Moshe Matalon - Academia.edu (original) (raw)
Papers by Moshe Matalon
Combustion Theory and Modelling, 2016
We examine the dynamics of premixed flames in long, narrow, adiabatic channels focusing, in parti... more We examine the dynamics of premixed flames in long, narrow, adiabatic channels focusing, in particular, on the effects of gas compressibility on the propagation. Recognising the importance of the boundary conditions, we examine and compare three cases: flame propagation in channels open at both ends, where the pressure must adjust to the ambient pressure at both ends and the expanding gas is allowed to leave the channel freely, and flame propagation in channels that remain closed at one of the two ends, where the burned/unburned gas remains trapped between the flame and one of the two walls. Earlier studies have shown that a flame accelerates when travelling down a narrow channel as a result of the combined effects of wall friction and thermal expansion. In the present work we show that compressibility effects enhance the transition to fast accelerating flames in channels open at both ends and in channels closed at the ignition end. In both situations, the accelerating flames could reach values that, depending on the effective Mach number, are as large as fifty times the laminar flame speed. In contrast, when the channel is closed at the far end, the acceleration is limited and the propagation speed is damped as the flame approaches the far boundary. Moreover, we show that, in channels closed at their ignition end, the flame in sufficiently long channels evolves into a steadily propagating compression-driven flame. The propagation speed of these flames depends exponentially on the constant-volume equilibrium temperature, which is higher than the (constant pressure) adiabatic flame temperature, and is therefore larger than for ordinary isobaric flames. Fast propagating compression waves cannot emerge in channels that remain open at their ignition end because of the reduced pressure forced by the open boundary.
48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010
The of dynamics of an edge ame con ned in a narrow channel is studied numerically within the cont... more The of dynamics of an edge ame con ned in a narrow channel is studied numerically within the context of a di usive-thermal model. Fuel and oxidizer, separated upstream by a thin plate, ow through a channel with a prescribed velocity. At the tip of the plate, the fuel and oxidizer mix and, when ignited, an edge ame is sustained at some distance from the plate. The objective in this work is to determine the e ect of lateral con nement on the stando distance, the ame shape, and the ame stability. We consider a wide range of widths, allow for di erential di usion and examine the e ect heat loss to the channel wall.
45th AIAA Aerospace Sciences Meeting and Exhibit, 2007
The problem under consideration is concerned with the mixing and combustion of two mutually perpe... more The problem under consideration is concerned with the mixing and combustion of two mutually perpendicular streams, one containing fuel and the other oxidizer. Combustion occurs in the corner in the form of an edge-∞ame which consists of one or two premixed segments and a trailing difiusion ∞ame. The focus in this work is on ∞ame stabilization, onset of oscillations and/or blowofi. At low ∞ow rates the edge-∞ame is always stabilized near the corner. For su‐ciently small Lewis numbers the ∞ame is blown ofi when the ∞ow rate exceeds a critical value, but for Lewis numbers that are su‐ciently large increasing the ∞ow rate leads to spontaneous oscillations. The oscillations do not always persist as the ∞ow rate continues to increase; they are often amplifled leading to blowofi, or damped leading to restabilization. To better understand the phenomenon of ∞ame restabilization we have carried out a complete stability analysis of a similar yet simpler problem, namely a ∞at ∞ame on a porous-plug burner, which also exhibits a similar behavior.
Proceedings of the Combustion Institute, 2011
ABSTRACT In this paper we numerically examine the stabilization of an edge flame in a confined mi... more ABSTRACT In this paper we numerically examine the stabilization of an edge flame in a confined mixing layer. Unlike most previous theoretical studies which have assumed, for convenience, that the density is constant and independent of temperature, the present work realistically accounts for density variations and their effect on the flow field. The focus is on the effect of lateral confinement on the flame structure and standoff distance, for both adiabatic conditions and in the presence of conductive losses to the channel walls. The inability of the reactants to diffuse outwards, as they would in an unlimited mixing layer, promotes mixing and as a result the premixed flame segment extends further in the transverse direction, stands farther away from the tip of the splitter plate, and the trailing diffusion flame is much shorter. In very narrow channels the resulting flame is a planar premixed flame that consumes all the supplied reactants. Heat losses cause a drop in temperature and as a result, the premixed flame segment is limited near the channel axis, with a shorter diffusion flame trailing behind.
