and natural gas distribution pressure. Specific power production of the cycle is 100.45 , thermal efficiency is 12.76%, exergy efficiency is 27.92%. By decreasing the total coefficient of heat transfer, the condensers of different stages of the cycle reduce the maximum output power of the cycle with different trends. The results show that by decreasing the distribution pressure of natural gas, specific power production, thermal efficiency and exergy efficiency increases. So that their optimal values at 6 bar are 290.87 , 25.63% and 39.12%, respectively.

The use of multi-generation systems is rapidly developing in the world. Although Sabalan geothermal field is one of the important geothermal fields of Iran, the possibility of using the multi-generation systems has not yet been performed. As an attempt to fulfill the gap in the field, a new cooling, hydrogen, oxygen and power multi-generation cycle for using Sabalan geothermal wells is proposed and analyzed. In the proposed system, the double flash configuration from the Sabalan geothermal wells as the heat source is used. An organic rankine cycle is used to generate power for proton exchange membrane for hydrogen production and a LiBr-H2O absorption refrigeration system is used for cooling production. First, a simulation was done by E.E.S software and then the effects of some design parameters, such as separators pressures, evaporator temperature, pinch point temperature difference in the Rankine evaporator, generator temperature and ambient temperature on the integrated system performance are studied. A parametric study shows that the value of the thermal efficiency and cooling continuously increase with separators pressures. Also, the hydrogen production continuously decreases with the first separator while the hydrogen production increases with the second separator. According to the results, the value of the net output power, hydrogen production, cooling and thermal and exergy efficiencies of the cogeneration system are obtained as 14739 kW, 13.25 kg/hr, 10925 kW, 22.34% and 50.62% respectively.

The compressor inlet air cooling, especially in hot climates, has significant effect on the power output augmentation of the gas turbines. The typical methods of inlet air cooling mainly consist of evaporative cooling and fogging systems and chillers. The water consumption in evaporative cooling and fogging systems and power consumption and investment cost in vapor absorption and compression refrigeration systems are high. So, the purpose of this article is to investigate the possibility of using the night sky radiative cooling system for cooling the intake air of the gas turbine compressor. This mode of refrigeration is among the renewable and passive cooling technologies. In this method, the circulating water used to cool the intake air of the compressor, at night and in the exchange of radiation with the sky, is cooled using flat plate solar collectors without glass and then returns to the storage tank for reuse on the next day. The effect of parameters such as the number of collectors and their arrangement in series or in parallel on the cooling capacity of system is investigated. Also, the required storage tank volume for returned water from collectors is calculated. The results show that, the compressor’s intake air cooling for six hours a day needs a water storage tank with a minimum capacity of 1000 cubic meters. Also, by using 400 parallel collectors and a 1000 cubic meter storage tank, the amount of power produced increases by an average of 2 MWh.

In this paper, the thermal analysis of the solar wall system equipped with photovoltaic cells and phase-change materials (PCM) has been numerically investigated. For the thermal modeling of the system, the energy balance for its various components, including photovoltaic cells, air channel, adsorbent plate, Phase-change material and room are written. The validation of the numerical results is consistent with the experimental data of previous studies. In parametric studies the effect of PCM thickness, inlet air flow rate, collector width and packing factor have been investigated on the increase of room temperature and average system energy efficiency in four consecutive days. Results show that the optimal PCM thickness is 0.05 m. Increasing the PCM thickness reduces the room temperature and the energy efficiency. Increasing the air flow rate decreases the photovoltaic cells temperature and increases electrical efficiency, thereby increasing energy efficiency. However, it reduces room temperature. Therefore, the optimum flow rate of air was obtained 0.04 kg/s. Increasing the collector width, despite increasing room temperature, reduces energy efficiency, so the optimum collector width was 0.7 m. The increase of the packing factor increases room temperature and reduces energy efficiency. Therefore, the optimum packing factor was 0.5.

In swimming pools with spectators’ stand, due to differences in skin wetness, metabolic rates and clothing type of the swimmers and spectators, providing thermal comfort conditions for all residents is very difficult. Accordingly, a reasonable idea to provide the mentioned different comfort conditions is using the air curtain to aerodynamically separate the pool hall and the spectators’ stand. In these conditions,it is possible to use two different ventilation systems for these two parts. In the present study, an Olympic-size swimming pool with the spectators’ stand is modeled and distribution of velocity, temperature, relative humidity and chlorine concentration have been determined.Also, the results have been analyzed in both cases: using air curtain and without air curtain. The results show that the air curtain can significantly reduce the influence of chlorine pollutants on the spectators section, so the concentration of chlorine at spectators’ stand with the air curtain is about 0.00016 mg/m3 less than the case without air curtain.In this study,65 multi node local thermal comfort model has been used to determine the thermal comfort of individuals.In case with the air curtain, the standard deviation of thermal comfort index for first to third rows are 0.26, 0.25, and 0.28 respectively; and in the absence of air curtain, for first to third rows thermal comfort quantities the standard deviation of thermal comfort index for first to third rows are 0.33, 0.39 and 0.35, respectively.These results indicate that using the air curtain can leads the thermal sensation to be more favorable and more uniform.

