Article: Liu, YC; Xu, YH; Hicks, MC; Avedisian, CT; “Comprehensive Study of Initial Diameter Effects and Other Observations on Convection-Free Droplet Combustion in the Standard Atmosphere for n-heptane, n-octane, and n-decan”, Combustion and Flame, 171:27-41
Abstract:
This paper reports the results of a comprehensive experimental study on the effect of initial droplet diameter (Do) over a very wide range (0.5 mm < D-0 < 5 mm) on the spherically symmetric droplet burning characteristics in the standard atmosphere of three alkanes – n-heptane, n-octane and n-decane – that are representative of components found in petroleum-based transportation fuels and their surrogates. Spherical symmetry in the burning process was promoted by carrying out the experiments in a reduced convection (stagnant ambience) and buoyancy (low gravity) environment using the facilities of a ground based drop tower for D-0 < 0.8 mm and a spaced-based platform (the International Space Station) for D-0 > 1.0 mm.
The results show that for Do greater than about 2 mm, K decreases with increasing Do in an early period of burning and with the data being correlated in the form K similar to D-0(-n) based on a scale analysis of an energy balance on the flame. For Do larger than approximately 2 mm the droplet flames often disappeared indicating an extinction mechanism that was speculated to be due to radiative losses from the flame.
Concurrently, measurements of wideband radiation dropped significantly and the burning rate gradually approached pure evaporation.
In some instances for n-heptane and n-octane radiative extinction was accompanied by droplet evaporation rates that were significantly higher than evaporation in a hot ambience which persisted for a significant fraction of the burning history before decreasing to evaporation in a cold ambience. An energy balance on the drop related the flame temperature to droplet diameter from which it was predicted that flame temperatures after ignition were greater than 1200 K before dropping to under approximately 800 K and remaining constant thereafter until eventually reaching near ambient conditions. This intermediate regime of burning was conjectured to be associated with a low temperature combustion process. The transition to this intermediate regime upon radiative extinction was occasionally accompanied by flame oscillations, the origin of which was uncertain but could have been initiated by motion of the droplet owing to the deployment process. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Funding Acknowledgement: National Administration of Space and Aeronautics (NASA) [NNX08AI51G]
Funding Text: This work was supported by the National Administration of Space and Aeronautics (NASA) under Grants NNX08AI51G to Cornell University (where the ground-based experiments were carried out). The authors are pleased to acknowledge Drs. Vedha Nayagam and Daniel Dietrich of NASA-Glenn who offered insights regarding data analysis and combustion physics of some of the observed trends and assistance with some of the reported experiments. Messrs Jeff Rah, Koffi Trenou, Wei-Chih Kuo and Anthony Savas of Cornell provided assistance with the experiments reported here and analyses of the data. The interest of F.A. Williams (UC-San Diego), F.L. Dryer (Princeton), T. Farouk (U. South Carolina), and B.D. Shaw (UC-Davis)) is also greatly appreciated.