EAGER: MINIMUM THERMAL CONDUCTIVITY AND THERMAL EXPANSION CERAMIC NANOCOMPOSITES FOR MICROCOMBUSTOR APPLICATION

University Of Maryland, College Park, College Park MD

Investigators

Abstract

In micro-scale liquid fuelled combustion systems a finite volume of the combustor along with a finite residence time is required for fuel vaporization and mixing. At the small scale this time can be significant compared to the gaseous premixed residence time because mixing and vaporization at low Reynolds numbers is generally poor. In both conventional and small scale combustors swirl is often imparted to the oxidizer flow, the fuel flow, or both, in order to stabilize the flame and broaden the extinction limits. At the small scale the swirl flow can be provided by tangential injection, swirl vanes, or step increases in the cross sectional area of the combustor. In order to minimize the characteristic length of the combustion chamber a fundamental understanding of droplet transport, vaporization, and mixing at low Reynolds number is required. The project objective is to develop efficient and non-polluting very small volume micro-scale combustor that can be operated with gas and liquid fuels. Successful demonstration at microscale will allow portable micro-power generation for use in portable devices, such as, laptops, and also for applications in micro-propulsion and micro-satellites. The project aims to develop fuel flexible and efficient micro-combustor (combustion volume of the order of 0.0018 cubic inches). Such size combustor is smaller than flame quenching distance. The fundamental understanding developed via experiments and calculations will allow efficient heat recirculation back into the combustor to help alleviate thermal quenching of the flame. The fuel will be injected through a porous heat recuperator for pre-vaporization with measures taken to avoid fuel coking and deterioration of the porous media that can limit the operational life of the combustor and micro-thruster. To achieve the project objectives, set of novel tasks will be developed and tested using experiments and calculations to enrich knowledge and provide wider applications. This project will explore means to develop and deposit a novel and innovative ceramic nanostructured composite materials on inside walls of micro-scale combustor with ultralow thermal conductivity and near-zero thermal expansion so that no or minimal heat exchange occurs where it is not desired while maintaining high heat transfer on the other walls to preheat the reactants prior to fuel-air mixture entering the combustion volume. Efficient thermal management with efficient heat exchange between the exhaust gases and fresh reactant mixture is critical. A recuperator type heat exchanger will be used for heat exchange between the exhaust gases and incoming fresh fuel-air mixture. For the case of liquid fuel the vaporization of fuel will be via porous heat recuperator. The emphasis will be placed on zirconium-based ceramics for micro-combustor for micro-scale power applications. The approach used will be to manufacture specially-configured Zirconium-based nano-composites in which thermal conductivity and expansion can be altered by introducing multi-scale phonon scatters, i.e., rattling atoms (atomic scale) and interfaces (nanometer scale). Nanosize particles of ultralow thermal conductivity will be deposited on defined wall of the combustor for near zero thermal conductivity while maintaining high heat exchange on the other walls to enhance the efficiency and performance. This project will influence the development of next generation miniature scale combustion and propulsion devices for use in terrestrial and space applications.