Measurements to determine the regimes of premixed flames in extreme turbulence

A new regime of extreme turbulence - defined as the ratio of turbulence intensity to laminar flame speed u'/S-L from 25 to 243 - was characterized for six premixed flames using a new piloted Bunsen burner (called Hi-Pilot). The flames studied had u'/S-L values several times larger than those of previous related studies and integral scales and turbulent Reynolds numbers as large as 41 mm and 99,000, respectively. Layer thicknesses were determined from planar laser-induced fluorescence (PLIF) images of OH and formaldehyde. Preheat layer thickness was found to increase to sixteen times the laminar value. Residence time of eddies in the flame appears to be important, since the flame tip had preheat regions that were thicker than at the flame base. Reaction layers were not broadened, remaining below twice the laminar value. Four of the cases were predicted to lie in the Broadened Preheat - Thin Reaction layer (BP-TR) regime and the measurements confirmed that they had a BP-TR structure. However, two cases went far beyond the predicted boundary for the Broken Reactions (BR) regime but measurements showed that they were not broken but retained their BP-TR structure. Thus the regime of BP-TR is measured to persist over a wider range than previously predicted. One explanation is that the turbulent eddies may become weakened by the thick, viscous preheat layer before they arrive at the reaction front. Distributed reactions were not observed in the six cases that were selected. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.