Research Papers

Background-Oriented Schlieren of Fuel Jet Flapping Under Thermoacoustic Oscillations in a Sequential Combustor

[+] Author and Article Information
Markus Weilenmann

CAPS Laboratory,
Department of Mechanical and
Process Engineering,
ETHZ, Zürich 8092, Switzerland
e-mail: wemarkus@ethz.ch

Yuan Xiong

CAPS Laboratory,
Department of Mechanical and
Process Engineering,
ETHZ, Zürich 8092, Switzerland
e-mail: xiyuan@ethz.ch

Mirko Bothien

Ansaldo Energia Switzerland Ltd,
Baden 5401, Switzerland
e-mail: mirko.bothien@ansaldoenergia.com

Nicolas Noiray

CAPS Laboratory,
Department of Mechanical and
Process Engineering,
ETHZ, Zürich 8092, Switzerland
e-mail: noirayn@ethz.ch

1Corresponding authors.

Manuscript received July 16, 2018; final manuscript received July 30, 2018; published online October 17, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(1), 011030 (Oct 17, 2018) (8 pages) Paper No: GTP-18-1494; doi: 10.1115/1.4041240 History: Received July 16, 2018; Revised July 30, 2018

This study deals with thermoacoustic instabilities in a generic sequential combustor. The thermoacoustic feedback involves two flames: the perfectly premixed swirled flame anchored in the first stage and the sequential flame established downstream of the mixing section, into which secondary fuel is injected in the vitiated stream from the first stage. It is shown that the large amplitude flapping of the secondary fuel jet in the mixing section plays a key role in the thermoacoustic feedback. This evidence is brought using high-speed background-oriented Schlieren (BOS). The fuel jet flapping is induced by the intense acoustic field at the fuel injection point. It has two consequences: first, it leads to the advection of equivalence ratio oscillations toward the sequential flame; second, it modulates the residence time of the ignitable mixture in the mixing section, which periodically triggers autoignition kernels developing upstream of the chamber. In addition, the BOS images are processed to quantify the flow velocity in the mixing section and these results are validated using particle image velocimetry (PIV). This study presents a new type of thermoacoustic feedback mechanism, which is peculiar to sequential combustion systems. In addition, it demonstrates how BOS can effectively complement other diagnostic techniques that are routinely used for the study of thermoacoustic instabilities.

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Grahic Jump Location
Fig. 1

(a) Background oriented Schlieren working principle. (b) and (c) Sequential combustor comprising of a premixed (air-natural gas) swirl stabilized first-stage flame, a mixing section where secondary fuel (natural gas) is injected, and a sequential combustion chamber. Dilution air is added to the vitiated stream of the first flame to reduce the sequential stage inlet temperature and increase the oxygen fraction. High-speed BOS and chemiluminescence imaging are performed simultaneously to provide density gradients in the mixing section and sequential flame snapshots. Dynamic pressure sensors give the acoustic pressure at several locations in the combustor. (d) Table.

Grahic Jump Location
Fig. 2

Background oriented Schlieren background pattern viewed through hot flow (combustor in operation) and cold flow (ambient temperature) in the mixing section for rather open and closed aperture

Grahic Jump Location
Fig. 3

(a) Timetrace of the spatially integrated OH CHL signal recorded in the sequential chamber. (b) Power spectrum of OH CHL signal. (c) Timetrace of the acoustic pressure measurements. The probability density function of the filtered (80 Hz to 150 Hz) acoustic pressure signals is also shown on the side. (d) Power spectrum of the acoustic pressure signal. The cycles indicated by the vertical dashed line in (a) and (c) are used in Fig.4 (b) to look at the instantaneous BOS results.

Grahic Jump Location
Fig. 4

(a) Phase-locked averages of the acquired BOS signal in the mixing section and OH CHL taken in the sequential combustion chamber (b) Left column: Instantaneous snapshots of the acquired BOS signal in the mixing section and OH CHL taken in the mixing section and in the sequential combustion chamber for three cycles. Right column: Second cycle shown with more phases to emphasize the propagation of autoignition kernels.

Grahic Jump Location
Fig. 5

Structure of the fuel stream within two consecutive BOS fields and resulting velocity vectors obtained by OF algorithm. The rectangle in the top right corner of the velocity field represents the area taken into account for the calculation of the bulk velocity. To show also the finer structures that are important for BOS velocimetry, the color map of the top and middle plot is different compared to Figs. 4 and 7, where the finer structures are hidden for clarity. The color map of the axial velocity is consistent with Fig. 7. The direction of the vectors is determined by u BOS (x,t)=[u BOS(x,t)  v BOS(x,t)]T, including also the vertical component of the velocity and x being the 2D position vector.

Grahic Jump Location
Fig. 6

Top: Phase-averaged percentage of effective area containing density gradient structures that result in a sufficiently strong signal for BOS velocimetry (inside the green rectangle shown in Fig. 5) Middle: Phase-averaged (ũ bulk) and instantaneous (ubulk) values of the bulk axial velocity, obtained by BOS velocimetry and further extraction employing the algorithm described in Sec. 3.3, are shown. The phase of the recorded pressure signal was used as reference for the phase-locking. A sinusoidal fit of ũ BOS,bulk was performed for further analysis. The error bars show the phase-averaged standard deviation of the axial velocities, spatially distributed inside the green box, calculated for each instant. Two instantaneous cycles are shown in green for comparison. Lower: Comparison between phase-averaged PIV and BOS-based velocities.

Grahic Jump Location
Fig. 7

(a) Instantaneous BOS results for one cycle (times increase from top to bottom). (b) Phase-averaged BOS results. (c) Phase-averaged axial velocity component ũ BOS(x,t), obtained by BOS velocimetry. The gray rectangles indicate the area used for the extraction of the bulk axial velocity ubulk. (d) Phase-averaged axial velocity component fields ũ PIV(x,t), obtained by PIV and the extracted bulk axial velocity ũ PIV,bulk(t) that is the spatial mean of ũ PIV(x,t) within the gray rectangle. For the color map of all axial velocity fields, please refer to Fig. 5.

Grahic Jump Location
Fig. 8

(a) Integrated instantaneous OH CHL for the 2 last selected cycles indicated in Fig. 3 and pressure of the same cycles as a function of phase. (b) Convective time delay τconv as a function of the starting phase (time for a perturbation to travel from secondary fuel injection to the combustion chamber inlet) and fitted phase-averaged bulk axial velocity.



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