Effects of fuel properties and free stream turbulence on characteristics of bluff-body stabilized flames

Abstract

An experimental investigation of the effect of fuel properties and different levels of free stream turbulence intensities on the structure of bluff body stabilized lean, premixed flames is reported. The diagnostic techniques involving simultaneous imaging of hydroxyl (OH) and formaldehyde (CH2O) by planar laser induced fluorescence and particle image velocimetry (PIV) were used to study the interaction between the flame and the flow field. CH2O fluorescence and the pixel-by-pixel multiplication of OH and CH2O fluorescence signals were utilized to mark preheat and heat release regions, respectively. As the turbulence intensity increased from 4% to 14%, pronounced formation of cusps and unburnt mixture fingers were observed along the flame front. For the intense turbulence conditions, different characteristics of the flame front were observed which strongly depended on the properties of fuel/air mixture. For lean methane/- and ethylene/air (ϕ = 0.85), localized extinctions along the flame sheet and flamelet merging were observed which created isolated pockets of reactants in the flame envelope with heat release regions along their boundary. In addition to these features, propane/- and ethylene/air (ϕ = 0.655) flames exhibited the occurrence of flame fragmentation events which created multiple islands of OH filled regions separated by thick layers of CH2O. The overall flame shape for these conditions was observed to change intermittently from symmetric to asymmetric mode with increasing turbulence intensity. Several properties were measured to characterize the effects of turbulence–flame interaction which includes the preheat and reaction zone thicknesses, 2-D strain rate, burning fraction, flame brush thickness, flame surface density and turbulent to laminar flame area ratio.