Proceedings of the Combustion Institute, 2002
The onset of instability and subsequent development of cells on spherically expanding flames is e... more The onset of instability and subsequent development of cells on spherically expanding flames is examined theoretically. The model used accounts for both hydrodynamic and diffusive-thermal effects and, in contrast to earlier theories, is valid for variable transport properties over a wide range of equivalence ratios. The analysis yields predictions for a number of flame properties, including growth rate of small disturbances, critical flame size for the instability onset, cell size beyond the threshold, and an estimate of the speed of the developing turbulent flame. It is shown that results using the more realistic temperature-dependent transport coefficients are more commensurate with experimental data concerning the critical conditions, that is, flame size or Peclet number, at the transition from one burning regime to another.
Proceedings of the Combustion Institute, 2007
ABSTRACT
Two streams, one containing fuel and the other oxidizer, are flowing into a relatively narrow cha... more Two streams, one containing fuel and the other oxidizer, are flowing into a relatively narrow channel where they mix and support an edge flame at some distance downstream. Our analysis is based on two models; one that fully couples the fluid dynamics and transport equations, used to determine the flame shape and location, and the other that assumes a constant-density flow, used to test the steady solutions for stability. It is found that in relatively wide channels the flame has a premixed, rounded edge with a trailing diffusion flame, but when the channel width decreases the flame is located further away from the supply and has a broader edge that can span the entire channel, when its width becomes comparable to the characteristic flame thickness. The effect of thermal expansion is to relocate the edge flame closer to the reactant supply. Heat losses at the channel walls cause a drop in the overall temperature and, as a result, the edge flame is confined to the center of the channe...
Combustion and Flame, 1999
ABSTRACT
Combustion and Flame, 2008
In recent studies of edge-flames it was found that when the characteristic gas velocity exceeds a... more In recent studies of edge-flames it was found that when the characteristic gas velocity exceeds a critical value the flame often undergoes spontaneous oscillations. The oscillations are amplified as the flow rate increases, reaching a maximum amplitude, and then decrease with further increasing flow rate until the flame restabilizes. In this paper we examine the concept of flame restabilization in a simpler but related problem-the planar premixed flame on a porous-plug burner-which is amenable to a full stability analysis. We show the dependence of all possible steady states on the relevant parameters, including the mass flow rate, the effective Lewis number of the mixture, the overall activation energy of the chemical reaction, and the extent of heat release. A linear stability analysis is then carried out to examine whether these steady states are stable to small disturbances. The analysis determines the critical conditions for the onset of instability, as well as the nature of the instability. In particular, we show that by decreasing the mass flow rate, the flame, which is at first stable, starts to oscillate back and forth for a limited range of gas velocities but is then restabilized by further decreasing the mass flow rate. We also show that the properties of the plug, such as the thickness of the plate and its porosity, play a significant role in flame stabilization.
Bulletin of the American Physical Society, 2017
The hydrodynamic instability which results from large density variations between the fresh mixtur... more The hydrodynamic instability which results from large density variations between the fresh mixture and the hot combustion products was discovered by Darrieus and Landau over seventy years ago, and has been named after its inventors. The instability, which prevents flames from being too flat, was thought to lead immediately to turbulent flames. Recent studies, initiated by weakly nonlinear analyses and extended by two-dimensional simulations suggest that this is not the case. It was established that the flame, beyond the onset of instability develops into a cusp-like structure pointing towards the burned gas region that propagates at a speed substantially large than the laminar flame speed. In this work, we present for the first time a systematic study of the bifurcation phenomena in the more realistic three-dimensional flow and extend this analysis to homogeneous isotropic turbulent flows. The computations are carried out within the context of the hydrodynamic theory where the flame is treated as a surface of density discontinuity separating burned gas from the fresh mixture, and propagates at a speed that depends on the local curvature and hydrodynamic strain rate. The asymptotic model derived from first principles exploits the multi-scale nature of the problem, specifically the difference between the flame thickness representing the diffusion length scale and the hydrodynamic length which is characteristic of the dimensions of the domain. The dependence of the local stretch rate experienced by the flame-a measure of the local flame surface curvature and the strain rate, is modulated by the Markstein length, which mimics effects of reaction and diffusion occurring inside the flame. This parameter is of the order of the flame thickness and for an experimental setting can be changed by varying the fuel type or its equivalence ratio or the ambient system pressure. A low Mach-number Navier-Stokes solver modified by an appropriate source term is used to determine the flow field that results from the gas expansion and the flame is tracked using a level-set methodology with a surface parameterization method employed to accurately capture the local velocity and stretch rate. Under laminar flow conditions, the hybrid numerical scheme is shown to recover the known exact solutions predicted in the weak gas expansion limit and corroborates the bifurcation results from linear stability analysis. The new conformations that evolve beyond the instability threshold have a sharp crest pointing towards the burned gas with ridges along the troughs, and propagate nearly 40% faster than planar flames.
