The effects of elevated pressure on the kinetics and extinction of hot, warm, and cool gaseous ethylene spherical diffusion flames

Previous studies have found that the elevation of ambient pressure helps facilitate cool and warm flame formation. In this work, the behavior and chemical kinetics of burner-supported C2H4 microgravity spherical diffusion flames at elevated pressure are examined. Experiments were carried out aboard the International Space Station at pressures of 0.5-3 bar, and thin-filament pyrometry measurements were used to measure peak flame temperature. Flame temperatures at extinction are shown to decrease with increasing pressure, and at a pressure of 3 bar the flame is shown to extend below the critical temperature of 1130 K and into the warm flame regime. Simulations were performed using a transient numerical model with detailed chemistry, transport, and radiation. This model incorporates the San Diego mechanism with 57 species and 270 reactions, including low-temperature chemistry. Species concentrations, temperatures, reaction rates, and heat release rates are examined. When the pressure is increased to 50 bar the flame smoothly transitions into the warm flame regime, and cool flame behavior is observed in the simulations after warm flame radiative extinction. The results for hot, warm, and cool flame combustion at elevated pressure are compared to the results for hot flame burning at atmospheric pressure. The flame is shown to move from stabilizing on the oxidizer side to the fuel side as temperature decreases. The HO2 radical becomes the dominant radical early in the flame lifetime and is largely responsible for the emergence of low-temperature chemistry.