صادق تابع جماعت

In this research, complementing previous studies that have focused on flame structure and pollutant levels, the combustion of biogas, composed of methane and carbon dioxide, in a micro gas turbine combustor is experimentally investigated. The influence of the presence of carbon dioxide and variations in its ratio in the fuel mixture on the performance parameters of the combustor and its produced pollutants is examined. The study is conducted at two different thermal powers and constant equivalence ratio for two biogases and compared with natural gas combustion. The results indicate that the carbon dioxide ratio in the fuel significantly affects the turbulent combustion flame structure and performance parameters. Increasing the carbon dioxide ratio in the fuel reduces the combustion rate, causing combustion delay and flame temperature reduction. According to the obtained results, although the addition of carbon dioxide leads to a decrease in the amount of NOx emissions, however, the produced CO in the combustor increases, resulting in an outlet temperature drop and consequently a decrease in the combustor efficiency. Based on these results, it is evident that the utilization of such fuels in combustion systems with swirl burners is feasible. However, measures need to be implemented to mitigate carbon monoxide emissions.

In this article, the influences of swirl intensity on the mixing pattern, successful ignition probability, and flame propagation manner in a non-premixed gas burner with natural gas fuel have been examined by using numerical simulation and high-speed digital imaging. The ignition success probability maps were scrutinized under changing spark location in axial and radial directions. The flow field and mixing pattern were further inspected with the aid of numerical simulation. High-speed digital imaging was employed to study the initial flame kernel propagation. The ignition success probability diagrams defined three areas. The first, dubbed the ineffective zone, holds less than a 20% ignition success chance. The second, known as the transitional zone, has an ignition success probability between 20% and 80%. Ignition success here significantly depends on the spark location. The third area—the high-probability zone—has over an 80% ignition success chance. Findings from the study demonstrate an increased swirl number promotes a higher mixing rate, but it doesn't enhance the distribution of successful ignition probability. Higher swirl numbers actually reduce the high-probability zone, whereas lower swirl numbers allow a broader distribution in the burner outlet and better ignition characteristics.

To achieve the zero carbon emissions goal, it is of great importance to replace conventional hydrocarbon fuels with carbon-free fuels. In terms of carbon-free fuel, ammonia has several advantages over hydrogen, despite of problems such as low flame speed and high NOx emission. The present experimental study investigates the effect of adding ammonia to methane and its ratio on the combustion performance of a can-type micro-turbine combustion chamber under atmospheric condition at two different heat powers. The results show that with an increase in the percentage of ammonia in the fuel mixture, maximum flame temperature has decreased and combustion has delayed until the end of chamber. Also, the addition of ammonia, despite not reducing CO, results in a sharp increase in the NOx production. In addition, although the outlet temperature of the chamber has not changed much except for one condition, combustion efficiency of the chamber has decreased with the increase of ammonia ratio, and pattern factor is more than the case of pure natural gas. Therefore, although the use of methane/ammonia fuel mixture with stable combustion conditions in the existing chamber is possible, it requires modifications to achieve optimal combustion performance.a
 
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In this study, the experimental investigation of oxygen-methane combustion has been done by investigating the effect of nitrogen dilution in a novel spiral-channel combustion chamber along with the recovery of thermal energy of the products.  The methane-oxygen non-premixed flame by a fixed equivalence ratio of 1.69, with nitrogen dilution percentages of 0%, 5%, 10%, and 18%, has been investigated with the spectroscopy method. The chamber is made of aluminum and created as a spiral-form channel to aid heat recovery. In this chamber, the combustion gases then pass a path parallel to the inlet flow, causing the inlet gases to be preheated. Also, oxygen is diluted and mixed with nitrogen before entering the chamber. Based on the results, by increasing of the diluent ratio the reduction of the H2O* radical radiation spectrum has been observed, where indicates complete combustion. In general, with the increase of the dilution ratio, the flame length increases, and in 5% and 10% dilution states, the best temperature distribution uniformity, and the completion of the combustion reaction chain are observed.

