Fabien Halter

Investigations on the Flamelet Inner Structure of Turbulent Premixed Flames

Planar Rayleigh scattering measurements were used to investigate the inner structure of flamelets in premixed turbulent Bunsen flames. Measurements were performed in a high pressure chamber, where pressure is varied from 0.3 to 0.9 MPa. Two lean equivalence ratios (0.6 and 0.7) of methane/air mixtures were studied. Local information regarding the thickness and the local curvature of the flamelets were obtained. It is observed that an increase of the equivalence ratio induces a decrease of the mean local flamelet thickness. When pressure is increased from 0.3 to 0.9 MPa, the mean flamelet thickness does not decrease as for laminar case. The different external phenomena (turbulence and stretch (curvature and strain rate)) which could explain this trend are investigated.

Characterization of Cellular Instabilities of a Flame Propagating in an Aerosol

This paper presents a fundamental study of two-phase combustion using a quasi mono-sized droplet aerosol configuration generated with the Wilson cloud chamber principle. Experiments were conducted under microgravity conditions to prevent droplet settling, using a dual chamber apparatus that ensures safe operation in these conditions. Two relevant parameters, the Sauter Mean Diameter and the number of droplets, were considered to describe the aerosols. In order to properly highlight the aerosol impact, the initial conditions were selected so as to avoid gaseous instabilities. It was found that the presence of an aerosol triggers cellular instabilities whereas the equivalent gaseous flame is totally stable. The cellularity of laminar flames increased both with the Sauter Mean Diameter and the number of droplets but differed in terms of cell size and flame shape. It was demonstrated that the aerosol flame speed can greatly exceed the gaseous flame speed. Conversely, the results also show that droplet evaporation during flame propagation can act as a thermal sink on the flame, and may thus reduce the flame speed.Copyright

Experimental Investigation of the Mechanisms of Cellular Instabilities Developing on Spherical Two-Phase Flames

ABSTRACT The presence of liquid fuel droplets in a flammable mixture causes cellular instabilities on the flame surface, which significantly enhances the flame speed when compared to the fully vaporized case. The prediction of the mechanisms responsible for the onset of cellularity for two-phase mixtures is essential to better understand spray combustion. The present study considers an innovative experimental strategy to isolate and investigate any potential mechanisms. The fuel droplets were replaced by inert water droplets in order to amplify the thermal sink effect, characterized by the absorption of part of the heat released by the flame, and to suppress the local enrichment of fuel formed around droplets. Spherical expanding flames with narrow-size distribution droplets were used and qualitative comparisons of the flame structure were performed with a shadowgraph system. The results have shown that the heat sink has no significant effect, whereas the local enrichment of fuel appears as a key phenomenon, which suggests that in the case of fuel droplet aerosols the onset of cellularities is triggered in the inhomogeneous part of the gaseous phase.

Effects of CO2, H2O, and Exhaust Gas Recirculation Dilution on Laminar Burning Velocities and Markstein Lengths of Iso-Octane/Air Mixtures

ABSTRACT A laminar burning velocities database, measured by a shadowgraph method on spherical expanding flames, is still required to improve numerical simulation of kinetic mechanism. Iso-octane/air mixtures at high temperatures (373 and 423 K), high pressure (1, 3, 5, and 10 bar), and high dilution rate were studied over a range of equivalence ratios. CO2, H2O, and exhaust gas recirculation (EGR) were used as diluent over dilution range from 5 to 25 vol%. An explicit correlation giving the laminar burning velocity from the initial pressure, the initial temperature, the dilution rate, and the equivalence ratio is proposed for those flames. Markstein length was also investigated under these conditions. A correlation depending on the same variables was also proposed. Particular attention was given to provide a Markstein length correlation depending on the dilution rate.

Modelling of the subgrid scale wrinkling factor for large eddy simulation of turbulent premixed combustion

We propose a model for assessing the unresolved wrinkling factor in the large eddy simulation of turbulent premixed combustion. It relies essentially on a power-law dependence of the wrinkling factor on the filter size and an original expression for the ‘active’ corrugating strain rate. The latter is written as the turbulent strain multiplied by an efficiency function that accounts for viscous effects and the kinematic constraint of Peters. This yields functional expressions for the fractal dimension and the inner cut-off length scale, the latter being (i) filter-size independent and (ii) consistent with the Damköhler asymptotic behaviours at both large and small Karlovitz numbers. A new expression for the wrinkling factor that incorporates finite Reynolds number effects is further proposed. Finally, the model is successfully assessed on an experimental filtered database.