The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Mehdi Bidabadi - One of the best experts on this subject based on the ideXlab platform.
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An asymptotic approach to heat recirculation in diffusion flames fueled by organic particles
Journal of Thermal Analysis and Calorimetry, 2020Co-Authors: Navid Malekian, Hesam Moghadasi, Mehdi BidabadiAbstract:Owing to their safety, stability and controllability, diffusion flames have found extensive applications in medicine and power generation. Regarding the significance of recirculation impact on micro-combustors, an efficient method should be developed for better analysis of the micro-combustors performance. In this paper, an asymptotic method is developed to model diffusion flames propagation through a biofuel in Counterflow Configuration with the consideration of heat recirculation effect. The flame structure includes pre-heat, post-vaporization and oxidizer zones. Micron-sized lycopodium particles and air can be regarded as biofuel and oxidizer, respectively. Mass and energy conservation equations are investigated in each zone. For evaluation of the thermal recirculation impact, a specific term is included in the energy conservation equation. Furthermore, the effects of changes in the flame temperature, mass fraction of the gaseous fuel and oxidizer (relative to fuel and oxidizer Lewis numbers), mass particle content, particle radius and equivalence ratio were examined considering and ignoring the thermal recirculation effect. The results indicate that increase of heat recirculation coefficient will rise the flame temperature and shift the flame position to the fuel nozzle side. Also, consideration of thermal heat recirculation will enhance the gaseous fuel production in the pre-heat and post-vaporization zones. Graphic abstract
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A simplified mathematical study of thermochemical preparation of particle oxide under Counterflow Configuration for use in biomedical applications
Journal of Thermal Analysis and Calorimetry, 2020Co-Authors: Amir Tabaei, Mehdi Bidabadi, Sadegh Sadeghi, Saman Hosseinzadeh, Qingang Xiong, Nader KarimiAbstract:This study mathematically presents a Counterflow non-premixed thermochemical technique for preparing a particle oxide used for cancer diagnosis and treatment. For this purpose, preheating, reaction, melting, and oxidation processes were simulated considering an asymptotic concept. Mass and energy conservation equations in dimensional and non-dimensional forms were solved using MATLAB^®. To preserve the continuity in the system and calculate the locations of melting and flame fronts, promising jump conditions were derived. In this research, variations in flame temperature, flame front location and mass fractions of the particle, particle oxide and oxidizer, with position, Lewis number and initial temperature of the particles were investigated. The simulation results were compared with those obtained from an earlier experimental study under the same conditions. Regarding the comparison, an appropriate compatibility was observed between the results. Based on the simulation results, flame temperature was found to be about 1310 K. Positions of flame and melting fronts were found to be − 1.8 mm and − 1.78 mm, respectively.
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Semi-analytical modeling of non-premixed Counterflow combustion of metal dust
Journal of Thermal Analysis and Calorimetry, 2019Co-Authors: S. A. Madani, Mehdi Bidabadi, Nafiseh Mohammadian Aftah, Abolfazl AfzalabadiAbstract:In this study, a semi-analytical model is developed for non-premixed combustion of metal dusts in Counterflow Configuration. Combustion domain is divided into three separate zones, each of which possesses corresponding mass and energy conservation equations as well as boundary and jump conditions. Metal dust, assumed to be aluminum, undergoes an Arrhenius-type reaction with oxidizer, when it is heated enough to reach the ignition temperature. Dimensionless forms of conservation equations are derived and utilized to elucidate the combustion characteristics. The effects of oxidizer Lewis number and fuel mass concentration on the flame position and temperature are discussed thoroughly. In addition, temperature distribution of the whole domain is calculated by numerically solving the system of partial differential equations. In order to track particles through combustion domain, Lagrangian equations of motion are solved either mathematically or numerically, considering thermophoretic, weight, buoyancy and drag forces. The effects of thermophoretic force on the particle path are investigated, and the deviation of particle from carrier neutral gas direction is obtained. The results showed a great agreement with the data reported in the literature highlighting the fact that the presented model is an efficient one to accurately model the non-premixed Counterflow combustion of metal dust.
