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Ed September 15, 2003 07:40

Real gas model with UDF
Hi All,

Reading this forum encouraged me to post a question. We are a small team and have the problem to simulate the flow of Ethylene through a let-down valve at very high pressure and velocity. Our first steps were to estimate the results by using a incompressible simulation, which is in fact not the reality but we tried to get a first impression of what will happen. Now we are at a stage to calculate the whole model by using Ethylene gas as a compressible fluid and noticed out of the phase diagram (and even the fluent support confirmed us this) that we cannot us the ideal-gas equiation for this problem. So we have to implement a real-gas model by using UDF. I know that there are some real-gas models already implemented at Fluent 6 but unfortunately no Ethylen gas.

My question to all users outside no is: does anyone have already such a UDF for Ethylene gas or knows how to implement this in a short time.

The last stage for us is to write a UDF with the proper equiations by ourself but this is time consuming as well.

Thanks all in advance for your comments.


sandeep September 17, 2003 13:38

Re: Real gas model with UDF
HI Ed, Did you try the NIST model database in fluent? They might have ethylene there. or if some other gas like propylene comes close in properties, you could use that from NIST model. I am undergoing a similar situation, where in I need to model a mildly compressible pure-liquid refregerant. Fluent's UDRGM is the only option I have( using my own equations) and that is not sufficient. Any ideas what I could do?--- sandeep

Ed September 18, 2003 02:12

Re: Real gas model with UDF
Hi sandeep,

thanks for your comments. I know that fluent has included several real gas models from NIST and at the present version (we have 6.1.22) there is no ethylene included. Searching at the NIST homepage ( I found out that the have modified their number of handled fluids. There is also a new software at their homepage available for different gases and mixtures to calculate the exact conditions at a given point of the phase diagram. Unfortunately there is also no ethylene included and I don't know how long it would need before this will be adapted at a new Fluent update. But maybe you can find some useful information for your problem there. Till now it seems to me that trying to solve the problem by developing an UDRGM is the only way. But I guess that this will need a longer time and effort to do. Thanks again for your ideas and comments and good luck for your work. If I would get any further information I would post it here.


sandeep September 18, 2003 09:10

Re: Real gas model with UDF
Hi Ed, about the software you mentioned at the NIST website, can it give me states( or points on phase diag) that are in the suprecooled region. If so I can get a number of points for say constant temp and thus get the relation for d(rho)/Dp at constant Temp to be inserted in UDRGM


Ed September 18, 2003 10:42

Re: Real gas model with UDF
hi sandeep ... sorry but I don't know if the software is able to give you the proper states. I guess it would but I don't have the software. I just saw it at the mentioned homepage. Maybe you just ask there.

cu Ed

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