Showing papers on "Laminar flame speed published in 2009"

Effects of Lewis number and ignition energy on the determination of laminar flame speed using propagating spherical flames

Abstract: The trajectories of outwardly propagating spherical flames initiated by an external energy deposition are studied theoretically, numerically, and experimentally by using hydrogen/air mixtures. Emphasis is placed on how to accurately determine the laminar flame speeds experimentally from the time history of the flame fronts for mixtures with different Lewis numbers and ignition energies. The results show that there is a critical flame radius only above which is the linear and non-linear extrapolation for flame speeds valid. It is found that the critical radius depends strongly on the Lewis number. At large Lewis numbers, the critical radius is larger than the minimum flame radius used in the experimental measurements, leading to invalid flame speed extrapolation. The results also show that there is a maximum Karlovitz number beyond which propagating spherical flame does not exist. The maximum Karlovitz number decreases dramatically with the increase of Lewis number. Furthermore, the results show that the ignition energy has a significant impact on the flame trajectories. It is found that the unsteady flame transition causes a flame speed reverse phenomenon near the maximum Karlovitz number with different ignition energies. The occurrence of flame speed reverse greatly narrows the experimental data range for flame speed extrapolation. The strong dependence of flame trajectory on ignition energy and the existence of the flame speed reverse phenomenon are also confirmed by experimental results. Published by Elsevier Inc. on behalf of The Combustion Institute.

Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in heated coflow: flame stabilization and structure

Abstract: Direct numerical simulation (DNS) of the near field of a three-dimensional spatially developing turbulent lifted hydrogen jet flame in heated coflow is performed with a detailed mechanism to determine the stabilization mechanism and the flame structure. The DNS was performed at a jet Reynolds number of 11,000 with over 940 million grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. A chemical flux analysis shows the occurrence of near-isothermal chemical chain branching preceding thermal runaway upstream of the stabilization point, indicative of hydrogen auto-ignition in the second limit. The Damkoehler number and key intermediate-species behaviour near the leading edge of the lifted flame also verify that auto-ignition occurs at the flame base. At the lifted-flame base, it is found that heat release occurs predominantly through ignition in which the gradients of reactants are opposed. Downstream of the flame base, both rich-premixed and non-premixed flames develop and coexist with auto-ignition. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base. In particular, the relative position of themore » flame base and the coherent flow structure induces a cyclic motion of the flame base in the transverse and axial directions about a mean lift-off height. This is confirmed by Lagrangian tracking of key scalars, heat release rate and velocity at the stabilization point.« less

Effects of compression and stretch on the determination of laminar flame speeds using propagating spherical flames

Abstract: The effects of flow compression and flame stretch on the accurate determination of laminar flame speeds at normal and elevated pressures using propagating spherical flames at constant pressure or constant volume are studied theoretically and numerically. The results show that both the compression-induced flow motion and flame stretch have significant impacts on the accuracy of flame speed determination. For the constant pressure method, a new method to obtain a compression-corrected flame speed (CCFS) for nearly constant pressure spherical bomb experiments is presented. Likewise, for the constant volume method, a technique to obtain a stretch-corrected flame speed (SCFS) at elevated pressures and temperatures is developed. The validity of theoretical results for both constant pressure and constant volume methods is demonstrated by numerical simulations using detailed chemistry for hydrogen/air, methane/air, and propane/air mixtures. It is shown that the present CCFS and SCFS methods not only improve the a...

