Multidimensional Reacting Flow Simulation with Coupled Homogeneous and Heterogeneous Models

The coupling of homogeneous (gas phase) and heterogeneous (solid phase) reactions is found in various phenomena, from the burning of wood in a fireplace to the ablating heat shield of the space shuttle flying at hypersonic speeds. With the goal of understanding the complex coupling mechanisms in such flows, this dissertation focuses developing a numerical model of low-Mach number reacting flows with coupled homogeneous and heterogeneous reactions. Numerical studies were performed in collaboration with the Air Force Institute of Technology, with the objective of guiding experimental design to investigate high-energy laser interactions with surfaces. The surfaces considered so far are porous carbon samples due to the well-established literature on heterogeneous reaction rates.

In this study, the reacting carbon surfaces considered were two flow configurations: parallel flow over a reacting boundary layer (flat plate) and impinging stagnation point flow (stagnation flow). Both configurations were modeled with derived-type boundary conditions for the conservation of reacting species at a surface. Rigorous interface boundary conditions for pressure and enthalpy were also developed. All multidimensional modeling was performed using a steady-state, laminar, finite volume method solver that was constructed using the OpenFOAM package with additional sub-models for finite rate chemistry, detailed transport quantities, and heterogeneous surface reactions. Surface temperatures considered ranged from 1600 to 2600 K and two semi-global heterogeneous reaction models were used, one for graphite surfaces and one for carbon surfaces with 25 percent porosity.

Both the flat plate and stagnation flow investigations focused on understanding the effects that momentum and surface temperatures have on the heterogeneous-homogeneous reaction coupling. These two configurations resulted in multidimensional reacting layers that are dependent on surface reactivity, free stream velocities, and geometric configuration. Reactions that contribute a negligible amount to CO production were identified by defining a sensitivity metric. Reactions with OH and O, despite their small concentrations, contributed on the order of fifty percent in carbon monoxide production for the nonporous carbon surface reactions. This indicates that trace species are important for predicting carbon surface reactions, despite their small concentrations. Additionally, for the nonporous model, reactions with CO2 were found to contribute less than one percent to the total production of carbon monoxide. However, it was found that neglecting this reaction caused significant differences in the multidimensional reacting layer structures. These findings particular affect past researchers who neglect surface reactions based on concentration as this study concludes that all surface reactions are important in predicting carbon surface reaction layers. Additionally, for the stagnation flow configuration, several metrics were developed that gave a conclusive optimal separation distance for using the quasi-one dimensional formulation as a predictor for parabolic, tube-jets flows impinging on reacting surfaces. Beyond these contributions, this works serves as a foundation for further studies into the multidimensional modeling of coupled homogeneous-heterogeneous reactions