Previous measurements of turbulent burning velocity (ST) have been reported by Gülder and colleagues for intense levels of turbulence, defined to be u' ⁄ SL values between 12 and 24, and normalized integral scales (Lx ⁄ δL) up to 46. The present work extends burning velocity measurements to much higher levels of turbulence than have been considered before: to extreme turbulence defined as u' ⁄ SL values from 25 to 163 and Lx ⁄ δL up to 114. These conditions are argued to be more representative of the turbulence found in certain engines. To do so, a new large, piloted Bunsen burner (called Hi-Pilot) was developed and OH and formaldehyde PLIF images provided the time-averaged contours of progress variable based on OH (cOH). The conventional global consumption speed (ST, GC, 1 ⁄ S L) is based on the cOH = 0.5 contour and it was found to exceed 25. Two other measured speeds are based on the leading edge (ST, GC, 2) and the component due to flamelet surface density (ST, F). Varying the integral scale had a significant effect on ST, GC, 2 but not on the other two burning velocities. The consumption speed ST, GC, 1 curve displayed "bending" in the range of extreme turbulence, while the flamelet surface density contribution (ST, F) curve instead flattened out and was independent of turbulence intensity. A possible explanation for these measured trends is based on the observed extensive broadening of the preheat zone. Preheat broadening depends on the integral scale and is believed to attenuate the turbulence that eventually interacts with the reaction zone. Preheat broadening was also found to cause a breakdown of the thin flamelet assumption; this appears to cause thermal diffusivity to dominate over the flame wrinkling mechanism.
A new regime of extreme turbulence dened as the ratio of turbulence intensity to laminar ame speed u'/S L from 25 to 243 was characterized for six premixed ames using a new piloted Bunsen burner (called Hi-Pilot). The ames studied had u'/S L values several times larger than those of previous related studies and integral scales and turbulent Reynolds numbers as large as 41 mm and 99,000, respectively. Layer thicknesses were determined from planar laser-induced uorescence (PLIF) images of OH and formaldehyde. Preheat layer thickness was found to increase to sixteen times the laminar value. Residence time of eddies in the ame appears to be important, since the ame tip had preheat regions that were thicker than at the ame base. Reaction layers were not broadened, remaining below twice the laminar value. Four of the cases were predicted to lie in the Broadened Preheat -Thin Reaction layer (BP-TR) regime and the measurements conrmed that they had a BP-TR structure. However, two cases went far beyond the predicted boundary for the Broken Reactions (BR) regime but measurements showed that they were not broken but retained their BP-TR structure. Thus the regime of BP-TR is measured to persist over a wider range than previously predicted. One explanation is that the turbulent eddies may become weakened by the thick, viscous preheat layer before they arrive at the reaction front. Distributed reactions were not observed in the six cases that were selected.
Structural features of highly turbulent piloted flames were acquired from simultaneous PLIF images of formaldehyde (CH2O) and OH. Both lean and near-stoichiometric (equivalence ratio ϕ = 0.75 and 1.05, respectively) methane-air flames were studied under twelve different flow conditions and at two different interrogation regions. The non-reacting conditions for these flames consist of turbulent Reynolds numbers (ReT), turbulence intensities (u'/SL), and integral length scales that range from 520 to 80,000; 5 to 185; and 6 mm to 37 mm, respectively. Eight of the twelve cases have u'/SL > 25 and thus are classified into a regime of extreme turbulence. Preheat and reaction zone thicknesses were measured in all twelve cases. The preheat zone thickness was interpreted from the CH2O PLIF images and the reaction zone thicknesses were obtained from the profiles derived from the pixel-by-pixel product of the OH and CH2O PLIF images. The preheat zones associated with a particular condition were classified as being "thickened" if the mean thickness for that condition exceeded two but not four times the measured laminar value (0.42 and 0.39 mm for lean and rich flames, respectively). If the average thickness was greater than four times the measured laminar value that preheat zone was deemed "primarily distributed." Ten of the twelve cases possessed "primarily distributed" preheat zones, while those in the two least turbulent cases were "thickened." The majority of the cases possessed average reaction layer thicknesses that are no thicker than twice the measured laminar value (0.39 and 0.38 mm for lean and rich flames, respectively); hence, they were identified as having "thin" reaction layers. Regardless of being categorized as "thin," the reaction zones in each case exhibited regions of both relatively thin and thick reaction layers. In fact the appearance of the observed reaction zones can best be described as resembling "chicken noodle soup." That is, in any given instantaneous image relatively thin, "noodle-like" reaction layers are generally accompanied by thicker "chunky-chicken-like" reaction regions. Furthermore, the observed reaction zone structures in a particular case often fail to correspond to those predicted by the turbulent premixed combustion regime diagram. This suggests that the regime diagram requires alterations if it is to properly forecast the appearance of a flame based on a simple set of operating conditions. The data set presented here is currently too limited to enable a thorough re-mapping of the regime diagram. However, based on their structural features, the cases considered here were categorized into appropriate regimes of combustion.
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'/S L ) of 185, an integral length scale (λ/δ F,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 (CH 2 O) 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 CH 2 O 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.
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