Showing papers on "Laminar flame speed published in 2008"

Dynamics of Laminar Premixed Flames Forced by Harmonic Velocity Disturbances

Abstract: This paper describes the dynamics of constant-burning-velocity premixed flames responding to harmonic velocity disturbances. Results are derived from analytical and computational solutions of the nonlinear G equation and compared with available experimental data. It is shown that the flame dynamics are controlled by the superposition of two waves propagating along the flame sheet: those originating at the flame-anchoring point and from flow nonuniformities along the flame. They may either constructively or destructively superpose, and so the overall linear flame response depends upon two Strouhal numbers, St 2 and Stc, related to the amount of time taken for a flow (St c ) and flame-front (St 2 ) disturbance to propagate the flame length, normalized by the acoustic period. The nonlinear flame response is controlled by flame propagation normal to itself, which smoothens out the wrinkles induced by the forcing at an amplitude-dependent rate. The flame's nonlinear response is shown to exhibit two qualitatively different behaviors. For parameter values at which these disturbances constructively interfere, the nonlinear flame response saturates. When the flame disturbances destructively interfere, the nonlinear transfer function may actually exceed its linear value before saturating. This result explains experimentally observed variations of the nonlinear flame response with frequency.

A priori direct numerical simulation assessment of algebraic flame surface density models for turbulent premixed flames in the context of large eddy simulation

Abstract: Flame surface density (FSD) based reaction rate closure is one of the most important approaches in turbulent premixed flame modeling. The algebraic models for FSD based on power laws often require information about the fractal dimension D and the inner cut-off scale ηi. In the present study, two three-dimensional direct numerical simulation (DNS) databases for freely propagating statistically planar turbulent premixed flames are analyzed among which the flame in one case belongs to the corrugated flamelet (CF) regime, whereas the other falls well within the thin reaction zone (TRZ) regime. It is found that D for the flame in the TRZ regime is greater than the value obtained for the flame in the CF regime. For the flame within the TRZ regime, the fractal dimension is found to be 7/3, which is the same as D for a material surface in a turbulent environment. For the flame in the CF regime, ηi is found to scale with the Gibson scale, whereas ηi is found to scale with the Kolmogorov length scale for the flame ...

Effect of Flame Structure on the Flame Transfer Function in a Premixed Gas Turbine Combustor

Abstract: The flame transfer function in a premixed gas turbine combustor is experimentally determined. The fuel (natural gas) is premixed with air upstream of a choked inlet to combustor. Therefore, the input to the flame transfer function is the imposed velocity fluctuations of the fuel/air mixture without equivalence ratio fluctuations. The inlet-velocity fluctuations are achieved by a variable-speed siren over the forcing frequency of 75–280 Hz and measured using a hot-wire-anemometer at the inlet to the combustor. The output function (heat release) is determined using chemiluminescence measurement from the whole flame. Flame images are recorded to understand how the flame structure plays a role in the global heat release response of flame to the inlet-velocity perturbation. The results show that the gain and phase of the flame transfer function depend on flame structure as well as the frequency and magnitude of inlet-velocity modulation and can be generalized in terms of the relative length scale of flame to convection length scale of inlet-velocity perturbation, which is represented by a Strouhal number. Non-linear flame response is characterized by a periodic vortex shedding from shear layer and the non-linearity occurs at lower magnitude of inlet-velocity fluctuation as the modulation frequency increases. However, for a given modulation frequency, the flame structure does not affect the magnitude of inlet-velocity fluctuation at which the non-linearity starts.Copyright © 2008 by ASME

Flame acceleration and DDT run-up distance for smooth and obstacles filled tubes

Abstract: The process of flame acceleration inside the tubes and channels depends on several parameters such as nature of the fuel involved, composition of the mixture and configuration of the enclosure itself. The wall roughness and the presence of obstacles in the flame path act as a turbulence generator causing continuous flame acceleration. In some situations the flame can reach a sufficiently high speed to allow the transition of the deflagration into a detonation. A considerably large amount of experimental data on flame speed and DDT run-up distance for several mixtures have been accumulated. Nevertheless simple relationships, based on the most relevant parameters governing the phenomenon, could be useful for design purpose and safety assessment. The present paper suggests some simplified formulas for the evaluation of flame speed and DDT run-up distance of flammable mixtures for both smooth and obstacles filled tubes.

