جستجوی مقالات مرتبط با کلیدواژه « احتراق » در نشریات گروه « مکانیک »

In the present study, the combustion of hydrogen-air premixed mixture in a circular cross-section swiss-roll micro combustion chamber, designed as a heat source for micro thermophotovoltaic systems, has been investigated. The governing equations were solved using a three-dimensional steady state CFD method, considering detailed chemical kinetics and conjugate heat transfer. The parameters examined in this study include the average inlet velocity, equivalence ratio, and wall thermal conductivity. The thermal performance of the combustion chamber was evaluated by calculating the temperature of the inlet and outlet paths, wall temperature uniformity index, and efficiency. Results indicated that increasing the average inlet velocity leads to a deviation from the optimal heat exchange between the inlet and outlet paths and affects the wall temperature distribution. Although the average temperature of the outer wall increases with inlet velocity, the temperature distribution becomes more non-uniform. This increase in the non-uniformity reduces efficiency with higher inlet velocities. The study on the effect of the equivalence ratio showed that increasing it optimizes heat exchange between the inlet and outlet paths and enhances system efficiency. The impact of increasing the wall thermal conductivity up to 12W/m.K was analyzed, showing a positive influence on all parameters related to the thermal performance of the combustion chamber and efficiency. The maximum total efficiency was calculated to be 10.18%, demonstrating that swiss-roll micro combustion chambers can compete with other micro combustor geometries for use in micro thermophotovoltaic systems.

In this study, a three dimensional numerical simulation of an industrial entrained flow combustion reactor has been conducted. The governing equations and reactions are implemented and the operating parameters are considered according to the experimental works. The results obtained from the numerical simulation are validated by comparing with existing experimental data and similar published papers. Four different devolatilization models are investigated and the simulation results are compared to each other. The obtained results show that although the Kobayashi model has a higher calculation time, it provides more accurate results compare to the experimental results. The effect of increasing/decreasing of injected coal particle sizes are studied. The results show that increasing of the coal particle sizes from 55 to 120 μm has led to a decrease in the gas temperature inside the reactor. By reducing the average coal particle size from 55 to 30μm, the gas temperature close to the flame has increased from 1800 K to 1900 K.

In this paper, the numerical simulation of combustion in one of the industrial furnaces of Kermanshah Oil Refining Company has been carried out in order to investigate the cause of high temperature damage to its damper and provide a solution to solve this problem. At first, the current situation has been investigated. For this purpose, the numerical model of furnace and burner has been produced and numerical simulations have been carried out to check the current situation. The agreement between the actual factory data and the numerical simulation data is acceptable. In the next step, by changing the burner and changing the shape of the flame, numerical simulations have been performed in order to solve the problem. The obtained results show that by changing the burner and using 4 smaller burners but with a longer flame, the shape of the flame becomes branched and with the increase of heat transfer caused by this situation, the temperature at the place where the damper is installed decreases by about 170 degrees Celsius. This decrease in temperature can solve the problem of damaging the damper and softening the structure connecting it to the flue.
 
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Numerical simulation of turbulent flames with laminar flamelet models is not easily possible under high turbulence intensity conditions. Experimental results and direct numerical simulations show that introducing strain effects in the production of flamelet tables can significantly increase the accuracy of modeling. In this work, implementing the strain effects in the model results in 30 mm increase in the flame height relative to the unstrained model. In this work, a strained flamlet model has been implemented and evaluated in the simulation of turbulent premixed flames. The premixed counterflow flame has been used to produce the flamelet tables. These tables are used in the computational fluid dynamics solver using two reaction progress variables and the presumed probability density function method. The model obtained in this research has been used in Reynolds-Averaged simulation of a turbulent piloted premixed flame in a bunsen burner. This burner utilized a novel approach to highly increase the input turbulence intensity. The results show that the strained flamelet model predicts the flame propagation speed and consequently the flame length, significantly better compared to the unstrained flamelet model.
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Combustion process improvement as one of the most important factors in the energy supplement for different devices has attracted great attention. Recently the concept of plasma assisted combustion as a new approach to increase efficiency and reliability of combustion based systems in the different working conditions has considered. In the present paper, two different experimental setups are used to investigate plasma system effects on the operational characteristics of combustion process. At the first setup, open and confined Bunsen burner is used to study premixed stagnated flame. In this structure flame properties are measured by optical schlieren photography method based on generation and stability of the flame cone. At the second setup, constant volume chamber is used to analyze propagating flame in combustible environment. In this structure high speed schlieren photography from propagating flame front and high frequency pressure measurement is used to study premixed propagating flame. Results indicate that, in the open stagnated flame plasma effects improve the laminar flame speed, in the confined stagnated flame plasma effects improve the flame stability and in the propagating flame plasma effects reduce the ignition delay, develop the lean flammability limit and increase laminar flame speed.