Characterization of acoustically forced swirl flame dynamics

Abstract: Lean premixed combustors are highly susceptible to combustion instabilities, caused by the coupling between heat release fluctuations and combustor acoustics In order to predict the conditions under which these instabilities occur and their limit cycle amplitudes, understanding of the amplitude dependent response of the flame to acoustic excitation is required This study presents an analysis of phase-locked OH PLIF images of acoustically excited swirl flames, to identify the key controlling physical processes and qualitatively discuss their characteristics This analysis suggests that the flame dynamics are controlled by a superposition of the following processes: (1) annular jet fluctuations, (2) oscillatory turbulent flame brush development, (3) flame stabilization, and (4) fluid mechanical instabilities of the backward facing step, jet column, swirl, and shear layer These results illustrate that the flame response is not controlled by any single physical process but, rather, by several simultaneously occurring processes which are potentially competing, and whose relative significance depends upon forcing frequency, amplitude of excitation, and flame stabilization dynamics

Propagation and extinction of premixed dimethyl-ether/air flames

Abstract: Laminar flame speeds and extinction strain rates of dimethyl-ether/air mixtures were measured at room temperature and atmospheric pressure over a wide range of equivalence ratios. The experiments were performed in the counterflow configuration, and included the use of digital particle image velocimetry and laser Doppler velocimetry. The laminar flame speeds were experimentally determined using a new nonlinear extrapolation technique, which utilizes simulations obtained using detailed chemistry and transport. Compared to literature experimental data, the measured laminar flame speeds were found to be in good agreement with the majority of measurements using spherically expanding flames, and they are lower compared to measurements reported by other groups using the stagnation flame technique. An updated kinetic model of dimethyl-ether oxidation is also proposed, which entails a number of adjustments to reactions involving methane chemistry. Compared to previous versions of the model, improved agreement was found with the experimental data. Simulations incorporated both the mixture-averaged and full multicomponent formulations to evaluate transport properties. Results revealed that the use of the mixture-averaged formulation results in negligible discrepancy in the calculated laminar flame speeds but can substantially overestimate the extinction strain rates particularly near stoichiometry. Sensitivity analyses with respect to reactions and binary diffusion coefficients were conducted to provide insight into the controlling physico-chemical processes. Additionally, reaction pathway analyses were used to interpret the results, and to identify the high-temperature reaction pathways of dimethyl-ether oxidation. Intermediates were shown to dominate high-temperature DME oxidation kinetics.

Pressure and preheat dependence of laminar flame speeds of H2/CO/CO2/O2/He mixtures

Abstract: Laminar flame speeds of lean H2/CO/CO2 (syngas) fuel mixtures have been measured for a range of H2 levels (20–90% of the fuel) at pressures and reactant preheat temperatures relevant to gas turbine combustors (15 atm and up to 600 K) A conical flame stabilized on a contoured nozzle is used for the flame speed measurement, which is based on the reaction zone area calculated from chemiluminescence imaging of the flame An O2:He mixture (1:9 by volume) is used as the oxidizer in order to suppress the hydrodynamic and thermo-diffusive instabilities that become prominent at elevated pressure conditions for lean H2/CO fuel mixtures All the measurements are compared with numerical predictions based on two leading kinetic mechanisms: a H2/CO mechanism from Davis et al and a C1 mechanism from Li et al The results generally agree with the findings of an earlier study at atmospheric pressure: (1) for low H2 content ( 60%) H2 content fuels, especially at very lean conditions At elevated pressure, however, the effect is less pronounced than at atmospheric conditions The exaggerated temperature dependence of the current models may be due to errors in the temperature dependence used for so-called “low temperature” reactions that become more important as the preheat temperature is increased There is also evidence of slight radiative heat transfer effects on the laminar flame speed for lean syngas mixtures associated with CO2 addition to the fuel (up to 40%) at elevated pressure and preheat temperature

Flame-sheet dynamics of bluff-body stabilized flames during longitudinal acoustic forcing

Abstract: Bluff-body stabilized flames are susceptible to combustion instabilities due to interactions between acoustics, vortical disturbances, and the flame. In order to elucidate these flow-flame interactions during an instability, an experimental and computational investigation of the flame-sheet dynamics of a harmonically excited flame was performed. It is shown that the flame dynamics are controlled by three key processes: excitation of shear layer instabilities by the axially oscillating flow, anchoring of the flame at the bluff body, and the kinematic response of the flame to this forcing. The near-field flame features are controlled by flame anchoring and the far-field by kinematic restoration. In the near-field, the flame response grows with downstream distance due to flame anchoring, which prevents significant flame movement near the attachment point. Theory predicts that this results in linear flame response characteristics as a function of perturbation amplitude, and a monotonic growth in magnitude of the flame-sheet fluctuations near the stabilization point, consistent with the experimental data. Farther downstream, the flame response reaches a maximum and then decays due to the dissipation of the vortical disturbances and action of flame propagation normal to itself, which acts to smooth out the wrinkles generated by the harmonic flow forcing. This behavior is strongly non-linear, resulting in significant variation in far-field flame-sheet response with perturbation amplitude.