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Thermal radiative study of Counterflow combustion of porous particles
Chemical Engineering and Processing, 2018Co-Authors: Hesam Moghadasi, Alireza Khoeini Poorfar, Navid Malekian, Mehdi BidabadiAbstract:Abstract The paper deals with modeling Counterflow, non-premixed combustion of porous fuel particles (lycopodium) with the consideration of thermal radiation effects. Assuming that the streams of fuel particles and air as oxidizer, move towards the stagnation plane from the two opposing nozzles in a Counterflow Configuration. It is presumed that particles first vaporize in order to yield a gaseous fuel, methane, which then reacts with the oxidizer which is air. In this research, conservation equations with certain boundary conditions are solved using mathematical methods with the consideration of radiation heat transfer in different regions and compared to cases in which radiation heat transfer is not considered. Furthermore, flame temperature and mass fraction profiles are presented in terms of oxidizer and fuel Lewis numbers. In addition, effects of particle porosity are investigated. As a result, with the increase of dust concentration and reduction of particles radius and porosity, it would lead to a rise in the flame temperature.
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Modeling multi regional counter flow combustion of lycopodium dust cloud with considering radiative heat loss
Journal of Central South University, 2017Co-Authors: Mehdi Bidabadi, Farzaneh Ebrahimi, Vahid BordbarAbstract:In this work, an analytical model is presented to simulate the combustion process of organic dust with considering radiative heat loss effect in Counterflow Configuration. A thermal model has been generated to estimate the flame propagation speed in various dust concentrations. The structure of premixed flame in a symmetric Configuration, containing uniformly distributed volatile fuel particles, with nonunity Lewis number is examined with strain rate issue. The flame structure is divided into six zones: first heating, drying, second heating, volatile evaporation, reaction and post-flame zones. At first, the governing equations of lycopodium combustion dust particles are written for each zone. Finally, boundary conditions and matching conditions are applied for each zone in order to solve the differential equations. The purpose of this article is to analyze radiation heat transfer on lycopodium flame propagation dust particles and characteristics to check the effect of parameters on combustion.
Hans J. Fahr - One of the best experts on this subject based on the ideXlab platform.
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On nonideal MHD properties of the partially ionized interstellar gas
Journal of Geophysical Research, 2003Co-Authors: V. B. Baranov, Hans J. FahrAbstract:[1] The Counterflow Configuration resulting from the motion of the heliosphere through the ambient interstellar medium meanwhile has been described by several hydrodynamic and magnetohydrodynamic (MHD) model approaches. The self-consistent inclusion of interstellar and solar magnetic fields in presently existing MHD approaches has been achieved assuming the validity of ideal magnetohydrodynamics where frozen-in magnetic field condition is fulfilled. This assumption is valid only, however, if the electrical conductivity of plasma is very high (i.e., magnetic Reynolds number Rem ≫ 1). As we are going to show here, the presence of neutral H atoms in the interstellar medium strongly questions this requirement and leads to a nonideal Ohm's law invalidating the frozen-in field condition. The MHD modeling of the heliospheric interface thus has to be carried out on a new basis presented in this paper.
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The interstellar gas flow through the heliospheric interface region
Space Science Reviews, 1996Co-Authors: Hans J. FahrAbstract:The relative motion of the solar system with respect to the ambient interstellar medium forms a plasma interface region where the eventually subsonic, interstellar and solar wind plasma flows adapt to each other. In this region ahead of the solar system magnetohydrodynamically perturbed plasma flows are formed which, however, can be penetrated by interstellar neutral atoms at their approach towards the inner heliosphere. Thereby the distribution function of neutral interstellar gas species by means of charge exchange processes in the heliosphere attain gas-specific imprints from the perturbed moments in this plasma region. In recent years one has become interested in the influence of this interface plasma on the helium-to-oxygen-to-hydrogen ratios since observational facts from pick-up ion and anomalous cosmic ray data have meanwhile become available shrinking down the inaccuracy in these ratios to fairly low numbers. The aim thus is to study these ratios as they result from alternative forms of interface structures under debate at present and then to identify the best-fitting interface model. The fact is stressed in this article here that a more conrect description of the interface needs a careful consideration of the magnetic fields and the plasma temperature anisotropies which are involved in the actually prevailing Counterflow Configuration.
