The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Ahmed F Ghoniem - One of the best experts on this subject based on the ideXlab platform.
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leading edge dynamics of lean premixed flames stabilized on a Bluff Body
Combustion and Flame, 2018Co-Authors: Dan Michaels, Ahmed F GhoniemAbstract:Abstract This paper examines the dynamics of the flame leading edge in a laminar premixed CH4/air flame stabilized on a Bluff Body in a channel. Harmonic fluctuations and step velocity change are used to simulate the flame response to acoustic oscillations, which are of primary importance in the study of thermo-acoustic instabilities. We use a fully resolved unsteady two-dimensional code with detailed chemistry and species transport, with coupled heat transfer to the Bluff Body. Calculations were conducted with different equivalence ratios, Body materials, and steady state inlet velocity with step or harmonic perturbations. Results reveal that the flame leading edge dynamics displays a peak response around St = 0.5 suggesting that the leading edge motion is mainly due to the advection of appropriate ignition conditions as a result of the excitement of the wake recirculating flow. There is considerable augmentation of the flame wrinkles generated by the flame leading edge motion as result of the flow–flame interaction. Additionally, we show that a flame that anchors on average further upstream leads to stronger damping of the shear layer vortices and thus weaker vortex-flame interaction and heat release fluctuations. Hence, we identify two different mechanisms by which the flame leading edge location and oscillation amplitude impact heat release fluctuations. The study suggests a stronger dependence of the overall flame wrinkling and heat release fluctuations on the flame leading edge dynamics than recognized previously and the potential role it plays in combustion dynamics.
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impact of the Bluff Body material on the flame leading edge structure and flame flow interaction of premixed ch4 air flames
Combustion and Flame, 2016Co-Authors: Dan Michaels, Ahmed F GhoniemAbstract:Abstract In this paper we investigate the interaction between the flame structure, the flow field and the coupled heat transfer with the flame holder of a laminar lean premixed CH 4 /air flame stabilized on a heat conducting Bluff Body in a channel. The study is conducted with a 2-D direct numerical simulation with detailed chemistry and species transport and with no artificial flame anchoring boundary conditions. Capturing the multiple time scales, length scales and flame-wall thermal interaction was done using a low Mach number operator-split projection algorithm, coupled with a block-structured adaptive mesh refinement and an immersed boundary method for the solid Body. The flame structure displays profiles of the main species and atomic ratios similar to previously published experimental measurements on an annular Bluff Body configuration for both laminar and turbulent flow, demonstrating generality of the resolved flame leading edge structure for flames that stabilize on a sudden expansion. The flame structure near the Bluff Body and further downstream shows dependence on the thermal properties of the Bluff Body. We analyze the influence of flow strain and heat losses on the flame, and show that the flame stretch increases sharply at the flame leading edge, and this high stretch rate, together with heat losses, dictate the flame anchoring location. By analyzing the impact of the flame on the flow field we reveal that the strong dependence of vorticity dilatation on the flame location leads to high impact of the flame anchoring location on the flow and flame stretch downstream. This study sheds light on the impact of heat losses to the flame holder on the flame–flow feedback mechanism in lean premixed flames.
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the blow off mechanism of a Bluff Body stabilized laminar premixed flame
Combustion and Flame, 2015Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the dynamics leading to blow-off of a laminar premixed flame stabilized on a confined Bluff-Body using high fidelity numerical simulations. We used unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. The flame–wall interaction between the hot reactants and the heat conducting Bluff-Body was accurately captured by incorporating the conjugate heat exchange between them. Simulations showed a shear-layer stabilized flame just downstream of the Bluff-Body, with a recirculation zone formed by the products of combustion. The flame was negatively stretched along its entire length, primarily dominated by the normal component of the strain. Blow-off was approached by decreasing the mixture equivalence ratio, at a fixed Reynolds number, of the incoming flow. A flame is stable (does not undergo blow-off) when (1) flame displacement speed is equal to the flow speed and (2) the gradient of the flame displacement speed normal to its surface is higher than the gradient of the flow speed along the same direction. As the equivalence ratio is reduced, the difference between the former and the latter shrinks until the dynamic stability condition (2) is violated, leading to blow-off. Blow-off initiates at a location where this is first violated along the flame. Our results showed that this location was far downstream from the flame anchoring zone, near the end of the recirculation zone. Blow-off started by flame pinching separating the flame into an upstream moving (carried within the recirculation zone) and a downstream convecting (detached from the recirculation zone) flame piece. Within the range of operating conditions investigated, the conjugate heat exchange with the Bluff-Body had no impact on the flame blow-off.
