Carbon combustion in the forward stagnation flowfield has been examined through experimental comparisons, by conducting aerothermochemical analyses, with the surface C-O2 and C-CO2 reactions and the gas-phase CO-O2 reaction taken into account. By virtue of the generalized species-enthalpy coupling functions, close coupling of those reactions has been elucidated. Explicit combustion-rate expressions by use of the transfer number in terms of the natural logarithmic term, just like that for droplet combustion, have further been obtained for the combustion response in the limiting situations. It has been confirmed that before the establishment of CO flame, the combustion rate can fairly be represented by the expression in the frozen mode, that after its establishment by the expression in the flame-attached or flame-detached modes, and that the critical condition derived by the asymptotics can fairly predict the surface temperature for its establishment. The formulation has further been extended to include the surface C-H2O and gas-phase H2-O2 reactions additionally, so as to evaluate the combustion rate in humid airflow. Since those expressions are explicit and have fair accuracy, they are anticipated to make various contributions not only for qualitative/quantitative studies, but also for various aerospace applications, including propulsion with high-energy-density fuels. 1. Introduction Carbon combustion has been a research subject, indispensable for practical utilization of coal/char combustion, aerospace applications with carbon-carbon composites (C/C-composites), ablative carbon heat shields, and/or propulsion with using high-energy-density fuels. Because of this practical importance, extensive research has been conducted not only experimentally but also theoretically/numerically, and several comprehensive reviews [1–12] summarize accomplishments in this field. Nevertheless, because of complexities involved, there still remain several problems indispensable for understanding the basic nature of the combustion. Some of them also command fundamental interest, because of simultaneous existence of surface and gas-phase reactions, interacting with each other. Generally speaking, the carbon combustion consists of the following processes:(1)diffusion of oxidizing species to the solid surface,(2)adsorption of molecules onto active sites on the surface,(3)formation of products from adsorbed molecules on the surface,(4)desorption of solid oxides into the gas phase, (5)migration of gaseous products through the boundary layer into the freestream. The slowest of
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