Combustion in microscale heat-recirculating burners

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 Damkšhler numbers typical of desired microscale values. Both Ò2DÓ Swiss Roll burners (basically a linearly extruded spiral shape) and fully Ò3DÓ 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.