Howard Pearlman

COOL FLAME PROPAGATION SPEEDS

Abstract Cool flames are studied at reduced-gravity in a closed, unstirred, spherical reactor to minimize complexities associated with natural convection. Under such conditions, transport is controlled by diffusive fluxes and the flames are observed to propagate radially outward from the center of the reactor toward the wall. Intensified video records are obtained and analyzed to determine the flame radius as a function of time for different vessel temperatures (593–623 K) and initial pressures (55.2–81.4 kPa) using an equimolar (φ = 5) propane-oxygen premixture. Polynomial-fits are applied to the data and differentiated to determine the cool flame propagation speeds. In nearly all cases considered, the flame decelerates monotonically and in some cases, subsequently retreats towards the center of the reactor. The flame speed is also tabulated as a function of the flame stretch rate. Extrapolation of the cool flame speeds to zero stretch is then performed to determine the “unstretched” cool flame propagation speeds.

The Role of Diffusive Transport on Low and Intermediate Temperature Hydrocarbon Oxidation: Numerical Simulations Using the Wang-Mou Mechanism

The spatio-temporal temperature and species concentration distributions associated with low and intermediate temperature hydrocarbon oxidation are computed using a global thermokinetic scheme augmented with diffusive transport. The scheme used for the computations was proposed by Wang and Mou and is extended to include diffusion of species and heat. The conservation equations for species and energy are then derived and solved for a one-dimensional and an axisymmetric, spherical domain for temperatures ranging from 540 to 660 K at subatmospheric pressures. The predictions are then used to develop ignition diagrams for different Lewis (Le) numbers. Increasing Le is found to promote oscillatory cool flames and two-stage ignition in the one-dimensional model, while the ratio of the mass diffusivity of the parent fuel to that associated with the autocatalytic chain carrier has a negligible effect on the structure of the ignition diagrams. In the spherical model, oscillatory cool flames and two-stage ignition were also predicted albeit at lower values of the Le.

The Role of Diffusive Transport on Low and Intermediate Temperature Hydrocarbon Oxidation: Closed Reactor Experiments Using Equimolar n–C4H10 + O2 Premixtures at Reduced-Gravity

Experiments were conducted in a closed, spherical reactor aboard NASAs KC-135 reduced-gravity aircraft using an equimolar n–C4H10 + O2 premixture (Le = 1.3) at subatmospheric pressures to compliment model predictions and further explore the reactive-diffusive structure of cool flames and ignitions. The pressure and radial temperature histories were recorded and analyzed for different initial conditions. In addition, the visible light emission from excited formaldehyde was recorded using an intensified video camera and was observed to be radially symmetric in all cases. Unexpectedly, however, the measured temperature distributions during (and after the passage of) the cool flames and ignitions were not parabolic as predicted by conduction models, which suggests the onset of weak convection at 10−2 g.

The Effects of Particle Size, Morphology, and Mass Loading on the Reactivity of Nanocatalytic Particles

Nanocatalytic particles of Gold (Au), Platinum (Pt), and Palladium (Pd) are highly reactive at room-temperature and can be used to generate heat in micro-scale devices for portable power generation. No pre-heating is required for light-off and high steady-state operating temperatures can be sustained with high density alcohol-air premixtures. Preliminary experiments conducted in our lab and those reported by Hu and co-workers at Oak Ridge National Lab have measured peak operating temperatures ∼ 300–500 degrees Celsius using near-stoichiometric methanol/air and ethanol/air premixtures at ambient initial temperature and atmospheric pressure. The effect of particle size, morphology, mass loading, and flow residence time are reported for different mixture stoichiometries. Temperature measurements and gas species analyses are also tabulated. Interestingly, smaller particles were observed to be less reactive than larger particles for the same mass loadings for select conditions. Materials characterization of the particles has also been conducted to characterize the specific surface area of the catalyst and evaluate the importance of particle sintering, morphology changes, and particle distribution.Copyright

Diffusion-Controlled Cool Flame Propagation Speeds

Cool flames in an equimolar propane-oxygen premixture have been studied at reduced gravity in a closed, unstirred, static, spherical reactor. Acquired visual records were then analyzed and used to determine the cool flame propagation speeds for different vessel temperatures and initial reactant pressures. Without complexities associated with natural convection, transport is governed by diffusion of heat and species and the reactive-diffusive flame front as evidenced by the peak visible light emission, which is observed to propagate from the center of the reactor towards the walls. Flame speed dependencies on the flame radius, mixture composition, temperature, and pressure are presented. Additionally, the very nature of the propagation mechanism is brought into question as the flame propagation speeds are roughly an order of magnitude higher than the diffusional speeds based on a simple scaling analysis. Perhaps, the apparent front propagation is a phase wave with temperature and species concentration profiles altered by diffusion. A first comparison between these experimental results and the light emission predicted using a reduced propane mechanism, augmented with diffusive transport, by Fairlie and co-workers shows a much steeper light intensity gradient in the experiments than in the model.

The Cool Flames Experiment: Recent Results at Reduced and Partial Gravity

Cool flames at Earth (1g), Martian (0.38g), Lunar (0.18g) and reduced-gravity (10g) have been studied experimentally in a closed, unstirred, static reactor to better understand the role of natural convection and diffusive transport on the induction period(s), flame shape, flame propagation speed, pressure history and temperature profile. Natural convection is known to play an important role in all terrestrial, unstirred, static reactor cool flame and auto-ignition experiments when the Rayleigh number, Ra≡βg∆TR3/να, exceeds 600 [2,3,6]. At 1g, typical values of the Ra are 10-10. In this paper, experimental results from static, unstirred reactor studies conducted at four different gravitational acceleration levels are reported for an equimolar propane-oxygen premixture. At 1g, the effects of natural convection dominate diffusive transport, the cool flame starts near the top of the vessel and subsequently propagates downward through the vessel. The flame is inherently two-dimensional. As the effective gravitational acceleration decreases, the associated Ra decreases linearly, convective transport weakens relative to diffusive fluxes of heat and species. At reduced-gravity, cool flames are observed to propagate radially outward from a centrally-located kernel without distortion owed to convective flow at a velocity that depends on the flame radius. HARDWARE DESCRIPTION AND OPERATION The static, unstirred reactor and support apparatus are described in detail in reference 5. The essential hardware components include a furnace (0-600C), a fused-silica spherical vessel (i.d.=10.2 cm) and a premixed gas mixing and delivery system. The temperature uniformity within the furnace is ± 10C. The pressure in the reaction vessel is recorded with a Setra Model 204 0-25 psia transducer (accuracy:±1.4 Torr) mounted on the gas inlet to the reactor. The radial gas temperature profile is measured at 5 discrete radial locations, using a horizontally-mounted, type-K thermocouple rake in the vessel. A Hamamatsu C5909 ICCD camera operating at maximum gain records the emitted light through an optical port on the side of the furnace and a Xybion intensified camera mounted on the top of the furnace records an orthogonal view. EXPERIMENTAL RESULTS The slow heat release that occurs during the initial induction period is expected to induce a slow toroidal convective flow in the closed vessel for Ra>600 (1g, 0.38g, and 0.18g cases) as the hot, less dense gas rises in the bulk and recirculates downward along the internal vessel wall as it cools [3,6]. The rise speed of the hot gas is expected to scale as g [5] and thus decrease as the effective gravitational acceleration (g) decreases. Relative to the 1g case, the manifestation of the slower convective flows at 0.38g and 0.18g on the flame shape and propagation is observed indirectly through the observed modifications in the flame shape and its evolution (see Fig. 1). To date, however, this toroidal flow field has not been quantified experimentally.