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SUMMARY:Global shear instabilities in flames and their effect on the respo
 nse to acoustic forcing. - Matthew Juniper\, CUED
DTSTART:20080926T124500Z
DTEND:20080926T134500Z
UID:TALK13781@talks.cam.ac.uk
CONTACT:Dr C. P.  Caulfield
DESCRIPTION:In industrial applications\, such as gas turbine combustion ch
 ambers\, global shear instabilities can be exploited to generate good mixi
 ng. These instabilities lead to large scale vortical structures\, secondar
 y instabilities and high turbulent kinetic energy. In this study\, we ask 
 whether global shear instabilities can also be used to disrupt global ther
 mo-acoustic instabilities. An internal combustion system consists of one o
 r more burners inside a combustion chamber. Thermo-acoustic instabilities 
 arise when pressure perturbations in the chamber are amplified by the burn
 er and cause the frequency of heat release to be in phase with one of the 
 acoustic eigenmodes of the chamber.  It is often said that global shear in
 stabilities at one frequency are insensitive to external forcing at other 
 frequencies\, except at very high forcing amplitudes. If this is the case\
 , then it should be possible to weaken the feedback loop that causes combu
 stion instability by designing a burner to have a natural frequency well a
 way from the acoustic eigenmodes of the chamber. In this experimental stud
 y\, we examine acoustically-forced buoyant jet diffusion flames. In these 
 flames\, a global shear instability can be turned on and off by careful ad
 justment of the composition of the fuel. We find that a flow that has a gl
 obal shear instability is not\, in fact\, insensitive to external forcing 
 at other frequencies. At all forcing amplitudes\, the natural and forced m
 ode co-exist. The growth rate of the forced mode\, however\, is lower in t
 he flame that contains a global shear instability. Furthermore\, with non-
 linear development\, the frequency of heat release will be determined by t
 he frequency of the mode that rolls up first. There is some evidence from 
 real combustion systems that this process reduces the amplitude of thermo-
 acoustic instabilities. 
LOCATION:Open Plan Area\, BP Institute\, Madingley Rise CB3 0EZ
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