The heat transfer coefficient of a fluid is one of the most important effective factors on the performance of a fluid in the heat transfer process. Due to the higher conductive heat transfer coefficient of metals than liquids, metal particles can be used to increase the heat transfer rate of liquids. Nano fluid is one of the novel and developing methods to improve heat transfer rate in heat exchangers. In this paper, the main effective parameters on increasing the convective heat transfer coefficient of carbon nano fluid compared to water as based fluid, including: flow rate and concentration, in the range of 7100 to 16700 for Reynolds which is known as turbulent flow, are investigated. The results illustrate that increasing in Re number leads to increase the Nusselt number and convective heat transfer coefficient, and moreover decrease the friction factor. It is also showed that in a constant Re, carbon nano fluid has been able to create up to 10.17% more convective heat transfer coefficient, compared with pure water as base fluid. It was found that adding Carbon nano-particles to water, initially leads to increase in the convective heat transfer coefficient of the nanofluid. This increase continues to a concentration of about 0.2 wt% of the nano-carbon, and then has a descending trend. In addition, the pressure drop due to changes in Re, was investigated too, and showed that the behavior of this curve is in agreement with Moody’s diagram.

In this research, the thermal and hydrodynamic behavior of a non-Newtonian nanofluid turbulent flow in the counterflow arrangement in a double pipe helical heat exchanger is numerically simulated. A solution of carboxymethyl cellulose powder in water with a mass percentage of 0.1% with a nanoparticle of aluminum oxide as a working fluid has been used. The CFD commercial software Fluent was used to solve the governing equations while the turbulent flow was simulated using k-ε turbulent model; the results were in a good agreement with experimental data. The effect of important parameters such as curvature, Reynolds number and volume percentage of aluminum oxide nanoparticles on the heat transfer has been investigated. The results show that as the curvature ratio increases in constant Dean (Dn) numbers, the Nu number and the coefficient of friction increases. The addition of nanoparticles of aluminum oxide to the base fluid for the flow with the constant Reynolds and Dn number increases the heat transfer and increases the pressure drop in the helically coiled tubes. The centrifugal force generated by the curvature of the coiled tube results in a secondary flow in the heat exchanger so that the heat transfer and pressure drop increased up to 35% and 30%, respectively, compared to the straight tubes. The effect of heat transfer enhancement methods on the hydrodynamic index has also been studied, so that in the helical coils, the amount of hydrodynamic index increased with decreasing curvature ratio and increasing the volume concentration of nanoparticles.

In this paper, the natural heat transfer of Rayleigh-Benard's non-Newtonian Pseudo-Plastics fluid in a tube heat exchanger with its left wall lined with a porous layer of a thickness of 0.1 is considered numerically for an unstable state of laminar. The lower wall of the heat exchanger is at constant temperature Th and the upper wall at Tc temperature (Th> Tc). The walls are left and right insulated. The equations governing the problem after dimensionless are solved by the finite element method and then the accuracy of the results is compared with previous studies. The results show that, in a large Rayleigh number (Ra=105), the average Nusselt number increases due to the fact that the natural heat transfer is more than conduction heat transfer. Also, in small Darcy numbers (Da = 10-4), the flow permeability is very low which causes a reduces the natural heat transfer convection. The results show that by decreasing the Power-law index, the non-dimensional temperature is reduced and the lowest non-dimensional temperature is obtained for the lowest Power-law index. On the other hand, with the increase of Power-law index in a constant Rayleigh number and the passage of time, the increase of natural heat transfer occurs in the tube. Also, the Rayleigh number decreases with the increase of the Power-law index to start the natural convection in the heat exchanger.

In this study, the incompressible laminar flow of non-Newtownian fluid was studied with a central thermal source in a chamber for non-field conditions and the application of a uniform magnetic field under special angles. Using similarity transformations, the system of governing the flow equations is converted to a nonlinear ordinary differential equations and then solved by finite difference based numerical method. In the present study, the effect of non-Newtonian of the fluid with the power law model with a uniform external magnetic field was investigated for power index values n=0.75, 1 and 1.4 and the results were compared with the absence of a magnetic field. It was found that the consideration of the Newtonian and Non- Newtonian index for the power-law model fluid in the presence of the magnetic field could lead to different results. The results also show that the magnetic field tends to affect the displacement flow and leads to a decrease in these flows. In this study, for the Newtonian model of power-law model, the effect of magnetic field on the average Nusselt decreases.

Heat pipes are one of the most attractive instruments for heat transfer processes. With the development of technology and reducing the dimensions of heat generating equipments, it is essential to provide solutions and innovations to increase the thermal performance of heat pipes. Thus, an experimental study has been investigated on straight heat pipes with double-ended cooling and middle heating. At first, heat pipes fabricated with sintered copper felt wick. Then, two cooling blocks installed on both end of heat pipes and a coil heater placed in the middle as the condenser and evaporator sections respectively. The experiments were conducted at inclinations 0° to 90° for heat inputs 20 to 80 W. The effects of heat input variation, cooling water flow rate and tilt angle on the thermal performance of heat pipes were studied. The obtained results for new structure of heat pipes compared with the conventional heat pipes which had one evaporator and one condenser. The results showed that by using the new cooling approach, the thermal resistance of heat pipes can be reduced significantly. Also, in low heat inputs, increasing the cooling water flow rate increases the thermal performance of heat pipes. The experimental results indicated that the tilt angle has a significant effect on the thermal performance of heat pipes. The minimum thermal resistance and the maximum effective thermal conductivity coefficient values are 0.2533 °C/W and 14072.65 W/m °C respectively, and they were observed at the tilt angle equal to 60° and for heat input of 60 W.