Spray Combustion In Porous Inert Medium

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.