Structure of a Planar Reacting Shear Layer Using Hydrocarbon Fuels

Pickett, L. Structure of a Planar Reacting Shear Layer Using Hydrocarbon Fuels. University of Wisconsin-Madison, 2000.

The flame structure and mixing behavior of a non-premixed, planar reacting shear layer were investigated using diluted propane and dimethyl ether fuels. Non-reacting passive scalar measurements were made using laser-induced fluorescence of acetone. In reacting conditions, fluorescence from OH was employed to mark the reaction zone while laser-induced incandescence of soot was used to mark the internal structures of the mixing layer.

The passive scalar measurements showed sensitivity to the state of the inlet high-speed boundary layer. When laminar, large-scale structures with fairly homogenous composition were found resulting in non-marching style probability distribution functions (PDFs). When turbulent, gradients in the cross-stream direction were found and the PDFs shifted to a hybrid style with a strong march from the high-speed side followed by a gradual drop-off near the low-speed side. The developing mixing layer showed significant mixing in laminar interfacial regions prior to the turbulence transition. The mixed fluid fraction was found to increase to an asymptotic value of 0.5 by Re ? ? 5000. The increase occurs largely before the onset of turbulence as a result of interfacial mixing.

Under reacting conditions, two distinct classes of test conditions were investigated: one with air as the high-speed fluid (standard case) and one with fuel as the high-speed fluid (flip case). In each case the mixing layer appeared to consist 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 mix with pyrolized fuel in a manner reminiscent of a two-stream shear layer. The structure spacing was significantly lengthened for the standard case and reduced for the flip case compared to non-reacting results. The change in structure size was caused by density effects on the instability at the splitter plate as all other heat release parameters were the same in the two cases. The effect of varying fuel type, concentration, air inlet temperature, and pressure was to alter the liftoff characteristics of the flame, but no discernable change in flame structure was observed.