Computational and experimental investigation of flow and combustion physics in porous media

As emission regulations become increasingly more stringent and policies evolve to combat global climate change impacts, reducing pollutant and greenhouse gas emissions emerge as one of the most important goals of combustion research. Techniques such as staged combustion, lean premixed combustion, catalytic combustion, and advanced mixing and fuel atomization are some of the methods examined to reduce emissions of pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons (UHCs). Porous media combustion represents an advanced combustion concept that is capable of achieving low emissions, enhanced flame stabilization, and improved fuel efficiency. Conventionally, Porous Media Burners (PMBs) utilize a two-zone ``step" burner design, which operates on the principal that the upstream high pore-density region serves as a flame arrestor and flame stability is observed at the interface between the two regions of high and low pore density. This dissertation contributes to the analysis of combustion in porous media, characterization of its performance in conventional PMBs, and the development and testing of a novel porous matrix design for enhanced combustion performance. First, a characterization of the combustion stability, pressure drop and pollutant emissions of conventional two-zone ``step" PMB is presented for a range of operating conditions and burner designs. Long-term material durability tests at steady-state and cycled on-off conditions were performed under operation with methane-fuel at atmospheric pressure. Thermocouple temperature measurements and pressure drop data are presented and compared to results obtained from 1D volume-averaged simulations. Additionally, the burner design with the maximum combustion stability regime was identified and tested in subsequent high-pressure experiments at 2, 8, and 20 bar with fully vaporized and preheated n-heptane and methane fuels, at fuel-lean equivalence ratios. Second, in an effort to expand the combustion stability regime beyond the capability of two-zone ``step" PMBs, a novel burner design having a spatially graded porous matrix is proposed, resulting from the theoretical analysis of the governing equations and constitutive relations. This analysis reveals the significance of the pore topology on interphase heat exchange and radiative heat transfer properties, quantified by the local Stanton number and optical depth, respectively. Gradation in topology (i.e. porosity, pore diameter, cell diameter, etc.) enables the flame to stabilize dynamically within the porous matrix and for a wider range of operating conditions. Computational stability maps, temperature profiles, and emissions data are presented for comparable two-zone ``step'' and ``graded" burner concepts, which predict significant performance enhancements in the latter. The theoretical and computational investigation of matrix gradation in PMBs as well as experiments reveal the potential for tailoring the internal heat transfer properties to optimize performance, and thus motivates the subsequent work in leveraging recent advancements in additive manufacturing to enable smoothly graded porous structures. The next part of this dissertation achieves the use of Lithography-based Ceramic Manufacturing for the fabrication of functionally graded matrix structures, designed using periodic surface equations. The manufactured samples were operated in a PMB over a range of operating conditions to test the feasibility and performance of additive manufactured materials in PMBs. Thermal and durability testing of the manufactured parts are characterized, along with combustion stability maps from the ``step" and ``graded" PMB experiment, which, consistent with the previous theoretical, computational, and experimental results, show significant performance improvements of the ``graded" burner.

Keywords: Mechanical Engineering.
Advisor: Ihme Matthias
Bowman Craig T. (Craig Thomas)
Kovscek Anthony R. (Anthony Robert) 
Mansour N. N. (Nagi N.)