• Energy Research

Abstract An experimental study of a spiral counterflow “Swiss roll” burner was conducted, with emphasis on the determination of extinction limits and comparison of results with and without bare-metal Pt catalyst. A wide range of Reynolds numbers ( Re ) were tested using propane–air mixtures. Both lean and rich extinction limits were extended with the catalyst, though rich limits were extended much further. With the catalyst, combustion could be sustained at Re as low as 1.2 with peak temperatures as low as 350 K. A heat transfer parameter characterizing the thermal performance of both gas-phase and catalytic combustion at all Re was identified. At low Re , the “lean” extinction limit was actually rich of stoichiometric, and rich-limit had equivalence ratios exceeded 40 in some cases. No corresponding behavior was observed without the catalyst. Gas-phase combustion, in general, occurred in a “flameless” mode near the burner center. With or without catalyst, for sufficiently robust conditions (high Re , near-stoichiometric) not requiring heat recirculation, a visible flame would propagate out of the center, but this flame could only be re-centered if the catalyst were present. Gas chromatography indicated that at low Re , even in extremely rich mixtures, CO and non-propane hydrocarbons did not form. For higher Re , where both gas-phase and catalytic combustion could occur, catalytic limits were slightly broader but had much lower limit temperatures. At sufficiently high Re , catalytic and gas-phase limits merged. It is concluded that combustion at low Re in heat-recirculating burners greatly benefits from catalytic combustion with the proper choice of mixtures that are different from those preferred for gas-phase combustion. In particular, the importance of providing a reducing environment for the catalyst to enhance O 2 desorption, especially at low Re where heat losses are severe thus peak temperatures are low, is noted.

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.

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 degrees C. A power output of approximately 350 mW (at 1.0 V) was obtained from a device with a total cathode area of only 1.42 cm2.

Abstract A two-dimensional numerical model of spiral counterflow heat recirculating combustors was developed including the effects of temperature-dependent gas and solid properties, viscous flow, surface-to-surface radiative heat transfer, heat conduction within the solid structure, one-step chemical reaction and heat loss from the combustor to its surroundings. A simplified model of heat loss in the 3rd dimension was implemented and found to provide satisfactory representation of such losses at greatly reduced computational cost compared to fully three-dimensional models. The model predicts broad reaction zones with structure decidedly different from conventional premixed flames. Extinction limits were determined over a wide range of Reynolds numbers (2 500, modeling of turbulent flow and transport was required to obtain such agreement. Heat conduction along the heat exchanger wall has a major impact extinction limits; the wall thermal conductivity providing the broadest limits is actually less than that of air. Radiative heat transfer between walls was found to have an effect similar to that of heat conduction along the wall. In addition to weak-burning extinction limits, strong-burning limits in which the reaction zone moves out of the combustor center toward the inlet were also predicted by the numerical model, in agreement with experiments. It is concluded that several physical processes including radiative transfer, turbulence and wall heat conduction strongly affect the performance of heat-recirculating combustors, but the relative importance of such effects is strongly dependent on Re.

A simple first-principles model of counter current heat-recirculating combustors is developed, including the effects of heat transfer from the product gas stream to the reactant stream, heat loss to ambient, and heat conduction in the streamwise direction through the dividing wall (and heat transfer surface) between the reactant and product streams. It is shown that streamwise conduction through the wall has a major effect on the operating limits of the combustor, especially at small dimensionless mass fluxes (M) or Reynolds numbers that would be characteristic of microscale devices. In particular, if this conduction is neglected, there is no small-M extinction limit because smaller M leads to larger heat recirculation and longer residence times that overcome heat loss if M is sufficiently small. In contrast, even a small effect of conduction along this surface leads to significantly higher minimum M. Comparison is made with an alternative configuration of a flame stabilized at the exit of a tube, where heat recirculation occurs via conduction through tube wall; it is found that the counter-current exchanger configuration provides superior performance under similar operating conditions. Implications for microscale combustion are discussed.

