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Title: Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames

Abstract

Simultaneous planar-laser induced fluorescence (PLIF) and particle image velocimetry (PIV) provide a comprehensive view of the molecular mixing and velocity fields in the stabilization region of turbulent, lifted jet diffusion flames. The Mie scattering medium for PIV is a glycerol-water fog, which evaporates at elevated temperatures and allows inference of the location of the high-temperature interface at the flame base. The jet Reynolds numbers vary from 4400 to 10,700. The mixing and velocity fields upstream of the flame base evolve consistently with nonreacting jet scaling. Conditional statistics of the fuel mole fraction at the instantaneous high-temperature interface show that the flame stabilization point does not generally correspond to the most upstream point on the interface (called here the leading point), because the mixture there is typically too lean to support combustion. Instead, the flame stabilization point lies toward the jet centerline relative to the leading point. Conditional axial velocity statistics indicate that the mean axial velocity at the flame front is {approx}1.8S{sub L}, where S{sub L} is the stoichiometric laminar flame speed. The data also permit determination of the scalar dissipation rates, {chi}, with the results indicating that {chi} values near the high-temperature interfaces do not typically exceed the quenchingmore » value. Thus, the flame stabilization process is more consistent with theories based on partial fuel-air premixing than with those dependent on diffusion flame quenching. We propose a description of flame stabilization that depends on the large-scale organization of the mixing field. (author)« less

Authors:
;  [1];  [2]
  1. Applied Fluid Imaging Laboratory, Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218 (United States)
  2. Thermosciences Division, Mechanical Engineering Department, Stanford University, Stanford, CA 94305 (United States)
Publication Date:
OSTI Identifier:
20700732
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 144; Journal Issue: 3; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; FLAMES; JETS; TURBULENCE; COMBUSTION KINETICS; STABILIZATION; VELOCITY

