Flame Stabilization Location Research Articles

Flame dynamics of a meso-scale heat recirculating combustor

The dynamics of premixed propane–air flame in a meso-scale ceramic combustor has been examined here. The flame characteristics in the combustor were examined by measuring the acoustic emissions and preheat temperatures together with high-speed cinematography. For the small-scale combustor, the volume to surface area ratio is small and hence the walls have significant effect on the global flame structure, flame location and flame dynamics. In addition to the flame–wall thermal coupling there is a coupling between flame and acoustics in the case of confined flames. Flame–wall thermal interactions lead to low frequency flame fluctuations (∼100 Hz) depending upon the thermal response of the wall. However, the flame–acoustic interactions can result in a wide range of flame fluctuations ranging from few hundred Hz to few kHz. Wall temperature distribution is one of the factors that control the amount of reactant preheating which in turn effects the location of flame stabilization. Acoustic emission signals and high-speed flame imaging confirmed that for the present case flame–acoustic interactions have more significant effect on flame dynamics. Based on the acoustic emissions, five different flame regimes have been identified; whistling/harmonic mode, rich instability mode, lean instability mode, silent mode and pulsating flame mode.

Flame propagation enhancement by plasma excitation of oxygen. Part I: Effects of O 3

The thermal and kinetic effects of O 3 on flame propagation were investigated experimentally and numerically by using C 3H 8/O 2/N 2 laminar lifted flames. Ozone produced by a dielectric barrier plasma discharge was isolated and measured quantitatively by using absorption spectroscopy. Significant kinetic enhancement by O 3 was observed by comparing flame stabilization locations with and without O 3 production. Experiments at atmospheric pressures showed an 8% enhancement in the flame propagation speed for 1260 ppm of O 3 addition to the O 2/N 2 oxidizer. Numerical simulations showed that the O 3 decomposition and reaction with H early in the pre-heat zone of the flame produced O and OH, respectively, from which the O reacted rapidly with C 3H 8 and produced additional OH. The subsequent reaction of OH with the fuel and fuel fragments, such as CH 2O, provided chemical heat release at lower temperatures to enhance the flame propagation speed. It was shown that the kinetic effect on flame propagation enhancement by O 3 reaching the pre-heat zone of the flame for early oxidation of fuel was much greater than that by the thermal effect from the energy contained within O 3. For non-premixed laminar lifted flames, the kinetic enhancement by O 3 also induced changes to the hydrodynamics at the flame front which provided additional enhancement of the flame propagation speed. The present results will have a direct impact on the development of detailed plasma-flame kinetic mechanisms and provided a foundation for the study of combustion enhancement by O 2( a 1 Δ g ) in part II of this investigation.

Experimental Study of Surface and Interior Combustion Using Composite Porous Inert Media

Combustion using silicon carbide coated, carbon–carbon composite porous inert media (PIM) was investigated. Two combustion modes, surface and interior, depending upon the location of flame stabilization, were considered. Combustion performance was evaluated by measurements of pressure drop across the PIM, emissions of NOx and CO, and the lean blow-off limit. Data were obtained for the two combustion modes at identical conditions for a range of reactant flowrates, equivalence ratios, and pore sizes of the PIM. Results affirm PIM combustion as an effective method to extend the blow-off limit in lean premixed combustion.

Stabilization mechanisms of lifted laminar flames in axisymmetric jet flows

Stabilization mechanisms of lifted laminar propane flames are investigated in an axisymmetric jet flow configuration. Detailed mixing and flow fields upstream of the flame lift-off heights measured by Chung and coworkers are calculated on a nonreacting flow basis. Jet exit velocity, nozzle diameter, coflow air velocity as well as partial jet premixing with air are varied to obtain the local stoichiometric axial velocity, U st , and scalar dissipation rate, χ st , at points that are upstream of the flame stabilization locations by a redirection region length. It is found that U st decreases consistently with increasing χ st when the centerline mixture fraction is higher than that of the rich flammability limit. Beyond this threshold at downstream locations, a rich-joined flame is formed with U st independent of the local scalar dissipation rate. Instead, U st decreases with increasing ξ st , the stoichiometric mixture fraction of the jet flow. For lifted flames stabilized close to the burner exit with an edge-flame appearance, the maximum attainable mixture fraction gradient approaches that calculated for extinction of stretched diffusion flames in the counterflow geometry. The flame propagation velocity remains positive. The trend of decreasing U st with increasing χ st is also found for numerical simulations with a unity Lewis number and for experimental data of lifted methane flames. These results corroborate the triple flame stabilization concept. An empirical formula for the triple flame propagation velocity is proposed with a nonlinear dependence on local scalar dissipation rate. In addition, stabilization mechanisms previously proposed for lifted turbulent diffusion flames are reconciled for lifted laminar flames, depending on the local flow/mixing conditions. Appropriate flame stability and blow-out criteria are derived and these predict the flame lift-off height and blow-out velocity accurately. Implications for flame stabilization in turbulent jet flows with such triple flame structures are also discussed.

Stabilization of lifted tribrachial flames in a laminar nonpremixed jet

The stabilization mechanism of lifted flames in a laminar nonpremixed jet has been analyzed and experimentally investigated. An analysis on the flame response by the perturbation from the tribrachial location shows that the lifted flame is unstable for Sc < 1, confirming the experimental findings of the nonexistence of stationary lifted flames for methane and ethane jets. Appreciable upstream mass diffusion from the nozzle exit for Sc < 0.5 explains the flame behaviour for hydrogen fuel such that a nozzle-attached flame exists even up to jet velocities comparable to the speed of sound. Flow-field measurements using laser Doppler velocimetry reveal that the flow velocity at the flame stabilization location for lifted flames is larger than the laminar burning velocity of the stoichiometric premixture, which confirms the increasing effect on flame speed by the flow redirection for tribachial flames. Also, it is demonstrated that the region between the nozzle exit and the anchoring point of the lifted flames can be treated as a cold jet.