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

High-Speed Imaging of Forced Ignition Kernels in Nonuniform Jet Fuel/Air Mixtures

[+] Author and Article Information
Sheng Wei

Ben T. Zinn Combustion Laboratory,
Aerospace Engineering Georgia Institute
of Technology,
Atlanta, GA 30332-0150
e-mail: sheng_wei@gatech.edu

Brandon Sforzo

Argonne National Laboratory,
9700 South Cass Avenue,
Lemont, IL 60493-4844
e-mail: bsforzo@anl.gov

Jerry Seitzman

Professor
Ben T. Zinn Combustion Laboratory,
Aerospace Engineering Georgia Institute
of Technology,
Atlanta, GA 30332-0150
e-mail: jerry.seitzman@aerospace.gatech.edu

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 10, 2017; final manuscript received August 27, 2017; published online May 29, 2018. Editor: David Wisler.

J. Eng. Gas Turbines Power 140(7), 071503 (May 29, 2018) (9 pages) Paper No: GTP-17-1335; doi: 10.1115/1.4038322 History: Received July 10, 2017; Revised August 27, 2017

This paper describes experimental measurements of forced ignition of prevaporized liquid fuels in a well-controlled facility that incorporates nonuniform flow conditions similar to those of gas turbine engine combustors. The goal here is to elucidate the processes by which the initially unfueled kernel evolves into a self-sustained flame. Three fuels are examined: a conventional Jet-A and two synthesized fuels that are used to explore fuel composition effects. A commercial, high-energy recessed cavity discharge igniter located at the test section wall ejects kernels at 15 Hz into a preheated, striated crossflow. Next to the igniter wall is an unfueled air flow; above this is a premixed, prevaporized, fuel–air flow, with a matched velocity and an equivalence ratio near 0.75. The fuels are prevaporized in order to isolate chemical effects. Differences in early ignition kernel development are explored using three synchronized, high-speed imaging diagnostics: schlieren, emission/chemiluminescence, and OH planar laser-induced fluorescence (PLIF). The schlieren images reveal rapid entrainment of crossflow fluid into the kernel. The PLIF and emission images suggest chemical reactions between the hot kernel and the entrained fuel–air mixture start within tens of microseconds after the kernel begins entraining fuel, with some heat release possibly occurring. Initially, dilution cooling of the kernel appears to outweigh whatever heat release occurs; so whether the kernel leads to successful ignition or not, the reaction rate and the spatial extent of the reacting region decrease significantly with time. During a successful ignition event, small regions of the reacting kernel survive this dilution and are able to transition into a self-sustained flame after ∼1–2 ms. The low-aromatic/low-cetane-number fuel, which also has the lowest ignition probability, takes much longer for the reaction zone to grow after the initial decay. The high-aromatic, more easily ignited fuel, shows the largest reaction region at early times.

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Figures

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Fig. 1

Schematic of the test facility (from Ref. [13])

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Fig. 2

Planar laser-induced fluorescence of A-2 showing spatially consistent fuel/air mixing in the main flow

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Fig. 3

Profile schematic of schlieren illumination and imaging configuration (not to scale); C2 is the schlieren imaging camera

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Fig. 4

Top view schematic of test section with relative position of the (C1) timing camera, (C2) schlieren camera, (C3) chemiluminescence camera, and (C4) PLIF camera. intensifiers on the (I1) chemiluminescence camera and (I2) PLIF camera are also depicted

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Fig. 5

Timing relations of the beginning of TTL signal (TO), laser signal, gate opening of intensifier for chemiluminescence camera (I1), gate opening of intensifier for the PLIF camera (I2), and gate opening of the schlieren camera (C2)

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Fig. 6

Depiction of edge-tracking algorithm used to obtain kernel velocities

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Fig. 7

Calibration images acquired before experiments for registration. Schlieren camera (C2) was used as reference for chemiluminescence (C3) and PLIF (C4) cameras.

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Fig. 8

Vertical velocity histories of the spark kernel, determined from the high-speed schlieren images, averaged over three successful and unsuccessful ignition events. For the A-2 fuel, the horizontal bars show the uncertainty in time after the spark discharge due to triggering of the camera.

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Fig. 9

Vertical velocity history of the spark kernels for each of the fuels tested, each averaged over three successful events

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Fig. 10

Intensity per pixel versus time for a successful ignition event and an unsuccessful event for fuel A-2

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Fig. 11

Sequence of simultaneously acquired emission (top), PLIF (middle) and schlieren (bottom) images for early times from a successful ignition event with the A-2 fuel; the event is the same used in Fig. 10. The crossflow direction is left to right in the images.

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Fig. 12

Sequence of simultaneously acquired emission (top), PLIF (middle) and schlieren (bottom) images for early times from a failed ignition event with the A-2 fuel; the event is the same used in Fig. 10

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Fig. 13

Sequence of PLIF images for later times for the successful A-2 ignition event depicted in Fig. 11

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Fig. 14

Sequence of simultaneously acquired emission (top), PLIF (middle), and schlieren (bottom) images for early times from a successful ignition event with the C-5 fuel

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Fig. 15

Sequence of simultaneously acquired emission (top), PLIF (middle), and schlieren (bottom) images for early times from a successful ignition event with the C-1 fuel

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Fig. 16

Spatially integrated and background corrected emission camera intensities for three fuels. Each data point represents the average over three successful ignition events.

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