An investigation of coherent structures in laminar boundary layer flames
Miller, C.H., Tang, W., Finney, M.A., McAllister, S.S., Forthofer, J.M., Gollner, M.J., Combustion and Flame, 181, 123-135
Observations of coherent structures in boundary layer flames, particularly wildland fires, motivated an investigation on instabilities in a boundary layer flame. This experimental study examined streaklike structures in a stationary diffusion flame stabilized within a laminar boundary layer. The incoming flow was characterized with a hotwire anemometer, and locations of the flame streaks were found to align with pre-existing velocity perturbations. These upstream disturbances enabled stabilization of flame streaks, which could then be probed with point measurements. Flame streaks were seen to amplify with the streamwise development of the fire, and this growth was quantified via thermocouple measurements. Temperature mapping of the flame streaks indicated a temperature rise in the flame streaks, while the region in between these streaks, the trough, was seen to decrease in temperature. The heat flux to the surface was measured with a total heat flux gauge. Measurements were taken below the flame streaks and below the regions between the streaks (i.e., troughs). At all measurement locations, the heat flux below the troughs was found to be higher. This was likely a function of the flame standoff distance, and indicated that the flame streaks were acting to modify the spanwise distribution of heat flux. The presence of instabilities had a significant effect on the spanwise distribution of heat transfer. Instantaneous properties in boundary layer combustion can vary significantly due to three-dimensional effects, and this may have significant implications for describing and modeling boundary layer combustion.
Overhead view of flames under different wind velocities and boundary layer development lengths. The top pictures were taken from experiments with a short boundary layer development length, resulting in a meandering lattice of spanwise waves and streamwise streaks. The lower pictures were taken from experiments with a longer boundary layer development length, which exhibited stable streamwise streaks. In order to highlight coherent structures, the flow rate of fuel in these pictures was 5.5 slpm (1.7×10−4g/s), slightly elevated above typical experimental conditions used in later analyses of the study. The width of the burner and the resultant flame is 25 cm, and the total distance between the white sidewalls is 32.5 cm.
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