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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

Intermittency Route to Combustion Instability in a Laboratory Spray Combustor

[+] Author and Article Information
Samadhan A. Pawar

Department of Aerospace Engineering,
Indian Institute of Technology Madras,
Chennai 600036, India
e-mail: samadhanpawar@ymail.com

R. Vishnu

Department of Aerospace Engineering,
Indian Institute of Technology Madras,
Chennai 600036, India
e-mail: agnithepower@gmail.com

M. Vadivukkarasan

Department of Applied Mechanics,
Indian Institute of Technology Madras,
Chennai 600036, India
e-mail: mvadivu@gmail.com

M. V. Panchagnula

Professor
Department of Applied Mechanics,
Indian Institute of Technology Madras,
Chennai 600036, India
e-mail: mvp@iitm.ac.in

R. I. Sujith

Professor
Department of Aerospace Engineering,
Indian Institute of Technology Madras,
Chennai 600036, India
e-mail: sujith@iitm.ac.in

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 3, 2015; final manuscript received August 18, 2015; published online October 21, 2015. Editor: David Wisler.

J. Eng. Gas Turbines Power 138(4), 041505 (Oct 21, 2015) (8 pages) Paper No: GTP-15-1389; doi: 10.1115/1.4031405 History: Received August 03, 2015; Revised August 18, 2015

In the present study, we investigate the phenomenon of transition of a thermoacoustic system involving two-phase flow, from aperiodic oscillations to limit cycle oscillations. Experiments were performed in a laboratory scale model of a spray combustor. A needle spray injector is used to generate a droplet spray having one-dimensional velocity field. This simplified design of the injector helps in keeping away the geometric complexities involved in the real spray atomizers. We investigate the stability of the spray combustor in response to the variation of the flame location inside the combustor. Equivalence ratio is maintained constant throughout the experiment. The dynamics of the system is captured by measuring the unsteady pressure fluctuations present in the system. As the flame location is gradually varied, self-excited high-amplitude acoustic oscillations are observed in the combustor. We observe the transition of the system behavior from low-amplitude aperiodic oscillations to large amplitude limit cycle oscillations occurring through intermittency. This intermittent state mainly consists of a sequence of high-amplitude bursts of periodic oscillations separated by low-amplitude aperiodic regions. Moreover, the experimental results highlight that during intermittency, the maximum amplitude of bursts, near to the onset of intermittency, is as much as three times higher than the maximum amplitude of the limit cycle oscillations. These high-amplitude intermittent loads can have stronger adverse effects on the structural properties of the engine than the low-amplitude cyclic loading caused by the sustained limit cycle oscillations. Evolution of the three different dynamical states of the spray combustion system (viz., stable, intermittency, and limit cycle) is studied in three-dimensional phase space by using a phase space reconstruction tool from the dynamical system theory. We report the first experimental observation of type-II intermittency in a spray combustion system. The statistical distributions of the length of aperiodic (turbulent) phase with respect to the control parameter, first return map and recurrence plot (RP) techniques are employed to confirm the type of intermittency.

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Figures

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Fig. 1

(a) Schematic of the spray combustion system and (b) and (c) schematic of the front and top view of the needle injector, respectively

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Fig. 3

Time series data acquired at different flame locations in the combustor. (a) Stable operation of the combustor at xf = 520 mm, (b)–(o) intermittency state, from xf = 540 mm to xf = 670 mm, (p) limit cycle oscillation at xf = 680 mm. These plots show the intermittency route to limit cycle oscillations in the spray combustion system. (q) A magnified portion of intermittency signal, obtained at xf = 550 mm, shows the low-amplitude aperiodic and the burst of high-amplitude periodic oscillations present in the signal.

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Fig. 2

Bifurcation plots of the acoustic pressure data, where (a) the global maximum and (b) rms values of the acoustic pressure signal are plotted against the location of flame inside the combustor. Each plot is divided into three regions. Region (i) low-amplitude aperiodic oscillations, region (ii) intermittency, and region (iii) limit cycle oscillations.

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Fig. 8

(a) First return map of the experimental data, for the intermittent phase, shows a scatter of points along the diagonal line. (b) and (c) indicate the spiraling behavior of trajectory, shown by plotting the evolution of some of the initial points present in (a), confirming the type-II intermittency.

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Fig. 7

Log–log plot of the average value of length of turbulent phase 〈T〉 versus normalized flame location (r) shows a scaling law behavior of type-II intermittency

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Fig. 6

Histogram of length of the turbulent phase at xf = 540 mm and amplitude threshold of 54 Pa shows an exponential tail

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Fig. 5

The intermittency signal consists of low-amplitude turbulent phases in between the consecutive high-amplitude laminar phases. A pressure threshold demarks the high-amplitude periodic oscillations from low-amplitude aperiodic oscillations.

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Fig. 4

Reconstructed phase portrait of the 0.5 s pressure time series data obtained at three different dynamical states present in the system: (a) stable combustion (xf = 520 mm; τ = 3 and E = 11), (b) intermittency (xf = 590 mm; τ = 10, E = 9), and (c) limit cycle oscillation (xf = 680 mm; τ = 10, E = 9)

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Fig. 9

(a) A portion of time signal displaying intermittency. (b)–(d) RP of the regions of intermittency signal. (c) RP shows the kitelike structure of type-II intermittency. (Data points (N) = 7500, τ = 3, E = 10, ϵ = 0.2 d, “ϵ” is the threshold (radius) based on 20% of the mean distance of the attractor).

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