O. C. Kwon

Flame/stretch interactions of premixed hydrogen-fueled flames: measurements and predictions

Abstract Fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) were considered both experimentally and computationally for laminar premixed flames. Mixtures of hydrogen and oxygen with nitrogen, argon and helium as diluents were considered to modify flame transport properties for computationally tractable reactant mixtures. Freely (outwardly)-propagating spherical laminar premixed flames were considered for fuel-equivalence ratios of 0.6 to 4.5, pressures of 0.3 to 3.0 atm, volumetric oxygen concentrations in the nonfuel gases of 0.21 to 0.36, and Karlovitz numbers of 0 to 0.5, at normal temperatures. For these conditions, both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), yielding constant Markstein numbers for particular reactant conditions. The present flames were very sensitive to flame stretch, exhibiting ratios of unstretched-to-stretched laminar burning velocities in the range 0.6 to 3.0 for levels of flame stretch well below quenching conditions. At fuel-lean conditions, increasing flame temperatures (by dilution with argon rather than nitrogen) tended to reduce flame sensitivity to stretch whereas increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior. At low pressures, helium-diluted flames had reduced tendencies toward preferential-diffusion instability behavior compared to nitrogen- and argon-diluted flames due to stabilization of flame properties by strong effects of preferential diffusion of heat. Predicted and measured flame properties exhibited encouraging agreement using contemporary reaction mechanisms. Finally, flame structure predictions suggest that H and OH radical production and transport are important aspects of preferential-diffusion/stretch interactions, reflecting the strong correlation between laminar burning velocities and H+OH radical concentrations for present test conditions.

Properties of Laminar Premixed Hydrocarbon/Air Flames at Various Pressures

Outwardly propagating spherical laminar premixed e ames were experimentally and computationally used to e nd the sensitivities of laminar burning velocities to e ame stretch, represented by Markstein numbers, and the fundamental laminar burning velocities of unstretched e ames. Conditions considered included ethane, ethylene, and propane/air e ames at fuel-equivalence ratios of 0.8 ‐1.6, and pressures of 0.5‐4.0 atm at normal temperatures. Predictions were limited to unstretched (plane) e ames using mechanisms based on GRI-Mech, e nding reasonably good agreement between measurements and predictions. An interesting experimental e nding was that Markstein numbers tended to become negative over broader ranges of fuel-equivalence ratios as the pressure increased, suggesting a greater propensity toward unstable combustion because of preferential-diffusion effects at the elevated pressures of interest for most practical applications.

Flame/Stretch Interactions of Premixed Fuel-Vapor/O/N Flames

Unstretched laminar burning velocities and e ame response to stretch (Markstein numbers ) were measured for outwardly propagating spherical laminar premixed e ames involving mixtures of hydrocarbon vapors, oxygen, and nitrogen. Experimental conditions consisted of vapors of several liquid fuels (n-hexane, n-heptane, iso-octane, methyl-alcohol, and ethyl-alcohol ), concentrations of oxygen in the nonfuel gases of 19 ‐33% by volume, pressures of 0.5‐2.0 atm, fuel-equivalence ratios of 0.80 ‐1.60, and reactant mixture temperatures of 298 § 5 K. The present e ames were very sensitive to e ame stretch, yielding ratios of unstretched to stretched laminar burning velocities in the range 0.4 ‐4.0 for levels of e ame stretch well below quenching conditions (Karlovitz numbers less than 0.2 ). At low pressures, the present hydrocarbon vapor e ames had positive Markstein numbers at fuel-lean conditions, which is consistent with classical preferential-diffusion ideas. Increasing pressures, however, reduced Markstein numbers and progressively decreased the fuel-equivalence ratio range where Markstein numbers were positive. Negative Markstein numbers were associated with the presence of preferential-diffusion instability as evidenced by the appearance of chaotically distorted (wrinkled) e ame surfaces early during the e ame propagation process.

Self-Preserving Properties of Unsteady Round Nonbuoyant Turbulent Starting Jets and Puffs in Still Fluids

The self-preserving properties of round nonbuoyant turbulent starting jets, puffs, and interrupted jets were investigated both experimentally and theoretically for flows in still and unstratified environments. The experiments involved dye-containing fresh water sources injected into still fresh water within a large windowed tank. Time-resolved video images of the flows were obtained using a CCD camera. Experimental conditions were as follows: jet exit diameters of 3.2 and 6.4 mm, jet exit Reynolds numbers of 3000-12,000, jet passage lengths in excess of 50 injector passage diameters, volume of injected fluid for puffs and interrupted jets up to 191 source diameters, and streamwise penetration lengths up to 140 source diameters. Near-source behavior varied significantly with source properties but the flows generally became turbulent within 5 source diameters from the source and self-preserving behavior was generally observed at distances greater than 20-30 source diameters from the source