Symposium (International) on Combustion, 1996
A diffusion flame may be characterized by the response of the burning rate to a properly defined ... more A diffusion flame may be characterized by the response of the burning rate to a properly defined Damkohler number, representing the ratio of diffusion time to chemical reaction time. Depending on the oxidant concentration in the ambient and on the Lewis numbers, the response may be an S-shaped or monotonic curve. While the S curve exhibits ignition and extinction phenomena, the monotonic curve indicates that there is a gradual transition from intense burning to a nearly frozen state and vice versa. Stability considerations indicate that spontaneous oscillations develop when the Lewis number is sufficiently large and/or heat losses are excessive. This suggests that the ignition/extinction conditions, normally associated with the turning points of the S curve, must be modified and related to the points of exchange of stability. For a monotonic response curve, the oscillations lead to extinction that is not predicted otherwise. The near-limit oscillations predicted here are qualitatively similar to those observed in the microgravity candle flame experiment, and the frequencies of oscillations predicted are of the same order of magnitude.
AIAA Journal, 1996
We examine the dynamic characteristics of the diffusion flame surrounding a droplet of liquid fue... more We examine the dynamic characteristics of the diffusion flame surrounding a droplet of liquid fuel in a reduced oxidant environment. The study is motivated by the experimental observations of candle flames in microgravity, which bear some similarity to the spherical diffusion flame that surrounds a burning liquid fuel droplet. The microgravity experiments reveal that, when the oxidant concentration in the chamber is sufficiently low, oscillations develop before extinction. A similar phenomenon is predicted here. An oscillatory state results for a sufficiently low oxidant concentration when radiative losses from the flame are appreciable. The frequencies of oscillations we predict are in good agreement with the experimental observations.
43rd AIAA Aerospace Sciences Meeting and Exhibit, 2005
Proceedings of the Combustion Institute, 2019
A fundamental study aimed at investigating the stabilization characteristics of edge flames estab... more A fundamental study aimed at investigating the stabilization characteristics of edge flames established in the near-wake of two merging streams, one containing fuel and the other oxidizer, is presented, with the main focus placed on the effects of the thermal interaction between the flame and the splitter plate. To this end, a diffusive-thermal model characterized by constant gas density and transport coefficients is used for conditions at which flame liftoff is likely to occur. It is assumed that the incoming streams are of equal strain rates, that the fuel and oxidizer are supplied in stoichiometric proportion, and that the mass diffusivities of the reactants are equal, such that the resulting combustion field is symmetric with respect to the centerline extending from the splitter plate. The results indicate that the plate has a negligible effect on the edge flame unless the tip of the plate intrudes into the preheat zone of the curved premixed flame segment forming the edge flame. In an overall adiabatic system, the heat conducted from the flame to the plate is completely recirculated back to the reactants via the lateral surfaces of the plate, thus supporting an excess enthalpy flame in the near-wake. The average output heat flux, defined as the total heat output through the lateral surfaces of the plate divided by the characteristic length associated with the temperature variation along the plate, is identified as an appropriate measure to characterize the heat recirculation efficiency.
