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Title: Liftoff characteristics of partially premixed flames under normal and microgravity conditions

Abstract

An experimental and computational investigation on the liftoff characteristics of laminar partially premixed flames (PPFs) under normal (1-g) and microgravity ({mu}-g) conditions is presented. Lifted methane-air PPFs were established in axisymmetric coflowing jets using nitrogen dilution and various levels of partial premixing. The {mu}-g experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated through a comparison of the flame reaction zone topologies, liftoff heights, lengths, and oscillation frequencies. The effects of equivalence ratio, gravity, jet velocity, and radiation on flame topology, liftoff height, flame length, base structure, and oscillation frequency are characterized. Both the simulations and measurements indicate that under identical conditions, a lifted {mu}-g PPF is stabilized closer to the burner compared with the 1-g flame, and that the liftoff heights of both 1-g and {mu}-g flames decrease with increasing equivalence ratio and approach their respective nonpremixed flame limits. The liftoff height also increases as the jet velocity is increased. In addition, the flame base structure transitions from a triple- to a double-flame structure as the flame liftoff heightmore » decreases. A modified flame index is developed to distinguish between the rich premixed, lean premixed, and nonpremixed reaction zones near the flame base. The 1-g lifted flames exhibit well-organized oscillations due to buoyancy-induced instability, while the corresponding {mu}-g flames exhibit steady-state behavior. The effect of thermal radiation is to slightly decrease the liftoff heights of both 1-g and {mu}-g flames under coflow conditions.« less

Authors:
; ;  [1];  [2];  [3];  [4]
  1. Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, IL 60607 (United States)
  2. Department of Mechanical & Aerospace Engineering, Princeton University, Princeton, NJ 08544 (United States)
  3. Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 (United States)
  4. National Center for Microgravity Research, Cleveland, OH 44135 (United States)
Publication Date:
OSTI Identifier:
20677727
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 143; 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; GRAVITATIONAL FIELDS; AIR; METHANE; FLAMES; COMBUSTION KINETICS; COMPUTERIZED SIMULATION; JETS; OSCILLATIONS

