The Experts below are selected from a list of 21204 Experts worldwide ranked by ideXlab platform
Ahmed F Ghoniem - One of the best experts on this subject based on the ideXlab platform.
-
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.
-
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.
-
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.
-
mechanisms of stabilization and blowoff of a Premixed Flame downstream of a heat conducting perforated plate
Combustion and Flame, 2012Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the Flame stabilization mechanism and the conditions leading to the blowoff of a laminar Premixed Flame anchored downstream of a heat-conducting perforated-plate/multi-hole burner, with overall nearly adiabatic conditions. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane-air combustion. Results show a bell-shaped Flame stabilizing above the burner plate hole, with a U-shaped section anchored between neighboring holes. The base of the positively curved U-shaped section of the Flame is positioned near the stagnation point, at a location where the Flame displacement speed is equal to the flow speed. This location is determined by the combined effect of heat loss and Flame stretch on the Flame displacement speed. As the mass flow rate of the reactants is increased, the Flame displacement speed at this location varies non-monotonically. As the inlet velocity is increased, the recirculation zone grows slowly, the Flame moves downstream, and the heat loss to the burner decreases, strengthening the Flame and increasing its displacement speed. As the inlet velocity is raised, the stagnation point moves downstream, and the Flame length grows to accommodate the reactants mass flow. Concomitantly, the radius of curvature of the Flame base decreases until it reaches an almost constant value, comparable to the Flame thickness. While the heat loss decreases, the higher Flame curvature dominates thereby reducing the displacement speed of the Flame base. For a stable Flame, the gradient of the Flame base displacement speed normal to the Flame is higher than the gradient of the flow speed along the same direction, leading to dynamic stability. As inlet velocity is raised further, the former decreases while the latter increases until the stability condition is violated, leading to blowoff. The Flame speed during blow off is determined by the feedback between the growing recirculation zone and the cooling burner plate.
-
response of a laminar Premixed Flame to flow oscillations a kinematic model and thermoacoustic instability results
Combustion and Flame, 1996Co-Authors: M Fleifil, Anuradha M Annaswamy, Z Ghoneim, Ahmed F GhoniemAbstract:Combustion instability is a resonance phenomenon that arises due to the coupling between the system acoustics and the unsteady heat release. The constructive feedback between the two processes, which is known to occur as a certain phase relationship between the pressure and the unsteady heat release rate is satisfied, depends on many parameters among which is the acoustic mode, the Flame holder characteristics, and the dominant burning pattern. In this paper, we construct an analytical model to describe the dynamic response of a laminar Premixed Flame stabilized on the rim of a tube to velocity oscillation. We consider uniform and nonuniform velocity perturbations superimposed on a pipe flow velocity profile. The model results show that the magnitude of heat release perturbation and its phase with respect to the dynamic perturbation depend primarily on the Flame Strouhal number, representing the ratio of the dominant frequency times the tube radius to the laminar burning velocity. In terms of this number, high-frequency perturbations pass through the Flame while low frequencies lead to a strong response. The phase with respect to the velocity perturbation behaves in the opposite way. Results of this model are shown to agree with experimental observations and to be useful in determining how the combustion excited mode is selected among all the acoustic unstable modes. The model is then used to obtain a time-domain differential equation describing the relationship between the velocity perturbation and the heat release response over the entire frequency range.
Mamoru Tanahashi - One of the best experts on this subject based on the ideXlab platform.
-
Flame propagation and heat transfer characteristics of a hydrogen air Premixed Flame in a constant volume vessel
International Journal of Hydrogen Energy, 2016Co-Authors: Basmil Yenerdag, Masayasu Shimura, Yuki Minamoto, Yoshitsugu Naka, Mamoru TanahashiAbstract:Abstract Direct numerical simulation of a stoichiometric hydrogen–air turbulent Premixed Flame in a rectangular constant volume vessel has been conducted to gain fundamental insights into turbulence–Flame interactions and heat loss characteristics under a pressure rising condition. The turbulent vortices are significantly weakened in the burnt side due to expansion and viscosity increase. The conditionally averaged turbulent kinetic energy is suppressed near the wall and it takes mostly constant in the rest of the domain which suggests that there is no significant turbulence production near the wall, and the wall just damps the turbulence in the present simulation's set up. It is found that the turbulent kinetic energy still takes a significant value in the burnt side due to the velocity induced by the expansion, not much inherent to the turbulent eddies. Temporal evolution of the wall heat loss characteristics is studied. Although the heat loss induced by the burnt gas dominates the total heat loss at the end of combustion, the heat loss induced by Flame impingements contributes to substantial portion of total heat loss. Finally, turbulence–Flame interactions modeling is studied using a conventional Flamelet model for reaction rate closure. The result shows that turbulence–Flame interaction mechanism does not change significantly under pressure rising conditions, suggesting that this model could be also used in a pressure-evolving combustion system without further modifications.
