Experimental investigation of OH*/CH* ratio variations in turbulent, disk stabilized, lean propane-air flames with inlet mixture preheat and stratification

Abstract

An experimental investigation of OH* and CH* chemiluminescence emissions from bluff-body stabilized, stratified, lean propane-air flames, operated under elevated inlet temperatures has been performed in this work. Simultaneous band-pass filtered emissions, at inlet mixture temperatures ranging from 300 to 573 K, have been acquired to evaluate the OH*/CH* ratio in an effort to identify a possible correlation between the chemiluminescence ratio and the equivalence ratio, for these flame configurations. Moreover, the impact of preheat on the disposition of the radially stratified equivalence ratio gradient, issuing from an upstream premixing section has been investigated. The employed premixer/burner set up consists of three concentric disks that form two consecutive premixing cavities and lead to a radially stratified equivalence ratio gradient, characterized by its peak profile value, at the inlet to the burner zone. Band-passed filtered chemiluminescence measurements and Fourier Transform Infrared Spectroscopy (FTIR) analysis have been performed to assess the OH*/CH* ratio and the mixing fields supporting the flames. The results revealed an altered premixedness behavior of the employed arrangement with elevated inlet preheat, when compared to a non-preheated case. Additionally, both maximum and spatially averaged OH*/CH* ratio values have been studied in an effort to assess the equivalence ratio by using line of sight chemiluminscence measurements. The maximum chemiluminscence ratio values have been proven to better compensate for the spatial inhomogenieties encountered in the turbulent, stratified set ups studied, thus better estimating the effective equivalence ratio value that is burned in the reacting shear layer, closely corroborating counterpart laminar flame investigations.