Flame/stretch interactions in laminar and turbulent premixed flames

The flame/stretch interactions of laminar and turbulent premixed flames are considered both experimentally and computationally. Potentially strong effects of flame/stretch interactions due to preferential-diffusion phenomena within practical turbulent premixed flames were suggested by experiments and numerical simulations of spherical outwardly propagating laminar premixed flames. These considerations were limited to conditions where ignition disturbances, pressure variations, intrinsic unsteadiness of propagating spherical flames, and radiative heat losses were small. Flame reactants consisting of H 2 /O 2 /N 2 and several light hydrocarbon/air mixtures were studied for various fuel-equivalence ratios and pressures of 0.5-4.0 atm at normal temperature (298±3K). The measurements and predictions yielded several interesting results, as follows: Flame response to stretch was linear using a local conditions hypothesis to define characteristic flame length and time scales, yielding constant Markstein numbers for given flame conditions; effects of stretch were surprisingly strong with up to 100 percent variations of laminar burning velocities resulting from rather modest stretch rates well below extinction conditions (i.e., Karlovitz numbers less than 0.5); there was a progressive tendency for greater ranges of unstable preferential-diffusion conditions (negative Markstein numbers) as pressures were increased for all reactant mixtures studied; and several contemporary detailed treatments of multicomponent transport and chemical reaction mechanisms yielded reasonably good predictions of laminar burning velocities and their sensitivity to flame stretch due to preferential-diffusion effects. The predictions suggest that the strong sensitivity of the present flames to stretch is mainly caused by preferential diffusion of light radicals and stable species relative to typical stable reaction products and heat, with increased preferential-diffusion instability at elevated pressures resulting from reduced radical concentrations in the reaction zone due to increased radical recombination rates. The potential practical importance of flame/stretch interactions was examined by considering the properties of strongly turbulent premixed flames. These measurements involved premixed H 2 /O 2 /N 2 and C 3 H 8 /air flames propagating in the thin wrinkled flamelet regime within isotropic turbulence. Test conditions included unstable, near-neutral, and stable flames with respect to effects of preferential-diffusion. The experiments yielded several interesting observations, as follows: 1) Rates of turbulent flame propagation progressively decreased as flame stability with respect to preferential-diffusion effects increased even through unstretched laminar burning velocities and turbulence properties were the same; 2) Distortion of the flame surfaces by turbulence as the flames grew caused their fractal dimensions to progressively increase from a value of 2.0, appropriate for a smooth surface, to asymptotic values in the range 2.3-2.4, irrespective of preferential-diffusion stability conditions; 3) Other parameters characterizing the extent of distortion of the flame surfaces showed no tendency to approach asymptotic values for available observation times, however, raising questions about the existence of steady turbulent flame propagation properties for the present test conditions; and 4) The extent of flame surface distortion progressively increased at a given flame diameter, but decreased at a given time of propagation, as preferential-diffusion stability was progressively increased even though unstretched laminar burning velocities and turbulence properties were the same. These flame/stretch interactions in turbulent flames can be explained by noting that stable (unstable) preferential-diffusion conditions tend to retard (enhance) distortion of the flame surface by turbulence for outwardly propagating spherical turbulent premixed flames in much the same way that preferential-diffusion effects interact with small disturbances to yield either stable (unstable) flames for nonturbulent conditions.

Self-Preserving Properties of Unsteady Round Buoyant Turbulent Plumes and Thermals in Still Fluids

The self-preserving properties of round buoyant turbulent starting plumes and starting jets in unstratified environments. The experiments involved dye-containing salt water sources injected vertically downward into still fresh water within a windowed tank. Time-resolved images of the flows were obtained using a CCD camera. Experimental conditions were as follows: source diameters of 3.2 and 6.4 mm, source/ambient density ratios of 1.070 and 1,150, source Reynolds numbers of 4,000-11,000, source Froude numbers of 10-82, volume of source fluid for thermals comprising cylinders having the same cross-sectional areas as the source exit and lengths of 50-382 source diameters, and stream-wise flow penetration lengths up to 110 source diameters and 5.05 Morton length scales from the source. Near-source flow properties varied significantly with source properties but the flows generally became turbulent and then became self-preserving within 5 and 20-30 source diameters from the source, respectively. Within the self-preserving region, both normalized streamwise penetration distances and normalised maximum radial penetration distances as functions of time were in agreement with the scaling relationships for the behavior of self-preserving round buoyant turbulent flows to the following powers: time to the 3/4 power for starting plumes and to the 1/2 power for thermals. Finally, the virtual origins of thermals were independent of source fluid volume for the present test conditions.