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On the motion of wind-driving stars relative to the ambient interstellar medium
Astrophysics and Space Science, 1995Co-Authors: Hans J. Fahr, Klaus SchererAbstract:In a series of foregoing papers we have argued that each wind-driving star in relative motion with respect to the ambient interstellar medium which has formed an adapted astropause is subject to the action of thrust forces exerted on its wind-generating central body. The instantaneous magnitude of the resulting net force depends on the actual geometry of the Counterflow Configuration of stellar and interstellar winds in the particular kinematic situation, especially depending on the instantaneous velocity relative to the ambient medium. We investigate how sensitive this Configuration is to whether the interstellar flow is sub- or supersonic. It will be demonstrated here that the resulting net force varies in a complicated, non-monotonic manner with the actual velocity, however, for subsonic motions it is generally of an accelerating nature, thus operating like a rocket thrust motor, whereas for supersonic motions at supercritical Mach numbers m _s≥ μ _s,cit is of a decelerating nature. For an adequate description of a time-dependent circumstellar flow Configuration, we shall use an analytic, hydrodynamic modelling of the Counterflow Configuration representing the case of a stellar wind system in an adiabatically adapted subsonic or supersonic motion with respect to the local interstellar medium. Hereby we assume irrotational and incompressible flows, after passage through the respective shocks, and can give quantitative numbers for the accelerating forces acting on the motion of such a star. We describe the long-period evolution of star motions and can give typical acceleration time periods for different stars.
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The adapted solar wind system as cause for a momentum transfer to the sun and its consequences for the orbital motions of keplerian objects
Astrophysics and Space Science, 1993Co-Authors: Hans J. Fahr, Klaus SchererAbstract:In the following paper we argue that each wind-driving star in relative motion with respect to the ambient interstellar medium experiences a force exerted on its central wind-generating body. The exact magnitude of this force depends on the actual geometry of the Counterflow Configuration of stellar and interstellar winds for a particular kinematic situation which is especially sensitive to whether the interstellar flow is subsonic or supersonic. It will, however, be demonstrated here that this force is of an accelerating nature, i.e., it operates like a rocket-motor, as long as the peculiar motion of the wind-driving star with respect to the ambient interstellar medium remains subsonic.
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The adapted solar wind system as cause for a momentum transfer to the sun and its consequences for the orbital motions of keplerian objects
Astrophysics and Space Science, 1993Co-Authors: Hans J. Fahr, Klaus SchererAbstract:In the following paper we argue that each wind-driving star in relative motion with respect to the ambient interstellar medium experiences a force exerted on its central wind-generating body. The exact magnitude of this force depends on the actual geometry of the Counterflow Configuration of stellar and interstellar winds for a particular kinematic situation which is especially sensitive to whether the interstellar flow is subsonic or supersonic. It will, however, be demonstrated here that this force is of an accelerating nature, i.e., it operates like a rocket-motor, as long as the peculiar motion of the wind-driving star with respect to the ambient interstellar medium remains subsonic. Here we use a specific analytical model to describe theoretically the specific Counterflow Configuration for the case of the solar system in a subsonic peculiar motion with respect to the local interstellar medium assuming irrotational and incompressible flows. We can work out a quantitative number for the accelerating force governing the Sun's motion at present. The net reaction force exerted on the solar body is then mediated by the asymmetric boundary conditions to which the distant solar wind field has to adapt. Next we study the indirect action of such a force on orbiting Keplerian objects like planets, planetesimals and comets. Since this force only influences the central solar body, but not the planets themselves, the problem is different from the treatment of a constant perturbation force perturbing the Keplerian orbits. We present a perturbation analysis treating the action of a corresponding position-dependent perturbation force resulting in secular changes of the orbital elements of Keplerian objects. It is found that changes are accumulating more rapidly in time the closer to the sun the orbiting bodies are. Main axis and perihelion distances are systematically increasing. Especially pronounced are changes in the perihelion position angle of the objects. For solar wind mass losses larger than the Sun's present value by a factor of 1000 (T-Tauri phase of the Sun,) the migration periods calculated for the planet Mercury are of the same order of magnitude as that for corresponding general relativistic migration.
