Paul D. Ronney

A thermally self-sustained micro solid-oxide fuel-cell stack with high power density

High energy efficiency and energy density, together with rapid refuelling capability, render fuel cells highly attractive for portable power generation. Accordingly, polymer-electrolyte direct-methanol fuel cells are of increasing interest as possible alternatives to Li ion batteries. However, such fuel cells face several design challenges and cannot operate with hydrocarbon fuels of higher energy density. Solid-oxide fuel cells (SOFCs) enable direct use of higher hydrocarbons, but have not been seriously considered for portable applications because of thermal management difficulties at small scales, slow start-up and poor thermal cyclability. Here we demonstrate a thermally self-sustaining micro-SOFC stack with high power output and rapid start-up by using single chamber operation on propane fuel. The catalytic oxidation reactions supply sufficient thermal energy to maintain the fuel cells at 500–600 °C. A power output of ∼350 mW (at 1.0 V) was obtained from a device with a total cathode area of only 1.42 cm2.

Near-limit flame structures at low Lewis number

The characteristics of premixed gas flames in mixtures with low Lewis numbers near flammability limits were studied experimentally using a low-gravity environment to reduce buoyant convection. The behavior of such flames was found to be dominated by diffusive-thermal instabilities. For sufficiently reactive mixtures, cellular structures resulting from these instabilities were observed and found to spawn new cells in regular patterns. For less reactive mixtures, cells formed shortly after ignition but did not spawn new cells; instead these cells evolved into a flame structure composed of stationary, apparently stable spherical flamelets. Experimental observations are found to be in qualitative agreement with elementary analytical models based on the interaction of heat release due to chemical reaction, differential diffusion of thermal energy and mass, flame front curvature, and volumetric heat losses due to gas and/or soot radiation.

Transient plasma ignition of quiescent and flowing air/fuel mixtures

Transient plasmas that exist during the formative phase of a pulse-ignited atmospheric pressure discharge were studied for application to ignition of quiescent and flowing fuel-air mixtures. Quiescent methane-air mixture ignition was studied as a function of equivalence ratio, and flowing ethane-air mixture was studied in a pulse detonation engine (PDE). The transient plasma was primarily comprised of streamers, which exist during approximately 50 ns prior to the formation of an equilibrated electron energy distribution. Results of significant reduction in delay to ignition and ignition pressure rise time were obtained with energy costs roughly comparable to traditional spark ignition methods (100-800 mJ). Reduction in delay to ignition by factors of typically 3 in quiescent mixes to >4 in a flowing PDE (0.35 kg/s), and other enhancements in performance were obtained. These results, along with a discussion of a pseudospark-based pulse generator that was developed for these applications, will be presented.

Extinction limits of catalytic combustion in microchannels

The limits to self-sustaining catalytic combustion in a microscale channel were studied computationally using a cylindrical tube reactor. The tube, 1 mm in diameter, 10 mm long, and coated with Pt catalyst, was assumed to be thermally thin, and the boundary condition on the wall was set to be either adiabatic or non-adiabatic with fixed heat transfer coefficient. Methane/air mixtures with average velocities of 0.0375–0.96 m/s (corresponding to Reynolds number, Re, ranging from 2.5 to 60) were used. When the wall boundary condition was adiabatic, the equivalence ratio at the extinction limit monotonically decreased with increasing Re. In contrast, for non-adiabatic conditions, the extinction curve exhibited U-shaped dual limit behavior, that is, the extinction limits increased/decreased with decreasing Re in smaller/larger Re regions, respectively. The former extinction limit is caused by heat loss through the wall, and the latter is a blow-off-type extinction due to insufficient residence time compared to the chemical timescale. These heat-losses and blow-off-type extinction limits are characterized by small/large surface coverage of Pt(s) and conversely large/small numbers of surface coverage of O(s). It was found that by diluting the mixture with N2 rather than air, the fuel concentration and peak temperatures at the limit decreased substantially for mixtures with fuel-to-oxygen ratios even slightly rich of stoichiometric because of a transition from O(s) coverage to CO(s) coverage. Analogous behavior was observed experimentally in a heat-recirculating “Swiss-roll” burner at low Re, suggesting that the phenomenon is commonplace in catalytic combustors near extinction. No corresponding behavior was found for non-catalytic combustion. These results suggest that exhaust-gas recirculation rather than lean mixtures are preferable for minimizing flame temperatures in catalytic microcombustors.