Citation Formats

Su, L.K., Sun, O.S., and Mungal, M.G.. Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames. United States: N. p., 2006. Web. doi:10.1016/j.combustflame.2005.08.010.
Su, L.K., Sun, O.S., & Mungal, M.G.. Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames. United States. doi:10.1016/j.combustflame.2005.08.010.
Su, L.K., Sun, O.S., and Mungal, M.G.. Wed . "Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames". United States. doi:10.1016/j.combustflame.2005.08.010.
@article{osti_20700732,
title = {Experimental investigation of stabilization mechanisms in turbulent, lifted jet diffusion flames},
author = {Su, L.K. and Sun, O.S. and Mungal, M.G.},
abstractNote = {Simultaneous planar-laser induced fluorescence (PLIF) and particle image velocimetry (PIV) provide a comprehensive view of the molecular mixing and velocity fields in the stabilization region of turbulent, lifted jet diffusion flames. The Mie scattering medium for PIV is a glycerol-water fog, which evaporates at elevated temperatures and allows inference of the location of the high-temperature interface at the flame base. The jet Reynolds numbers vary from 4400 to 10,700. The mixing and velocity fields upstream of the flame base evolve consistently with nonreacting jet scaling. Conditional statistics of the fuel mole fraction at the instantaneous high-temperature interface show that the flame stabilization point does not generally correspond to the most upstream point on the interface (called here the leading point), because the mixture there is typically too lean to support combustion. Instead, the flame stabilization point lies toward the jet centerline relative to the leading point. Conditional axial velocity statistics indicate that the mean axial velocity at the flame front is {approx}1.8S{sub L}, where S{sub L} is the stoichiometric laminar flame speed. The data also permit determination of the scalar dissipation rates, {chi}, with the results indicating that {chi} values near the high-temperature interfaces do not typically exceed the quenching value. Thus, the flame stabilization process is more consistent with theories based on partial fuel-air premixing than with those dependent on diffusion flame quenching. We propose a description of flame stabilization that depends on the large-scale organization of the mixing field. (author)},
doi = {10.1016/j.combustflame.2005.08.010},
journal = {Combustion and Flame},
number = 3,
volume = 144,
place = {United States},
year = {Wed Feb 01 00:00:00 EST 2006},
month = {Wed Feb 01 00:00:00 EST 2006}
}
  • Planar imaging measurement of CH, CH[sub 4], and temperature are used to evaluate current models for flame stabilization in lifted, turbulent CH[sub 4]-jet flames. The experimental system consists of two cameras that simultaneously record the instantaneous distributions of species concentration and temperature. Measurements were made in two flames with fuel-jet Reynolds numbers of 7,000 and 12,100. the results show that the fuel and air are premixed and within the CH[sub 4] flammability limits at the flame stabilization point. The flame zone also falls considerably outside the region where scalar dissipation is significant (dissipation levels are well below the critical valuemore » for extinction). It is concluded that local stoichiometry, and not scalar dissipation, is the primary factor controlling flame stability. Considerable interaction exists between the large-scale structure associated with the central fuel jet and the instantaneous flame zone, which is consistent with both premixed flame propagation and the concept of large-scale motion resulting in the ignition of unreacted fuel by hot combustion products.« less
  • Particle image velocimetry was used to study the velocity field in the stabilization region of lifted, turbulent CH{sub 4}-jet flames over a range of Reynolds numbers from 7,000 to 19,500. Measured velocities at the flame base are considerably below the turbulent flame speeds derived from previous studies and show a dependence on the Reynolds number. The average velocity at the stabilization point is nearly a factor of five below the premixed laminar burning velocity at the lowest Reynolds number and asymptotes to a value about 20% higher as the Reynolds number is increased. Planar images of OH show that themore » flame zone structure near the stabilization point is also highly dependent on the Reynolds number. Comparison of the present OH images with previous CH{sub 4} Raman imaging results shows that the flame thickness is determined by the width of the flammable region. At a low Reynolds number, the flame is stabilized near the jet exit where the flammable layer is thin, resulting in a thin flame zone. At an increased Reynolds number, the stabilization point is located farther downstream where the flammable region is wider, resulting in a correspondingly wider flame zone. It is proposed that the lower velocities observed at the flame base are related to thinning of the flame zone at low Reynolds, which results in greater curvature of the flame base. The increased flame curvature effectively defocuses the transport of heat and flame radicals to reactants upstream of the propagating flame front, resulting in reduced burning velocities. The implications of these results for mechanisms controlling turbulent flame stabilization, with an emphasis on the applicability of triple flame concepts to turbulent flows, are discussed.« less
  • To reduce fuel consumption or the pollutant emissions of combustion (furnaces, aircraft engines, turbo-reactors, etc.), attempts are made to obtain lean mixture combustion regimes. These lead to poor stability of the flame. Thus, it is particularly interesting to find new systems providing more flexibility in aiding flame stabilization than the usual processes (bluff-body, stabilizer, quarl, swirl, etc.). The objective is to enlarge the stability domain of flames while offering flexibility at a low energy cost. Evidence is presented that the stabilization of a turbulent partially premixed flame of more than 10 kW can be enhanced by pulsed high-voltage discharges withmore » power consumption less than 0.1% of the power of the flame. The originality of this work is to demonstrate that very effective stabilization of turbulent flames is obtained when high-voltage pulses with very short rise times are used (a decrease by 300% in terms of liftoff height for a given exit jet velocity can be reached) and to provide measurements of minimum liftoff height obtained with discharge over a large range of the stability domain of the lifted jet flame.« less
  • Liftoff height and velocity measurements are presented for turbulent, lifted methane and ethylene flames. A range of Reynolds numbers from 3,800 to 22,000 is investigated, and the effect of coflow velocity is examined. The mean liftoff height is shown to increase with jet-exit and coflow velocity, while the rms fluctuation about the mean increases with distance from the jet exit. Particle image velocimetry (PIV) is used to provide instantaneous, two-dimensional velocity fields in the region of the lifted flame base. The results show that the instantaneous flame base is anchored primarily in the low-velocity regions of the jet, with axialmore » and radial movement of the flame to meet this criterion. The fluid velocity conditioned on the instantaneous flame base location is less than three times the laminar flame speed, and the velocity profiles through the flame base are similar in nature to those predicted by recent simulations of triple flames.« less
  • Abstract not provided.