In the design of the combustion chamber, various parameters should be considered. These parameters include uniform temperature distribution at the outlet of the chamber, more flame stability, lower pollution, higher combustion efficiency, lower wall temperature, and lower pressure drop in the chamber. Regarding to the complex condition of the flow in the combustion chamber due to the various effects of turbulence and mixing of flows as well as the behavior of turbulent flames, predicting the performance of flow in the combustion chambers is very complicated. In this paper, it is tried to study and optimize the combustion chamber of Amirkabir University of Technology in terms of swirler. It is done by using the numerical method and finally the selected swirler in the numerical method is tested in the experimental setup to investigate optimization method .According to the studies, swirler with an angle of 60 degrees, 12 blades, and a thickness of 0.75 mm is selected as the final case. In the experimental results, the amount of CO pollution has significantly reduced. The output temperature, the pattern factor and unburned hydrocarbon have reduced in the final case. However, the temperature uniformity inside the chamber has increased.

The non-premixed autoignition characteristic of a hot turbulent jet ejecting into the quiescent air was studied experimentally. The jet was the combustion product of fuel rich natural gas/air mixture burn in a thermally isolated combustion chamber. The combustion products discharge through a nozzle (13 and 20 mm in diameter) into the ambient air. The pre-chamber equivalence ratio, jet Reynolds number, and temperature are the governing parameters which are controlled by mass flow controller and thermocouples. The results of these experiments provide insight into the temporal and spatial non-premixed autoignition of the high-temperature combustible jet. Observations of ignition-flame formation –extinction behaviors of these jets are categorized into 1-no ignition 2- anchored diffusion flame 3- lifted diffusion flame 4- unstable diffusion flame 5- random autoignition kernel. The high-speed imaging helps to measure the location, lifetime and frequency of autoignition kernel using the in-house image processing developed code. Furthermore, the CH chemiluminescence imaging via an optical band-pass filter ensures that the jet composition is chemically quenched at the nozzle tip. The autoignition kernel can occur in radial and axial domain determined by the jet temperature and Reynolds number. The high-speed imaging shows that the most probable zone for autoignition kernel formation is on the jet axis and the jet Reynolds number has the major effect on axial distance. The jet temperature development merely extends this domain in the axial direction. Moreover, the temperature directly affects the lifetime and frequency of autoignition kernel.
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An experimental study on the effects of methane-oxygen partially-premixed input flow characteristics in a mesoscale reactor with constant length and geometry was investigated in the present work. For this research, two partially-premixed ratios of 25% and 50% are considered. The reactor is mounted horizontally, made from quartz material and its geometric characteristics are internal diameter: 5 mm, wall thickness: 1 mm, and length: 10 cm. In this research, we have tried to determine the factors affecting flame regimes. The range of flame regimes, flame dynamics, the outer wall temperature distribution of the reactor, frequency, and oscillation of oscillating flames, along with the intensity of the Repetitive Extinction and ReIgnition (RERI) extinguishing sound, were analyzed and reported. This flame's dynamics are more affected by changes in mixing ratio, oxygen volume flow rate, and fuel volume flow rate, causing changes in inlet flow velocity and equivalence ratio, respectively. Examination of the results of acoustic oscillations indicates an increase in oscillating flame velocity with increasing volumetric flow and mixing ratio. Loud extinguishing sound of flames when quenching is caused by converting a portion of the thermal energy of the flame into sound in the flame arrestor and the acoustic vibration waves resulting from the extinguishing of the flame and the difference in gas velocity.

In this paper, the experimental study of partially premixed combustion of methane and oxygen in a 5 mm mesoscale quartz reactor with 1 mm wall thickness and 5, 10, and 15 cm lengths. The partially premixed for 25%, 50%, and 75% mixing ratios paid. Experimental results including the factor of affecting flame regimes, formation range, flame dynamics, the outer wall temperature distribution of the reactor had been analyzing and reporting. The above tests were performing in an asymmetrically centered cylinder combustion chamber and a laminar flow regime. In most partial pre-mixing combustion experiments, the oscillation regime, which had an optimal heat distribution throughout the reactor, had been observed. The flame dynamics were more effect by changes in mixing ratio, reactor length, oxygen flow rate, and finally fuel flow rate (equivalence ratio). Also observed that by increasing the reactor length due to the appropriate time for homogenization of the mixture, differences in the flame formation interval were reducing in the different ratios of the pre-mixes.

