Analysis of lean premixed turbulent combustion using coherent anti-Stokes Raman spectroscopy temperature measurements

An existing CARS instrument was modified to use a new dual dye laser that produces simultaneously CARS spectra of N₂, CO, CO₂ and O₂. These CARS spectra yield simultaneous values for the gas temperature from N₂ and concentrations of CO, CO₂ and O₂. The dual-dye laser was generated using a mixture of pyromethene 650 and 597 dyes (both commercially available) dissolved in ethanol. Calibration studies in a tube furnace showed that the modified CARS instrument using the dual dye laser has good accuracy and acceptable precision for gas temperature measurements. However, the modified instrument had limited capabilities in measuring the concentrations of O₂ and CO₂ in the concentration ranges of interest, possibly due to the approximations involved in the CARS reduction algorithm used in this work.; The modified CARS instrument was successfully applied to obtain detailed measurements of instantaneous gas temperature for four lean premixed combustion conditions, with varying inlet stoichiometry and swirl level, in a Laboratory Scale Gas Turbine Combustor (LSGTC). The temperature data were used to produce iso-contour plots of averaged and normalized standard deviations. Probability Density Function (PDF) plots were also produced at several measurement locations.; Additional insights regarding the flame behavior were obtained by examining the spatial variation in the shape of the PDFs for all the combustion conditions. Several zones were identified in all four cases where the nature of the PDFs seems to be determined by different factors. At lower heights, near the bottom of the combustor, the swirl level seems to be the predominant factor determining the nature of the turbulent fluctuations, whereas near the wall and around z = 50 mm, the equivalence ratio appears to predominate. For the rest of the flame, the combined effect of the equivalence ratio and swirl level seems to determine the nature of the temperature fluctuations.; A preliminary analysis of combined LDA, PLIF and CARS data was presented for the most stable combustion case. The combined analysis made it possible to identify a potential zone of ignition, started by the mixing with the hot gases from both the central and the side recirculation zones.; Analysis of the gas temperature data has revealed new insights into the complex nature of the mixing and reaction processes that take place in lean premixed swirling flames. Comparison with LDA velocities and PLIF OH images suggests that the side recirculation zone may play an important role in the stabilization of the flame, along with the central recirculation zone. In addition, a flow reversal observed in the velocity components suggests the existence of eddies throughout the flame.