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the response of a harmonically forced premixed flame stabilized on a heat conducting Bluff Body
Proceedings of the Combustion Institute, 2015Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the unsteady response of a Bluff-Body stabilized laminar premixed flame to harmonic inlet velocity excitation. A time series analysis was performed to analyze the physical sequence of events at a fixed longitudinal forcing frequency of 100 Hz for cases with (1) two different equivalence ratios and (2) two different thermal properties of the stabilizing Bluff-Body. It was observed that conjugate heat exchange between the heat conducting Bluff-Body and the surrounding reacting flow has a crucial impact on the dynamic response. The flame area and anchoring location, the net conjugate heat transfer and the total heat release underwent significant oscillations. The latter was mean shifted and had multiple frequencies. The burning velocity varied significantly along the flame length and the recirculation zone underwent complex changes in its shape and size during an unsteady cycle. The lower equivalence ratio case exhibited vortex shedding after an initial symmetric response with periodic flame extinction and re-ignition along its surface, unlike the higher equivalence ratio case. The metal/ceramic Bluff-Body showed a net heat transfer directed from/to the Bluff-Body, to/from the reacting flow during an unsteady cycle, resulting in a significantly different flame response for the two otherwise equivalent cases.
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the anchoring mechanism of a Bluff Body stabilized laminar premixed flame
Combustion and Flame, 2014Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the mechanism of the laminar premixed flame anchoring near a heat-conducting Bluff-Body. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. No artificial flame anchoring boundary conditions were imposed. Simulations show a shear-layer stabilized flame just downstream of the Bluff-Body, with a recirculation zone formed by the products of combustion. A steel Bluff-Body resulted in a slightly larger recirculation zone than a ceramic Bluff-Body; the size of which grew as the equivalence ratio was decreased. A significant departure from the conventional two-zone flame-structure is shown in the anchoring region. In this region, the reaction zone is associated with a large negative energy convection (directed from products to reactants) resulting in a negative flame-displacement speed. It is shown that the premixed flame anchors at an immediate downstream location near the Bluff-Body where favorable ignition conditions are established; a region associated with (1) a sufficiently high temperature impacted by the conjugate heat exchange between the heat-conducting Bluff-Body and the hot reacting flow and (2) a locally maximum stoichiometry characterized by the preferential diffusion effects.
Kushal S Kedia - One of the best experts on this subject based on the ideXlab platform.
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the blow off mechanism of a Bluff Body stabilized laminar premixed flame
Combustion and Flame, 2015Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the dynamics leading to blow-off of a laminar premixed flame stabilized on a confined Bluff-Body using high fidelity numerical simulations. We used unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. The flame–wall interaction between the hot reactants and the heat conducting Bluff-Body was accurately captured by incorporating the conjugate heat exchange between them. Simulations showed a shear-layer stabilized flame just downstream of the Bluff-Body, with a recirculation zone formed by the products of combustion. The flame was negatively stretched along its entire length, primarily dominated by the normal component of the strain. Blow-off was approached by decreasing the mixture equivalence ratio, at a fixed Reynolds number, of the incoming flow. A flame is stable (does not undergo blow-off) when (1) flame displacement speed is equal to the flow speed and (2) the gradient of the flame displacement speed normal to its surface is higher than the gradient of the flow speed along the same direction. As the equivalence ratio is reduced, the difference between the former and the latter shrinks until the dynamic stability condition (2) is violated, leading to blow-off. Blow-off initiates at a location where this is first violated along the flame. Our results showed that this location was far downstream from the flame anchoring zone, near the end of the recirculation zone. Blow-off started by flame pinching separating the flame into an upstream moving (carried within the recirculation zone) and a downstream convecting (detached from the recirculation zone) flame piece. Within the range of operating conditions investigated, the conjugate heat exchange with the Bluff-Body had no impact on the flame blow-off.
