On the Low Temperature and Low Pressure Regime Diagram of n-Heptane Droplet Burning in Microgravity

The present study investigates the n-heptane droplet burning with ambient conditions 600-1000 K and 0.1 2 bar through numerical simulations. Experimental data (initial droplet size 0.7 mm, pressure down to 1 bar) from microgravity experiments offer validation of our 1-D modeling for characteristic time scales identified throughout the droplet lifetime. The motivation of this study is the seemingly narrowing cool flame regime towards the low-pressure conditions presented by prior experimental and numerical works, as contrast to the diverging negative temperature coefficient region for lower pressures. Results for sub-atmospheric pressure show that the cool flame area on the regime diagram converges with decreasing pressure due to major contribution from the ceiling temperature effect. Moreover, the physics trigger at various chemical time scales are well bounded by the droplet lifetime such that very long ignition delay becomes irrelevant to droplet problems.