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Old   February 14, 2005, 13:44
Default Flame strain-rate
  #1
flamingo
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i know that the local rate of strain (a) effects the flame characteristics, i.e. extinction etc.

but which direction of strain (e.g. S11, S22, S33, S12, S13, S23) in an LES or DNS simualation actually has the effect. is it the magnitude of all strain directions or one in particular?!?!?

basically i think i want to know the relationship between 'a' and 'Sij'.

thank you.

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Old   February 15, 2005, 03:38
Default Re: Flame strain-rate
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Tom
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Hi,

The strain-rate direction which is alinged with the scalar gradient affects the flame. Basically, turbulent strain steepens the scalar gradient (and increases heat transfer) if this strain component is negative, but it can also decrease heat transfer if it is positive.More details can be found in papers by Pumir (Phys. Fluids 1996) and Brethouwer et al. (JFM, vol. 474) I believe.

Tom
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Old   February 15, 2005, 06:09
Default Re: Flame strain-rate
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Bob.
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Hello,

How can turbulent strain increase the scalar (what is the scalar in this case?) gradient in a premixed flow? Also, why is it that large strain rate can delay burning of a flame in a shear layer for example and therefore help in the formation of a Kelvin Helmholtz instability? In essence, why does burning suppress the formation of a Kelvin Helmholtz instability?

Also, can anyone explain the darius-landau instability and where it is seen.

Thank you all.
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Old   February 15, 2005, 10:16
Default Re: Flame strain-rate
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Tom
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Hi,

In case of premixed combustion strain can increase the temperature gradient and therefore increase heat transfer. A large strain rate can thus cool down a flame and eventually quench it.

Tom
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Old   February 15, 2005, 10:33
Default Re: Flame strain-rate
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Bob.
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If, as is seen in reaction progess variable codes, the temperature is a function of the local reaction progress variable, are you saying that the fluid strain will increase the reaction progress variable gradient between burnt and unburnt gas? Surely it would have the effect of broadening the zone between C=0 and C=1 and therefore lower the gradient? I understand that an increase in gradient of temperature will increase heat removal form the flame zone, but I don't see how strain rate will increase the gradient of the reaction progress variable. It seems a sort of catch 22 situation.
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