A new concept is advanced for the miniaturization of direct-injection liquid-fueled combustors wherein the fuel is flowed in a wall film that reduces heat losses, optimizes vaporization rate, and inhibits quenching. The new alternative for high surface/volume combustors is the delivery of the liquid fuel as a film on the combustor surfaces. This delivery simultaneously cools the combustor walls and exposes the fuel for vaporization. A simple analysis indicates that if the combustor were part of a miniature engine, power levels from ten watts to ten kilowatts would be achievable with combustor volumes varying from a few hundred cubic millimeters to a few cubic centimeters and fuel flow rates varying from about a milligram per second to a gram per second. The liquid-film burning concept extends beyond compact power devices and applies to any combustor where high surface-to-volume ratio is imposed or desired. Studies analyze the combustor over a range of laminar and turbulent conditions using both experimental and computational methods. The goals are to maximize the combustor efficiency and stability while minimizing size and emissions. Although proof-of-concept studies are promising, a substantial amount of research is needed to bring the film-combustion strategy forward. This includes fundamental issues regarding swirl flows interacting with filming surfaces, heat transfer and vaporization under these conditions, fuel/air mixing in the chamber, and minimizing reaction times. A range of technical issues are also examined, including fuel filming approaches (e.g., porous cylinders or wall sprays), moving to high pressure conditions, flame stability, ignition strategies, and tolerance of different fuel types. Small combustors are quite sensitive to operating conditions, so it is likely that control of the system will be important. Control of the fuel and air flow rates, heat flux control through wall cooling, and swirl control are examples of potentially important system inputs needed to maintain optimal combustor performance.
The growing market of ideas that require personal power ranges from electronic and telecommunication equipment (e.g., cellular telephones and laptop computers) to small, mobile reconnaissance robots that can safely explore potentially hazardous environments. Many of these lightweight devices demand tens of watts of power for durations on the order of tens of hours, thereby driving the power source considerations towards those with highest energy density. Because internal combustion has the potential to simultaneously provide high power density and high energy density, it is natural to explore this method of power generation. The micro-gas turbine (combustor volume 0.04cc), the mini (0.078 cc displacement) and micro (0.0017 cc displacement) rotary engine , the microrocket (0.1 cc combustion chamber) , and the micro Swiss roll burner are examples of such exploration. These devices are not yet performing at efficiencies that make them competitive with the best available batteries, but they have demonstrated the plausibility of internal combustion as a personal power source.