Nanostructured additives have been widely used in various industries especially power industry in order to decrease both specific fuel consumption (fuel consumption per unit of electricity production, SFC) and pollutants emission. The most important fuel properties are fuel density, kinematic viscosity, flash point, cetane number, and calorific value. Calorific value of a fuel is the thermal energy released per unit quantity of fuel when the fuel is burned completely and the products of combustion are cooled back to the initial temperature of the combustible mixture. It measures the energy content in a fuel. The calorific value of a fuel can be measured in a bomb calorimeter. The most frequently used fuel additives for diesel engines are oxygenated additives, antioxidants, cold flow improvers, lubricity improvers, cetane improvers, metal-based additives, and nanomaterials. In recent years, various nanomaterials, acting as diesel engine fuel additives, gained substantial attention. According to investigation results obtained so far, these additives look to be extremely promising. This follows from the wide range of options they offer in influencing practically all important engine characteristics by affecting many processes from fuel combustion to exhaust treatment. In this study, hybrid nanoparticles were dispersed in mazut and diesel fuels as hybrid nanofuels and the physico-chemical properties of manufactured nanofuels were evaluated. Dispersion of the nanoparticles were carried out in two stages. They were weighed and mixed to surfactants and added to fuel. Then the mixture was dispersed by using an ultrasonic device.The results indicated that the rate of calorific values of mazut-based nanofuels and diesel-based nanofuels were improved to 13.41% and 14.02%, respectively, in proportion to pure mazut and pure diesel fuels. Therefore, metal oxide nanoparticles and multi-walled carbon nanotubes had a remarkable impact on combustion properties of fuels. The final price of nanofuel was up to 20-30% more than pure fuels which can be reduced when nanofules are produced in large scale. It can be concluded that the optimized samples of mazut-based and diesel-based nanofuels had all properties of an ideal fuel for using in a combustion process and could be considered as suitable alternatives to pure mazut and pure diesel fuels.

Increasing the propagation speed of the flame due to centrifugal force can lead to reducing the length of the combustion chamber and increasing the thrust-to-weight ratio in air gas turbine engines. The effect of centrifugal force on the propagation of the premixed flame has been investigated. For this purpose, the large eddy simulation of premixed combustion of the air-propane mixture in two straight and curved ducts with a step in the outer wall as a flame holder was performed using OpenFoam software and compared with the experimental data. The ducts have an inlet and outlet, and the averaged temperature and the wrinkling (the ratio of laminar to turbulent flame speed) were investigated for two different inlet velocities. It was observed that the curved duct inducing centrifugal force to the fuel and air mixture causes better mixing and wrinkling, increases the area of the flame, and as a result, the speed of flame propagation was improved. Also, the curved duct can withstand increasing the inlet velocity to higher values. To study the effect of fluid circulation, a new duct geometry for more mixture circulation was designed and analyzed. The comparison of temperature and wrinkling parameters in the outlet section for two initial curve ducts (C2) and the new one (C3) showed that the increase in the rotation due to the increase in centrifugal force improved the average temperature and wrinkling parameters in the outlet.