Combustion chamber has a crucial role in gas turbines and has a significant effect on the pollution and efficiency of them. Due to the complicated flow in combustion chambers because of high turbulence intensity, flow mixing, and flame behavior, prediction of the performance of such chambers is very complicated. There is a vital need for experimental investigations to study and understand the flame behavior in combustors. This experimental study was performed using a can type combustion chamber and LPG fuel at atmospheric conditions. First, stability curve, temperature distribution in the combustion chamber, and its exit plane in 6 flow conditions and then flow behavior were evaluated. The pollution at the outlet was obtained in different conditions and equivalence ratios. The results show that the flame tends to go downstream of the combustion chamber when the fuel mass flow rate increases (or in other words, by increasing the equivalence ratio) in constant air mass flow rate and finally exits from the chamber. By increasing the air mass flow rate in constant fuel mass flow rate, CO pollution is increased, and NOx pollution is decreased.

In this study, oxy-combustion is numerically investigated under MILD conditions. This novel combination is ‎simulated in a laboratory-scale furnace with parallel fuel and oxidant jets and a recirculating flow. With nitrogen ‎removal from the oxidant stream, zero NOx emission for gaseous fuel systems is expected. Combustion field is ‎modelled using a well-stirred reactor and computational fluid dynamics. In the CFD modelling, RANS equations ‎are solved using RNG k-ε and EDC model is employed to model the turbulence-chemistry interaction. The effect ‎of oxidizer preheating and CO2 dilution on the combustion field as well as flue gas composition is investigated. ‎The results indicate that the flame maximum temperature does not experience a significant increase when the ‎preheat temperature rises, positively affecting the temperature distribution at the cost of CO emission which is a ‎direct consequence of higher recirculation and mixing rate. Also, a kinetic study on the chemical effect of CO2 ‎presence reveals that CO production path through CH3O radical is more strengthened compared to the main path ‎through formaldehyde when increasing the dilution level. When diluting the oxidant, methylene’s role become ‎more influential in CO formation than when pure oxygen is used, contributing to higher CO emission. CO2 defeats ‎CH4 and O2 to absorb the free H radicals, causing higher levels of CO production.‎

The flamelet library in FGM model is constructed based on mixture fraction and progress variable. This library facilitates to model the reactive flow precisely only in the case of two fuel/oxidizer in-flow streams. For the case of more in-flow streams, e.g. Sandia piloted flames fuel jet/ pilot/ air, it is needed to extend dimensions of the FGM library. This extension increase the size of stored data leads to higher computational cost. In this paper, the main motivation is to develop the Ignition Mixing Layer method (IML) to the Variable Ignition Mixing Layer (VIML) as the novel approach to generate the 2D FGM table for multi in-flow streams. Large Eddy Simulations of the Sandia D, E, and F with Reynolds number of 22400, 33600, and 44800 were conducted to validate this method. The LES results show a good agreement on the mean and variance of the mixture fraction and temperature. Therefore, the VIML is a successful model to simulate multi in-flow streams with 2D FGM library.

A gas turbine combustor has been investigated experimentally in this paper. The effect of air and fuel flow rates on the on combustor performance and lean blow out at atmospheric and steady condition is the goal. The combustor is a can type with swirl pressure fed injector. An axial swirler with swirl No. equal to 0.8 has been installed. The kerosene has been used as fuel while the air temperature at combustor inlet is equal to 315 K. The combustor Stability loop has been determined with several tests. Then variation of temperature inside the combustor and at combustor exit with respect to the injector back pressure and air flow rate are measured. 4 operating conditions have been selected due to detail investigation of flame temperature contour inside the combustor. The results show that the flame holds near the walls. Also, the pattern factor show that the best condition is when the flame is totally inside the combustor while the air flow rate is minimum. Furthermore, it is cleared that the lean blow out is not uniform. It means that with increasing the air flow rate, upper section of flame has been quenched and then the lower section of flame.