Large Eddy Simulation and PIV Measurements of Unsteady Premixed Flames Accelerated by Obstacles

Abstract: In gas explosions, the unsteady coupling of the propagating flame and the flow field induced by the presence of blockages along the flame path produces vortices of different scales ahead of the flame front. The resulting flame–vortex interaction intensifies the rate of flame propagation and the pressure rise. In this paper, a joint numerical and experimental study of unsteady premixed flame propagation around three sequential obstacles in a small-scale vented explosion chamber is presented. The modeling work is carried out utilizing large eddy simulation (LES). In the experimental work, previous results (Patel et al., Proc Combust Inst 29:1849–1854, 2002) are extended to include simultaneous flame and particle image velocimetry (PIV) measurements of the flow field within the wake of each obstacle. Comparisons between LES predictions and experimental data show a satisfactory agreement in terms of shape of the propagating flame, flame arrival times, spatial profile of the flame speed, pressure time history, and velocity vector fields. Computations through the validated model are also performed to evaluate the effects of both large-scale and sub-grid scale (SGS) vortices on the flame propagation. The results obtained demonstrate that the large vortical structures dictate the evolution of the flame in qualitative terms (shape and structure of the flame, succession of the combustion regimes along the path, acceleration-deceleration step around each obstacle, and pressure time trend). Conversely, the SGS vortices do not affect the qualitative trends. However, it is essential to model their effects on the combustion rate to achieve quantitative predictions for the flame speed and the pressure peak.

Characterization of forced flame response of swirl-stabilized turbulent lean-premixed flames in a gas turbine combustor

Abstract: Flame transfer function measurements of turbulent premixed flames are made in a model lean-premixed, swirl-stabilized, gas turbine combustor. OH∗, CH∗, and CO2∗ chemiluminescence emissions are measured to determine heat release oscillation from a whole flame, and the two-microphone technique is used to measure inlet velocity fluctuation. 2D CH∗ chemiluminescence imaging is used to characterize the flame shape: the flame length (LCH∗ max) and flame angle (α). Using H2-natural gas composite fuels, XH2=0.00–0.60, a very short flame is obtained and hydrogen enrichment of natural gas is found to have a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of M flames is much smaller than that of V flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St=LCH∗ max/Lconv), the flame length (LCH∗ max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the nondimensional parameters, which collapse all relevant input conditions into the flame shape and the Strouhal number.

Lower limit of weak flame in a heated channel

Abstract: Stationary and non-stationary flame responses in a heated meso-scale channel were investigated both experimentally and computationally. Special attention was paid to flame stabilities, particularly the existence of lower limit of weak flame regime. Previous microcombustion methodology with an external heat source to form stationary temperature gradient in the channel wall was employed. Normal stable flames at high and low velocity regimes, and non-stationary dynamic flames (FREI) at moderate velocity regime were confirmed experimentally. In addition to them, the lower limit of weak flame regime was experimentally identified for the first time. Measured temperature increase in such weak flame was found to be approaching to nearly zero degree. It was found that the flame temperature at the lower limit of the weak flame regime corresponds to ignition temperature of the employed mixture. One-dimensional computations with detailed chemistry and transport properties exhibited e -shaped curve which has additional lowest velocity regime with previous S-shaped curve. Computational results comprehensively support and interpret the present experimental results indicating that the lower limit of weak flame regime is induced by the weakened reaction due to less frequent molecular collisions by diffusive mass dissipation at extremely low velocity regime.