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Title: Interaction of turbulent premixed flames with combustion products: Role of stoichiometry

Abstract

Stabilization methods of turbulent flames often involve mixing of reactants with hot products of combustion. The stabilizing effect of combustion product enthalpy has been long recognized, but the role played by the chemical composition of the product gases is typically overlooked. We employ a counterflow system to pinpoint the effects of the combustion product stoichiometry on the structure of turbulent premixed flames under conditions of both stable burning and local extinction. To that end, a turbulent jet of lean-to-rich, CH 4/O 2/N 2-premixed reactants at a turbulent Reynolds number of 1050 was opposed to a stream of hot products of combustion that were generated in a preburner. While the combustion product stream temperature was kept constant, its stoichiometry was varied independently from that of the reactant stream, leading to reactant-to-product stratification of relevance to practical combustion systems. The detailed structure of the turbulent flame front was analyzed in two series of experiments using laser-induced fluorescence (LIF): joint CH 2O LIF and OH LIF measurements and joint CO LIF and OH LIF measurements. Results revealed that a decrease in local CH 2O+OH and CO+OH reaction rates coincide with the depletion of OH radicals in the vicinity of the combustion product stream.more » These critical combustion reaction rates were more readily quenched in the presence of products of combustion from a stoichiometric flame, whereas they were favored by lean combustion products. As a result, stoichiometric combustion products contributed to a greater occurrence of local extinction. Furthermore, they limited the capacity of premixed reactants to ignite and of the turbulent premixed flames to stabilize. In contrast, lean and rich combustion products facilitated flame ignition and stability and reduced the rate of local extinction. The influence of the combustion product stream on the turbulent flame front was limited to a zone of approximately two millimeters from the gas mixing layer interface (GMLI) of the product stream. As a result, flame fronts that were separated from the GMLI by larger distances were unaffected by the product stream stoichiometry.« less

Authors:
ORCiD logo [1];  [1];  [2]
  1. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
  2. Yale Univ., New Haven, CT (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-CA), Livermore, CA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1335516
Alternate Identifier(s):
OSTI ID: 1425685
Report Number(s):
SAND-2016-12326J
Journal ID: ISSN 0010-2180; 649699
Grant/Contract Number:
AC04-94AL85000; AC04-94-AL85000
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Combustion and Flame
Additional Journal Information:
Journal Volume: 170; Journal Issue: C; Journal ID: ISSN 0010-2180
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; turbulent counterflow; premixed flames; stratification; extinction

Citation Formats

Coriton, Bruno Rene Leon, Frank, Jonathan H., and Gomez, Alessandro. Interaction of turbulent premixed flames with combustion products: Role of stoichiometry. United States: N. p., 2016. Web. doi:10.1016/j.combustflame.2016.04.020.
Coriton, Bruno Rene Leon, Frank, Jonathan H., & Gomez, Alessandro. Interaction of turbulent premixed flames with combustion products: Role of stoichiometry. United States. doi:10.1016/j.combustflame.2016.04.020.
Coriton, Bruno Rene Leon, Frank, Jonathan H., and Gomez, Alessandro. Mon . "Interaction of turbulent premixed flames with combustion products: Role of stoichiometry". United States. doi:10.1016/j.combustflame.2016.04.020. https://www.osti.gov/servlets/purl/1335516.
@article{osti_1335516,
title = {Interaction of turbulent premixed flames with combustion products: Role of stoichiometry},
author = {Coriton, Bruno Rene Leon and Frank, Jonathan H. and Gomez, Alessandro},
abstractNote = {Stabilization methods of turbulent flames often involve mixing of reactants with hot products of combustion. The stabilizing effect of combustion product enthalpy has been long recognized, but the role played by the chemical composition of the product gases is typically overlooked. We employ a counterflow system to pinpoint the effects of the combustion product stoichiometry on the structure of turbulent premixed flames under conditions of both stable burning and local extinction. To that end, a turbulent jet of lean-to-rich, CH4/O2/N2-premixed reactants at a turbulent Reynolds number of 1050 was opposed to a stream of hot products of combustion that were generated in a preburner. While the combustion product stream temperature was kept constant, its stoichiometry was varied independently from that of the reactant stream, leading to reactant-to-product stratification of relevance to practical combustion systems. The detailed structure of the turbulent flame front was analyzed in two series of experiments using laser-induced fluorescence (LIF): joint CH2O LIF and OH LIF measurements and joint CO LIF and OH LIF measurements. Results revealed that a decrease in local CH2O+OH and CO+OH reaction rates coincide with the depletion of OH radicals in the vicinity of the combustion product stream. These critical combustion reaction rates were more readily quenched in the presence of products of combustion from a stoichiometric flame, whereas they were favored by lean combustion products. As a result, stoichiometric combustion products contributed to a greater occurrence of local extinction. Furthermore, they limited the capacity of premixed reactants to ignite and of the turbulent premixed flames to stabilize. In contrast, lean and rich combustion products facilitated flame ignition and stability and reduced the rate of local extinction. The influence of the combustion product stream on the turbulent flame front was limited to a zone of approximately two millimeters from the gas mixing layer interface (GMLI) of the product stream. As a result, flame fronts that were separated from the GMLI by larger distances were unaffected by the product stream stoichiometry.},
doi = {10.1016/j.combustflame.2016.04.020},
journal = {Combustion and Flame},
number = C,
volume = 170,
place = {United States},
year = {Mon May 30 00:00:00 EDT 2016},
month = {Mon May 30 00:00:00 EDT 2016}
}

