Combustion rates of graphite rods in the forward stagnation field of the high-temperature, humid airflow

Relevant to High-Temperature Air Combustion, an experiment has been conducted to study carbon combustion in the high-temperature oxidizer-flow, by use of a graphite rod set in the forward stagnation field. Effects of humid air and O₂-reduced oxidizer have been examined, after re-confirming that the combustion rate in the high-temperature oxidizer-flow is enhanced, because of elevated transport properties when the mass flow rate of oxidizer is the same, and that it is suppressed, because of reduced mass transfer rates through the thickened boundary layer when the velocity gradient is the same. It is found that high H₂O mass-fraction is favorable for the enhancement of combustion rate at high surface temperatures (>2000 K), because of its participation in the surface C–H₂O reaction, while it is not the case at medium surface temperatures (1400–1700 K), because of the suppressing effect, caused by the establishment of CO flame. As for O₂ and CO₂ concentrations in the high-temperature oxidizer-flow, it is found that O₂ mass-fraction can be reduced without reducing combustion rate in the room-temperature airflow, and that it can further be reduced in existence of enough CO₂ that can be an oxidizer in carbon combustion. Theoretical works have also been conducted for the system with three surface reactions and two global gas-phase reactions. It is found that the Frozen mode without CO flame and the Flame-detached mode with infinitely fast gas-phase reaction can fairly represents combustion behavior before and after the establishment of CO flame, respectively, as far as the trend and the approximate magnitude are concerned. It is also shown that a new mode with suppressing H₂ ejection from the surface can fairly represent the combustion rate, experimentally obtained in humid airflow with relatively low velocity gradient when the surface temperature is high.