The presence of liquid fuel droplets in a flammable mixture causes cellular instabilities on the flame surface.These perturbations of the flame surface are known to significantly enhance the flame speed when compared to the equivalent homogeneous gaseous flame.The prediction and identification of the mechanisms responsible for the onset of cellularity for two-phase mixtures is essential to better understand spray combustion.The present study considers premixed combustion with inert particles(liquid and solid)in order to isolate and determine any potential mechanisms.The liquid fuel droplets were first replaced with water to investigate the fuel gradient effect forming around evaporating droplets.Three mixtures(a stoichiometric CH4-air mixture,a lean and a rich C3H8-air mixtures)were used to select and control the diffusive balance between mass and thermal diffusion(quantified by the Lewis number Le)which is caused by the gaseous fuel gradient.A dual chamber design with an internal spherical combustion chamber was employed to study expanding centrally ignited spherical flames.A classical shadowgraphy system was used to visualize the flame morphology.Homogeneously distributed and narrow-sized droplets were generated using the condensation technique of expansion cooling.Temperature was set at 350 K and various pressures and water quantities,corresponding to different sizes and concentrations,were explored.Qualitative comparisons of the flame structures revealed no cellular instabilities with the methane-air and lean propane-air with aerosol water mixtures.On the other hand,cellular instabilities appeared with the rich propaneair-water aerosol mixtures.These results suggest that the diffusive balance property of the gaseous environment is predominant and that the presence of droplets triggers cell development only in the case where the flame is already potentially unstable.To complement those results,the effect of obstruction on the propagating flame front will be explored,using hollow glass microspheres as solid particles.