In gas furnaces based on oxyfuel combustion, radiative heat transfer is an important part of the heat flux and plays an important role in the flame temperature distribution. Different parameters affect the radiant heat transfer of furnaces. In this study, the effect of wall emissivity coefficient, oxidizer compound, and inlet flow swirl number in a Harwell gas furnace was investigated. k-ε standard, discrete ordinate, and eddy dissipation model were utilized to model turbulence, radiation, and combustion process, respectively. The radiative properties of the gaseous medium were determined using the weighted-sum-of-gray-gases model. The results showed that with increasing the swirl number, the maximum flame temperature moves upwards and approaches the inlet. This causes the heat flux of the walls to increase and the axial heat flux to decrease. By changing the oxidizer composition, the radiant activity of the gaseous medium changes. This causes a change in the temperature distribution in the whole field and axial and wall heat fluxes. The use of nitrogen in the oxidizer causes the maximum temperature to move towards the walls, while the use of carbon dioxide causes the flame to concentrate in the central axis, although the increase of the mass percentage of oxygen in the oxidizer improves flame diffusion. Increasing the wall emissivity coefficient causes the flame to become more concentrated and its maximum temperature to move upwards.

In this paper, numerical simulation of ignition of fuel-air mixture in a cylinder of a v-type engine has been done by the method of computational fluid dynamics and premixed mixture. The premixed mixture consists of iso-octane and air with an equivalence ratio of 0.9. The existing simulation is performed using the finite volume method and combustion of the premixed mixture. Besides, the dynamic mesh in the piston motion is also used. In this regard, the trend of temperature and pressure changes with respect to the crank angle has been investigated. In order to validate, the results obtained by mixed mixture method were compared to that of a research engine. The simulation results show maximum error of only 2.2% when compared to the experimental results. Therefore, the proposed numerical method can be used in the design of spark ignition engines. In order to optimize the combustion and better spreading the flame front in the pent-roof type combustion chamber, the location of the spark plug has been investigated with several simulations at different spark locations. According to the data of pressure and temperature and either flame front contours and their comparison in different analyzes, the optimal range was achieved.

In the present study, with the aid of computational fluid dynamics tools three different radiant tubes including straight, u-shape and w-shape were compared. After designing the geometry, the grids generated for geometries in Gambit software. Grid independence was also performed for the W-shaped geometry, which was the most complex geometry. Firstly, the results showed that the amount of nitrogen oxide emission in the U-shaped radiant tube is the highest compared to direct and W-shaped radiant tubes and the lowest was in the W-shaped type. W-shaped radiant tube had the highest efficiency (55.3%) among all radiant tubes. Besides, the ratio of radiant heat transfer from the tube wall surface to the total heat transfer from it in all three types of radiant tubes was high and more than 90%. By moving away from the wall of the radiant tube in all tubes, the amount of radiation per unit area decreases. Furthermore, the highest amount of radiation intensity was devoted to the W-shaped tube, then the U-shaped tube, and finally the lowest amount was related to the direct tube.

Increasing the flame propagation speed with aid of centrifugal force proposed by Lewis is a new challenge that can reduce the length of the combustion chamber and thus increase the thrust to weight ratio. In this study, large eddy simulation of premixed combustion of air-propane mixture in a two-dimensional tube with closed ends have been implemented in OpenFoam Software to investigate the effect of centrifugal force on the flame propagation speed. Comparison of numerical solution results with experimental data showed about 8% error in the most critical centrifugal acceleration (3000g). To investigate the effect of the turbulence model, the combustion simulation using the k turbulence model for 3000g, was compared with the LES model and it was observed that the LES model, investigate propagation speed and flame wrinkling with higher accuracy. Considering the flame surface wrinkling parameter, it was observed that at a centrifugal acceleration equal to 4000g, the flame surface wrinkling, suddenly increased and then decreased rapidly that indicates the extinction of the flame. Then, the effect of pipe length on flame propagation speed and flame wrinkling at 2000g was investigated and it was observed that increasing pipe length and distance from the origin of rotation, the induced centrifugal force increased and as a result, propagation speed and wrinkling Increases.

