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A numerical study of auto-ignition in turbulent lifted flames issuing ...

A numerical study of auto-ignition in turbulent lifted flames issuing ...

A numerical study of auto-ignition in turbulent lifted flames issuing

Combustion Theory and ModellingVol. 11, No. 3, June 2007, 351–376A numerical study of auto-ignition in turbulent lifted flamesissuing into a vitiated co-flowR. L. GORDON ∗ †, A. R. MASRI†, S. B. POPE‡ and G. M. GOLDIN§†School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney,NSW 2006, Australia‡Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, USA§Fluent Inc, Lebanon, New Hampshire, USA(Received 8 August 2005)This paper presents a numerical study of auto-ignition in simple jets of a hydrogen–nitrogen mixtureissuing into a vitiated co-flowing stream. The stabilization region of these flames is complex and,depending on the flow conditions, may undergo a transition from auto-ignition to premixed flamepropagation. The objective of this paper is to develop numerical indicators for identifying such behavior,first in well-known simple test cases and then in the lifted turbulent flames. The calculationsemploy a composition probability density function (PDF) approach coupled to the commercial CFDcode, FLUENT. The in-situ-adaptive tabulation (ISAT) method is used to implement detailed chemicalkinetics. A simple k–ε turbulence model is used for turbulence along with a low Reynolds numbermodel close to the solid walls of the fuel pipe.The first indicator is based on an analysis of the species transport with respect to the budget ofconvection, diffusion and chemical reaction terms. This is a powerful tool for investigating aspects ofturbulent combustion that would otherwise be prohibitive or impossible to examine experimentally.Reaction balanced by convection with minimal axial diffusion is taken as an indicator of auto-ignitionwhile a diffusive–reactive balance, preceded by a convective–diffusive balanced pre-heat zone, is representativeof a premixed flame. The second indicator is the relative location of the onset of creation ofcertain radical species such as HO 2 ahead of the flame zone. The buildup of HO 2 prior to the creationof H, O and OH is taken as another indicator of autoignition.The paper first confirms the relevance of these indicators with respect to two simple test casesrepresenting clear auto-ignition and premixed flame propagation. Three turbulent lifted flames arethen investigated and the presence of auto-ignition is identified. These numerical tools are essentialin providing valuable insights into the stabilization behaviour of these flames, and the demarcationbetween processes of auto-ignition and premixed flame propagation.Keywords: Turbulent; Non-premixed flames; Lifted flames; Auto-ignition1. IntroductionA long-term objective of combustion research is to develop computational models thatmay be used as engineering tools to optimize combustor designs. Such a goal is graduallyapproached through the development of improved physical sub-models, more efficient numericalalgorithms and enhanced representations of detailed chemical kinetics. This processshould involve regular comparisons with established databases that help validate relevant∗ Corresponding author. E-mail: robert.gordon@aeromech.usyd.edu.auCombustion Theory and ModellingISSN: 1364-7830 (print), 1741-3559 (online) c○ 2007 Taylor & Francishttp://www.tandf.co.uk/journalsDOI: 10.1080/13647830600903472

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