Flammability limit
 

Hello,

I simulate a Butan leckage in a room and I use Butan/Air as the material in domains with different mass fraction. I would like to know, how can I show in cfx Post the Flammability range of my gas. I know the Butan Flammability limit is between 1.6 to 8.4 % .

Exactly I say I would like to know which parameter such as Mass Fraction, Molar Fraction, Mass Concentration and etc shows in an Isosurface the percentage of Flammability limit .

regards,

This depends on how the percentages for the Flammability limits are defined. Better look it up.
However, my first guess is that it would be volume percentage i.e. Molar fraction as long as ideal gas law is applicable.

Hi,

I think the percentages are the volume percentage. So it means I can use the Molar fraction. Isn´t it?

bests,

Molar Fraction is equal to Volume Fraction for ideal gas, so for this purpose, they are the same. If you modeled the gases as ideal gases, they are the exact same. Flammability limits are normally listed as volume fraction, probably just because VolFrac is more commonly used than MolFrac. For non-ideal cases, it is likely the mole fraction is the true variable of interest, since this is chemistry. So listing them as volume fraction is the simplification, and valid since they are essentially the same except for very specific situations, like a gas near its critical point.

Hello again:)

I would like to be sure, when we talk about a range of Molar Fraction from 0 to 1 in Cfd-Post, It means from 0 to 100 percent of Volume fraction. Isn´t it?

wishes,

Yes. As long as they are ideal gasses, this is true. As an engineer, you can assume that this is the case at room temperature and atmospheric pressure.

As a scientist in chemical engineering, I have some remarks.
At low molar fractions butane, you have almost pure air. This can be treated as ideal gas at STP.
At high molar fractions, you have almost pure Butane. I'm not fully sure if 100% Butane can be treated as idaal gas. The Butane molecule is not too large and not too complicated. But there are two types of Butane, making it a possible mixture of two components. Then things might become tricky. Molecualr interactions might be ideal, may be not. I don't know if this is relevant for your case. I would say that in engineering applications around explosion limits, this is irrelevant. But you really need to find out yourself.