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the response of a harmonically forced premixed flame stabilized on a heat conducting Bluff Body
Proceedings of the Combustion Institute, 2015Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the unsteady response of a Bluff-Body stabilized laminar premixed flame to harmonic inlet velocity excitation. A time series analysis was performed to analyze the physical sequence of events at a fixed longitudinal forcing frequency of 100 Hz for cases with (1) two different equivalence ratios and (2) two different thermal properties of the stabilizing Bluff-Body. It was observed that conjugate heat exchange between the heat conducting Bluff-Body and the surrounding reacting flow has a crucial impact on the dynamic response. The flame area and anchoring location, the net conjugate heat transfer and the total heat release underwent significant oscillations. The latter was mean shifted and had multiple frequencies. The burning velocity varied significantly along the flame length and the recirculation zone underwent complex changes in its shape and size during an unsteady cycle. The lower equivalence ratio case exhibited vortex shedding after an initial symmetric response with periodic flame extinction and re-ignition along its surface, unlike the higher equivalence ratio case. The metal/ceramic Bluff-Body showed a net heat transfer directed from/to the Bluff-Body, to/from the reacting flow during an unsteady cycle, resulting in a significantly different flame response for the two otherwise equivalent cases.
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the anchoring mechanism of a Bluff Body stabilized laminar premixed flame
Combustion and Flame, 2014Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the mechanism of the laminar premixed flame anchoring near a heat-conducting Bluff-Body. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. No artificial flame anchoring boundary conditions were imposed. Simulations show a shear-layer stabilized flame just downstream of the Bluff-Body, with a recirculation zone formed by the products of combustion. A steel Bluff-Body resulted in a slightly larger recirculation zone than a ceramic Bluff-Body; the size of which grew as the equivalence ratio was decreased. A significant departure from the conventional two-zone flame-structure is shown in the anchoring region. In this region, the reaction zone is associated with a large negative energy convection (directed from products to reactants) resulting in a negative flame-displacement speed. It is shown that the premixed flame anchors at an immediate downstream location near the Bluff-Body where favorable ignition conditions are established; a region associated with (1) a sufficiently high temperature impacted by the conjugate heat exchange between the heat-conducting Bluff-Body and the hot reacting flow and (2) a locally maximum stoichiometry characterized by the preferential diffusion effects.
Baki M Cetegen - One of the best experts on this subject based on the ideXlab platform.
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lean blowoff behavior of asymmetrically fueled Bluff Body stabilized flames
Combustion and Flame, 2013Co-Authors: Steven G Tuttle, Swetaprovo Chaudhuri, Michael W Renfro, Baki M Cetegen, Kristin M Koppvaughan, Trevor R Jensen, Jeffrey M CohenAbstract:Abstract Bluff-Body stabilized flames were studied in an enclosed, asymmetrically-fueled duct with a two-dimensional triangular flame holder. Acetone laser-induced fluorescence was used to characterize the fuel distribution for both uniform and non-uniform fuel profiles. Flame dynamics were captured with high-speed chemiluminescence imaging during stable operation and near blow off conditions for three cases with varying fuel–air gradients across the flame holder. Particle imaging velocimetry was used to measure the velocity field. It was discovered that for a given velocity, increased fuel profile asymmetry caused an increase in the blowoff equivalence ratio, produced greater vortex shedding coherence, and for lower velocities resulted in dynamic coupling between the heat release and the duct acoustics. High-speed imaging of the acoustically uncoupled cases revealed the same flame blowoff process as previously observed in uniformly fueled cases. The blow off process in the acoustically coupled cases was dominated by acoustically influenced velocity straining the flame adjacent to the wake stagnation zone causing local extinction and rapid entrainment of reactants into the recirculation zone. From the Mie scattering images gathered for PIV, density transition contours were extracted and used as flame contours to calculate local aerodynamic strain rates and curvature. Statistics revealed conditional relationships between the local strain, wake geometry and fluid mechanics.
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time resolved blowoff transition measurements for two dimensional Bluff Body stabilized flames in vitiated flow
Combustion and Flame, 2012Co-Authors: Steven G Tuttle, Swetaprovo Chaudhuri, Stanislav Kostka, Baki M Cetegen, Kristin M Koppvaughan, Trevor R Jensen, Michael W RenfroAbstract:Abstract Flame holding and blowoff characteristics of Bluff-Body stabilized, turbulent flames were measured in an enclosed rectangular duct with a triangular flame holder in vitiated, premixed flows. Blowoff stability margins were characterized with chemiluminescence measurements performed by high-speed imaging to capture flame dynamics during the approach to flame blow off. As the equivalence ratio was decreased, local extinctions along the flames interacting with shear layers surrounding the Bluff Body recirculation zone occurred with greater frequency and proximity to the wake stagnation zone. Decreased equivalence ratio resulted in extinction events at the trailing edge of the stagnation zone, which allowed reactants to be convected into the recirculation zone and burned behind the Bluff Body. Increasing reactant dilution of the recirculation zone eventually resulted in flame lift-off or extinction of the flame in the neighboring shear layer. These near field shear layer flames convected to the wake stagnation zone, and were eventually quenched. Simultaneous particle imaging velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) measurements captured the flame edge location and aerodynamic behavior as blowoff was approached. Two-dimensional hydrodynamic stretch along the flame front and flow field vorticity maps were extracted from the combined PIV/OH PLIF data. The distribution of flame stretch shifted to greater values as the equivalence ratio decreased and is believed to be the cause of local flame extinction in the wake stagnation zone that starts the blowoff process.
