Experimental Investigation on the Effects of Free Stream Turbulence and Fuel Type on Structure and Blowoff Characteristics of Turbulent Premixed Bluff-body Stabilized Flames

An experimental investigation on the effect of different levels of turbulence intensity and properties of the fuel/air mixture on the structure and characteristics of lean flames stabilized on an axisymmetric bluff body is described in this thesis. 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. In addition, high-speed chemiluminescence imaging was performed to understand the time resolved characteristics of the flame. The first part of the thesis focuses on the characteristics of stably burning lean methane/-, propane/- and ethylene/air flames when subjected to low (4 %), moderate (14 %) and intense (24 and 30%) levels of free stream turbulence. The flame front structure was observed to be strongly dependent on the free stream turbulence level of the incoming fuel/air mixture as well on the properties of the fuel/air mixture. Formation of cusps and unburnt mixture fingers were observed as the turbulence intensity was increased from 4 to 14 % but, the heat release region remained continuous. Under intense turbulence conditions, methane/- and ethylene/air (f = 0.85) flames exhibited localized extinctions along the flame sheet and flamelet merging events which created isolated pockets of reactants in the flame envelope. In addition to these features, propane/- and ethylene/air (f=0.655) flames exhibited the occurrence of flame fragmentation events and the general shape of these flames were observed to intermittently switch from a symmetric (varicose) to asymmetric (sinuous) mode. Several properties were measured to characterize the effects of turbulence – flame interaction which includes the average preheat and reaction zone thicknesses, strain rates and curvature along the flame front, burning fraction, flame brush thickness, flame surface density, area ratio and turbulent flame speed. The next part of the thesis focuses on blowoff dynamics of lean methane/-, propane/- and ethylene/air flames for mean velocities of 5, 10 and 15 m/s and subjected to free stream turbulence levels from 4 to 30%. Apart from the propane/air flames at an apporach velcoity of 5 m/s and turbulence intensity of 30 %, increasing turbulence intensity was found to reduce the flame stability. The blowoff equivalence ratios of propane/air flames was observed to be higher than methane/- and ethylene/air flames. As blowoff was approached, the flame front and shear layer vortices entangled inducing high local strain rates on the flame front that exceed the extinction strain rate resulting in significant breaks along the reaction zone. At conditions near blowoff, significant increase in the frequency of breaks along the reaction zone was observed for low and moderate turbulence conditions. For the higher turbulence conditions, fragmentation of the flame along with the presence of sinuous wakes was observed which aided in the penetration of reactants into the recirculation zone. Velocity vectors near the flame holes indicate the penetration of the reactants into the recirculation zone. Mostly similar sequence of events was observed for methane/-, propane/- and ethylene/air flames near blowoff. Several properties weremeasured to characterize the near blowoff flames which include the strain rate and curvature statistics along the flame front, burning fraction, asymmetric index and the average duration of the blowoff event. Based on the observation from the experiments, turbulent flame speed was attributed to be the primary factor in governing the blowoff equivalence ratio. This point of view was examined by comparing the mean strain rate of methane/- and ethylene/air flames at the equivalence ratio corresponding to near blowoff for propane/air flames