Effect of fuel-to-air equivalence ratio on combustion process in the furnace of Razi petrochemical company’s ammonia unit is numerically analyzed in this paper. Three-dimensional geometry of the furnace has been modelled based on non-premixed fuel and air assumption. Modeling of steady-state compressible reactive-turbulent combustion is presented by the basic conservation equations considering the effects of conductive, convective and radiative heat transfers. Zeldovich and Fenimore mechanisms have been employed to account thermal and prompt NOx emission. Numerical results obtained with 99% accuracy compared to experimental data on a computational grid possessing 3165349 elements. Results showed that reduction of equivalence ratio causes the maximum flame temperature to approach inlet fuel nozzle and decrease flame length. High temperature at the furnace outlet is caused by better mixing of flows in great equivalence ratios. The production of NOx pollutants in the equivalence ratios of 0.833, 0.714 and 0.625 increases by 50.62, 53.23 and 56.2 percentages respectively with respect to the stoichiometric state.

In this study, the computational fluid dynamics (CFD) simulation of an experimental 350 kg cast iron rotary furnace was conducted for the aim of optimizing its fuel consumption and pollutants reduction. The furnace is divided into 3 distinct simulation zones: a) solid charge zone with liquid-solid phase, b) combustion zone with gas phase, and c) solid rotating zone or furnace refractory wall. These three zones are three-dimensionally and transiently modeled in terms of the leading phenomena within each zones and interfaces. The simulation in each region is based on the simultaneous solution of hydrodynamic equations, including vortex dissipation and chemical reaction kinetics equations. The simulation results for the outside wall temperature of the furnace body are in close agreement with the data obtained from experimental units. Furthermore, melting rate, NOx and CO pollutant generation, specific fuel consumption, rotating speed, preheating of combustion air, excess air percentage, and pollutant production were all evaluated in this simulation. Changing the furnace configuration decreases fuel consumption by 5%, which is important in terms of improving fuel consumption and alloy product quality in this furnace.  The results of this study can be used to optimize the industrial rotary furnace operations.

Numerical modeling of space engines aside the experimental test is routine. In the design step of such engines, low-cost softwares are vital. In this paper, small-scale space engine thrust chamber analysis code will be developed. In this code, propellant injection and evaporation distribution will be modelled. 1D Combustion solver calculates the reactions of distributed fuel and oxidizer through the thrust chamber axis by chemical mechanisms. Then the cooling solver computes the heat transfer from hot gases to the film cooling layer and the outer surroundings. Validation shows acceptable errors in the modelling of processes. By this developed code, the performance of the Astrium bi-propellant thruster with MonoMethylHydrazine and NitrogenTetrOxide and distributed chemical reaction has been investigated. Results show that hot gas temperature inside the combustor is not uniform and has a peak. Furthermore, the evaporation of the propellant droplets is continued to the nozzle throat.

In this research, the geometric parameters of a preheated furnace burner in the press line of Esfarayen Industrial Complex have been studied, experimentally and numerically. To improve the combustion process in the burner, three different diameters (10, 20 and 30 mm) are provided for the nozzle diameter and three different lengths (225, 250 and 300 mm) for the mixing length of the burner. Ansys-Fluent software and RNG k-ε turbulence model have been used for the simulation and the modeling results show that the applied method has good accuracy and with a maximum error of 23% higher than the experimental values for temperature. This temperature difference is due to the lack of accurate measurement of inlet air flow and also point measurement of temperature in the burner. In the study of the effect of nozzle diameter, it was observed that by increasing the nozzle diameter from 10 to 30 mm, the maximum temperature inside the burner increased by 6%, which against a slight increase in nitrogen oxides (Nox) pollutants inside the burner, the 30 mm diameter for optimum design is selected among the tested diameters. Also, the results of the study for the effect of mixing length on burner performance have shown that by increasing the length, the amount of heat produced decreases slightly, which due to more favorable stability of NOx pollutants due to more space and complete reaction, length of 300 mm has been chosen for optimal design of burner.

