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Pargaonkar Vijay May 4, 1999 02:04

Significance of damkohler number
I am searching for the significance of damkohler number which essentially one gets if there is simultaneous chemical reactions, heat transfer and forced convection. We say that for natural convection the Rayleigh number less than 1708 signifies that the buoyancy effect is negligible. Thus what is the significance of damkohler number.

Also I am in search of such number which will throw light on as to when to consider soret effect and when to neglect it. (Soret effect is mass transfer due to temperature gradient.)

Please guide me and suggest some books or good papers which will make me conversant with this number.

Please reply. Thanking you.

Patrick Godon May 4, 1999 08:12

critical Rayleigh number
Hi, pay attention that the value of Ra=1708 is when you assume (for example) a temperature difference between two plates. This value can be different (lower, around 700) when you make a different assumption at the boundaries. PG.

Anil Shenoy May 4, 1999 13:41

Re: Significance of damkohler number
Hello Vijay,

A detailed description of Damkohlers number can be found in (1) Combustion theory - Willams F. A. (2) Fundamentals of Combustion- Linan (3) Chemistry problems in combustion (something like that)-

S. Penner Also a recent paper by Katta, V. et. al in combustion and flame/combustion Symposium discusses Soret effects in hydrogen flames.

There is another good paper by Penner, something like "Similarity Parameters in..." was published in a Combustion Symposium couple of years back. I will try to find the exact title.

My understanding is that Damkohlers number (Da)can be defined as factor that determines if the reaction rate is chemical controlled or diffusion controlled.

Da = (mixing time scale)/(chemical reaction time scale)

(a) in premixed combustion mixing time scale << chemical reaction time scale. Da tends to 0. So in premixed combustion rate of heat generated depends on the chemical time scale.

(b) similarly in non-premixed combustion reaction mixing time scale >> chemical reaction time scale. Da tends to infinity. So non-premixed combustion is diffusion controlled.

According to Willams (see combustion theory) Soret effect has little influence on diffusion flames. But it was been reported by Katta et. all that it does have an significant effect.


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