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  • The local scalar statistics of premixed flames in turbulent opposed streams has been studied by sheet laser tomography. The statistics collected on these flame edges provide information on the mean flame position, and the mean and standard deviations of local flamelet orientation and curvature. Emphasis is given to how these parameters vary through the flame brush as the flames are pushed toward extinction. The flames are essentially planar in the mean and the probability density function (pdf) of flamelet orientation in symmetric about this mean orientation. The standard deviation of flame angle is essentially constant throughout the flame brush, butmore » varies strongly at the leading and trailing flame edges. The mean curvature of these flame is positive (i.e., concave to products) at the leading edge of the flame and negative at its trailing edge. Similar to the flame angle, the standard deviation of flame curvature is also constant throughout most of the central portion of the flame brush. As the mean nozzle exit velocity and the turbulence intensity are increased to bring the flame nearer to extinction, the individual flame brushes thicken as much as 50%. The standard deviations of flame angle an curvature also increase but more modestly. An unexpected result of the data collected is the differences between the upper and lower flames, which is probably an effect of buoyancy. The lower flame is consistently and significantly more wrinkled than the upper flame, resulting in the lower brush being thicker by as much as 25% and having larger standard deviations of flame angle and curvature than the upper flame.« less
  • The mechanism of unburnt pocket formation in an unsteady two-dimensional premixed lean methane-air flame is investigated using direct numerical simulations. Theoretical results for nonlinear diffusion equations combined with analytical examples are used to interpret some of the results. Flame structure and propagation show three distinct stages of pocket formation: (1) flame channel closing involving head-on quenching of flames, (2) cusp recovery, and (3) pocket burnout. The flame channel closing and subsequent pocket burnout are mutual annihilation events that feature curvature, diffusion normal to the flame front, unsteady strain rate effects, and singularities in flame propagation and stretch rate. The resultsmore » show that during channel closing and pocket burnout thermo-diffusive and chemical interactions result in the acceleration of the flames prior to annihilation; the time scales associated with the final stage of mutual annihilation and the initial stage of cusp recovery are significantly smaller than diffusive and convective time scales. Peak radical concentrations resulting from flame channel closing and pocket burnout exceed peak laminar values by as much as 25%. After the merging of the fuel consumption layers, radical production and flame structure shifts more towards an H{sub 2}/CO/O{sub 2} system at the expense of hydrocarbon reactions. Species thermodiffusive interaction times are shorter than the unstrained one-dimensional counterpart due to unsteady strain and convection. Curvature effects on the flame propagation are prominent during pocket burnout and cusp recovery. The recovery stage shows strong dependence on diffusion of radicals left from the channel closing stage. This diffusion is amplified by the strong curvature of the flame cusp.« less
  • No abstract prepared.