Liquid fuel microcombustor using microfabricated multiplexed electrospray sources

The use of hydrocarbons is very appealing for the realization of high energy density power sources, so long as the fuel can be stored in the liquid phase. As a result, except for the most volatile hydrocarbons, a miniaturized combustor must rely on a good design of the fuel atomizer, which should yield small, rapidly evaporating droplets to generate the fuel vapor promptly, mix it with the oxidizer and subsequently burn it with the attending heat release. To achieve this goal, we relied on the use of multiplexed electrosprays and a catalytic reactor for fuel conversion consisting of a pack of catalyst impregnated meshes (Microliths^®). Fuel dispersion was achieved by microfabricating the fuel distributor in Si using deep reactive ion etching. Tests were performed using JP-8 as the liquid fuel. Preliminary experiments in a 0.8 cm³ optically accessible combustor, enabling the measurements of droplet size and velocity, revealed that the spreading of the electrospray by Coulombic repulsion is the phenomenon controlling the volume of the mixing/evaporation chamber. Droplet evaporation occurs in the thin (Peclet number dependent) thermal layer preceding the catalytic section of the combustor. Subsequent system optimization in a fully ceramic combustor yielded a volumetric heat release rate as large as 270 MW/m³, a value that is of the same order as that of conventional gas turbines. The small overall combustor volume, at only 0.22 cm³, suggests that the large volumetric energy density was achieved despite the device large surface-to-volume ratio and attending heat losses. The fuel was fully oxidized, with CO/CO₂ ratios well under 1% over a range of equivalence ratios. Inspection of the combustor inner walls after operating continuously for 10 h, revealed no traces of deposits. The design has the potential of being scaled either up or down, depending on power needs.