A radical index for the determination of the chemical kinetic contribution to diffusion flame extinction of large hydrocarbon fuels Sang Hee Won ⇑ , Stephen Dooley, Frederick L. Dryer, Yiguang Ju ⇑ Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA article info Article history: Received 7 June 2011 Received in revised form 24 August 2011 Accepted 24 August 2011 Available online 15 September 2011 Keywords: Radical index Extinction limit Diffusion flame Transport-weighted enthalpy Surrogate fuel Chemical kinetics abstract The extinction limits of diffusion flames have been measured experimentally and computed numerically for fuels of three different molecular structures pertinent to surrogate fuel formulation: n-alkanes, alkyl benzenes, and iso-octane. The focus of this study is to isolate the thermal and mass transport effects from chemical kinetic contributions to diffusion flame extinction, allowing for a universal correlation of extinc- tion limit to molecular structure. A scaling analysis has been performed and reveals that the thermal and mass transport effects on the extinction limit can be normalized by consideration of the enthalpy flux to the flame via the diffusion process. The transport-weighted enthalpy is defined as the product of the enthalpy of combustion per unit mole of fuel and the inverse of the square root of fuel molecular weight. The chemical kinetic contribution provided by the specific fuel chemistry has thus been elucidated for tested individual component and multi-component surrogate fuels. A chemical kinetic flux analysis for n-decane flames shows that the production/consumption rates of the hydroxyl (OH) radical govern the heat release rate in these flames and therefore play significant roles in defining the extinction limit. The rate of OH formation has been defined by considering the OH concentration, flame thickness, and flow strain rate. A fuel-specific radical index has been introduced as a concept to represent and quantify the kinetic contribution to the extinction limit owing to the fuel-specific chemistry. A relative radical index scale, centered on the radical index of a series of n-alkanes which are observed and fundamentally explained to be common, is established. A universal correlation of the observed extinction limits of all tested fuels has been obtained through a combined metric of radical index and transport-weighted enthalpy. Finally, evidence as to the validity of the fundamental arguments presented is provided by the success of the universal correlation in predicting the extinction limits of the multi-component mix- tures typical of surrogate fuels. Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Extinction limits of diffusion flames in the counterflow configu- ration have been rigorously studied for various fuel classes from hydrocarbon to oxygenated fuels [1–18]. Since the extinction limit of diffusion flames is not only governed by chemical kinetics but also by thermal and mass transport in a coupled manner, it has been utilized as a fundamental target by which to validate kinetic and transport models for a particular fuel [7–18]. This rich coupling ef- fect of chemical kinetics and mass transport has also resulted in the utilization of diffusion flames to evaluate the performance of surro- gate fuel mixtures in emulating the behavior of real fuels [11–18]. As the diffusion flame extinction limit is affected simultaneously by many parameters such as fuel mass fraction, flame temperature, mass transport, and fuel chemistry, little progress has been made in understanding the role of fuel chemistry in dictating diffusion flame phenomena, despite extensive experimental study. This lack of fun- damental knowledge of the relation between diffusion flame extinction limit and fuel chemistry limits an understanding of the impact of molecular structure on flame extinction. Thus the funda- mental design of surrogate fuel models of targeted real fuels has been impeded. From asymptotic analysis with one-step global chemistry, the extinction limit or the extinction Damköhler number of a diffusion flame has been elucidated to be governed by the ratio of chemical heat release from the reaction zone to heat losses to the fuel and oxidizer sides [1–6]. Extinction occurs when this ratio is below a critical value as is the case when the flame temperature becomes so low that the rate of chemical heat release is not sufficient to overcome the rate of heat loss. Consequently, chemical reaction may not be sustained [1–6]. However in a detailed chemistry anal- ysis, the chemical heat release rate is not only governed by the flame temperature but also by fuel chemistry. Recent studies of the kinetic coupling between alkanes and aromatics demonstrated 0010-2180/$ - see front matter Ó 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2011.08.020 ⇑ Corresponding authors. E-mail addresses: sangwon@princeton.edu (S.H. Won), yju@princeton.edu (Y. Ju). Combustion and Flame 159 (2012) 541–551 Contents lists available at SciVerse ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame