Effect of Lewis number on ball-like lean limit flames

Abstract

The lean limit flames for three different fuel compositions premixed with air, representing three different mixture Lewis numbers, stabilized inside a tube in a downward flow are examined by experiments and numerical simulations. The CH * chemiluminescence distribution in CH 4 -air and CH 4 -H 2 -air flames and the OH * chemiluminescence distribution in H 2 -air flames are recorded in the experiments. Cell-like flames are observed for the CH 4 -air mixture for all tested equivalence ratios. However, for CH 4 -H 2 -air and H 2 -air flames, ball-like lean limit flames are observed. Flame temperature fields are measured using Rayleigh scattering. The experimentally observed lean limit flames are predicted qualitatively by numerical simulation with the mixture-averaged transport model and skeletal mechanism of CH 4 . The results of the simulations show that the entire lean limit flame of CH 4 -H 2 -air and H 2 -air mixtures are located inside a recirculation zone. However, for the lean limit CH 4 -air flame, only the leading edge is located inside the recirculation zone.A flame structure with negative flame displacement speed is observed for the leading edges of the predicted lean limit flames with all three different fuel compositions. As compared with 1D planar flames, the fuel transport caused by convection is less significant in the present 2D lean limit flames for the three different fuel compositions. For the trailing edges of the three predicted lean limit flames, a diffusion dominated flame structure is observed.

Flame Stabilization and Blow-Off of Ultra-Lean H2-Air Premixed Flames

“…At the upstream region, the flame burns less strongly than at the flame tip but is again positively curved. The enclosed surface of the flame inside a RZ is very similar to ball-like flames observed in [8][9][10]. For further investigation, diffusive and convective fluxes of H 2 are plotted in Figure 10b along with the iso-level of progress variable Y = 0.8.…”

“…For fuels with Lewis numbers less than unity, positive stretch rates can induce strong changes in local stoichiometry, resulting in an increase in the local flame speed [7]. Due to these effects, it is possible to stabilize H 2 /air flames near the lean flammability limit and even beyond that limit [8][9][10]. Such ultra-lean flames are of fundamental interest for investigation of limits of blow-off, formation of unique flames such as flame balls and validation of kinetics.…”

“…Recently, Jimenez et al studied the shedding of flame balls from the residual flame in a time-dependent simulation [15] highlighting the complex nature of the physical phenomenon. Stabilization of unique ball-like flames beyond flammability conditions has attracted the interest of many researchers [8][9][10]. These flames stabilize due to strong curvature effects and have been a subject of fundamental interest.…”

Numerical and experimental studies of torus-like flame around the vortex filament in a premixed reactant flow

“…At the same time, near-uniform steady spherical flames may exist in premixed reactants even in the presence of weak convective flows. Such flames have been observed for example in tubes in normal gravity experiments (Hernández-Pérez et al, 2015;Shoshin et al, 2011;Zhou et al, 2017Zhou et al, , 2018 or in counterflow microgravity experiments around the stagnation point (Takasea et al, 2013). As well as microgravity flame balls, flame balls in the presence of convection flows were also observed for small Lewis number mixtures.…”

Experimental and numerical study of cap-like lean limit flames in H2-CH4-air mixtures

“…The details on the Rayleigh scattering experiments and data processing can be found in our previous publication [17] . The effective Rayleigh scattering cross section for the present lean limit flames is evaluated using the numerically predicted distribution of species in the studied flames, as shown in Fig.…”