Klaus Scherer - One of the best experts on this subject based on the ideXlab platform.
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On the motion of wind-driving stars relative to the ambient interstellar medium
Astrophysics and Space Science, 1995Co-Authors: Hans J. Fahr, Klaus SchererAbstract:In a series of foregoing papers we have argued that each wind-driving star in relative motion with respect to the ambient interstellar medium which has formed an adapted astropause is subject to the action of thrust forces exerted on its wind-generating central body. The instantaneous magnitude of the resulting net force depends on the actual geometry of the Counterflow Configuration of stellar and interstellar winds in the particular kinematic situation, especially depending on the instantaneous velocity relative to the ambient medium. We investigate how sensitive this Configuration is to whether the interstellar flow is sub- or supersonic. It will be demonstrated here that the resulting net force varies in a complicated, non-monotonic manner with the actual velocity, however, for subsonic motions it is generally of an accelerating nature, thus operating like a rocket thrust motor, whereas for supersonic motions at supercritical Mach numbers m _s≥ μ _s,cit is of a decelerating nature. For an adequate description of a time-dependent circumstellar flow Configuration, we shall use an analytic, hydrodynamic modelling of the Counterflow Configuration representing the case of a stellar wind system in an adiabatically adapted subsonic or supersonic motion with respect to the local interstellar medium. Hereby we assume irrotational and incompressible flows, after passage through the respective shocks, and can give quantitative numbers for the accelerating forces acting on the motion of such a star. We describe the long-period evolution of star motions and can give typical acceleration time periods for different stars.
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The adapted solar wind system as cause for a momentum transfer to the sun and its consequences for the orbital motions of keplerian objects
Astrophysics and Space Science, 1993Co-Authors: Hans J. Fahr, Klaus SchererAbstract:In the following paper we argue that each wind-driving star in relative motion with respect to the ambient interstellar medium experiences a force exerted on its central wind-generating body. The exact magnitude of this force depends on the actual geometry of the Counterflow Configuration of stellar and interstellar winds for a particular kinematic situation which is especially sensitive to whether the interstellar flow is subsonic or supersonic. It will, however, be demonstrated here that this force is of an accelerating nature, i.e., it operates like a rocket-motor, as long as the peculiar motion of the wind-driving star with respect to the ambient interstellar medium remains subsonic.
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The adapted solar wind system as cause for a momentum transfer to the sun and its consequences for the orbital motions of keplerian objects
Astrophysics and Space Science, 1993Co-Authors: Hans J. Fahr, Klaus SchererAbstract:In the following paper we argue that each wind-driving star in relative motion with respect to the ambient interstellar medium experiences a force exerted on its central wind-generating body. The exact magnitude of this force depends on the actual geometry of the Counterflow Configuration of stellar and interstellar winds for a particular kinematic situation which is especially sensitive to whether the interstellar flow is subsonic or supersonic. It will, however, be demonstrated here that this force is of an accelerating nature, i.e., it operates like a rocket-motor, as long as the peculiar motion of the wind-driving star with respect to the ambient interstellar medium remains subsonic. Here we use a specific analytical model to describe theoretically the specific Counterflow Configuration for the case of the solar system in a subsonic peculiar motion with respect to the local interstellar medium assuming irrotational and incompressible flows. We can work out a quantitative number for the accelerating force governing the Sun's motion at present. The net reaction force exerted on the solar body is then mediated by the asymmetric boundary conditions to which the distant solar wind field has to adapt. Next we study the indirect action of such a force on orbiting Keplerian objects like planets, planetesimals and comets. Since this force only influences the central solar body, but not the planets themselves, the problem is different from the treatment of a constant perturbation force perturbing the Keplerian orbits. We present a perturbation analysis treating the action of a corresponding position-dependent perturbation force resulting in secular changes of the orbital elements of Keplerian objects. It is found that changes are accumulating more rapidly in time the closer to the sun the orbiting bodies are. Main axis and perihelion distances are systematically increasing. Especially pronounced are changes in the perihelion position angle of the objects. For solar wind mass losses larger than the Sun's present value by a factor of 1000 (T-Tauri phase of the Sun,) the migration periods calculated for the planet Mercury are of the same order of magnitude as that for corresponding general relativistic migration.