Ignition and Flame Speed Kinetics of Two Natural Gas Blends With High Levels of Heavier Hydrocarbons

Abstract: High-pressure experiments and chemical kinetics modeling were performed to generate a database and a chemical kinetic model that can characterize the combustion chemistry of methane-based fuel blends containing significant levels of heavy hydrocarbons (up to 37.5% by volume). Ignition delay times were measured in two different shock tubes and in a rapid compression machine at pressures up to 34 atm and temperatures from 740 to 1660 K. Laminar flame speeds were also measured at pressures up to 4 atm using a high-pressure vessel with optical access. Two different fuel blends containing ethane, propane, n-butane, and n-pentane added to methane were studied at equivalence ratios varying from lean (0.3) to rich (2.0). This paper represents the most comprehensive set of experimental ignition and laminar flame speed data available in the open literature for CH4 /C2 H6 /C3 H8 /C4 H10 /C5 H12 fuel blends with significant levels of C2+ hydrocarbons. Using these data, a detailed chemical kinetics model, based on current and recent work by the authors, was compiled and refined. The predictions of the model are very good over the entire range of ignition delay times, considering the fact that the data set is so thorough. Nonetheless, some improvements to the model can still be made with respect to ignition times at the lowest temperatures and for the laminar flame speeds at pressures above 1 atm and rich conditions.Copyright © 2008 by ASME

Laboratory Studies of the Flow Field Characteristics of Low-Swirl Injectors for Adaptation to Fuel-Flexible Turbines

Abstract: The low-swirl injector (LSI) is a simple and cost-effective lean premixed combustion method for natural-gas turbines to achieve ultralow emissions (<5 ppm NO x and CO) without invoking tight control of mixture stoichiometry, elaborate active tip cooling, or costly materials and catalysts. To gain an understanding of how this flame stabilization mechanism remains robust throughout a large range of Reynolds numbers, laboratory experiments were performed to characterize the flowfield of natural-gas flames at simulated partial load conditions. Also studied was a flame using simulated landfill gas of 50% natural gas and 50% CO 2 . Using particle image velocimetry, the nonreacting and reacting flowfields were measured at five bulk flow velocities. The results show that the LSI flowfield exhibits similarity features. From the velocity data, an analytical expression for the flame position as function of the flowfield characteristics and turbulent flame speed has been deduced. It shows that the similarity feature coupled with a linear dependency of the turbulent flame speed with bulk flow velocity enables the flame to remain relatively stationary throughout the load range. This expression can be the basis for an analytical model for designing LSIs that operate on alternate gaseous fuels such as slower burning biomass gases or faster burning coal-based syngases.

Formation and role of cool flames in plasma-assisted premixed combustion

Abstract: The structure of a plasma-assisted laminar premixed flame is studied numerically. The initial radical yield generated by a nonequilibrium discharge serves as the boundary condition for a one-dimensional flame code predicting the formation of a cool flame which pilots the premixed methane/air combustion. The ignition of the surrounding unactivated methane-air mixture by this cool flame is modeled as an opposed diffusion flame. Our findings indicate that the nonequilibrium discharge is an in situ reformer of the fuel for the production of the cool flame, producing primarily H2 and CO, thus, facilitating the burning of the lean methane-air mixture.

Reduced Reaction Mechanisms for Methane and Syngas Combustion in Gas Turbines

Abstract: Two reduced reaction mechanisms were established that predict reliably for pressures up to about 20 bar the heat release for different syngas mixtures including initial concentrations of methane. The mechanisms were validated on the base of laminar flame speed data covering a wide range of preheat temperature, pressure, and fuel-air mixtures. Additionally, a global reduced mechanism for syngas, which comprises only two steps, was developed and validated, too. This global reduced and validated mechanism can be incorporated into CFD codes for modeling turbulent combustion in stationary gas turbines.