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

在微尺度液体燃料燃烧系统中，燃料蒸发和混合需要有限体积的燃烧室和有限的停留时间。在小尺度下，这个时间与气体预混的停留时间相比是重要的，因为在低雷诺数下混合和汽化通常很差。在传统和小型燃烧器中，为了稳定火焰和扩大熄灭极限，通常将涡流传递给氧化剂流、燃料流或两者。在小尺度上，旋涡流动可以通过切向喷射、旋涡叶片或燃烧室横截面积的阶跃增加来提供。为了最小化燃烧室的特征长度，需要对低雷诺数下的液滴传输、汽化和混合有基本的了解。该项目的目标是开发可使用气体和液体燃料的高效、无污染的非常小体积微型燃烧器。在微型尺度上的成功示范将使便携式微型发电能够用于便携式设备，例如笔记本电脑，也可用于微型推进和微型卫星。该项目旨在开发燃料灵活高效的微型燃烧器（燃烧体积约为0.0018立方英寸）。这种尺寸的燃烧器比火焰的熄灭距离小。通过实验和计算得出的基本理解将允许有效的热量再循环回到燃烧室，以帮助减轻火焰的热淬。燃料将通过多孔热回收器喷射进行预汽化，并采取措施避免燃料结焦和多孔介质的变质，从而限制燃烧器和微推进器的使用寿命。为了实现项目目标，将开发一套新的任务，并使用实验和计算来测试，以丰富知识并提供更广泛的应用。本项目将探索开发和沉积一种新型的陶瓷纳米结构复合材料的方法，该材料具有超低导热性和接近零的热膨胀，从而在不需要的地方不发生热交换或很少发生热交换，同时在燃料-空气混合物进入燃烧体积之前在其他壁上保持高传热以预热反应物。有效的热管理与废气和新鲜反应物混合物之间的有效热交换至关重要。一个回热式热交换器将用于废气和进入的新鲜燃料-空气混合物之间的热交换。对于液体燃料，燃料的蒸发将通过多孔热回热器。重点将放在用于微尺度功率应用的微燃烧器的锆基陶瓷。所使用的方法将是制造特殊配置的锆基纳米复合材料，其中可以通过引入多尺度声子散射，即嘎嘎原子（原子尺度）和界面（纳米尺度）来改变导热性和膨胀。超低导热的纳米颗粒将沉积在燃烧室的指定壁面上，使其导热系数接近于零，同时在其他壁面上保持高的热交换，以提高效率和性能。该项目将影响用于地面和空间应用的下一代微型燃烧和推进装置的发展。

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.