Abstract Three-dimensional (3D) numerical simulations of spiral counterflow Swiss roll heat-recirculating combustors were performed including gas-phase conduction, convection and chemical reaction of propane–air mixtures, solid-phase conduction and surface-to-surface radiation. These simulations showed that in 3D, results are surprisingly similar with or without a turbulence model activated because without turbulence, Dean vortices form in the curved channels which enhance heat transport and thus heat recirculation by nearly the same amount as turbulence does. Turbulence enhances the apparent viscosity to the point that, when the turbulence model is activated, the Dean vortices are not formed at the moderate Reynolds numbers studied in this work. Predictions of both 3D models are in good agreement with experiments with respect to extinction limits and temperature distributions. Comparing 3D to two-dimensional (2D) simulations employing a simple model for the out-of-plane heat losses, the turbulence model is found to be essential for accurate predictions in 2D because Dean vortices cannot form in 2D simulations. Predictions obtained using a 1-step chemical reaction model with the pre-exponential term adjusted to obtain agreement between model and experiments at one test condition are compared to a 4-step reaction model developed for flow reactors. The latter is found to provide good agreement with experiments with no adjustable parameters, which is argued to be plausible because the conditions inside heat-recirculating combustors are closer to those of flow reactors than propagating flames.

Abstract The rates of flame spread and minimum oxygen concentrations supporting flame spared over thermally thick fuel beds were determined at earth gravity and microgravity for varying concentrations of a gaseous extinguishing agent (N2, CO2, or He) added to “baseline” O2-N2 atmospheres. Drop towers were used to obtain microgravity, and foam fuels were employed to obtain sufficiently rapid flame spread in these short-duration facilities. At earth gravity, CO2 was the most effective on a molar basis at reducing spread rate whereas at microgravity, He was the most effective at reducing spread rate and causing flame extinction. These findings are proposed to result from the transition from a high-speed blow off-type limit at earth gravity to a low-speed radiative heat loss-induced limit at microgravity. CO2 is less effective as an extinguishment agent at microgravity because it reabsorbs radiation from flame and reradiates it back to the fuel bed at microgravity, whereas with N2 and He this phenomena is not evident at microgravity because these two gases are radiatively nonparticipating. He is particularly effective at microgravity because its high thermal diffusivity leads to much larger flame thicknesses resulting in much greater volume for radiative heat losses. Radiative effects are unimportant at earth gravity because convective flow is significantly faster, leading to thinner flames with much lower ratios of radiative to conductive/convective heat transfer. These results are particularly noteworthy considering that the International Space Station employs CO2 as fire extinguishers; our results suggest that helium may be a better suppressant agent on both mass and mole bases at microgravity even though CO2 is much better on a mole basis at earth gravity.

The thermodynamic theory governing the absolute maximum efficiency of energy conversion by thermoelectric devices that operate as part of the heat recycle in regenerative burners is examined. Comparison with a series of elementary Carnot cycles helps to address the question of whether higher system efficiencies are realizable by rejecting the unconverted heat to the cold surroundings or to the incoming reactants as part of the recycle. While for the second law (Carnot) heat engine cycles the maximum power that can be extracted is independent of the layout, in the case of irreversible thermoelectric assemblies a particular combination of both in a novel configuration is shown to be most advantageous. This heat exchanger/thermoelectric converter configuration consists of a coaxial assembly of many annular thermoelectric elements in series and a section in which heat is rejected to the incoming reactants that is followed by a second section, which discards unconverted heat to the cold surroundings. It is shown that the efficiencies of such devices could substantially exceed the maximum efficiencies of the best present-day thermolectric of such devices could substantially exceed the maximum efficiencies of the best present-day thermoelectric conversion systems, and the theory suggests practical designs for small, combustion-driven, power supplies.

A simple analysis of linear and spiral counterflow heat-recirculating combustors was conducted to identify the dimensionless parameters expected to quantify the performance of such devices. A three-dimensional (3D) numerical model of spiral counterflow ‘Swiss roll’ combustors was then used to confirm and extend the applicability of the identified parameters. It was found that without property adjustment to maintain constant values of these parameters, at low Reynolds number (Re) smaller-scale combustors actually showed better performance (in terms of having lower lean extinction limits at the same Re) due to lower heat loss and internal wall-to-wall radiation effects, whereas at high Re, larger-scale combustors showed better performance due to longer residence time relative to chemical reaction time. By adjustment of property values, it was confirmed that four dimensionless parameters were sufficient to characterise combustor performance at all scales: Re, a heat loss coefficient (α), a Damkohler number (...