The structure and stability of nonadiabatic flame balls

Abstract Recent experiments in microgravity suggest the possibility of stationary spherical premixed flames (flame balls) in which the only fluxes are diffusional. We construct stationary solutions of this nature, starting with simple model equations and using activation energy asymptotics. Sufficiently large volumetric heat losses quench the flame, and for heat losses less than the quenching value there are two possible solutions, a small flame, and a large flame. For vanishing heat loss the small solution is identical to one constructed by Zeldovich, and is known to be unstable, whereas the large solution is characterized by a flame of infinite radius. We examine the linear stability of these stationary solutions, and show that all small flames are unstable to one-dimensional (radial) perturbations. Large flames are unstable to three-dimensional perturbations, but only if they have a radius greater than some critical value. Thus there is a band of large flames, lying between the quenching point and unstable flames, that are stable.

Effects of Radiative Emission and Absorption on the Propagation and Extinction of Premixed Gas Flames

Premixed gas flames in mixtures of CH 4 , O 2 , N 2 and CO 2 were studied numerically using detailed chemical and radiative emission-absorption models to establish the conditions for which radiatively-induced extinction limits may exist independent of the system dimensions. It was found that reabsorption of emitted radiation led to substantially higher burning velocities and wider extinction limits than calculations using optically-thin radiation models, particularly when CO 2 , a strong absorber, is present in the unburned gas. Two heat loss mechanisms that lead to flammability limits even with reabsorption were identified. One is that for dry hydrocarbon-air mixtures, because of the differences in the absorption spectra of H 2 O and CO 2 , most of the radiation from product H 2 O that is emitted in the upstream direction cannot be absorbed by the reactants. The second is that the emission spectrum of CO 2 is broader at flame temperatures than ambient temperature, thus some radiation emitted near the flame front cannot be absorbed by the reactants even when they are seeded with CO 2 . Via both mechanisms some net upstream heat loss due to radiation will always occur, leading to extinction of sufficiently weak mixtures. Downstream loss has practically no influence. Comparison to experiment demonstrates the importance of reabsorption in CO 2 -diluted mixtures. It is concluded that fundamental flammability limits can exist due to radiative heat loss, but these limits are strongly dependent on the emissionabsorption spectra of the reactant and product gases and their temperature dependence, and cannot be predicted using gray-gas or optically-thin model parameters. Applications to practical flames at high pressure, in large combustion chambers and with exhaust-gas or flue-gas recirculation are discussed. Published in the Proceedings of the Twenty-Seventh International Symposium on Combustion , Combustion Institute, Pittsburgh, 1998, pp. 2619-2626.

A theoretical study of propagation and extinction of nonsteady spherical flame fronts

Freely propagating expanding spherical flame fronts are studied analytically using the method of activation energy asymptotics. Effects of Lewis number, curvature, heat loss, and thermal expansion, on propagation rates and extinction conditions are examined. The behavior of spherical flame fronts is found to be determined primarily by the values of the Lewis number (Le) and a heat loss parameter. Results are compared to recent experimental observations of spherical flame fronts in mixtures near the flammability limits; for

Effect of gravity on laminar premixed gas combustion I: Flammability limits and burning velocities

{\text{Le}} 1

Nonpremixed edge flames in spatially varying straining flows

, unrealistic dynamic behavior is predicted due to the nature of diffusive-thermal instabilities under these conditions.

The structure and stability of nonadiabatic flame balls: II. Effects of far-field losses

Abstract Fuel-lean flammability limits and burning velocities in a closed vessel were measured for methaneair mixtures burning at earth gravity (one-g) and zero-gravity (zero-g) at initial pressures of 50–1500 Torr. The zero-g flammability limit was found to be between the one-g upward and one-g downward flammability limits. For sublimit mixtures burning at zero-g, an extinguishment phenomenon unlike any found at one-g was observed. For fast burning mixtures (Su > 15 cm/s), burning velocities were identical at one-g and zero-g. For slower burning but still flammable mixtures, only the zero-g observations could be interpreted to obtain burning velocity data because at one-g natural convection caused severe flame front distortion. Zero-g burning velocities for these mixtures were in good agreement with existing models of laminar flame propagation and with extrapolation of current and prior one-g data. The main conclusions are that the one-g upward flammability limit occurs at a mixture which has a burning velocity which is so low that flame propagation is impractical, that the one-g downward flammability limit is related to the inability of the flame front to propagate downward against buoyant forces, and that near-limit flame propagation at zero-g is mostly independent of the experimental apparatus. Because of the unusual nature of the extinguishment process for sublimit mixtures burning at zero-g, further experiments are required to determine the cause of the zero-g flammability limit.