-
dns of turbulent swirling Premixed Flame in a micro gas turbine combustor
Proceedings of the Combustion Institute, 2011Co-Authors: Shoichi Tanaka, Masayasu Shimura, Naoya Fukushima, Mamoru Tanahashi, Toshio MiyauchiAbstract:Abstract Direct numerical simulation (DNS) of hydrogen–air turbulent Premixed Flame in swirling flow is conducted to investigate hydrodynamic behaviors, characteristics of Flame structures and heat transfer properties of a micro gas turbine combustor. The micro combustor is assumed to have a cuboid combustion chamber which is 15.0 mm in the streamwise direction with 10.0 mm × 10.0 mm cross-section. To investigate swirling effects on flow and combustion field, DNS of both cold and reactive flow is carried out under two swirl number conditions: S = 0.6 and 1.2 . Large-scale coherent structures such as ring-shaped or helical structures are generated near the inlet of combustion chamber, and a lot of small-scale vortices appear downstream. Small scale eddies play an important role in Flame wrinkling which causes high-frequency oscillations of pressure and outer recirculation zone in downstream region, whereas Flame front of turbulent swirling Premixed Flame tends to be entrained by large-scale coherent structures. The center positions of time-averaged central recirculation zone appear in a fixed streamwise location without dependency on swirl number in combustion flow. For both swirl number cases, the ratio of total heat loss on the walls against total heat release converges to almost a constant value and it reaches approximately 25.0% for the present micro gas turbine combustor configuration.
-
local structure and fractal characteristics of h2 air turbulent Premixed Flame
Proceedings of the Combustion Institute, 2011Co-Authors: Youngsam Shim, Mamoru Tanahashi, Shoichi Tanaka, Toshio MiyauchiAbstract:Abstract Direct numerical simulation of a hydrogen–air turbulent Premixed Flame propagating in homogeneous turbulence was conducted to investigate the local Flame structure and the fractal characteristics of the Flame front at different Reynolds number turbulences. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions was used to represent the hydrogen–air reaction in turbulence. At high Reynolds number turbulence, the multilayer and the multiply folded Flame structures were observed. Flame peninsulas toward unburned mixture were also created and they tended to diminish due to the heat conduction to the surrounding unburned mixture at low temperature. The local extinction of hydrogen–air turbulent Flames was frequently generated at the leading edge of corrugated Flame fronts at high Reynolds number turbulence. Fractal features of the Flame surface were also investigated by the three-dimensional box counting method. Fractal dimensions of the Flame surface were 2.3–2.5 and were independent of Reynolds number and equivalence ratio. A strong correlation between the inner cutoff in Kolmogorov length units and the ratio of the diameter of the coherent fine scale eddy, which is a universal fine scale structure of turbulence, to the laminar Flame thickness ( D / δ F ) was shown, and an inner cutoff scale expression based on D / δ F was proposed.
Kushal S Kedia - One of the best experts on this subject based on the ideXlab platform.
-
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.
-
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.
-
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.
-
mechanisms of stabilization and blowoff of a Premixed Flame downstream of a heat conducting perforated plate
Combustion and Flame, 2012Co-Authors: Kushal S Kedia, Ahmed F GhoniemAbstract:Abstract The objective of this work is to investigate the Flame stabilization mechanism and the conditions leading to the blowoff of a laminar Premixed Flame anchored downstream of a heat-conducting perforated-plate/multi-hole burner, with overall nearly adiabatic conditions. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane-air combustion. Results show a bell-shaped Flame stabilizing above the burner plate hole, with a U-shaped section anchored between neighboring holes. The base of the positively curved U-shaped section of the Flame is positioned near the stagnation point, at a location where the Flame displacement speed is equal to the flow speed. This location is determined by the combined effect of heat loss and Flame stretch on the Flame displacement speed. As the mass flow rate of the reactants is increased, the Flame displacement speed at this location varies non-monotonically. As the inlet velocity is increased, the recirculation zone grows slowly, the Flame moves downstream, and the heat loss to the burner decreases, strengthening the Flame and increasing its displacement speed. As the inlet velocity is raised, the stagnation point moves downstream, and the Flame length grows to accommodate the reactants mass flow. Concomitantly, the radius of curvature of the Flame base decreases until it reaches an almost constant value, comparable to the Flame thickness. While the heat loss decreases, the higher Flame curvature dominates thereby reducing the displacement speed of the Flame base. For a stable Flame, the gradient of the Flame base displacement speed normal to the Flame is higher than the gradient of the flow speed along the same direction, leading to dynamic stability. As inlet velocity is raised further, the former decreases while the latter increases until the stability condition is violated, leading to blowoff. The Flame speed during blow off is determined by the feedback between the growing recirculation zone and the cooling burner plate.
Toshio Miyauchi - One of the best experts on this subject based on the ideXlab platform.
