Structure of a reacting hydrocarbon-air planar mixing layer

The structure of a reacting hydrocarbon-air two-stream planar mixing layer was investigated experimentally with non-premixed reactants under pressurized conditions. Propane and dimethyl ether (DME) diluted with argon or nitrogen was used as the fuel stream while heated air was used as the oxidizer. Experiments were performed at a range of Reynolds numbers in both the pre- and post-mixing transition portions of the mixing layer under conditions where the lean reactant (air) was placed in either the high-speed (AHS) or low-speed stream (FHS). The reacting mixing layer was visualized using a combined OH LIF/soot LII technique, wherein the reaction zone and the region of parent fuel entrainment and decomposition were simultaneously imaged. In both AHS and FHS cases at all Reynolds numbers, examined, the mixing layer consisted of two regions: a high temperature reaction zone with a laminar appearance found on the oxidizer side of the mixing layer and an 'internal' mixing layer in which products mixed with pyrolized fuel in a manner reminiscent of a two-stream non-reacting mixing layer. The location and dynamics of the soot formed within the mixing layer were closely related to the mixing behavior of the large-scale structures. The regions of highest soot volume fraction were found in low temperature regions near the location of raw fuel entrainment. There was no significant broadening of the high-temperature reaction zone or increase in flame area under turbulent conditions due to the lack of dilution of the freestream conditions, unlike previous observations in jet flames. Changes in the inlet streams which affected chemistry did not appear to cause significant changes in the overall mixing layer structure shown in the OH/LII images. However, finite-chemistry effects were discernable with temperature measurements and indicated that reduced product formation was observed with reductions in a characteristic Damkohler number, Da. The point of flame lift-off was shown to occur at Da < I over a wide range of operating conditions that caused changes in mixing or chemistry. (C) 2003 The Combustion Institute. All rights reserved.