A new approach to model turbulent lifted CH 4 /air flame issuing in a vitiated coflow using conditional moment closure coupled with an extinction model Rudra N. Roy a , Sudarshan Kumar b , Sheshadri Sreedhara a,⇑ a Department of Mechanical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India b Department of Aerospace Engineering, Indian Institute of Technology Bombay, Powai, Mumbai, India article info Article history: Received 13 April 2013 Received in revised form 12 August 2013 Accepted 12 August 2013 Available online 5 September 2013 Keywords: Conditional moment closure Flame lift-off height Methane flame Extinction model abstract In this article, conditional moment closure model (CMC) with detailed chemistry is used to model lifted turbulent methane flame in a high temperature and vitiated coflow and to predict flame lift-off height. The flow and mixing field are predicted by a 2D in-house code employing a k–e turbulence model (RANS) with modified constant C e2 . The first-order CMC model on its own could not capture the behavior of the lifted flame. Large eddy simulations (LES) coupled with second-order CMC model would be a promising alternative but the objective here was to improve low-cost simulations based on RANS and first-order CMC to address realistic problems. Hence, an extinction model has been incorporated in the first-order CMC to improve its predictions and is referred in this paper as CMCE. In the CMCE model, flame is assumed to be extinguished when the ratio of flow time scale to the chemical time scale falls below a critical value. Predicted lift-off height by the CMCE model agrees very well with the experimental results. There is a significant improvement in temperature and species distributions in both axial and radial directions with the implementation of the CMCE model. Further, the model is extended to predict the flame lift-off height for various coflow temperatures and jet velocities by using scaling ratios. With these modifications, the lift-off heights predicted by the CMCE model match well with the experimental results for a wide range of jet velocities and coflow temperatures. Results from both CMC and CMCE models are compared against the experimental data to show the importance of the extinction model. Flame stabil- ization process indicates that flame stabilizes on the contour of mean stoichiometric mixture fraction where axial mean velocity equals the turbulent burning velocity. Ó 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Turbulent lifted jet diffusion flame can generally be observed when the velocity of the fuel jet discharging into surrounding cold or hot air exceeds beyond a critical value. This type of flame has been found in wide range of applications such as in gas turbine combustors and commercial burners. In these situations, some of the existing combustion models generally fail to predict the flame lift-off height accurately, and understanding the stabilization mechanisms which govern the flame lift-off becomes difficult. Hence, a new modeling approach is necessary or at least a suitable modification for these models is required for the numerical inves- tigation of the lifted flame. Proper prediction of flame structure, such as radial and axial distributions of temperature and species mass fractions is also very important. Various modeling ap- proaches have been proposed previously to capture the behavior of these types of flames. Lyons [1] had discussed recent progress in understanding tur- bulent lifted hydrocarbon jet flames and the conditions under which they stabilize. A premixed model was proposed by Van- quickenborne and Tiggelen [2] consisting of balance between lo- cal turbulent burning velocity and local time averaged axial velocity which leads to stabilization of a lifted flame. A lifted flame stabilizes at a region where stoichiometric mixture is formed. Ignoring the partial premixing of air and fuel upstream of the flame base, Peters and Williams [3] argued that quenching of laminar diffusion flamelets results in stabilization of lifted flame. The reaction zones shifts to the downstream locations where the value of scalar dissipation rate is not high enough to extinguish the flame. Later, Peters [4] acknowledged the fact that stabilization at the lift-off height occurs due to premixed flame propagation theory and not by diffusion flamelet quench- ing. To support this Watson et al. [5] carried out measurements of scalar dissipation rate for lifted flame using laser Rayleigh 0010-2180/$ - see front matter Ó 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.combustflame.2013.08.007 ⇑ Corresponding author. Fax: +91 22 2572 6875. E-mail address: sreedhara.s@iitb.ac.in (S. Sreedhara). Combustion and Flame 161 (2014) 197–209 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame