Fluid dynamics of boundary layer combustion

Thumbnail Image
Publication or External Link
Miller, Colin
Gollner, Michael J
Reactive flows within a boundary layer, representing a marriage of thermal, fluid, and combustion sciences, have been studied for decades by the scientific community. However, the role of coherent structures within the three-dimensional flow field is largely untouched. In particular, little knowledge exists regarding streamwise streaks, which are consistently observed in wildland fires, at the base of pool fires, and in other heated flows within a boundary layer. The following study examines both the origin of these structures and their role in influencing some of the macroscopic properties of the flow. Streaks were reproduced and characterized via experiments on stationary heat sources in laminar boundary layer flows, providing a framework to develop theory based on both observed and measured physical phenomena. This first experiment, performed at the University of Maryland, examined a stationary gas burner located in a laminar boundary layer with stationary streaks which could be probed with point measurements. The gas temperature within streaks increased downstream; however, the gas temperature of the regions between streaks decreased. Additionally, the heat flux to the surface increased between the streaks while decreasing beneath the streaks. The troughs are located in a downwash region, where counter-rotating vortices force the flame sheet towards the surface, increasing the surface heat flux. This spanwise redistribution of surface heat flux confirmed that streaks can, at least instantaneously, modify important heat transfer properties of the flow. Additionally, the incoming boundary layer was established as the controlling mechanism in forming streaks, which are generated by pre-existing coherent structures. Finally, the amplification of streaks was determined to be compatible with quadratic growth of Rayleigh-Taylor Instabilities, providing credence to the idea that the downstream growth of streaks is strongly tied to buoyancy. The next phase of the project was performed at the Missoula Fire Sciences Laboratory, where a hot plate in a laminar boundary layer was examined. In addition to manipulating the wind speed, the local buoyant force was controlled via the surface temperature of the hot copper plate. Infrared thermography was employed to detect streaks by means of local surface temperature fluctuations, and a novel and consistent method for tracking streaks and quantifying important properties was developed. Streak spacing was seen to be lognormally distributed, and the initial spacing, which was consistently between 60-70 dimensionless wall units, was shown to be governed by the incoming boundary layer. Streak spacing increased downstream of the plate, with higher plate temperatures resulting in larger magnitudes of spanwise fluctuations in surface temperature. Finally, streak behavior became more chaotic downstream, as streaks would meander rapidly and persist for shorter durations. The final phase of the study, performed at the Missoula Fire Sciences Laboratory, examined a saturated fuel wick in the same experimental configuration as the hot plate. Streaks were detected in the flame via high speed video, and tracked using the previous developed algorithm. Streak spacing was lognormally distributed, with the initial spacing (60-75 wall units) again being controlled by the incoming boundary layer. Spacing between coherent structures increased downstream, likely due to buoyant amplification. The width of streaks grew to an apparent assymptote, indicating a settling of length scale controlled by the time and rate of growth. Further downstream, coherent structures no longer resembled well-ordered streaks but more complex structures resulting from streak aggregation. Overall, trends for streaks are consistent in both the hot plate and the flame, indicating that the behavior of streaks is governed by similar mechanisms in both scenarios. Although the initial instabilities are governed by the incoming wind, buoyant forces cause the growth and aggregation of these structures. These local instabilities are capable of affecting macroscopic properties of the flow, including heat transfer to the surface, indicating that a two-dimensional assumption may fail to adequately describe heat and mass transfer during flame spread and other reacting boundary layer flows.