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blowoff dynamics of Bluff Body stabilized turbulent premixed flames
Combustion and Flame, 2010Co-Authors: Swetaprovo Chaudhuri, Stanislav Kostka, Michael W Renfro, Baki M CetegenAbstract:This article concerns the flame dynamics of a Bluff Body stabilized turbulent premixed flame as it approaches lean blowoff. Time resolved chemiluminescence imaging along with simultaneous particle image velocimetry and OH planar laser-induced fluorescence were utilized in an axisymmetric Bluff Body stabilized, propane-air flame to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental results. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. For a stably burning conical flame away from blowoff, the flame front envelopes the shear layer vortices. Near blowoff, the columnar flame front and shear layer vortices overlap to induce high local stretch rates that exceed the extinction stretch rates instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following shear layer extinction, fresh reactants can pass through the shear layers to react within the recirculation zone with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for a few milliseconds and can reignite the shear layers such that the entire flame is reestablished for a short period. This extinction and reignition event can happen several times before final blowoff which occurs when the flame kernel fails to reignite the shear layers and ultimately leads to total flame extinguishment.
Simone Hochgreb - One of the best experts on this subject based on the ideXlab platform.
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temperature measurements of the Bluff Body surface of a swirl burner using phosphor thermometry
Combustion and Flame, 2014Co-Authors: Matthias Euler, Ruigang Zhou, Simone Hochgreb, A DreizlerAbstract:Abstract Flames are often stabilised on Bluff-bodies, yet their surface temperatures are rarely measured. This paper presents temperature measurements for the Bluff Body surface of the Cambridge/Sandia Stratified Swirl Burner. The flame is stabilized by a Bluff Body, designed to provide a series of turbulent premixed and stratified methane/air flames with a variable degree of swirl and stratification. Recently, modellers have raised concerns about the role of surface temperature on the resulting gas temperatures and the overall heat loss of the burner. Laser-induced phosphorescence is used to measure surface temperatures, with Mg4GeO6F:Mn as the excitation phosphor, creating a spatially resolved temperature map. Results show that the temperature of the Bluff Body is in the range 550–900 K for different operating conditions. The temperature distribution is strongly correlated with the degree of swirl and local equivalence ratio, reflecting the temperature distribution obtained in the gas phase. The overall heat loss represents only a small fraction (
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effects of preferential transport in turbulent Bluff Body stabilized lean premixed ch4 air flames
Combustion and Flame, 2012Co-Authors: R S Barlow, Matthew J Dunn, Mark Sweeney, Simone HochgrebAbstract:Abstract Preferential species diffusion is known to have important effects on local flame structure in turbulent premixed flames, and differential diffusion of heat and mass can have significant effects on both local flame structure and global flame parameters, such as turbulent flame speed. However, models for turbulent premixed combustion normally assume that atomic mass fractions are conserved from reactants to fully burnt products. Experiments reported here indicate that this basic assumption may be incorrect for an important class of turbulent flames. Measurements of major species and temperature in the near field of turbulent, Bluff-Body stabilized, lean premixed methane–air flames (Le = 0.98) reveal significant departures from expected conditional mean compositional structure in the combustion products as well as within the flame. Net increases exceeding 10% in the equivalence ratio and the carbon-to-hydrogen atom ratio are observed across the turbulent flame brush. Corresponding measurements across an unstrained laminar flame at similar equivalence ratio are in close agreement with calculations performed using Chemkin with the GRI 3.0 mechanism and multi-component transport, confirming accuracy of experimental techniques. Results suggest that the large effects observed in the turbulent Bluff-Body burner are cause by preferential transport of H 2 and H 2 O through the preheat zone ahead of CO 2 and CO, followed by convective transport downstream and away from the local flame brush. This preferential transport effect increases with increasing velocity of reactants past the Bluff Body and is apparently amplified by the presence of a strong recirculation zone where excess CO 2 is accumulated.
