A computational study of spherical diffusion flames in microgravity with gas radiation Part I: Model development and validation Songtao Tang, Melissa K. Chernovsky 1 , Hong G. Im * , Arvind Atreya Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109-2125, United States article info Article history: Received 26 March 2009 Received in revised form 9 September 2009 Accepted 9 September 2009 Available online 8 October 2009 Keywords: Microgravity Diffusion flame Extinction Radiation abstract The overarching goal of this study is to improve our understanding of the extinction characteristics of spherical diffusion flames in microgravity. In particular, one of the key objectives is to assess the effects of gas radiation as a means to promote flame extinction. To investigate these phenomena, a one-dimen- sional computational model was developed to simulate the evolution of a spherical diffusion flame with consideration of detailed chemistry and transport properties. The model formulation was described along with the detailed numerical method. Radiation model was discussed with two aspects: radiation property model and radiative transfer model. Various levels of radiation models were implemented and the results were compared with experimental measurements of flame radius and temperature profiles. It was shown that the statistical narrow band model (SNB) combined with the discrete ordinate method (DOM) repro- duced the experimental results with highest accuracy, and this combination of the radiation models were adopted in the subsequent parametric studies in Part II. Computational issues to optimize numerical accuracy and efficiency are also discussed. Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Microgravity combustion research has contributed significantly to a better understanding of fundamental combustion characteris- tics. In microgravity, the absence of buoyancy-induced flows sig- nificantly increases the residence time of gaseous products that are accumulated in the reaction zone, which provides an idealized environment to unravel many key sub-processes that can explain the highly complex combustion phenomena such as extinction/ ignition and pollutant formation. A systematic understanding of flame characteristics in microgravity conditions can subsequently be extended to understand and predict combustion behavior in a more complex system. Besides fundamental scientific significance, the investigation of microgravity flame behavior and extinction mechanisms is also important for fire safety applications. Various substances serve as extinguishing agents for a number of different reasons. Thermal suppression of flames may result from the increased specific heat (thereby lowering the flame temperature), or from the increased conductive or radiative heat loss. To this end, it is important to understand which mode of thermal suppression is most effective in order to determine the appropriate choice of diluent gases. While the jet flame has been extensively used in the study of stretch-induced flame extinction, the spherical diffusion flame has served as an alternative model problem in which flame extinc- tion occurs at low-stretch conditions, thereby allowing a system- atic investigation on the role of various heat losses on flame extinction. Various studies have reported that flame extinction oc- curs at low-stretch conditions due to the enhanced radiative heat loss [1–6]. Atreya and coworkers [7–12] have further investigated the effect of radiative heat loss on spherical microgravity diffusion flames by comprehensive experimental measurements of flame ra- dius, temperature, radiation intensity, and soot formation. In par- ticular, the effects of different diluents added to fuel and/or oxidizer side on the flame growth and extinction were studied. For all the dilution cases considered, however, flame extinction was not observed during the 2.2 s of the drop tower experiments. Furthermore, due partly to the difficulties in the experimental implementation of a wide range of dilution level, some observed results were not consistent in terms of the qualitative effects of dilution in the fuel and oxidizer side. For example, Fig. 3.1 in Cher- novsky [12] shows that 40% fuel side dilution has a remarkably lar- ger influence on flame radius evolution compared with 60% dilution, which is inconsistent regarding the qualitative effects of dilution. Several other researchers have also investigated spherical diffu- sion flame characteristics in the NASA 2.2-s drop tower facility. Tse et al. [4] performed experiments on burner-generated spherical diffusion flames with H 2 /CH 4 /inert mixtures issued into atmosphere 0010-2180/$ - see front matter Ó 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2009.09.010 * Corresponding author. Fax: +1 734 615 5152. E-mail address: hgim@umich.edu (H.G. Im). 1 Present address: Exponent, 17000 Science Drive, Suite 200, Bowie, MD 20715, United States. Combustion and Flame 157 (2010) 118–126 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame