Experimental measurements of laminar flame speeds for highly N2-diluted ethanol flames under microgravity conditions

Nitrogen has been used as an alternative for CO2 in the fire suppression industry. The high nitrogen concentrations added to a reactive mixture slows down its flame propagation in case of a fire outbreak. To describe quantitatively the flame spread as a function of nitrogen dilution, it is mandatory to evaluate the mixture’s laminar flame speed. Performing experiments under earth gravity using the RWTH high-pressure, high-temperature spherical combustion chamber showed that the laminar flame speed extraction is impossible for such highly dilute conditions due to severe flame-front distortions. To overcome the influence of buoyancy, experiments were carried out under microgravity conditions in a parabolic flight campaign onboard the Airbus A310 Zero-G. The setup consists of a pressure-release-type dual chamber adequate for aircraft-related safety requirements, and a conventional shadowgraphy system to visualize the flame morphology under quasi-isobaric pressure. Only a limited number of liquid energy carriers are allowed onboard, and ethanol was chosen because of its short molecular structure and low boiling point compared to longer chain liquid alkanes such as n-heptane. Results showed a flame speed reduction induced by radiation in the order of 10% to 45% for near-stoichiometric to very rich conditions, respectively. To correct laminar flame speeds for radiation effects, an empirical correlation presented by Yu et al. (Combust. Flame 161 (2014) 2815–2824) is applied. Excellent agreement between radiation-corrected experimental data and simulations using the chemical mechanism of Cai et al. (Combust. Flame 37 (2019) 639–647) is found. This indicates that high dilution levels with nitrogen can be captured well by kinetic schemes derived for fast-burning flames. The unique experimental data obtained here, which were extremely difficult to measure and post-processed very carefully, can be used as a reference to validate/improve existing and newly developed kinetic mechanisms.