Cédric Galizzi

Effect of Reactant Preheating on the Stability of Non-Premixed Methane/Air Flames

This study details the influence of reactant temperature on the stability of non-premixed CH4 /air co-flow jet flames. Flame characteristics have been studied for five temperature levels (from 295 K to 600 K). The hysteresis zone formed by the limits between attached and lifted flame translates towards higher methane jet velocities with an increase of initial temperature, independently of the air velocity range. Moreover, critical velocities vary linearly with initial temperature. In addition, attachment and lift-off heights have been obtained from CH* chemiluminescence visualization. Results point out that the attachment height decreases significantly with temperature. Observations also indicate that the intrinsic process of lifting is modified. Pre-lifting anchored flame local extinctions, not observed at room-temperature, appear at higher initial temperatures; their occurrence increases with temperature. The lift-off height of turbulent lifted flames is also reduced with temperature. Overall, results show that increasing local temperature in the stabilization zone enhances flame stability.© 2010 ASME

Lifting and Splitting of Nonpremixed Methane/Air Flames Due to Reactant Preheating

In order to assess the impact of initial reactant temperature on the occurrence of local extinction (LE) and the subsequent lifting process of non-premixed attached flames with increasing fuel injection velocity, hydroxyl radical planar laser-induced fluorescence (OH-PLIF) and high-speed CH*-chemiluminescence visualizations were conducted in a methane/air jet-flame, with preheating up to 1000 K. LE occurrence probability increases when approaching lifting, and the preheating level (Tox,ref) affects the probability density function (PDF) shape of LE axial origin. At low Tox,ref, partial lifting events occur near the burner lip, eventually leading the flame to lift directly from the very flame base. At higher Tox,ref, partial lifting events no longer occur, and LE is mostly witnessed in the flame breakpoint zone (axially from 1 to 3 jet diameters), resulting in a breakpoint lifting process. For very high Tox,ref (1000 K), local extinctions become widespread in the breakpoint zone so that a stable split flame is achieved prior to the lifted regime.

Burning Rates and Surface Characteristics of Hydrogen-Enriched Turbulent Lean Premixed Methane-Air Flames

The burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methaneeair flames were experimentally studied by laser tomography visualization method using a V-shaped flame configuration. Turbulent burning velocity was measured and the variation of flame surface characteristics due to hydrogen addition was analyzed. The results show that hydrogen addition causes an increase in turbulent burning velocity for lean premixed CH4eair mixtureswhenturbulent level in unburnedmixtureisnot changed.Moreover, the increase of turbulent burning velocity is faster than that of the corresponding laminar burningvelocityatconstantequivalenceratio,suggestingthatthekineticseffectisnotthesole factor that results in the increase in turbulent burning velocity when hydrogen is added. The further analysis of flame surface characteristics and brush thickness indicates that hydrogen addition slightly decreases local flame surface density, but increases total flame surface area because of the increased flame brush thickness. The increase in flame brush thickness that results in the increase in total surface area may contribute to the faster increase in turbulent burning velocity, when hydrogen is added. Besides, the stretched local laminar burning velocity may be enhanced with the addition of hydrogen, which may also contribute to the faster increase rate of turbulent burning velocity. Both the variation in flame brush thickness andtheenhancementinstretchedlocallaminarburningvelocityareduetothedecreasedfuel Lewisnumberwhenhydrogenisadded.Therefore,theeffectsoffuelLewisnumberandstretch

A Numerical Study on a V-Shaped Laminar Stratified Flame

A V-shaped laminar stratified flame was investigated by numerical simulation. The primitive variable method, in which the fully elliptic governing equations were solved with detailed chemistry and complex thermal and transport properties, was used. The results indicate that in addition to the primary premixed flame, the stratified charge in a combustor causes the formation of a diffusion flame. The diffusion flame is located between the primary premixed flame branches. The fuel is fully decomposed and converted to some intermediate species, like CO and H2 , in the primary premixed flame branches. Due to the shortage of oxygen, the formed CO and H2 in the fuel rich region of the premixed flame branch is further transported to the downstream until they meet the oxygen from the fuel lean region. This leads to the formation of the diffusion flame. There is an interaction between the diffusion flame and the primary premixed flame branches. The interaction intensifies the burning speed of the primary premixed flame. Both the heat transfer and the diffusion of hydrogen and some radicals cause the interaction. With the increase of the stratified charge region, the diffusion flame zone is enlarged and the interaction is enhanced.Copyright