Combustion Theory and Modelling, 2016
We examine in this study the structure and dynamic properties of an edge flame formed in the near... more We examine in this study the structure and dynamic properties of an edge flame formed in the near-wake of two initially separated shear flows, one containing fuel and the other oxidiser. A comprehensive study is carried out within the diffusive-thermal framework where the flow field, computed a-priori, is used for the determination of the combustion field. Our focus is on the effects of three controlling parameters: the Damköhler number controlling the overall flow rate, the oxidiser-to-fuel strain rate ratio of the supply streams that determines the extent of oxidiser entrainment towards the mixing zone, and the Lewis number, assumed equal for the fuel and oxidiser, that depends on the mixture composition. Response curves, representing the edge flame standoff distance as a function of Damköhler number, exhibit two distinct shapes: C-shaped and U-shaped curves characterising the response of low and high Lewis number flames, respectively. Stability considerations show that the upper solution branch of the C-shaped response curve is unstable and hence corresponds to physically unrealistic states, but due to heat conduction toward the cold plate the lower solution branch is always stable. The states forming this solution branch correspond to flame attachment, where the edge flame remains practically attached to the tip of the plate until it is blown off by the flow when the velocity exceeds a critical value. The U-shaped response, on the other hand, consists of equilibrium states that are globally stable. Thus, high Lewis number flames can be always stabilised near the splitter plate, with the edge held stationary or undergoing a back and forth motion, or lifted and stabilised downstream by the flow. Insight into the distinct stabilisation characteristics, exhibited by the different Lewis number cases, is given by examining the relationship between the local flow velocity and the edge propagation speed.
Physics of Fluids A: Fluid Dynamics, 1993
The thermocapillary motion generated within a spinning vaporizing droplet is described. Rotation ... more The thermocapillary motion generated within a spinning vaporizing droplet is described. Rotation induces a swirling flow in the surrounding gas. This in turn establishes a nonuniform vapor concentration and temperature at the droplet surface. An internal circulation is thus created from the interfacial temperature gradients. This internal motion, described in the limits of small Reynolds and Marangoni numbers, appears as a pair of toroidal vortices. Depending on whether the gaseous Lewis number, Le, is less than or greater than one, the temperature along the surface peaks at either the poles or the equator of the droplet. Consequently, the direction of the internal circulation is from the poles to the equator or vice versa.
ABSTRACT We employ a newly-derived diffusional-thermal flame model to examine the extinction beha... more ABSTRACT We employ a newly-derived diffusional-thermal flame model to examine the extinction behavior of a premixed flame in counterflow under near-stoichiometric conditions. The extinction criteria are found to depend on the strain rate, the mixture's equivalence ratio and two distinct Lewis numbers, corresponding to the fuel and oxidant. For a flame situated in a straining flow, the rate at which a reactant reaches the reaction zone is strongly affected by its molecular diffusivity. We demonstrate that, for such flames, it is possible for the initially excess reactant to be consumed at the flame if it is the less mobile of the two species, while the initially deficient species leaks through. This can have important implications on the extinction characteristics of the flame. We calculate critical values at extinction for flame standoff distance and strain rate. Our predicitions are found to be in good agreement with experimental results.
Submitted for the DFD08 Meeting of The American Physical Society Flame propagation in dusty gases... more Submitted for the DFD08 Meeting of The American Physical Society Flame propagation in dusty gases NICHOLAS POOLE, Northwestern University, MOSHE MATALON, University of Illinois at Urbana-Champaign-The combustion of finely atomized dust particles is of great importance to many practical technologies. While the study of flame propagation through gaseous fuel-air mixtures is relatively well developed, there currently exists a lack of fundamental understanding on the mechanisms that govern flame propagation through dust clouds. Unlike homogeneous gas flames, the study of metal particle combustion is highly complex, involving chemical and physical processes occurring in multiple phases. The mathematical description requires particular importance to be placed on the processes occurring in the condensed phase, for they present novel challenges in dust combustion modeling and ultimately provide its most distinguishing characteristics. Typical reaction zones are confined to a thin region of space and necessitate the use of perturbation methods with distinguished limits in order to capture the essential behavior of the system. Through matched asymptotic expansions we are able to arrive at an expression for the speed of the propagating flame front in terms of important combustion parameters such as mixture strength, particle loading, and particle size.
