Flame Stabilization and Blow-Off of Ultra-Lean H2-Air Premixed Flames (original) (raw)
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Anomalous blow-off behavior of laminar inverted flames of ultra-lean hydrogen–methane–air mixtures
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An experimental study of rod-stabilized laminar inverted flames of ultra-lean hydrogen-methane-air mixtures has been performed. For mixtures with high hydrogen content, anomalous stabilization and blow-off behavior has been observed. Flames in those mixtures could be stabilized at equivalence ratios below the lean flammability limit for a zero-stretch planar flame. Stabilization of such flames was possible only when the mixture velocity exceeded some critical value. Flames were blown off when the mixture velocity was reduced below this value. The stand-off distance above the flame holder for those flames decreased and heat transfer from the flame base to the flame holder became more intense when the mixture velocity was increased. This is opposite to the regular behavior of inverted flames. These observed unusual phenomena were attributed to the combination of a strong flame stretch and preferential diffusion effects and to the negative value of the Markstein length in mixtures with high hydrogen content. According to the suggested explanation, increasing the velocity results in an increase of the flame stretch rate at the flame base. This, in turn leads to higher flame temperatures and higher burning velocities, making the survival of the flame below the flammability limits possible. Along with the anomalous blow-off behavior, normal blow-off occurring at increased velocity was observed for mixtures with high hydrogen content at the lowest tested equivalence ratios. The observation of the flame shape evolution showed that, when the normal blow-off limit is approached, a flame narrows slightly above the flame base, forming a ''neck''. The flame fronts merge at the ''neck'' location and flame breaks there, leading to complete flame extinction or leaving a very small flamelet near the flame holder. It is suggested that the flame local extinction in that case occurs as the result of the excessive flame stretch at the flame ''neck'' which leads to the flame fronts merging and to the incomplete reaction.
Local flame displacement speeds of hydrogen-air premixed flames in moderate to intense turbulence
Combustion and Flame, 2022
Comprehensive knowledge of local flame displacement speed, S d , in turbulent premixed flames is crucial towards the design and development of hydrogen fuelled next-generation engines. Premixed hydrogen-air flames are characterized by significantly higher laminar flame speed compared to other conventional fuels. Furthermore, in the presence of turbulence, S d is enhanced much beyond its corresponding unstretched, planar laminar value S L. In this study, the effect of high Karlovitz number (Ka) turbulence on densityweighted flame displacement speed, S d , in a H 2-air flame is investigated. Recently, it has been identified that flame-flame interactions in regions of large negative curvature govern large deviations of S d from S L , for moderately turbulent flames. An interaction model for the same has also been proposed. In this work, we seek to test the interaction model's applicability to intensely turbulent flames characterized by large Ka. To that end, we investigate the local flame structures: thermal, chemical structure, the effect of curvature, along the direction that is normal to the chosen isothermal surfaces. Furthermore, relative contributions of the transport and chemistry terms to S d are also analyzed. It is found that, unlike the moderately turbulent premixed flames, where enhanced S d is driven by interactions among complete flame structures, S d enhancement in high Re t and high Ka flame is predominantly governed by local interactions of the isotherms.
Numerical study of the quenching of a laminar premixed hydrogen flame
MATEC Web of Conferences
A numerical analysis of the quenching of a laminar, premixed hydrogen-air flame is presented. A global and a detailed reaction mechanism are considered. First, one-dimensional flame propagation is analyzed and the models are validated based on the predicted flame speed. Subsequently, the quenching near a solid wall of a duct is analyzed, within a two-dimensional, steady-state formulation. Finally, propagation of a flame front through a quenching mesh, within an unsteady, two-dimensional analysis is considered. It is observed that the global mechanism does not predict a quenching of the flame by the mesh, whereas the detailed mechanism does.
Flow, Turbulence and Combustion, 2020
Two-dimensional direct numerical simulations were conducted to investigate the effects of differential diffusion on flame stabilization and blow-off dynamics of lean premixed hydrogen-air and syngas-air flames stabilized on a meso-scale bluff-body in a square channel. The unity Lewis number for all species was imposed to isolate the effects of differential diffusion. Four sets of simulation cases were conducted. Two different inflow temperature with unity Lewis number were applied to examine distinct levels of hydrodynamic instability. Each unity Lewis number case was compared with the non-unity Lewis number case to investigate how differential diffusion affects the overall flame responses, instabilities, and blow-off mechanism. For all cases, the overall flame dynamics were observed in several distinct modes as the inflow velocity approaches blow-off limit. One of the primary effects of unity Lewis number was an increased level of hydrodynamic instability due to the lower flame temperature and thus a lower density ratio. The lower gas temperature also led to a weakening of the re-ignition of the quenched local mixture by the product gas entrainment. The combined effects were manifested as suppression of the re-ignition events, leading to a revised conclusion that the ultimate blow-off behavior at high velocity conditions are mainly controlled by the onset of local extinction.
An evaluation of different contributions to flame stretch for stationary premixed flames
Combustion and Flame, 1997
The concept of flame stretch is extended to study stationary premixed flames with a finite thickness. It is shown that the analysis results in additional contributions to the stretch rate due to changes in the flame thickness and due to density variations along the flame. Extended expressions are derived that describe the effect of stretch on variations in scalar quantities, such as the enthalpy. These expressions are used to determine local variations in the flame temperature, and it is shown that known results are recovered when a number of approximations are introduced. The extended stretch formalism might be useful to analyze and quantify the different flame stretch contributions and their effects in numerical flame studies. Finally, the different contributions to the total stretch rate and the effects thereof on the flame stabilization are numerically computed for the flame tip of a two-dimensional Bunsen flame as illustration.