-
dns of turbulent swirling Premixed Flame in a micro gas turbine combustor
Proceedings of the Combustion Institute, 2011Co-Authors: Shoichi Tanaka, Masayasu Shimura, Naoya Fukushima, Mamoru Tanahashi, Toshio MiyauchiAbstract:Abstract Direct numerical simulation (DNS) of hydrogen–air turbulent Premixed Flame in swirling flow is conducted to investigate hydrodynamic behaviors, characteristics of Flame structures and heat transfer properties of a micro gas turbine combustor. The micro combustor is assumed to have a cuboid combustion chamber which is 15.0 mm in the streamwise direction with 10.0 mm × 10.0 mm cross-section. To investigate swirling effects on flow and combustion field, DNS of both cold and reactive flow is carried out under two swirl number conditions: S = 0.6 and 1.2 . Large-scale coherent structures such as ring-shaped or helical structures are generated near the inlet of combustion chamber, and a lot of small-scale vortices appear downstream. Small scale eddies play an important role in Flame wrinkling which causes high-frequency oscillations of pressure and outer recirculation zone in downstream region, whereas Flame front of turbulent swirling Premixed Flame tends to be entrained by large-scale coherent structures. The center positions of time-averaged central recirculation zone appear in a fixed streamwise location without dependency on swirl number in combustion flow. For both swirl number cases, the ratio of total heat loss on the walls against total heat release converges to almost a constant value and it reaches approximately 25.0% for the present micro gas turbine combustor configuration.
-
local structure and fractal characteristics of h2 air turbulent Premixed Flame
Proceedings of the Combustion Institute, 2011Co-Authors: Youngsam Shim, Mamoru Tanahashi, Shoichi Tanaka, Toshio MiyauchiAbstract:Abstract Direct numerical simulation of a hydrogen–air turbulent Premixed Flame propagating in homogeneous turbulence was conducted to investigate the local Flame structure and the fractal characteristics of the Flame front at different Reynolds number turbulences. A detailed kinetic mechanism including 12 reactive species and 27 elementary reactions was used to represent the hydrogen–air reaction in turbulence. At high Reynolds number turbulence, the multilayer and the multiply folded Flame structures were observed. Flame peninsulas toward unburned mixture were also created and they tended to diminish due to the heat conduction to the surrounding unburned mixture at low temperature. The local extinction of hydrogen–air turbulent Flames was frequently generated at the leading edge of corrugated Flame fronts at high Reynolds number turbulence. Fractal features of the Flame surface were also investigated by the three-dimensional box counting method. Fractal dimensions of the Flame surface were 2.3–2.5 and were independent of Reynolds number and equivalence ratio. A strong correlation between the inner cutoff in Kolmogorov length units and the ratio of the diameter of the coherent fine scale eddy, which is a universal fine scale structure of turbulence, to the laminar Flame thickness ( D / δ F ) was shown, and an inner cutoff scale expression based on D / δ F was proposed.
R S Cant - One of the best experts on this subject based on the ideXlab platform.
-
stretch rate effects on displacement speed in turbulent Premixed Flame kernels in the thin reaction zones regime
Proceedings of the Combustion Institute, 2007Co-Authors: Nilanjan Chakraborty, M Klein, R S CantAbstract:Abstract Direct numerical simulation (DNS) of three-dimensional turbulent Premixed Flame kernels is carried out in order to study correlations of displacement speed with stretch rate for a range of different Flame kernel radii. A statistically planar back-to-back Flame is also simulated as a special case of a Flame kernel with infinite radius of curvature. In all the cases the joint pdf of displacement speed with stretch rate shows two distinct branches, whose relative strength is found to be dependent on the mean Flame curvature. A positive (negative) correlation is prevalent when the Flame is curved in a convex (concave) sense towards the reactants. The observed non-linearity of the displacement speed-stretch rate correlation, qualitatively consistent with previous two-dimensional DNS with complex chemistry, is shown to exist purely due to fluid-dynamical interactions even in the absence of detailed chemistry. It is demonstrated that the combined contribution of reaction and normal diffusion is important in the response of Flame propagation to stretch rate, and the effect is found to increase with decreasing mean kernel radius. The implications of the stretch rate dependence of displacement speed are discussed in detail in the context of the Flame surface density (FSD) approach to turbulent combustion modelling.
-
Curvature and wrinkling of Premixed Flame Kernels comparison between planar laser induced fluorescence (PLIF) and 3D direct numerical simulations (DNS)
2004Co-Authors: Gashi S, R S Cant, Hult J, Kw Jenkins, Cf KaminskiAbstract:A direct comparison between time resolved PLIF measurements of OH and two dimensional slices from a full three dimensional DNS data set of turbulent Premixed Flame kernels in lean methane/air mixture was presented. The local Flame structure and the degree of Flame wrinkling were examined in response to differing turbulence intensities and turbulent Reynolds numbers. Simulations were performed using the SEGA DNS code, which is based on the solution of the compressible Navier Stokes, species, and energy equations for a lean hydrocarbon mixture. For the OH PLIF measurements, a cluster of four Nd:YAG laser was fired sequentially at high repetition rates and used to pump a dye laser. The frequency doubled laser beam was formed into a sheet of 40 mm height using a cylindrical telescope. The combination of PLIF and DNS has been demonstrated as a powerful tool for Flame analysis. This research will form the basis for the development of sub-grid-scale (SGS) models for LES of lean-Premixed combustion systems such as gas turbines. This is an abstract of a paper presented at the 30th International Symposium on Combustion (Chicago, IL 7/25-30/2004)
