Article: Xu, YH; Hicks, MC; Avedisian, CT; “The Combustion of iso-octane Droplets with Initial Diameters from 0.5 to 5 mm: Effects on burning rate and flame extinction”, Proceeding of the Combustion Institute, 36 (2):2541-2548
Abstract: The burning characteristics of iso-octane droplets in the standard atmosphere are reported over a large range of initial droplet diameters (0.5 mm < D-o < 5 mm) for near one-dimensional droplet flames as promoted by low gravity. A ground-based drop tower and a space-based platform (International Space Station) were used to provide an environment to examine the influence of D-o that encompasses regimes where radiation does not have an effect on burning to where it does. For D-o < 2.7 mm, the droplets burned to completion without extinction.
Larger droplets evidenced extinction due to radiative emissions from the flames to the ambience that produced a two stage burning process. The burning rate (K) was nearly constant in the first stage and then gradually decreased. Concurrently, radiation emissions dropped by two orders of magnitude and ‘flickering’ flames were noted during the transition. After extinction, burning entered a regime where K gradually decreased with time and approached values commensurate with evaporation in a heated environment (i. e., no combustion). Energy balances based on scale analyses related K and flame temperature (T-f) to D-o : a balance at the flame including radiation losses led to the scaling K similar to D-o(-n) in reasonable agreement with the data for D-o > 2 mm; an energy balance at the droplet surface showed that T-f quickly dropped from similar to 1600 K to similar to 700 K after flame extinction, followed by a gradual reduction of T-f to near ambient temperature. The trends in the data are also discussed for potentially revealing a possible low temperature combustion regime. (C) 2016 by The Combustion Institute.
Published by Elsevier Inc.
Funding Acknowledgement: National Aeronautics and Space Administration [NNX08AI51G]
Funding Text: This research was funded by the National Aeronautics and Space Administration under Grants NNX08AI51G. The authors thank D.L. Dietrich (NASA) and V. Nayagam (Case Western Reserve University) for some assistance with the ISS experiments. The help of Hee Dae Tak, Meilin Dong, Do Hyun Chung, Nan Wei, and Yiren Shen with some of the experiments and video image analysis is appreciated. We also appreciate the interest of F.A. Williams (UCSD), F.L. Dryer (Princeton), and T. Farouk (U South Carolina) in this study.