Swetaprovo Chaudhuri - One of the best experts on this subject based on the ideXlab platform.
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lean blowoff behavior of asymmetrically fueled Bluff Body stabilized flames
Combustion and Flame, 2013Co-Authors: Steven G Tuttle, Swetaprovo Chaudhuri, Michael W Renfro, Baki M Cetegen, Kristin M Koppvaughan, Trevor R Jensen, Jeffrey M CohenAbstract:Abstract Bluff-Body stabilized flames were studied in an enclosed, asymmetrically-fueled duct with a two-dimensional triangular flame holder. Acetone laser-induced fluorescence was used to characterize the fuel distribution for both uniform and non-uniform fuel profiles. Flame dynamics were captured with high-speed chemiluminescence imaging during stable operation and near blow off conditions for three cases with varying fuel–air gradients across the flame holder. Particle imaging velocimetry was used to measure the velocity field. It was discovered that for a given velocity, increased fuel profile asymmetry caused an increase in the blowoff equivalence ratio, produced greater vortex shedding coherence, and for lower velocities resulted in dynamic coupling between the heat release and the duct acoustics. High-speed imaging of the acoustically uncoupled cases revealed the same flame blowoff process as previously observed in uniformly fueled cases. The blow off process in the acoustically coupled cases was dominated by acoustically influenced velocity straining the flame adjacent to the wake stagnation zone causing local extinction and rapid entrainment of reactants into the recirculation zone. From the Mie scattering images gathered for PIV, density transition contours were extracted and used as flame contours to calculate local aerodynamic strain rates and curvature. Statistics revealed conditional relationships between the local strain, wake geometry and fluid mechanics.
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time resolved blowoff transition measurements for two dimensional Bluff Body stabilized flames in vitiated flow
Combustion and Flame, 2012Co-Authors: Steven G Tuttle, Swetaprovo Chaudhuri, Stanislav Kostka, Baki M Cetegen, Kristin M Koppvaughan, Trevor R Jensen, Michael W RenfroAbstract:Abstract Flame holding and blowoff characteristics of Bluff-Body stabilized, turbulent flames were measured in an enclosed rectangular duct with a triangular flame holder in vitiated, premixed flows. Blowoff stability margins were characterized with chemiluminescence measurements performed by high-speed imaging to capture flame dynamics during the approach to flame blow off. As the equivalence ratio was decreased, local extinctions along the flames interacting with shear layers surrounding the Bluff Body recirculation zone occurred with greater frequency and proximity to the wake stagnation zone. Decreased equivalence ratio resulted in extinction events at the trailing edge of the stagnation zone, which allowed reactants to be convected into the recirculation zone and burned behind the Bluff Body. Increasing reactant dilution of the recirculation zone eventually resulted in flame lift-off or extinction of the flame in the neighboring shear layer. These near field shear layer flames convected to the wake stagnation zone, and were eventually quenched. Simultaneous particle imaging velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) measurements captured the flame edge location and aerodynamic behavior as blowoff was approached. Two-dimensional hydrodynamic stretch along the flame front and flow field vorticity maps were extracted from the combined PIV/OH PLIF data. The distribution of flame stretch shifted to greater values as the equivalence ratio decreased and is believed to be the cause of local flame extinction in the wake stagnation zone that starts the blowoff process.
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blowoff dynamics of Bluff Body stabilized turbulent premixed flames
Combustion and Flame, 2010Co-Authors: Swetaprovo Chaudhuri, Stanislav Kostka, Michael W Renfro, Baki M CetegenAbstract:This article concerns the flame dynamics of a Bluff Body stabilized turbulent premixed flame as it approaches lean blowoff. Time resolved chemiluminescence imaging along with simultaneous particle image velocimetry and OH planar laser-induced fluorescence were utilized in an axisymmetric Bluff Body stabilized, propane-air flame to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental results. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. For a stably burning conical flame away from blowoff, the flame front envelopes the shear layer vortices. Near blowoff, the columnar flame front and shear layer vortices overlap to induce high local stretch rates that exceed the extinction stretch rates instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following shear layer extinction, fresh reactants can pass through the shear layers to react within the recirculation zone with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for a few milliseconds and can reignite the shear layers such that the entire flame is reestablished for a short period. This extinction and reignition event can happen several times before final blowoff which occurs when the flame kernel fails to reignite the shear layers and ultimately leads to total flame extinguishment.