The propagation and shape of methane-air premix flame in an enclosure with dimensions of 50 × 11 × 8 cm and the effect of porous obstacle with a porosity percentage of 95 with 20 cavities per square inch in the flow path has been studied in a laboratory. The study of flame behavior has been done with high speed camera photography. One side of the enclosure is made of transparent plexiglas with dimensions of 8 x 50 cm for photographing. The pressure variations inside the enclosure are recorded with the help of a pressure sensor and converter located on top of the enclosure. The results for the unobstructed enclosure correctly represent the process of forming the classical tulip flame inside the enclosure. The range of pressure variations has been adapted to the reference samples and has been validated. The location of the porous obstacle has been tested for four different spacings of 5 cm, 10 cm, 15 cm and 20 cm from the spark location. In the chamber without obstacle, the tulip flame at a location of 25 cm from the spark and 50 ms from the time of sparking was created. The results for the four porous obstacle states indicate that the turbulence created in the flow field can change the location of formation of the tulip flame, as well as its formation time. For a distance of 20 cm the porous obstacle from the spark location, the turbulence of the flow field is at its maximum.

In piston engines, as the engine speed increases, the combustion cycle time decreases. Reducing the engine combustion cycle time, the opportunity for complete fuel combustion is reduced. As a result, in order to complete the fuel combustion, at high engine speeds, the spark plug must ignite sooner. Now that the natural gas flame is slower than gasoline, this problem is becoming more noticeable. As a result, the spark ignition timing should be increased further in CNG fuel mode than in the case of gasoline combustion. This leads to an increase in negative work during the compression phase, resulting in reduced cycle efficiency. Researchers have suggested adding hydrogen to improve natural gas combustion (HCNG). By adding hydrogen, the flammability properties of natural gas are improved. In this study, using Hydrogen generator, hydrogen and oxygen (HHO) gas is produced and consumed water by electrolysis method. The gas (HHO) produced together with the gas (CNG) is combined with the air entering the engine. Then the effect of gas (HHO-CNG) on the parameters of efficiency, power and pollution has been investigated. The results show that the thermal efficiency increases with the addition of HHO. Under these conditions, CO and UHC levels have decreased. This reduces contamination and increases efficiency due to reduced need for spark plug advance and complete fuel combustion.

The scarceness of non-renewable energies, rising fossil fuels prices, as well as strict environmental regulations have forced many researchers to focus on utilizing environmentally friendly renewable energies such as biogas in internal combustion engines. Variety in raw materials and process of biogas production results in different components and percentages of various elements, including methane. These differences make it hard to control the combustion, effectively, in internal combustion engines. In this research, by means of 3D-CFD model, the effect of reformed biogas (R.BG) was investigated on the combustion characteristics, emissions and performance of a RCCI engine. Validation of the model was done on a single-cylinder compression ignition engine in conventional diesel and dual-fuel operations. The combustion model of the RCCI engine was simulated by replacing diesel with 20%, 40% and 60% of R.BG as low reactivity fuel. The results showed that by increasing the R.BG energy share with a constant equivalence ratio of 0.43, the average combustion temperature decreases to 1300 K and the maximum in-cylinder pressure increases up to 22.63%. Instead, it results in 2.3%, 7.9%, and 14.5% engine output losses, respectively. Also, the emission of NOx, against CO, is reduced by 50%. Soot and UHC emissions were found to be slightly decreased while using R.BG more than 40%.

Nowadays, premixed flames are broadly utilized in various residential/industrial applications. Therefore, simultaneously increasing the quality of combustion and reducing the pollution of these flames is of great importance. In this study, collisions of two flame jets of H2/CH4 have been simulated; and flame structure, temperature, and NOx emission are reported at different conditions. The main purpose of this work was to study the effect of angle between the burners and the effect of air and fuel preheating on the key design factors of impinging jets. It was observed that by increasing the angle between two burners, mixing and recirculating flow is enlarged, and the maximum flame temperature and NOx production is increased accordingly. By increasing the angle from 0 to 180 degrees, the maximum flame temperature of methane and hydrogen increases by 11.5 and 12.4%, respectively. Preheating showed that with 400 K increase in the fuel and inlet air temperature, the maximum flame temperature for methane and hydrogen increases 6% and 4%, respectively, which eventually results in a higher NOx.