Ignition process in a shear-less mixing layer has been studied in this paper. The effect of initial temperature on the flame propagation phase of ignition process is the main goal. The investigation is done by large eddy simulation method coupled with thickened flame approach and DRM_19 chemical mechanism. Mean and RMS axial velocities from both coarse and fine grids and mean mixture fraction have been validated against experimental results. Most upstream and downstream positions of flame edge are in good agreement with experimental. By increasing the initial temperature from 323 K to 1000 K, the mean edge flame propagation velocity increase from 1 to 4.5 m/s. The same trend exists for flame kernel volume. Comparison between calculated edge flame propagation velocity and laminar flame speed and its root density correction show that corrected laminar flame propagation can better predicts the edge flame propagation velocity. Also, by increasing the initial temperature, bibrachial edge flame convert to a triple flame.

In this paper, the design and construction of a gas turbine combustor test rig and the experimental results of a sample combustor sector at atmospheric conditions are described. This test rig can be used to evaluate the effects of geometric variations on the performance of the combustion chamber. Flammability, stability and ignition maps, exhaust gas composition and temperature profile, and liner wall temperature can be studied. This rig has the potentiality of performing combustion tests with maximum air flow rate of 800m3/h and preheated air up to 1000K, as well as different types of liquid or gas fuels. A single swirler sector of an annular combustor is tested at different air and fuel mass flow rates. The results show that the exhaust gas temperature has a non-linear corelation with the fuel to air ratio. Using a one-dimensional model, the exhaust temperature of the combustion chamber is predicted and compared with the experimental results. The model results show good agreement with the experimental results.

Spar ignition in turbulent methane-air jet is studied by a compressible 3D Large Eddy Simulation in OpenFOAM code. The thicened flame model with the one-step chemical mechanism is used for methane combustion. The spar is modeled by artificial enthalpy source in energy equation at the spar location. The validation of the calculations is performed using the experimental results of the turbulent jet of airflow and non reacting mixing of methane jet. The ignition phenomenon and flame propagation are investigated in details for different conditions of initial jet temperature. The results show that the flame propagation speed, the flame lift-off and the flame distance to the axis change with changing the initial temperature.

In this study, the combustion of a fuel jet with supersonic air flow has been studied numerically.The injected gas was a preburned mixture of hydrogen and air with equivalence ratio of 4.5. The flow field was investigated with and without incident oblique shock wave. The simulations have been performed using openFoam software in a three-dimensional field. A two-equation turbulence model, SST-kω, and the PaSR combustion model were used. The aim of this study was to examine the impact of the shock wave on flame stabilization. According to the results, the reaction occurs at low rates without the oblique shock. However, when the incident shock hits the bottom surface downstream of the injection slot, reaction rate increases, indicating the flame stabilization in this case. These findings are in good agreement with experimental results.

A large eddy simulation (LES) of premixed flame-turbulence interaction is performed with special emphasis on computing the instantaneous chemical species reaction rates with the recently developed approach of artificial neural networks (ANNs) for chemical kinetics. Training of the neural network is based on an independent flame study using linear eddy mixing technique. An analysis of computational performance, considering CPU time and a comparison between the performance of artificial neural network technique and other conventional methods is used to represent the chemical kinetics such as direct integration (DI) - and the ability of neural networks to model the highly non-linear and stiff chemistry ODEs is illustrated. The sub-grid combustion model of the LES is based on a linear eddy mixing model while a skeletal multi-species, multi-step chemical kinetic mechanism is applied for the combustion. A feed-forward, multi-layer architecture is chosen for the neural network and the training algorithm is based on a back-propagation gradient descent rule with adaptive learning rate and individual momentum factors for the weight coefficients. The flow field distribution and the flame characteristics obtained by LES with neural network based chemical kinetics tabulation, are in reasonable agreement with previous direct numerical simulation (DNS) study of the flame. The results show if the neural network is trained accurately, it can predict the instantaneous chemical species reaction rates in LES framework.

Effects of the fuel inlet Reynolds number and the oxygen mass fraction in the oxidizer stream on the flame structure were studied. In this regard، the simulation was performed for two Reynolds numbers of 5000 and 10،000 and three oxygen mass fractions of 3، 6 and 9%. Numerical results were compared with experimental measurements of Dally. The turbulence-chemistry interaction in numerically unresolved scales was modeled using the PaSR model and the full mechanism GRI-2. 11 was used to precisely represent the methanehydrogen reactions. Accuracy of the LES simulation results was evaluated by a set of criteria indicating acceptable predictions. The results show that increasing the oxygen mass fraction in the oxidizer stream decreases the flame thickness، limits the species fluctuations to a smaller zone near the nozzle’s exit plane، decreases local extinctions in the flame structure and limits the partially premixed to a zone near the shear layer line and in summary improves the flame stability especially near the nozzle exit zone. Since decreasing the Reynolds number increases the residence time of fuel and oxidizer elements، it has a favorable effect on flame stability and decreasing its susceptibility to combustion instability.

