Aaron W. Skiba

Measurements of premixed turbulent combustion regimes of high reynolds number flames

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.

On the Resonance and Shelf/Open-Ocean Coupling of the Global Diurnal Tides

AbstractThe resonance of diurnal tidal elevations is investigated with a forward ocean tide model run in a realistic near-global domain and a synthesis of free oscillations (normal modes) computed for realistic global ocean geometries and ocean physics. As a prelude to performing the forward ocean tide simulations, the topographic wave drag, which is now commonly employed in forward ocean tide models, is tuned specifically for diurnal tides. The synthesis of global free oscillations predicts reasonably well the forward ocean diurnal tide model sensitivity to changes in the frequency, zonal structure, and meridional structure of the astronomical diurnal tidal forcing. Three global free oscillations that are important for understanding diurnal tides as a superposition of forced-damped, resonant, free oscillations are identified. An admittance analysis of the frequency sweep experiments demonstrates that some coastal locations such as the Sea of Okhotsk are resonant to diurnal tidal forcing. As in earlier wo...

Experimental assessment of premixed flames subjected to extreme turbulence

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.

Experimental investigation of premixed turbulent combustion in high reynolds number regimes using PLIF

Premixed turbulent flame structures are imaged with simultaneous formaldehyde and OH PLIF. A new piloted burner was designed to achieve high turbulent Reynolds numbers (Ret) up to 68,000 and low Damkohler numbers (Dat). Primary reaction zones are identified by the overlap of the OH and formaldehyde signals and preheat zones of low temperature secondary reactions are identified from the formaldehyde signal. At low Ret of 600, the primary reaction zones are continuous and products do not mix with reactants. This results in thin preheat layers and relatively thin flamelets. As Ret increases, the primary reaction zones become shredded and disconnected. This allows mixing of the hot products with the reactants and broadens the preheat/secondary reaction zones. Additionally, the reaction layers are typically 4-5 times thicker than those in a laminar flamelet. Interestingly, as Ret increases further, the thickness of the reaction layers only increases slowly, but the total area of reaction regions grows rapidly.