Combustion Theory and Modelling, 2016
We examine the dynamics of premixed flames in long, narrow, adiabatic channels focusing, in parti... more We examine the dynamics of premixed flames in long, narrow, adiabatic channels focusing, in particular, on the effects of gas compressibility on the propagation. Recognising the importance of the boundary conditions, we examine and compare three cases: flame propagation in channels open at both ends, where the pressure must adjust to the ambient pressure at both ends and the expanding gas is allowed to leave the channel freely, and flame propagation in channels that remain closed at one of the two ends, where the burned/unburned gas remains trapped between the flame and one of the two walls. Earlier studies have shown that a flame accelerates when travelling down a narrow channel as a result of the combined effects of wall friction and thermal expansion. In the present work we show that compressibility effects enhance the transition to fast accelerating flames in channels open at both ends and in channels closed at the ignition end. In both situations, the accelerating flames could reach values that, depending on the effective Mach number, are as large as fifty times the laminar flame speed. In contrast, when the channel is closed at the far end, the acceleration is limited and the propagation speed is damped as the flame approaches the far boundary. Moreover, we show that, in channels closed at their ignition end, the flame in sufficiently long channels evolves into a steadily propagating compression-driven flame. The propagation speed of these flames depends exponentially on the constant-volume equilibrium temperature, which is higher than the (constant pressure) adiabatic flame temperature, and is therefore larger than for ordinary isobaric flames. Fast propagating compression waves cannot emerge in channels that remain open at their ignition end because of the reduced pressure forced by the open boundary.
48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, 2010
The of dynamics of an edge ame con ned in a narrow channel is studied numerically within the cont... more The of dynamics of an edge ame con ned in a narrow channel is studied numerically within the context of a di usive-thermal model. Fuel and oxidizer, separated upstream by a thin plate, ow through a channel with a prescribed velocity. At the tip of the plate, the fuel and oxidizer mix and, when ignited, an edge ame is sustained at some distance from the plate. The objective in this work is to determine the e ect of lateral con nement on the stando distance, the ame shape, and the ame stability. We consider a wide range of widths, allow for di erential di usion and examine the e ect heat loss to the channel wall.
45th AIAA Aerospace Sciences Meeting and Exhibit, 2007
The problem under consideration is concerned with the mixing and combustion of two mutually perpe... more The problem under consideration is concerned with the mixing and combustion of two mutually perpendicular streams, one containing fuel and the other oxidizer. Combustion occurs in the corner in the form of an edge-∞ame which consists of one or two premixed segments and a trailing difiusion ∞ame. The focus in this work is on ∞ame stabilization, onset of oscillations and/or blowofi. At low ∞ow rates the edge-∞ame is always stabilized near the corner. For su‐ciently small Lewis numbers the ∞ame is blown ofi when the ∞ow rate exceeds a critical value, but for Lewis numbers that are su‐ciently large increasing the ∞ow rate leads to spontaneous oscillations. The oscillations do not always persist as the ∞ow rate continues to increase; they are often amplifled leading to blowofi, or damped leading to restabilization. To better understand the phenomenon of ∞ame restabilization we have carried out a complete stability analysis of a similar yet simpler problem, namely a ∞at ∞ame on a porous-plug burner, which also exhibits a similar behavior.
Proceedings of the Combustion Institute, 2011
ABSTRACT In this paper we numerically examine the stabilization of an edge flame in a confined mi... more ABSTRACT In this paper we numerically examine the stabilization of an edge flame in a confined mixing layer. Unlike most previous theoretical studies which have assumed, for convenience, that the density is constant and independent of temperature, the present work realistically accounts for density variations and their effect on the flow field. The focus is on the effect of lateral confinement on the flame structure and standoff distance, for both adiabatic conditions and in the presence of conductive losses to the channel walls. The inability of the reactants to diffuse outwards, as they would in an unlimited mixing layer, promotes mixing and as a result the premixed flame segment extends further in the transverse direction, stands farther away from the tip of the splitter plate, and the trailing diffusion flame is much shorter. In very narrow channels the resulting flame is a planar premixed flame that consumes all the supplied reactants. Heat losses cause a drop in temperature and as a result, the premixed flame segment is limited near the channel axis, with a shorter diffusion flame trailing behind.
