Simultaneous PIV and Formaldehyde PLIF Measurements in the Broadened Preheat - Thin Reactions layer regime

摘要

This work addresses the question of what changes occur when a turbulent premixed flame is subjected to "extreme" turbulence levels that are ten times larger than that of previous studies (u'/S_L up to 243 in this study). Our previous paper showed that increased turbulence causes a broadening of the preheat layers but, surprisingly, there is essentially no change in the reaction layer structure. Also, extreme turbulence causes a bending of the consumption speed curve, and it causes the curve to become sensitive to the integral scale. To help explain these trends, the present work reports conditioned velocity field statistics. Simultaneous PIV-formaldehyde PLIF identifies, for the first time, the gas velocity in each image as a function of η, the distance from the reaction zone. This yields the necessary conditioned averages of the turbulence when only the reactants are present. Results indicate that the turbulence level, strain rate, and enstrophy do not decay within the preheat layer. This indicates that the conditions upstream of the broadened preheat layers are approximately equal to the conditions impinging on the reaction surface. In contrast, there is a decay in all measured variables downstream of the reaction zone, indicating that the flame is positioned just upstream of the shear layer that exists between the co-flow and the central jet.