Citation Formats

Lock, Andrew J., Briones, Alejandro M., Aggarwal, Suresh K., Qin, Xiao, Puri, Ishwar K., and Hegde, Uday. Liftoff characteristics of partially premixed flames under normal and microgravity conditions. United States: N. p., 2005. Web. doi:10.1016/j.combustflame.2005.05.011.
Lock, Andrew J., Briones, Alejandro M., Aggarwal, Suresh K., Qin, Xiao, Puri, Ishwar K., & Hegde, Uday. Liftoff characteristics of partially premixed flames under normal and microgravity conditions. United States. doi:10.1016/j.combustflame.2005.05.011.
Lock, Andrew J., Briones, Alejandro M., Aggarwal, Suresh K., Qin, Xiao, Puri, Ishwar K., and Hegde, Uday. Tue . "Liftoff characteristics of partially premixed flames under normal and microgravity conditions". United States. doi:10.1016/j.combustflame.2005.05.011.
@article{osti_20677727,
title = {Liftoff characteristics of partially premixed flames under normal and microgravity conditions},
author = {Lock, Andrew J. and Briones, Alejandro M. and Aggarwal, Suresh K. and Qin, Xiao and Puri, Ishwar K. and Hegde, Uday},
abstractNote = {An experimental and computational investigation on the liftoff characteristics of laminar partially premixed flames (PPFs) under normal (1-g) and microgravity ({mu}-g) conditions is presented. Lifted methane-air PPFs were established in axisymmetric coflowing jets using nitrogen dilution and various levels of partial premixing. The {mu}-g experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A time-accurate, implicit algorithm that uses a detailed description of the chemistry and includes radiation effects is used for the simulations. The predictions are validated through a comparison of the flame reaction zone topologies, liftoff heights, lengths, and oscillation frequencies. The effects of equivalence ratio, gravity, jet velocity, and radiation on flame topology, liftoff height, flame length, base structure, and oscillation frequency are characterized. Both the simulations and measurements indicate that under identical conditions, a lifted {mu}-g PPF is stabilized closer to the burner compared with the 1-g flame, and that the liftoff heights of both 1-g and {mu}-g flames decrease with increasing equivalence ratio and approach their respective nonpremixed flame limits. The liftoff height also increases as the jet velocity is increased. In addition, the flame base structure transitions from a triple- to a double-flame structure as the flame liftoff height decreases. A modified flame index is developed to distinguish between the rich premixed, lean premixed, and nonpremixed reaction zones near the flame base. The 1-g lifted flames exhibit well-organized oscillations due to buoyancy-induced instability, while the corresponding {mu}-g flames exhibit steady-state behavior. The effect of thermal radiation is to slightly decrease the liftoff heights of both 1-g and {mu}-g flames under coflow conditions.},
doi = {10.1016/j.combustflame.2005.05.011},
journal = {Combustion and Flame},
number = 3,
volume = 143,
place = {United States},
year = {Tue Nov 01 00:00:00 EST 2005},
month = {Tue Nov 01 00:00:00 EST 2005}
}
  • The authors measured emission indices for NO{sub x}, CO, and HC for turbulent partially premixed flames formed by injecting rich methane/air mixtures through a central burner tube into a co-flow of air. The operating conditions included a broad range of burner tube equivalence ratios ({Phi}{sub B}) including the limiting cases of diffusion and stoichiometric premixed flames. At sufficiently high levels of partial premixing, a double flame structure consisting of a rich premixed inner flame and an outer diffusion flame is established similar to that previously observed in laminar flames. EINO{sub x} values remain approximately constant for 5 {le} {Phi}{sub B}more » {le} {infinity} and then decrease slightly with decreasing {Phi}{sub B} at 3.5 {le} {Phi}{sub B} {le} 5. For 3.5 {le} {Phi}{sub B} {le} 1.5, EINO{sub x} decreases further with decreasing {Phi}{sub B} to a minimum at around {Phi}{sub B} = 1.5, followed by a sharp increase as {Phi}{sub B} approaches unity. The reduction in EINO{sub x} at {Phi}{sub B} = 1.5 as compared with that for the diffusion flame is approximately 25% and as compared with that for the stoichiometric premixed flame is approximately 35%. In addition, the authors measured temperature distributions and found that temperatures increase continuously with increasing partial premixing. They also estimated global residence times ({tau}{sub 0}) from flame length measurements and average velocities. The observed changes in temperatures and residence times are not sufficient to explain the observed changes in EINO{sub x}.« less
  • Visible spectral characteristics of cross-sectional emissions from a partially premixed methane/air flame and a propane/air flame have been investigated. An optical train with a two-axis scanning mirror system was used to record line-of-sight emission spectra from 354 nm to 618 nm, and inversion technique was applied to obtain cross-sectional emission spectra. By analyzing the reconstructed emission spectra, cross-sectional intensities of CH and C{sub 2} radicals were separated from the background emission. The blue flame edge and yellow flame edge were also obtained by image processing technique for edge detection with color photographs of the flames. These edges were compared withmore » radial distributions of CH, C{sub 2} radicals and background emission. The CH radicals were observed at the blue flame edge. The background emission was generated by soot precursors at upstream of the flame and by soot at downstream of the flame. The C{sub 2} radicals in the propane/air flame were more noticeable than those in the methane/air flame.« less
  • Cited by 1
  • This article presents experimental findings on the blowoff characteristics of conical premixed flames anchored at their apex by three different flame holders (rod, disk, and cone) in the presence of upstream velocity oscillations. Experiments were performed with propane-air mixtures at mixture velocities approaching the flame holder of 5, 10, and 15 m/s. The flow speed was modulated sinusoidally at frequencies up to 400 Hz with a constant-velocity modulation amplitude of u{sub rms}/U{sub m}=0.08 upstream of the flame holder. It was found that the blowoff equivalence ratio exhibits a dependence on the flow modulation frequency. Specifically, at low approach velocities (5more » m/s), the effect of upstream flow modulation is to improve flame stability as evidenced by lower flame blowoff equivalence ratios for all three types of flame holders considered. At higher approach velocities (10 and 15 m/s), the disk- and cone-shaped flame holders exhibit less stability with increasing excitation frequency. The rod-shaped flame holder behavior is different at these higher velocities in that the flow modulation still provides enhanced flame stability. The flame stability results are supplemented with a detailed analysis of the flow field in the flame stabilization zone obtained by particle image velocimetry.« less