【作者】 林博颖；

【导师】 陈义良；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2008， 博士

【摘要】 针对日益突出的能源和环境问题,为发展清洁低能耗的燃烧技术,本文用数值模拟的方法研究了燃料液雾在惰性多孔介质内的流动燃烧过程,分析其燃烧特性。在多孔介质内组织燃烧可以提高燃烧系统的效率,同时降低污染。多孔介质燃烧器有其独有的特点:结构紧凑,超绝热火焰温度,拓宽的贫燃极限,低污染,燃烧过程更易控制等。对文献资料中出现的有关多孔介质的几何结构和其中发生的输运过程的研究成果做了系统整理,以便对多孔介质的性质有个准确把握,为建立准确的数学模型奠定基础。介绍了孔隙率、比面、弯曲率、相关函数、孔径分布、逾渗概率等几何结构特征参数和毛细管模型、隙缝模型、颗粒模型、孔隙网络模型、逾渗模型等几何结构模型;介绍了比流量、渗透率、流体动力弥散等流体力学输运性质,界面张力、湿润性、毛细压力等界面属性和传热传质输运特性。阐明了模拟多相多尺度问题的连续介质方法的基本思想和表征体元、孔隙率等基本概念,介绍了连续介质方法中的唯象方法和上升尺度方法。详细介绍了经典混合物理论和经典不可逆热力学。对唯象方法中的不溶混混合物理论,上升尺度方法中的平均理论作了简要介绍。应用上升尺度方法中的杂化混合物理论和经典不可逆热力学原理,推导得出了存在化学反应多相混合物体系的较为严格的杂化混合物理论模型。宏观守恒方程是将微观守恒方程在表征体元上体积平均得到,将微观热力学第二定律的熵不等式和微观Gibbs方程等状态函数之间的热力学关系也在表征体元上体积平均,得到宏观熵不等式和宏观热力学关系。用Lagrange乘子将宏观熵不等式、守恒方程、热力学关系组合成增广熵不等式,选取Lagrange乘子削去增广熵不等式中的物质导数项,确认出熵流部分和熵产部分。依据经典不可逆热力学的一般原则,将热力学流表示成热力学力的函数式,即找出本构关系,完成方程组的封闭。数值工作首先从较为简单的零维模拟开始,采用详细化学反应机理,应用零维情形的杂化混合物理论模型,模拟了微小空腔内气体的予页混燃烧,通过有无填充多孔介质的对比,分析了多孔介质在微小尺度燃烧领域的应用前景。研究了填充多孔介质对微小型化学推进器性能的影响。微小尺度下系统内部很容易达到热平衡,体现不出多孔介质的回热作用;但多孔介质的大热容量却能保证系统具有良好的热稳定性。通过用详细化学反应机理研究定容条件下正庚烷液雾在多孔介质中的瞬态着火过程,模拟了液体燃料在内燃机中的点火过程。多孔介质内燃机的点火延迟期会比传统柴油机显著缩短,而且受工况影响很小,有利于内燃机的点火控制。将杂化混合物理论模型具体应用于正庚烷液雾在惰性多孔介质中的流动、蒸发、燃烧过程。作一维假定,考虑了液雾与气体之间的对流换热过程、液雾与多孔介质固体骨架之间的碰撞接触传热过程、多孔介质对液雾的辐射加热过程。采用了自适应网格的Eluer-Lagrange数值解法,辐射传递方程采用离散坐标法求解。对程序代码进行了较系统地检验。考察了迭代过程中残差的下降程度,确定了迭代收敛判据。通过在经由系统加密的网格上进行计算,分析了单调收敛、收敛阶数、网格无关性等离散误差特征,应用Richardson外插对离散误差作了定性地估计。计算结果验证了超绝热火焰温度现象;辐射加热、与固体壁面的碰撞传热会显著影响到较大液滴的蒸发过程,使其比经典的直径平方律更快;ZrO2多孔介质中的辐射场基本是各向同性的;多孔介质中的辐射传热对液雾的燃烧过程有重要影响。更多还原

