Extinguishment of hydrogen laminar diffusion flames by water vapor in a cup burner apparatus

• The interaction of hydrogen flame and water steam was simulated with full chemistry.
• A simplified PSR model was proposed to predict extinguishing limit of water vapor.
• The reaction H + O2 + M = HO2 + M is crucial for the flame extinction.
• The extinguishing limits of water vapor are insensitive to the temperature and velocity changes.
• Thermal cooling and chemical kinetic effect are the dominant mechanisms for water vapor suppressing hydrogen cup-burner flame.

Transient computations with full hydrogen chemistry were performed to reveal the flame structure and extinguishment process of co-flow, hydrogen diffusion flame suppressed by water vapor. As the concentration of water vapor was increased, the flame detached away from the burner brim and formed an edge flame at the flame base. Water vapor showed larger chemical inhibition effect than nitrogen when extinguishing hydrogen flame, which was attributed to its enhanced third body effect in the reaction H + O2 + M = HO2 + M. The minimum extinguishing concentration (MEC) of water vapor and nitrogen was predicted by Senecal formula and perfectly stirred reactor (PSR) model respectively. The MECs predicted by PSR model agree with the MECs calculated by Fluent, which shows that 1) the flame extinction is controlled by the flame base, and 2) radiation absorption is negligible. The measured MECs are in a reasonable agreement with the values calculated by Fluent, which demonstrates the accuracy of the CFD model. A simple model was used to investigate the relative importance of extinguishing mechanisms of water vapor. The results show that in a co-flow configuration the thermal cooling and chemical inhibition effect are the main extinguishing mechanisms in suppressing hydrogen diffusion cup burner flame.