Matthias Ihme - One of the best experts on this subject based on the ideXlab platform.
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analysis of segregation and bifurcation in turbulent spray flames a 3d Counterflow Configuration
Proceedings of the Combustion Institute, 2015Co-Authors: Benedetta Franzelli, Hai Wang, Tianfeng Lu, Matthias IhmeAbstract:Abstract The understanding of spray combustion processes is of primary importance, as it is encountered in a wide range of industrial applications. In the present work, mesoscale-resolved simulations of a 3D turbulent Counterflow spray Configuration are conducted. Primary focus is on examining the effect of the coupling between turbulence, evaporation, mixing, and combustion. By considering different initial droplet diameters and through comparisons with turbulent and laminar Configurations at the same operating condition, it is shown that preferential concentration can lead to conditions of locally high mixture-fraction composition. In addition, local variability in strain rate and droplet diameter introduces a bifurcation of the spray flame. This bifurcation consists of spray flame structures exhibiting single-reaction or double-reaction structures. It is shown that this bimodal behavior is linked to the existence of a hysteresis in the laminar spray flame structure for droplet diameter variations, as well as the occurrence of a bifurcation for strain rate variations. These results have direct implications for flamelet-based tabulation methods, since identifying the appropriate flamelet structure in turbulent spray flames would require informations about boundary conditions and the flamelet history.
Fokion N. Egolfopoulos - One of the best experts on this subject based on the ideXlab platform.
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Ignition of non-premixed Counterflow flames of octane and decane isomers
Proceedings of the Combustion Institute, 2020Co-Authors: S. Mani Sarathy, Charles K. Westbrook, Fokion N. EgolfopoulosAbstract:Abstract Ignition temperatures of non-premixed flames of octane and decane isomers were determined in the Counterflow Configuration at atmospheric pressure, a free-stream fuel/N2 mixture temperature of 401 K, a local strain rate of 130 s−1, and fuel mole fractions ranging from 1% to 6%. The experiments were modeled using detailed chemical kinetic mechanisms for all isomers that were combined with established H2, CO, and n-alkane models, and close agreements were found for all flames considered. The results confirmed that increasing the degree of branching lowers the ignition propensity. On the other hand, increasing the straight chain length by two carbons was found to have no measurable effect on flame ignition for symmetric branched fuel structures. Detailed sensitivity analyses showed that flame ignition is sensitive primarily to the H2/CO and C1–C3 hydrocarbon kinetics for low degrees of branching, and to fuel-related reactions for the more branched molecules.
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Propagation and extinction of cyclopentadiene flames
Proceedings of the Combustion Institute, 2020Co-Authors: Chunsheng Ji, Runhua Zhao, Bo Li, Fokion N. EgolfopoulosAbstract:Abstract Laminar flame speeds and extinction strain rates of cyclopentadiene/air mixture were determined in the Counterflow Configuration at atmospheric pressure, unburned mixture temperature of 353 K, and for a wide range of equivalence ratios. The experiments were modeled using recently developed kinetic models. Sensitivity analyses showed that both flame propagation and extinction of cyclopentadiene/air mixtures flames depend notably on the fuel kinetics and subsequent intermediates such as cyclopentadienyl, cyclopentadienone, and cyclopentadienoxy. Analyses of the computed flame structures revealed that the high temperature oxidation of cyclopentadiene depends in general on the kinetics of first few intermediates in the oxidation process following the fuel consumption. The potential reaction pathways of the consumption of cyclopentadienyl radicals were discussed and further investigation and validation is recommended for two relevant reactions that could improve the high temperature oxidation kinetic model of cyclopentadiene. The experimental flame data of this study are the first ones to be reported.
