burning surface

Scat · February 28, 2013, 07:53

I’d like to simulate a room fire with CFX. So that my combustible surface starts to burn, I decided to create a wall boundary with a source condition that has a mass flow of combustible gas. My mass flow begins when the wall boundary has a specific average temperature. For that, I use a known CEL function “areaAve(T)@...” .
Everything works. I have a combustion – so far so good.

But, if you have large surfaces, like a carpet, it isn’t realistic that the whole surface starts to outlet a combustible gas overall.

Therefore, my idea was to create a mass flow expression for every mesh cell on my boundary. That means, if one cell has a specific temperature the area-related mass flow starts for this cell. Unfortunately I found no kind of CEL function that allow me to access a mesh cell of my boundary and nothing in the documentation that help me to realize my idea.

Maybe you have a kind of idea to solve my problem. Thanks at all.

ghorrocks · February 28, 2013, 16:42

In CEL you do not have access to cell information, you just write the equation as a continuum and let CFX take care of it.

So a simple function like if(T>threshold,source_term=something,source_term=0 ) should do the trick.

Scat · March 1, 2013, 08:05

Thanks for response.
An IF – function was also an idea of mine. But it still the same problem, because I have also a fixed value for the source term (e.g. kg/s) above my whole boundary (wall).

What I need is a function or a command that says my boundary when you’ve got a specific temperature in one cell or at a special point on this boundary, you have to start with the outflow only in this point/cell . And when another region of the boundary reaches this temperature it has to begin with the same outflow.

Maybe CFX has no solution for this kind of special simulation.

ghorrocks · March 3, 2013, 05:15

I do not see your point. I cannot see why you think the simple equation I quoted will not work. Can you explain it again?

Scat · March 4, 2013, 04:52

My bad. Sorry.
I tried some other simulations with a bigger surface, so I had a better temperature profile on my surface. The result was a mass outflow of every surface mesh cell which reaches the specific temperature. So your if-function was the right way and I thank you very much for your help.

February 28, 2013, 07:53	burning surface	#1
Scat New Member Join Date: Feb 2013 Posts: 4 Rep Power: 13	I’d like to simulate a room fire with CFX. So that my combustible surface starts to burn, I decided to create a wall boundary with a source condition that has a mass flow of combustible gas. My mass flow begins when the wall boundary has a specific average temperature. For that, I use a known CEL function “areaAve(T)@...” . Everything works. I have a combustion – so far so good. But, if you have large surfaces, like a carpet, it isn’t realistic that the whole surface starts to outlet a combustible gas overall. Therefore, my idea was to create a mass flow expression for every mesh cell on my boundary. That means, if one cell has a specific temperature the area-related mass flow starts for this cell. Unfortunately I found no kind of CEL function that allow me to access a mesh cell of my boundary and nothing in the documentation that help me to realize my idea. Maybe you have a kind of idea to solve my problem. Thanks at all.

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February 28, 2013, 16:42		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,715 Rep Power: 143	In CEL you do not have access to cell information, you just write the equation as a continuum and let CFX take care of it. So a simple function like if(T>threshold,source_term=something,source_term=0 ) should do the trick.

March 1, 2013, 08:05		#3
Scat New Member Join Date: Feb 2013 Posts: 4 Rep Power: 13	Thanks for response. An IF – function was also an idea of mine. But it still the same problem, because I have also a fixed value for the source term (e.g. kg/s) above my whole boundary (wall). What I need is a function or a command that says my boundary when you’ve got a specific temperature in one cell or at a special point on this boundary, you have to start with the outflow only in this point/cell . And when another region of the boundary reaches this temperature it has to begin with the same outflow. Maybe CFX has no solution for this kind of special simulation.

March 3, 2013, 05:15		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,715 Rep Power: 143	I do not see your point. I cannot see why you think the simple equation I quoted will not work. Can you explain it again?

March 4, 2013, 04:52		#5
Scat New Member Join Date: Feb 2013 Posts: 4 Rep Power: 13	My bad. Sorry. I tried some other simulations with a bigger surface, so I had a better temperature profile on my surface. The result was a mass outflow of every surface mesh cell which reaches the specific temperature. So your if-function was the right way and I thank you very much for your help.