Proceedings of the Combustion Institute, 2002
The onset of instability and subsequent development of cells on spherically expanding flames is e... more The onset of instability and subsequent development of cells on spherically expanding flames is examined theoretically. The model used accounts for both hydrodynamic and diffusive-thermal effects and, in contrast to earlier theories, is valid for variable transport properties over a wide range of equivalence ratios. The analysis yields predictions for a number of flame properties, including growth rate of small disturbances, critical flame size for the instability onset, cell size beyond the threshold, and an estimate of the speed of the developing turbulent flame. It is shown that results using the more realistic temperature-dependent transport coefficients are more commensurate with experimental data concerning the critical conditions, that is, flame size or Peclet number, at the transition from one burning regime to another.
Proceedings of the Combustion Institute, 2007
ABSTRACT
Two streams, one containing fuel and the other oxidizer, are flowing into a relatively narrow cha... more Two streams, one containing fuel and the other oxidizer, are flowing into a relatively narrow channel where they mix and support an edge flame at some distance downstream. Our analysis is based on two models; one that fully couples the fluid dynamics and transport equations, used to determine the flame shape and location, and the other that assumes a constant-density flow, used to test the steady solutions for stability. It is found that in relatively wide channels the flame has a premixed, rounded edge with a trailing diffusion flame, but when the channel width decreases the flame is located further away from the supply and has a broader edge that can span the entire channel, when its width becomes comparable to the characteristic flame thickness. The effect of thermal expansion is to relocate the edge flame closer to the reactant supply. Heat losses at the channel walls cause a drop in the overall temperature and, as a result, the edge flame is confined to the center of the channe...
Combustion and Flame, 1999
ABSTRACT
Combustion and Flame, 2008
In recent studies of edge-flames it was found that when the characteristic gas velocity exceeds a... more In recent studies of edge-flames it was found that when the characteristic gas velocity exceeds a critical value the flame often undergoes spontaneous oscillations. The oscillations are amplified as the flow rate increases, reaching a maximum amplitude, and then decrease with further increasing flow rate until the flame restabilizes. In this paper we examine the concept of flame restabilization in a simpler but related problem-the planar premixed flame on a porous-plug burner-which is amenable to a full stability analysis. We show the dependence of all possible steady states on the relevant parameters, including the mass flow rate, the effective Lewis number of the mixture, the overall activation energy of the chemical reaction, and the extent of heat release. A linear stability analysis is then carried out to examine whether these steady states are stable to small disturbances. The analysis determines the critical conditions for the onset of instability, as well as the nature of the instability. In particular, we show that by decreasing the mass flow rate, the flame, which is at first stable, starts to oscillate back and forth for a limited range of gas velocities but is then restabilized by further decreasing the mass flow rate. We also show that the properties of the plug, such as the thickness of the plate and its porosity, play a significant role in flame stabilization.
Bulletin of the American Physical Society, 2017
The hydrodynamic instability which results from large density variations between the fresh mixtur... more The hydrodynamic instability which results from large density variations between the fresh mixture and the hot combustion products was discovered by Darrieus and Landau over seventy years ago, and has been named after its inventors. The instability, which prevents flames from being too flat, was thought to lead immediately to turbulent flames. Recent studies, initiated by weakly nonlinear analyses and extended by two-dimensional simulations suggest that this is not the case. It was established that the flame, beyond the onset of instability develops into a cusp-like structure pointing towards the burned gas region that propagates at a speed substantially large than the laminar flame speed. In this work, we present for the first time a systematic study of the bifurcation phenomena in the more realistic three-dimensional flow and extend this analysis to homogeneous isotropic turbulent flows. The computations are carried out within the context of the hydrodynamic theory where the flame is treated as a surface of density discontinuity separating burned gas from the fresh mixture, and propagates at a speed that depends on the local curvature and hydrodynamic strain rate. The asymptotic model derived from first principles exploits the multi-scale nature of the problem, specifically the difference between the flame thickness representing the diffusion length scale and the hydrodynamic length which is characteristic of the dimensions of the domain. The dependence of the local stretch rate experienced by the flame-a measure of the local flame surface curvature and the strain rate, is modulated by the Markstein length, which mimics effects of reaction and diffusion occurring inside the flame. This parameter is of the order of the flame thickness and for an experimental setting can be changed by varying the fuel type or its equivalence ratio or the ambient system pressure. A low Mach-number Navier-Stokes solver modified by an appropriate source term is used to determine the flow field that results from the gas expansion and the flame is tracked using a level-set methodology with a surface parameterization method employed to accurately capture the local velocity and stretch rate. Under laminar flow conditions, the hybrid numerical scheme is shown to recover the known exact solutions predicted in the weak gas expansion limit and corroborates the bifurcation results from linear stability analysis. The new conformations that evolve beyond the instability threshold have a sharp crest pointing towards the burned gas with ridges along the troughs, and propagate nearly 40% faster than planar flames.
