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Observations on the Role of Autoignition in Flame Stabilization in Turbulent Non-premixed Jet Flames in Vitiated Coflow

[+] Author and Article Information
Aravind Ramachandran

Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695
aramach4@ncsu.edu

Venkat Narayanaswamy

Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695
vnaraya3@ncsu.edu

Kevin Lyons

Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695
lyons@ncsu.edu

1Corresponding author.

ASME doi:10.1115/1.4042807 History: Received March 08, 2018; Revised February 05, 2019

Abstract

Turbulent combustion of non-premixed jets issuing into a vitiated coflow is studied at coflow temperatures that do not significantly exceed the fuel autoignition temperatures, with the objective of observing the global features of lifted flames in this operating temperature regime and the role played by autoignition in flame stabilization. Three distinct modes of flame base motions are identified, which include a fluctuating lifted flame base (Mode A), avalanche downstream motion of the flame base (Mode B) and the formation and propagation of autoignition kernels (Mode C). Reducing the confinement length of the hot coflow serves to highlight the role of autoignition in flame stabilization when the flame is subjected to destabilization by ambient air entrainment. The influence of autoignition is further assessed by computing ignition delay times for homogenous CH_4/Air mixtures using chemical kinetic simulations and comparing them against the flow transit time corresponding to mean flame liftoff height of the bulk flame base. It is inferred from these studies that while autoignition is an active flame stabilization mechanism in this regime, the effect of turbulence may be crucial in determining the importance of autoignition towards stabilizing the flame at the conditions studied. An experimental investigation of autoignition characteristics at various jet Reynolds numbers reveals that turbulence appears to have a suppressing effect on the active role of autoignition in flame stabilization.

Copyright (c) 2019 by ASME
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