A numerical study of transient ignition and flame characteristics of diluted hydrogen versus heated air in counterflow

Journal Article · Sat Nov 15 00:00:00 EST 2008 · Combustion and Flame

DOI:https://doi.org/10.1016/J.COMBUSTFLAME.2008.07.012· OSTI ID:21116122

Yoo, Chun Sang; Chen, Jacqueline H; Frank, Jonathan H ^[1]

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1. Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94551-0969 (United States)

Combined experimental and numerical studies of the transient response of ignition to strained flows require a well-characterized ignition trigger. Laser deposition of a small radical pool provides a reliable method for initiating ignition of mixtures that are near the ignition limit. Two-dimensional direct numerical simulations are used to quantify the sensitivity of ignition kernel formation and subsequent edge-flame propagation to the oxidizer temperature and the initial width and amplitude of O-atom deposition used to trigger ignition in an axisymmetric counterflow of heated air versus ambient hydrogen/nitrogen. The ignition delay and super-equilibrium OH concentration in the nascent ignition kernel are highly sensitive to variations in these initial conditions. The ignition delay decreases as the amplitude of the initial O-atom deposition increases. The spatial distribution and the magnitude of the OH overshoot are governed by multi-dimensional effects. The degree of OH overshoot near the burner centerline increases as the diameter of the initial O-atom deposition region decreases. This result is attributed to preferential diffusion of hydrogen in the highly curved leading portion of the edge flame that is established following thermal runaway. The edge-flame speed and OH overshoot at the leading edge of the edge flame are relatively insensitive to variations in the initial conditions of the ignition. The steady edge-flame speed is approximately twice the corresponding laminar flame speed. The rate at which the edge flame approaches its steady state is insensitive to the initial conditions and depends solely on the diffusion time scale at the edge flame. The edge flame is curved toward the heated oxidizer stream as a result of differences in the chemical kinetics between the leading edge and the trailing diffusion flame. The structure of the highly diluted diffusion flame considered in this study corresponds to Linan's 'premixed flame regime' in which only the oxidizer leaks through the reaction zone such that the flame is located at fuel lean rather than stoichiometric mixture fraction conditions. (author)

• OSTI ID: 21116122
• Journal Information: Combustion and Flame, Vol. 155, Issue 3; Other Information: Elsevier Ltd. All rights reserved; ISSN 0010-2180
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
LAMINAR FLAMES
HYDROGEN
AIR
IGNITION
DEPOSITION
OXYGEN
DIFFUSION
MIXTURES
ATOMS
NITROGEN
OXIDIZERS
TRANSIENTS
VELOCITY
AXIAL SYMMETRY
SPATIAL DISTRIBUTION
AMPLITUDES
VARIATIONS
WIDTH
FLAME PROPAGATION
TWO-DIMENSIONAL CALCULATIONS
SIZE
EQUILIBRIUM
COMBUSTION KINETICS
HYDROXYL RADICALS
SENSITIVITY
COMPUTERIZED SIMULATION
TEMPERATURE DEPENDENCE
COUNTERFLOW SYSTEMS
HEATING
TIME DELAY
Edge flame
Nonpremixed counterflow