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Effects of confinement, geometry, inlet velocity profile, and Reynolds number on the asymmetry of opposed-jet flows
Theoretical and Computational Fluid Dynamics, 2018Co-Authors: Abtin Ansari, Kevin K. Chen, Robert R. Burrell, Fokion N. EgolfopoulosAbstract:The opposed-jet Counterflow Configuration is widely used to measure fundamental flame properties that are essential targets for validating chemical kinetic models. The main and key assumption of the Counterflow Configuration in laminar flame experiments is that the flow field is steady and quasi-one-dimensional. In this study, experiments and numerical simulations were carried out to investigate the behavior and controlling parameters of Counterflowing isothermal air jets for various nozzle designs, Reynolds numbers, and surrounding geometries. The flow field in the jets’ impingement region was analyzed in search of instabilities, asymmetries, and two-dimensional effects that can introduce errors when the data are compared with results of quasi-one-dimensional simulations. The modeling involved transient axisymmetric numerical simulations along with bifurcation analysis, which revealed that when the flow field is confined between walls, local bifurcation occurs, which in turn results in asymmetry, deviation from the one-dimensional assumption, and sensitivity of the flow field structure to boundary conditions and surrounding geometry. Particle image velocimetry was utilized and results revealed that for jets of equal momenta at low Reynolds numbers of the order of 300, the flow field is asymmetric with respect to the middle plane between the nozzles even in the absence of confining walls. The asymmetry was traced to the asymmetric nozzle exit velocity profiles caused by unavoidable imperfections in the nozzle assembly. The asymmetry was not detectable at high Reynolds numbers of the order of 1000 due to the reduced sensitivity of the flow field to boundary conditions. The cases investigated computationally covered a wide range of Reynolds numbers to identify designs that are minimally affected by errors in the experimental procedures or manufacturing imperfections, and the simulations results were used to identify conditions that best conform to the assumptions of quasi-one-dimensional modeling.
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flame ignition in the Counterflow Configuration reassessing the experimental assumptions
Combustion and Flame, 2016Co-Authors: Abtin Ansari, Fokion N. EgolfopoulosAbstract:Abstract The Counterflow Configuration is widely used to study experimentally premixed and non-premixed flame ignition, with the advantage being that the data can be modeled using quasi one-dimensional codes. In this study, experiments and direct numerical simulations were carried out in order to assess the validity of the assumptions of the one-dimensional formulation. Experimentally, particle image velocimetry, shadowgraph, and a high-speed camera were employed to characterize the flow field before ignition, and to capture the ignition position and further evolution of the flame. The modeling involved axisymmetric numerical simulations using detailed molecular transport and chemical kinetic models. Both experiments and simulations revealed that if solid surfaces are present in the vicinity of the jets exit, the flow separates generating recirculation zones that are unstable and result in the bifurcation of the flow field. As a result, for a given set of boundary conditions at the burners’ exits, there exists two possible stable states of the flow field which have different velocity and scalars distribution, and the fuel concentration at which ignition occurs was determined to differ for these two states. A novel approach is proposed to correct for the unavoidable radial non-uniformity of the temperature profile at the exit of the heated jet and the conditions that do not result in bifurcation are outlined, so that the results from one-dimensional codes can be compared to the data with confidence.
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Ignition of Non-Premixed Flames of Ethylene/n-Dodecane Blends
Journal of Propulsion and Power, 2015Co-Authors: Fokion N. EgolfopoulosAbstract:Ignition temperatures of non-premixed flames of ethylene/n-dodecane blends were measured and modeled in the Counterflow Configuration at atmospheric pressure and an unburned fuel-carrying stream temperature of 453 K. Ethylene is an important product of the thermal decomposition of large molecular weight n-alkanes such as n-dodecane that produces ethylene via β scission. Thermal decomposition is expected in scramjet applications in which the fuel is also used as the vehicle and engine coolant. Thus, the ignition process of the mixtures of the parent molecule and products of decomposition could be among the controlling factors of operation in hypersonic propulsion. In the present study, laser Doppler velocimetry was used to measure local strain rates and thermocouples were used to measure ignition temperatures. Simulations of the experiments were performed using four kinetic models, and comparisons were made against the experimental data. Notable discrepancies were found between the data and the predictions...
