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Old   April 23, 2003, 08:35
Default source problem
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John
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Hi: I want to calculate the toxic gas pollution problem in a room. For example, there is toxic gas source in the room and the concentration of toxic gas in the entire room will varie with time increases. How can we defined the pollution source? How can we calculate the mass transfer problem? what is the meaning of parameter 'C1'?

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Old   April 24, 2003, 12:55
Default Re: source problem
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John Ludwig, CHAM
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The following extract describing how to model smoke sources is taken from the FAQ pages of the CHAM Support web pages. If you do not have access to the Support pages but would like it, please contact CHAM User Support at support@cham.co.uk and they will tell you how to get to them.

Question: - How do I model smoke movement?

Answer:

The PHOENICS scalar variable C1 (or alternatively SMOK in FLAIR) is solved to represent the smoke concentration.

The obvious ways to introduce a fire are:

to have a blockage of the fluid material, usually air, with a heat release and associated source of smoke

to have an inlet of fluid, carrying with it a smoke concentration and an appropriate temperature (or having a fixed flux of heat instead for any combustion release).

In either case, the units for the C1 variable are to some extent up to the user. The conserved quantity is C1*density, so a natural choice is C1 = mass fraction of smoke, with units of kg/kg, but this is not essential: 'anything'/kg will do, provided that all sources are consistently treated.

Many users choose the first modelling approach above, and use the 'Fixed Value' setting to specify the value of C1 to 1.0 in the fire: C1 values elsewhere therefore give the smoke 'density' (smoke/kg) relative to that in the fire. An alternative is to use the 'Fixed Flux' setting to specify a rate of smoke creation, either for the whole fire or per unit volume. In this way the C1 value can represent a variable for which a creation rate is known (although the absence of a mass source makes this somewhat artificial). Of course, if there are several fires they can have different sources to represent their different behaviour. The C1 value elsewhere in the domain will then represent the cumulative effect of the different contributions. (If necessary, different scalars can be used for the smoke from each fire, say C2, C3 etc: in that way, it is possible to tell how much each one contributes in different locations.)

In the same way, with the second approach the value of C1 carried into the domain by the fluid (combustion product) is often set to 1.0; in that case, C1 field values should be interpreted relative to that (possibly arbitrary) value. Alternatively, a more realistic value can be set so that 'mass inflow rate times incoming C1 value' represents the mass flowrate of smoke created by the fire. (Note that the value of C1 in the fire will be rather lower than the incoming value, because the smoke will be diluted by the air in the domain as soon as it appears.)

If C1 is defined as a mass fraction (units of kg of smoke per kg of gas mixture), it is possible to derive the local smoke density (units of kg of smoke per unit volume) by multiplying C1 by the fluid density. In general, this requires storage of the density (DEN1) and another concentration variable (say CSMO); then, in Group 19 Section 6 of GROUND, the smoke density is calculated by:

CALL FN21(LBNAME('CSMO'),C1,DEN1,0.0,1.0) Alternatively the PLANT or INFORM options of PHOENICS can be used instead of using FORTRAN coding in GROUND.Thus, with INFORM, the following PIL statement in the Q1 file achieves the desired result:

(STORED VAR CSMO IS DEN1*C1)
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