Laser Diagnostics Applied to Lean Premixed Swirling Flames - Simultaneous Flow Field and Scalar Measurements

Abstract: The work reported here includes experimental studies of lean premixed swirling flames. Various laser-based optical diagnostic techniques were used to collect information under different operational conditions. Combined flow- and scalar field measurements were performed at high repetition rates for detailed studies of the turbulent flames. A large part of the work presented concerns experimental studies of lean turbulent premixed flames stabilized by a low-swirl burner. The unconfined flames that were studied were propagating in a divergent flow field and interacted with flow structures in the shear layers surrounding them. Particle Image Velocimetry (PIV) and stereo-PIV were employed for velocity measurements in a given plane. Volumetric Velocimetry (3D PIV) was performed in order to assess the full 3D characteristics of the flow field. Flame-front visualization and fuel distribution measurements were obtained by use of Planar Laser Induced Fluorescence (PLIF). PLIF of OH radicals, created in regions where high temperature reactions occurred, were used to determine the position, fluctuation, overall shape and wrinkling of the flame front. Simultaneous measurements, performed at repetition rates of several thousand images per second, of the velocity field and of flame location enabled information concerning the flow strucutres emanating from the nozzle and the flame flow interaction. The results obtained were used for validation of Large Eddy Simulations (LES), carried out by collaborating groups in parallel with the experiments. The influence of the inflow boundary conditions were investigated using both experimental data and LES of the flow through the swirler and nozzle. An investigation of flames close to flashback was carried out in a burner having a variable degree of swirl and equipped with an transparent nozzle. PIV and high-speed video of the flame luminosity were used to study the transition to flashback when the degree of swirl was increased. The observed flashback velocity was much higher than the estimated burning velocity of the flame and the flashback was attributed a convection directed upstream. A demonstration of simultaneous measurements of the flow field and the temperature field was also included. Here, thermographic phosphor particles were used both as flow tracers for PIV and as temperature sensitive particles. For the temperature measurements a Nd:YAG laser at 355 nm was used to excite the particles following a heated air flow. The subsequent phosphorescence from two emission lines was collected and an intensity-ratio method was then employed for assessing the gas temperature field.