Symposium (International) on Combustion, 1996
A diffusion flame may be characterized by the response of the burning rate to a properly defined ... more A diffusion flame may be characterized by the response of the burning rate to a properly defined Damkohler number, representing the ratio of diffusion time to chemical reaction time. Depending on the oxidant concentration in the ambient and on the Lewis numbers, the response may be an S-shaped or monotonic curve. While the S curve exhibits ignition and extinction phenomena, the monotonic curve indicates that there is a gradual transition from intense burning to a nearly frozen state and vice versa. Stability considerations indicate that spontaneous oscillations develop when the Lewis number is sufficiently large and/or heat losses are excessive. This suggests that the ignition/extinction conditions, normally associated with the turning points of the S curve, must be modified and related to the points of exchange of stability. For a monotonic response curve, the oscillations lead to extinction that is not predicted otherwise. The near-limit oscillations predicted here are qualitatively similar to those observed in the microgravity candle flame experiment, and the frequencies of oscillations predicted are of the same order of magnitude.
AIAA Journal, 1996
We examine the dynamic characteristics of the diffusion flame surrounding a droplet of liquid fue... more We examine the dynamic characteristics of the diffusion flame surrounding a droplet of liquid fuel in a reduced oxidant environment. The study is motivated by the experimental observations of candle flames in microgravity, which bear some similarity to the spherical diffusion flame that surrounds a burning liquid fuel droplet. The microgravity experiments reveal that, when the oxidant concentration in the chamber is sufficiently low, oscillations develop before extinction. A similar phenomenon is predicted here. An oscillatory state results for a sufficiently low oxidant concentration when radiative losses from the flame are appreciable. The frequencies of oscillations we predict are in good agreement with the experimental observations.
43rd AIAA Aerospace Sciences Meeting and Exhibit, 2005
Proceedings of the Combustion Institute, 2019
A fundamental study aimed at investigating the stabilization characteristics of edge flames estab... more A fundamental study aimed at investigating the stabilization characteristics of edge flames established in the near-wake of two merging streams, one containing fuel and the other oxidizer, is presented, with the main focus placed on the effects of the thermal interaction between the flame and the splitter plate. To this end, a diffusive-thermal model characterized by constant gas density and transport coefficients is used for conditions at which flame liftoff is likely to occur. It is assumed that the incoming streams are of equal strain rates, that the fuel and oxidizer are supplied in stoichiometric proportion, and that the mass diffusivities of the reactants are equal, such that the resulting combustion field is symmetric with respect to the centerline extending from the splitter plate. The results indicate that the plate has a negligible effect on the edge flame unless the tip of the plate intrudes into the preheat zone of the curved premixed flame segment forming the edge flame. In an overall adiabatic system, the heat conducted from the flame to the plate is completely recirculated back to the reactants via the lateral surfaces of the plate, thus supporting an excess enthalpy flame in the near-wake. The average output heat flux, defined as the total heat output through the lateral surfaces of the plate divided by the characteristic length associated with the temperature variation along the plate, is identified as an appropriate measure to characterize the heat recirculation efficiency.