【Abstract】 As energy and environmental issues become more and more emergent, it is important to develop clean combustion technology with high efficiency. Numerical simulations have been performed to investigate spray combustion in porous inert media. Higher efficiency can be achieved by inserting porous inert media into combustion systems, as well as lower emissions. Porous media burners have their own particular characteristics, such as compact structure, super-adiabatic flame temperature, extended lean flaming limit, lower emissions, more flexible combustion control, etc.As a basis for model formulation and to grasp the essence of porous media character, the fundamental concepts concerning geometric porous structure and transport phenomena in porous media are reviewed comprehensively. Geometric characteristics such as porosity, specific area, tortuosity, correlation funtion, pore diameter distribution, percolation probability, etc., fluid transport characteristics such as specific flow rate, permeability, fluid dynamic dispersion, etc., interface characteristics such as surface tension, wettability, capillary pressure, etc., as well as heat and mass transfer characteristics of porous systems are comprehensively reviewed.Fundamental ideas of continuum theories and the basic concepts such as representative volume element and porosity are clarified. The phenomenological methods and the upscaling methods for modeling transport processes in porous media are then reviewed and reformulated, with emphasis on the mixture theories and averaging techniques. The classical irreversible thermodynamics is also reviewed as it is a fundamental component of mixture theories.The hybrid mixture theory, a kind of upscaling technique, equipped with the classical irreversible thermodynamics, is employed to deduct a precise model of multi-phase multi-component mixtures. The macroscopic model equations are derived from the corresponding microscopic ones through volume averaging on the representative volume element. The macroscopic entropy equation and the thermodynamic relations are derived similarly. The entropy equation is then augmented by adding the conservative equations and thermodynamic relations which are multiplid by Lagrange multipliers. By choosing the appropriate Lagrange multipliers, the material derivatives are eliminated and left the entropy fluxes and entropy productions. According to the principles of classical irreversibly thermodynamics, the therdynamic forces and fluxes are then identified. Then the constitutive equations are formulated, and the model is closed.Simulations start with relatively simple zero-dimensional models. Premixed combustions in micro/mini-chambers filled with porous media are simulated, utilizing detailed chemical mechanisms. By comparison with cases that have no porous media filled with, the prospects of applying porous media into micro-combustion and its effects on micro/mini- chemical thrusters have been analyzed. It is relatively easy to attain thermal equilibria in micro/mini-system, so the energy feedback of porous media is not significant. But its large heat capacity garanteed the thermal stability of combustion systems.The constant volume ignition of n-heptane droplets in porou media has been studied using detailed chemical kinetics, which simulated the spray ignition process in intermal combustion engines. Analyses are performed for ignition delay and emission controls. The ignition delay inside porous media is significantly shortened, which is a desiring feature for ignition control inside internal combustion engines. The emission levels are relatively low.A simplified one-dimensional example of the pre-formulated hybrid mixture theory model is employed to simulate the evaporation and combustion processes of sprays while flowing through porous inert media. Heat transfer processes among spray, gas and porous skeleton have been accounted for, as well as the radiative heating of droplets. The Euler-Lagrange model equipped with self-adaptive mesh method is adopted for the numerical realization. The radiative heat tranfer equation is solved using discrete ordinate method. Systematic code validation has been performed. The iterative convergence criteria is established through analyzing the residual falling behavior. Computations are performed on a series of systematically refined grids, and the grid independence, monotone convergence and order of convergence of the solution are analyzed. Richardson extrapolation method is employed to estimate the discretization error in the finest grid. Results show that the super-adiabatic temperature is significant, than the radiative heating and heat transfer during impact with solid walls significantly accelerate the evaporation rate of large droplets and that the radiation field inside ZrO2 is somewhat isotropic. The radiative heat transfer affects the spray combustion significantly.更多还原