Simultaneous measurement of localized heat-release with OH/CH2O–LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

Abstract

In practical flames such as gas turbine combustors, spatially-integrated OH∗ chemiluminescence (CL) is frequently used as a heat release rate (HRR) indicator – which has been questioned by some authors to be restricted to flames of a limited range of equivalence ratios and low Reynolds numbers – while in lab flames the approach of combined detection of OH and H2CO via LIF is an accepted diagnostic technique. Even when using specialized optics with limited acceptance angle the first method is spatially integrating while the second one allows for spatially resolved imaging. In the present work we retrieved simultaneously HRR-based information via both techniques from the same spatial flame volume, i.e., OH∗–CL radiation is collected exclusively from within the light sheet volume cutting through the flame for LIF imaging. Turbulent premixed swirl flames were investigated with a thermal power up to 30kW to shed light on the still unresolved question if correlations exist between signal intensities derived from both methods in turbulent flames. Measurements were performed in methane/air flames with Reynolds numbers between 6900 and 10,000, equivalence ratios between 0.8 and 1.2, and with a replacement of 20vol% of methane by hydrogen. Although scatter plots of HRR vs. CL intensities cluster in certain regions depending on flame conditions, their large scatter shows that correlations are weak, probably caused by flame stretch and curvature. Depending on flame conditions, correlation coefficients to characterize the scatter plots range between 0.45 and 0.81.