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Review Article

Acoustically Coupled Combustion of Liquid Fuel Droplets

[+] Author and Article Information
Ann R. Karagozian

Department of Mechanical
and Aerospace Engineering,
University of California,
Los Angeles, CA 90095-1597
e-mail: ark@seas.ucla.edu

Manuscript received August 28, 2015; final manuscript received May 25, 2016; published online July 7, 2016. Editor: Harry Dankowicz.

Appl. Mech. Rev 68(4), 040801 (Jul 07, 2016) (11 pages) Paper No: AMR-15-1098; doi: 10.1115/1.4033792 History: Received August 28, 2015; Revised May 25, 2016

The dynamics of oscillatory flames is relevant to acoustically coupled combustion instabilities arising in many practical engineering systems. This paper reviews fundamental studies that pertain to the combustion of single liquid fuel droplets in an acoustically resonant environment. This flow field is not only an idealized model for the study of the fundamental interaction of reactive, evaporative, acoustic, and other transport-based timescales, but it may also be used to identify relevant phenomena in more complex or practical geometries that require a focus for future combustion control efforts. The nature of these phenomena is discussed in detail, in addition to their implications for broader issues associated with combustion instabilities.

Copyright © 2016 by ASME
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References

Figures

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Fig. 1

Schematic diagram of combustion instability created via feedback of acoustic disturbances

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Fig. 2

OH* chemiluminescence images of various liquid fuel droplets burning in air. The droplets are suspended from a fine capillary and burn under atmospheric, normal gravity conditions. (a) Ethanol, (b) methanol, (c) JP-8, and (d) FT. (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier).

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Fig. 3

Experimental setup of the acoustic waveguide and droplet feed system, from the normal gravity experiments. (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier.) Sample pressure and velocity amplitude distributions are shown for a standing wave configuration.

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Fig. 4

In microgravity, the effect of SPL (in dB) on methanol droplet mean burning rate constant K, normalized by its mean value under unforced conditions (SPL corresponding to negative infinity), where the droplet is situated in the vicinity of a PN. PN conditions created by excitation frequencies of 240 Hz, 290 Hz, 670 Hz, and 770 Hz are shown. (Reprinted with permission from Dattarajan et al. [43]. Copyright 2006 by Elsevier).

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Fig. 5

In microgravity, the effect of n-hexadecanol droplet location, relative to a PN (or velocity antinode), on flame deflection. (Reprinted with permission from Tanabe et al. [57]. Copyright 2000 by Elsevier).

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Fig. 6

Instantaneous video frames for a droplet situated precisely at a PN and burning in microgravity, subject to acoustic excitation at 770 Hz and 140 dB. The different photographs show the temporally evolving behavior of the flame, corresponding to times (a) 0.63 s, (b) 1.03 s, (c) 1.5 s, (d) 1.76 s, and (e) 1.97 s after the commencement of the microgravity experiment. A burning droplet in the absence of acoustic excitation is shown for comparison in (f). (Reprinted with permission from Dattarajan et al. [43]. Copyright 2006 by Elsevier).

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Fig. 7

For burning ethanol droplets exposed to acoustic forcing at fa = 1500 Hz and pmax′  = 150 Pa: (a) Mean OH* chemiluminescence images at different waveguide locations x, with corresponding measurements (symbols) and theoretical prediction (lines) of local pressure perturbation amplitude. (b) Measured [20] and theoretically predicted [57] acoustic acceleration ga as a function of x. (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier).

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Fig. 8

OH* chemiluminescence image of an ethanol droplet burning in the presence of acoustic excitation. With acoustic excitation, both buoyancy force Fb and acoustic radiation force Fa affect flame orientation so that orientation angle ϕf≠0. θ measures position about the droplet and δf measures local flame standoff distance. The droplet radius rs and the flame radius rf are functions of θ in the imposed polar coordinate system, whose origin coincides with the center of the droplet. u′ represents the horizontal perturbation velocity. (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier).

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Fig. 9

Estimated experimental acoustic accelerations ga as a function of the droplet displacement x, in units of wavelength λ associated with forcing frequencies 332 Hz (), 898 Hz (·), and 1500 Hz (). Data are shown, from top to bottom, for ethanol, methanol, JP-8, and FT fuels. (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier).

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Fig. 10

For ethanol droplet combustion exposed to acoustic excitation at a maximum pressure perturbation amplitude pmax′  = 150 Pa, results of phase-locked measurements, as a function of temporal phase, of: normalized OH* chemiluminescence intensity I′, pressure perturbation p′, and flame horizontal standoff distance δf. Data pertain to a droplet located at x/λ ≈ −0.02 for fa = 332 Hz (data extracted from Refs. [73] and [74]).

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Fig. 11

Horizontal flame standoff distance oscillation amplitudes δf′ for all fuels and excitation frequencies for droplets located at various locations x scaled by wavelength λ. Fuels shown: ethanol (), methanol (·), JP-8 (), FT (). (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier).

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Fig. 12

Rayleigh index G(x) for burning ethanol droplets situated at different waveguide locations x and for different excitation frequencies fa. (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier).

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Fig. 13

Average burning rate constant K (scaled with respect to the value of Kunf for a nonforced burning droplet) as a function of the droplet displacement x, in units of wavelength λ associated with forcing frequencies 332 Hz (), 898 Hz (), and 1500 Hz (). Data are shown, from top to bottom, for ethanol, methanol, JP-8, and FT fuels. (Reprinted with permission from Sevilla-Esparza et al. [20]. Copyright 2014 by Elsevier).

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