In the present study، numerical modeling of the combustion of a fully laminar stoichiometric premixed reactive flow of methane-air inside a 2D micro-combustion chamber was investigated. The aim of this study is the investigation of occurance، phenomenology and effective parameters such as flame stability on combustion process in MEMS devices for energy or propulsion generation on small scales for space exploration applications. The results show that flame stability in a micro- combustion chamber strongly depends on the combustion chamber wall thickness Lw، the combustion chamber wall thermal conductivity coefficient Kw، the combustion chamber width L، the outer wall convective heat transfer coefficient hout and reactive mixture velocity Vin.

In this work, an experimental investigation of stability limits of natural city gas (NG) in a non-premixed swirling burner has been conducted. Air swirl motion is imparted through 30º, 45º and 60º vane swirlers to produce low and high swirling air streams. Air dilution with nitrogen (N2) was also performed because of the importance of dilution in practical applications. N2 was added to the swirling air to examine the stability limits under diluted air conditions. The purpose of this study is to examine the stability limits including lift and blow-out velocity in low swirl conditions and liftoff height as well as liftoff velocity in high swirl conditions. It was observed that swirl increased the stability limits of NG while it decreased the negative effects of dilution on flame stability.

Mild is an acronym for moderate or intense low-oxygen dilution and refers to oxidizer high preheating and dilution. The Mild combustion has features which set it apart from other combustion systems, such as higher reaction zone volume, more uniform temperature distribution, lower reaction rate, lower heat release rate and lower Damkohler number in comparison with the ordinary combustion regime. In this paper, characteristics of the Mild regime were studied numerically. The experimental conditions of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154] were used for modelling in the present study. The EDC model was used to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI 2.11 full mechanism were used to represent the chemical reactions of H2/methane jet flame. The distribution of temperature, some species, reaction rates and also the importance of molecular diffusion for various O2 levels and jet Reynolds numbers were investigated. The results show that the molecular diffusion in Mild combustion cannot be ignored in comparison with the turbulent transport. Also, the method of inclusion of molecular diffusion in combustion modelling has a considerable effect on the accuracy of numerical modelling of Mild combustion. By decreasing the jet Reynolds number and reducing the oxygen concentration in the airflow, the influence of molecular diffusion on Mild combustion increases. Moreover, the effect of the molecular diffusion reduces on reaction zone when moving away from the nozzle. Although the agreement between numerical and experimental results are very satisfactory, by distancing from the nozzle, the rate of reactions increases while deviation from Mild condition and numerical errors grows.

The stability behavior of jet non-premixed flame developing in a coflowing stream is studied experimentally, using city gas and propane as fuel gases. Effects of oxidant and fuel stream velocities and oxidant stream dilution have been studied. Two types of stability limits are observed. Blow-off of the rim-stabilized flame is the first stability limit. The second is the break-off or extinction of the turbulent portion of the flame at the transition point from laminar to turbulent flow. Oxidant and fuel streams are in environmental temperature. In dilution experiments, oxidant primary stream is oxygen as diluted with nitrogen or carbon dioxide. In the other experiments, oxidant is environmental air.

In the present study, propane/ oxygen and city gas/ oxygen diffusion flames within a laminar regime have been investigated in two parts. First, the effect of oxygen dilution with nitrogen and carbon dioxide gases has been investigated. In this section, stability and flame structure variations against the dilution process are studied. In the second part, oxidant stream preheating up to 480 K, and contemporaneous diluting with nitrogen or carbon dioxide are investigated and the results are compared against non-preheating tests. Preheating causes more flame stability in comparison to the dilution process. Because of the temperature rise in the combustion of such products during preheating, these flames are more luminous in comparison with normal temperature flames.