Measurements of Premixed Turbulent Combustion Regimes of High Reynolds Number Flames

Abstract

The goal of this research is to empirically identify the boundaries between different regimes of premixed turbulent combustion that appear on the diagrams of Borghi and Williams. To date, four conditions have been extensively studied. The most intense of the four conditions possesses a turbulence level (u’/SL) of 185, an integral length scale (λ/δF,L) of 46, and a turbulent Reynolds number of 69,000. At present, the data set is too limited to plot boundaries on the regime diagrams. However, the four conditions have been categorized into their appropriate regimes. The structure and the thicknesses of the reaction zones were determined from simultaneous PLIF images of formaldehyde (CH2O) and OH. Locally distributed reactions and shredded (i.e. broken) flamelets were observed in these images. The burning fraction varied between 0.75 and 1.0, indicating that up to 25% of the reaction layer was locally extinguished where “holes” were formed. The reaction or preheat zones associated with a particular condition were classified as being “globally distributed” if the mean thickness for that condition exceeded four times the laminar value. If a particular reaction zone is both four times thicker than the laminar value and its length to thickness ratio is less than four it is identified as being “locally distributed.” In contrast, if this ratio exceeds four or the zone is not locally four times thicker than the laminar value it is considered to be thickened. While none of the cases were identified as being “globally distributed;” some of the cases were “partially distributed;” this is defined to occur when more than 25% of the reaction surface consists of “locally distributed” reaction zones. The preheat zone thickness was deduced from the CH2O PLIF images. Three of the four conditions, in which the turbulent Reynolds number exceeded 20,000, were found to have “globally distributed” preheat zones. Thickening of the preheat zone is believed to be enhanced when “holes” allow hot products to rapidly mix with the reactants. Previous studies conducted at much lower turbulent Reynolds numbers rarely observed local extinction within the reaction layer.