Azimuthal flame response and symmetry breaking in a forced annular combustor

Rotationally symmetric annular combustors are of practical importance because they generically resemble combustion chambers in gas turbines, in which thermoacoustically driven oscillations are a major concern. We focus on azimuthal thermoacoustic oscillations and model the fluctuating heat release rate as being dependent only on the local pressure in the combustion chamber. We study the dynamics of the annular combustor with a finite number of compact flames equispaced around the annulus, and characterize the flames’ response with a describing function. We discuss the existence, amplitude and the stability of standing and spinning waves, as a function of: (i) the number of the burners; (ii) the acoustic damping in the chamber; (iii) the flame response. We present the implications for industrial applications and the future direction of investigations. We then present as an example the first theoretical study of thermoacoustic triggering in annular combustors, which shows that rotationally symmetric annular chambers that are thermoacoustically unstable do not experience only stable spinning solutions, but can also experience stable standing solutions. We finally test the theory on one experiment with good agreement.

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Low-Order Network Modeling for Annular Combustors Exhibiting Longitudinal and Circumferential Modes

Volume 4B: Combustion, Fuels, and Emissions ◽

10.1115/gt2018-76506 ◽

2018 ◽

Cited By ~ 1

Author(s):

Dong Yang ◽

Aimee S. Morgans

Keyword(s):

Boundary Conditions ◽

Acoustic Waves ◽

Acoustic Boundary Conditions ◽

Thermoacoustic Instability ◽

Longitudinal Modes ◽

Before And After ◽

Flame Response ◽

Annular Combustor ◽

Set Up ◽

Low Order

Modern gas turbine combustors often have annular geometries. These are able to sustain thermoacoustic modes which vary in both the longitudinal and circumferential directions. Effects such as nonlinearity of the flame response to perturbations and differing burners around the annulus lead to the coupling of acoustic modes with different circumferential mode numbers. Such coupling renders differing spatial patterns of instability possible — for example purely longitudinal modes, circumferential standing modes, circumferential spinning modes, mixed modes and slanted modes. Accurately predicting the spatial pattern of limit cycle oscillations resulting from thermoacoustic instability remains an open challenge. This work develops a frequency domain low-order thermoacoustic network model for annular combustors which is notable in (i) accounting for both longitudinal and circumferential modes and (ii) allowing for generic acoustic boundary conditions at either end of the network. Linear acoustic waves are considered, with the different circumferential wavenumbers decoupled for sections both before and after the flames. Modal coupling occurs only at the flames, and is accounted for by summing all modal contributions prior to application of the flame models, and decomposing back into circumferential modes after application of flow conservation equations across the flames. By applying acoustic boundary conditions at either end of the network, an eigenvalue system is established which allows the thermoacoustic modes of the whole combustion system to be analysed. This low order modelling approach is applied to a simplified annular combustor set-up and is demonstrated to be able to capture limit cycles exhibiting longitudinal modes, circumferential spinning modes, circumferential standing modes and even the recently identified slanted modes.

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Symmetry breaking in convergent-beam diffraction

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100154378 ◽

1989 ◽

Vol 47 ◽

pp. 480-481

Author(s):

D.J. Eaglesham

Keyword(s):

Symmetry Breaking ◽

Point Group ◽

X Ray Diffraction ◽

Multiple Diffraction ◽

Space Groups ◽

Atomic Displacements ◽

Small Probe ◽

Phase Sensitive ◽

Convergent Beam

Convergent Beam Electron Diffraction is now almost routinely used in the determination of the point- and space-groups of crystalline samples. In addition to its small-probe capability, CBED is also postulated to be more sensitive than X-ray diffraction in determining crystal symmetries. Multiple diffraction is phase-sensitive, so that the distinction between centro- and non-centro-symmetric space groups should be trivial in CBED: in addition, the stronger scattering of electrons may give a general increase in sensitivity to small atomic displacements. However, the sensitivity of CBED symmetry to the crystal point group has rarely been quantified, and CBED is also subject to symmetry-breaking due to local strains and inhomogeneities. The purpose of this paper is to classify the various types of symmetry-breaking, present calculations of the sensitivity, and illustrate symmetry-breaking by surface strains.CBED symmetry determinations usually proceed by determining the diffraction group along various zone axes, and hence finding the point group. The diffraction group can be found using either the intensity distribution in the discs

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Centrosome Aurora A gradient ensures single polarity axis in C. elegans embryos

Biochemical Society Transactions ◽

10.1042/bst20200298 ◽

2020 ◽

Vol 48 (3) ◽

pp. 1243-1253 ◽

Cited By ~ 1

Author(s):

Sukriti Kapoor ◽

Sachin Kotak

Keyword(s):

Symmetry Breaking ◽

Cell Fate ◽

Cell Cortex ◽

Axis Formation ◽

Aurora A Kinase ◽

Aurora A ◽

C Elegans ◽

Cell Embryo ◽

Polarity Establishment

Cellular asymmetries are vital for generating cell fate diversity during development and in stem cells. In the newly fertilized Caenorhabditis elegans embryo, centrosomes are responsible for polarity establishment, i.e. anterior–posterior body axis formation. The signal for polarity originates from the centrosomes and is transmitted to the cell cortex, where it disassembles the actomyosin network. This event leads to symmetry breaking and the establishment of distinct domains of evolutionarily conserved PAR proteins. However, the identity of an essential component that localizes to the centrosomes and promotes symmetry breaking was unknown. Recent work has uncovered that the loss of Aurora A kinase (AIR-1 in C. elegans and hereafter referred to as Aurora A) in the one-cell embryo disrupts stereotypical actomyosin-based cortical flows that occur at the time of polarity establishment. This misregulation of actomyosin flow dynamics results in the occurrence of two polarity axes. Notably, the role of Aurora A in ensuring a single polarity axis is independent of its well-established function in centrosome maturation. The mechanism by which Aurora A directs symmetry breaking is likely through direct regulation of Rho-dependent contractility. In this mini-review, we will discuss the unconventional role of Aurora A kinase in polarity establishment in C. elegans embryos and propose a refined model of centrosome-dependent symmetry breaking.

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