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Bouyancy and chemical reaction

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Old   July 2, 2002, 11:08
Default Bouyancy and chemical reaction
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Felix Nyffenegger
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Hello,

I am modelling the effects of natural convection in an open chemical reactor. So far I had nice results of the air flux which is driven by the heated walls of the reactor.

One product of the reaction is CO2 which I expect to be transported out of the reactor. The reactands are brought into the reactor with a constant flux at the bottom. Now the CO2 stays there at the bottom where it is produced and neighter bouyancy nor the airflux seem to have any effect on it.

Thank you for any hint how I could get the CO2 behave as expected.

Felix
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Old   July 4, 2002, 06:25
Default Re: Bouyancy and chemical reaction
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Greg Perkinsg
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I'm modelling something similar to this.

The nature of the natural convection and the transportation of the species will depend on the dennity gradients created either by changes in temperature or changes in concentration. Thus you have a doubly diffusive convection problem.

For relatively heavy molecules such as CO2 (vs say CO) the concentration effect makes them want to move downward due to gravity. On the other hand, if CO2 was produced by an exothermic reaction, the high temps would cause a decrease in the density and lead to them becoming more buoyant and thus moving up (as you might expect in pure natural convection).

So it looks like the temperature effect is not dominant in your runs. In this case, modelling and experiment have also shown that the stable layer of heavy gas makes the system have very low velocities and tend to be laminar, rather than the tubulent system which results from a temperature dominant mode. You'll also find hystersis effects if you look in the literature.

Please tell me what sort of reactor you're modelling

Greg

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