Combustion Theory and Modelling, 2016
We examine in this study the structure and dynamic properties of an edge flame formed in the near... more We examine in this study the structure and dynamic properties of an edge flame formed in the near-wake of two initially separated shear flows, one containing fuel and the other oxidiser. A comprehensive study is carried out within the diffusive-thermal framework where the flow field, computed a-priori, is used for the determination of the combustion field. Our focus is on the effects of three controlling parameters: the Damköhler number controlling the overall flow rate, the oxidiser-to-fuel strain rate ratio of the supply streams that determines the extent of oxidiser entrainment towards the mixing zone, and the Lewis number, assumed equal for the fuel and oxidiser, that depends on the mixture composition. Response curves, representing the edge flame standoff distance as a function of Damköhler number, exhibit two distinct shapes: C-shaped and U-shaped curves characterising the response of low and high Lewis number flames, respectively. Stability considerations show that the upper solution branch of the C-shaped response curve is unstable and hence corresponds to physically unrealistic states, but due to heat conduction toward the cold plate the lower solution branch is always stable. The states forming this solution branch correspond to flame attachment, where the edge flame remains practically attached to the tip of the plate until it is blown off by the flow when the velocity exceeds a critical value. The U-shaped response, on the other hand, consists of equilibrium states that are globally stable. Thus, high Lewis number flames can be always stabilised near the splitter plate, with the edge held stationary or undergoing a back and forth motion, or lifted and stabilised downstream by the flow. Insight into the distinct stabilisation characteristics, exhibited by the different Lewis number cases, is given by examining the relationship between the local flow velocity and the edge propagation speed.
Physics of Fluids A: Fluid Dynamics, 1993
The thermocapillary motion generated within a spinning vaporizing droplet is described. Rotation ... more The thermocapillary motion generated within a spinning vaporizing droplet is described. Rotation induces a swirling flow in the surrounding gas. This in turn establishes a nonuniform vapor concentration and temperature at the droplet surface. An internal circulation is thus created from the interfacial temperature gradients. This internal motion, described in the limits of small Reynolds and Marangoni numbers, appears as a pair of toroidal vortices. Depending on whether the gaseous Lewis number, Le, is less than or greater than one, the temperature along the surface peaks at either the poles or the equator of the droplet. Consequently, the direction of the internal circulation is from the poles to the equator or vice versa.
ABSTRACT We employ a newly-derived diffusional-thermal flame model to examine the extinction beha... more ABSTRACT We employ a newly-derived diffusional-thermal flame model to examine the extinction behavior of a premixed flame in counterflow under near-stoichiometric conditions. The extinction criteria are found to depend on the strain rate, the mixture's equivalence ratio and two distinct Lewis numbers, corresponding to the fuel and oxidant. For a flame situated in a straining flow, the rate at which a reactant reaches the reaction zone is strongly affected by its molecular diffusivity. We demonstrate that, for such flames, it is possible for the initially excess reactant to be consumed at the flame if it is the less mobile of the two species, while the initially deficient species leaks through. This can have important implications on the extinction characteristics of the flame. We calculate critical values at extinction for flame standoff distance and strain rate. Our predicitions are found to be in good agreement with experimental results.
Submitted for the DFD08 Meeting of The American Physical Society Flame propagation in dusty gases... more Submitted for the DFD08 Meeting of The American Physical Society Flame propagation in dusty gases NICHOLAS POOLE, Northwestern University, MOSHE MATALON, University of Illinois at Urbana-Champaign-The combustion of finely atomized dust particles is of great importance to many practical technologies. While the study of flame propagation through gaseous fuel-air mixtures is relatively well developed, there currently exists a lack of fundamental understanding on the mechanisms that govern flame propagation through dust clouds. Unlike homogeneous gas flames, the study of metal particle combustion is highly complex, involving chemical and physical processes occurring in multiple phases. The mathematical description requires particular importance to be placed on the processes occurring in the condensed phase, for they present novel challenges in dust combustion modeling and ultimately provide its most distinguishing characteristics. Typical reaction zones are confined to a thin region of space and necessitate the use of perturbation methods with distinguished limits in order to capture the essential behavior of the system. Through matched asymptotic expansions we are able to arrive at an expression for the speed of the propagating flame front in terms of important combustion parameters such as mixture strength, particle loading, and particle size.