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-   -   constant tracer gas injection (http://www.cfd-online.com/Forums/star-ccm/97863-constant-tracer-gas-injection.html)

hiddenbunny February 27, 2012 13:46

constant tracer gas injection
 
Hi there,

The questions is related to replicating a constant injection tracer test for an indoor environment. The experiment involved injecting a tracer gas with a known constant flow rate next to a window, while the air coming into the space has variable flow rate.

I have tried to do this using multi-compnent flow in Star CCM+ but the only setting allowed at the opening boundary is either mass or molar fraction. Since the tracer gas is injected at a constant rate but the the flow rate through the opening boundary is not constant, neither method will work. I thought about writing a user function automatically adjust the fraction as the flow rate changes, but I don't see an option to link the input to any functions.

I have also tried to see whether I can write a user field function to describe this by isolating a small region near the window as the source. However, unlike what I can do with heatflux, species did not allow me to select a field function.

If anyone has a suggestion on how I might approach this different it will be greatly appreciated.

thank you.

abdul099 March 4, 2012 17:15

I assume, the tracer gas is injected through some pipes, tubes, hoses or whatever, right? In reality, the tracer gas will not occur in a magic way in the middle of the air...

So why you don't model the pipes, put a mass flow inlet there and set the volume fraction to be just the tracer gas?

hiddenbunny March 5, 2012 10:38

Thanks for the reply.

The reasons we did not model the tracer inlet are because:

1. For some too complicated to discussed reason, the model was given to us as a already completed volume mesh, which we need to use instead model it on our own.

2. We will need to do some parametric analysis for various position of the tracer releasing points, if we do remodel and add each possible position of release then the model would fill with bunch of unwanted geometry. Modeling it case by case would add possibly hundred of hours if we needed to go that route.

Due to these reasons, its for our best interest if we can change this programmatically and using existing boundaries.

as a program tho, it does not make sense to just allow partial volume heat flux input but no species input

hiddenbunny March 5, 2012 10:42

Quote:

Originally Posted by abdul099 (Post 347598)
I assume, the tracer gas is injected through some pipes, tubes, hoses or whatever, right? In reality, the tracer gas will not occur in a magic way in the middle of the air...

So why you don't model the pipes, put a mass flow inlet there and set the volume fraction to be just the tracer gas?

and also, a lot of time you DO want to have the source appear in the thin air magically to fulfill a certain condition. The simplest example is to achieve well-mixed condition of a certain concentration. Its best to release tracer into the whole volume "magically" than release a point source then model a fan to run forever to achieve the well-mixed condition.

abdul099 March 7, 2012 17:46

Fair enough. I agree there might be reasons not to model the inlet pipes. But that doesn't necessarily mean, there is a simple solution for your problem just because you WANT a solution to occur in a magic way.

So be happy there is an option you might use for your problem.

Switch the species source input to "composite". For each component you can specify a field function.

But you don't even need to split it up in several regions, you also might create a cell set and use that as a condition for your species field function. Something like $CellSet1>=0.5?1:0
That will help you a lot when the position of the tracer gas injectors will change since you don't need to split up your model again and again. Of course you still need to redefine your cell set, but that's less work than splitting the mesh.


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