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Title: Turbulent combustion of premixed flames in closed vessels

Abstract

An extensive series of experiments on spark-ignited explosion of propane-air mixtures in both turbulent and quiescent conditions in a cubic closed vessel is described. Turbulence was produced by a moving grid, and the development with time of pressure and of flame area (from light emission) recorded. The effects of grid-hole diameter, grid velocity, spark timing after passage of the grid, equivalence ratio, and initial pressure were investigated. Estimates of the rate of strain in the unburnt gases were derived from hot-wire anemometry. Results indicated that rate of strain was a major factor governing the rate of combustion. Theoretical simulations of explosions with a simple model were made, in which turbulence was characterized solely by the rate of strain, and in which the decay of turbulence during explosions and the effect of changes in pressure on both burning velocity and flow field were taken into account. The simulations were compared with experimental results, and reinforced the idea that turbulence has the dual effect of causing wrinkling of flames and, especially for weaker mixtures, reducing the burning velocity. An empirical relationship was found in which the logarithmic rate of wrinkling was proportional to the square root of the rate of strain. Somemore » simple conclusions are drawn regarding practical application of the results.« less

Authors:
 [1];  [2]
  1. (Univ. of Alberta, Edmonton (Canada). Mechanical Engineering)
  2. (Univ. of Liverpool (United Kingdom). Mechanical Engineering)
Publication Date:
OSTI Identifier:
5211098
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; (United States); Journal Volume: 96:4
Country of Publication:
United States
Language:
English
Subject:
03 NATURAL GAS; 33 ADVANCED PROPULSION SYSTEMS; PROPANE; COMBUSTION KINETICS; SPARK IGNITION ENGINES; MATHEMATICAL MODELS; TURBULENT FLOW; ALKANES; CHEMICAL REACTION KINETICS; ENGINES; FLUID FLOW; HEAT ENGINES; HYDROCARBONS; INTERNAL COMBUSTION ENGINES; KINETICS; ORGANIC COMPOUNDS; REACTION KINETICS; 034000* - Natural Gas- Combustion; 330101 - Internal Combustion Engines- Spark-Ignition

Citation Formats

Checkel, M.D., and Thomas, A. Turbulent combustion of premixed flames in closed vessels. United States: N. p., 1994. Web. doi:10.1016/0010-2180(94)90104-X.
Checkel, M.D., & Thomas, A. Turbulent combustion of premixed flames in closed vessels. United States. doi:10.1016/0010-2180(94)90104-X.
Checkel, M.D., and Thomas, A. 1994. "Turbulent combustion of premixed flames in closed vessels". United States. doi:10.1016/0010-2180(94)90104-X.
@article{osti_5211098,
title = {Turbulent combustion of premixed flames in closed vessels},
author = {Checkel, M.D. and Thomas, A.},
abstractNote = {An extensive series of experiments on spark-ignited explosion of propane-air mixtures in both turbulent and quiescent conditions in a cubic closed vessel is described. Turbulence was produced by a moving grid, and the development with time of pressure and of flame area (from light emission) recorded. The effects of grid-hole diameter, grid velocity, spark timing after passage of the grid, equivalence ratio, and initial pressure were investigated. Estimates of the rate of strain in the unburnt gases were derived from hot-wire anemometry. Results indicated that rate of strain was a major factor governing the rate of combustion. Theoretical simulations of explosions with a simple model were made, in which turbulence was characterized solely by the rate of strain, and in which the decay of turbulence during explosions and the effect of changes in pressure on both burning velocity and flow field were taken into account. The simulations were compared with experimental results, and reinforced the idea that turbulence has the dual effect of causing wrinkling of flames and, especially for weaker mixtures, reducing the burning velocity. An empirical relationship was found in which the logarithmic rate of wrinkling was proportional to the square root of the rate of strain. Some simple conclusions are drawn regarding practical application of the results.},
doi = {10.1016/0010-2180(94)90104-X},
journal = {Combustion and Flame; (United States)},
number = ,
volume = 96:4,
place = {United States},
year = 1994,
month = 3
}
  • The Bray-Moss-Libby model of premixed turbulent combustion is modified to account for unsteady problems with variable enthalpy. Nonconstant thermodynamic properties are also included. A numerical solution procedure is applied to the model equations and a one-dimensional test problem is solved. Four separate test cases are used to demonstrate the integrity of both model and solution procedure. Countergradient transport and flame-generated turbulence are both predicted to occur as a result of an interaction between density fluctuations and the mean pressure gradient. However, in the cases studied, these effects are found to be smaller than in stationary unconfined planar flames.
  • 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.
  • Cited by 7