In order to optimize the design of microscale combustors, macroscale spiral counterflow heat-recirculating "Swiss Roll" burners were constructed and tested using hydrocarbon fuels at the Reynolds and Damk hler numbers typical of desired microscale values. Both 2D Swiss Roll burners (basically a linearly extruded spiral shape) and fully 3D Swiss Roll burners (in which the spiral is extruded in a circular pattern to create a toroidal geometry) were built using a ceramic "rapid prototyping" technique. It was found that combustion could be sustained in a low-temperature "flameless" mode in which no visible flame occurs. Mixtures well below the conventional lean flammability limit could be burned even at mean flow velocities 30 times the stoichiometric laminar burning velocity. The addition of catalytic materials in the combustion region was found to either increase or decrease the range of flammable mixtures, by substantial amounts in both cases, depending on the Reynolds number. The possibility of using fuels that are selfstarting (i.e. require no external ignition source) on catalytic surfaces was also explored. Preliminary numerical simulations compared rather poorly with the experimental results, most likely due to inaccurate heat loss and chemical reaction rate (both gasphase and surface) sub-models. It is concluded that combustion in microscale burners is feasible, however, heat recirculation, catalysis and careful management of heat losses are essential to the success of such designs.
Extinction Limits of Catalytic Combustion in Micro-channels
AbstractThe limits to self-sustaining catalytic combustion in a micro-scale channel was 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 to 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 chemical time scale. These heat-loss type 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 N 2 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 due to 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 micro-combustors.
Gas-potentiometric analysis using oxide-ion-conducting solid electrolytes as stabilized zirconia is a worthwhile method for the investigation of combustion processes. In the case of gas and oil flames specific parameters like the flame contour, the degree of burn-out and mixing can be determined and information about flame turbulence and reaction density can be gained from the temporal resolution of the sensor signal. Measurements carried out with solid electrolyte oxygen sensors in a fluidized bed show that combustion processes of solid fuels are also analyzable. This analysis results in fuel specific burn-out curves finally leading to burn-out times and to parameters of a macrokinetics of the combustion process as well as to ideas about the burn-out mechanism. From the resulting constants of the effective reaction rate a reactivity relative to bituminous coal coke can be given for any solid fuel.
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