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chemical reaction - howto

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Old   February 21, 2000, 09:43
Default chemical reaction - howto
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Dan Sorensen
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I have a simple (it seems to me) chemical reaction scheme to setup in STARCD and run into problems.

Generally, what I want to calculate:

A ventilated room which has two inlets. One with air and a very small amount of chemical no 1 (S1) and one with air and a very small amount of chemical no 2 (S2). The reaction of S1 and S2 then creates a product (P1) at a certain rate (following the Arrhenius type).

S1 + S2 -> P1

So, I try to setup a chemical reaction scheme.

To get the stoichiometric equation correct, I define the molecular weight of S1 equal to S2 and the molecular weight of P1 to twice the molecular weight of S1.

The concentrations of S1 and S2 are much smaller than the concentration of air in the room (in the parts per billion range). So, I can neglect any changes in density, any heat generated from the reaction etc. In other words, the chemical species and the reaction does not in any way alter the flowfield.

In picture form, I try something like:

___________________________ Air + S1 ]

[ ]

[ ]

[ outlet, Air + S1 + S2 + P1

[ ]

[reaction takes some of ]

[S1 and S2 and creates P1 ]

[ ] Air + S2 ]

[__________________________

If the flowfield is steady, so is the reaction (and reaction rate).

Since I have some problems, could I please have some hints on how to setup this problem. Specifically:

- Can the Air be treated as a standard material or do I have to set it up as an active scalar with a large concentration in the inlets?

- Should S1, S2 and P1 be declared as PASSIVE or ACTIVE. They should not influence the flowfield of air in any way, but they should be included in a chemical reaction.

- I have used S1 as the leading reactant. At the inlet which injects S1, I get a concentration of S2 which is zero, but a concentration of P1 (in number of molecules) which equals S1, BUT IS NEGATIVE. At the inlet which injects S2, the concentration of S1 is zero and so are the concentration of P1 (as expected). Actually, everywhere in the domain, the concentration of P1 is less than or equal to zero? It seems that some algebraic equation determines P1?

- Although I specify that a transport equation should be used to solve for the product, STARCD alters this to an INTERNAL solution. Should this be so?

If required, I will send further info.

Regards and thanks in advance

Dan Sorensen
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Old   February 21, 2000, 10:20
Default Re: chemical reaction - howto
  #2
Dan Sorensen
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Well, due to word-wrap, my picture of the setup was mixed-up. So, here is a description:

A room with two inlets

Inlet 1: Air plus a small fraction of Reactant S1

Inlet 2: Air plus a small fraction of Reactant S2

In the room, some products is produced and thus the amount of S1 and S2 becomes smaller. At the outlet I then have some Air, S1, S2 and P1.

Regards

DAN
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