# Equation of state

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 November 29, 2005, 03:06 Equation of state #1 Ali Guest   Posts: n/a Hi all, I want to know wether there is any possibility to give an equation of state for density of a real gas or not. I saw a UDF option in density drop-down list in materials panel of Fluent V6.1. How does it work? Does it need any special format? Thanks a lot

 November 29, 2005, 04:28 Re: Equation of state #2 Viatcheslav Guest   Posts: n/a FLUENT 6.2 UDF Manual: 4.3.14 DEFINE_PROPERTY Description You can use DEFINE_PROPERTY to specify a custom material property in FLUENT for single-phase and multiphase flows. When you are writing a user-defined mixing law UDF for a mixture material, you will need to use special utilities to access species material properties. These are described below. If you want to define a custom mass diffusivity property, you must use DEFINE_DIFFUSIVITY instead of DEFINE_PROPERTY. See Section 4.3.3. Some of the properties you can customize using DEFINE_PROPERTY are: density (as a function of temperature) ...

 November 29, 2005, 06:25 Re: Equation of state #3 Ali Guest   Posts: n/a Hi Viatcheslav, Thank you for your kind attention. I saw this notion in the Fluent 6.0 UDF manual: !! If you plan to define density using a UDF, note that the solution convergence will become poor as the density variation becomes large. Specifying a compressible law (density as a function of pressure) or multiphase behavior (spatially varying density) may lead to divergence. It is recommended that you restrict the use of UDFs for density to weakly compressible flows with mild density variations. So, what i deduce from this comment is that we cant give a complete equation of state for example Benedict-Webb-Rubbin for the density of a real gas as rho = rho(P,T). Anyway, Im gonna check it out by myself.

 November 29, 2005, 14:36 Re: Equation of state #4 Viatcheslav Guest   Posts: n/a FLUENT 6.2 User's Guide: 8.15 Real Gas Models Some engineering problems involve fluids that do not behave as ideal gases. For example, high-pressure flows (e.g., the flow of a refrigerant through a compressor) cannot typically be modeled using the ideal-gas law. The real gas model allows you to solve fluid flow and heat transfer problems where the working fluid is a real gas. The FLUENT coupled solvers provide two methods for solving this type of flow: -------------------------------------------------------------------------------- 8.15.1 The NIST Real Gas Model 8.15.2 The User-Defined Real Gas Model

 November 30, 2005, 03:31 Re: Equation of state #5 Ali Guest   Posts: n/a Well, i dont have the Fluent 6.2 user's guide. I havent read the section 8.15.2 and dont know exactly which kind of user defined real gas model fluent supports! Is it a library file for gas properties or can it be a UDF cpp function?

 November 30, 2005, 04:45 Re: Equation of state #6 Viatcheslav Guest   Posts: n/a I have not understood how you could read the section 8.15.2 without having the Fluent 6.2 user's guide… Probably it is UDF cpp function that serve as interface to a library file: \$ cd /Fluent.Inc/fluent6.2.16 \$ ls client cortex cpropep help info isat KINetics lib lnx86 multiport realgas src tgrid \$ ls realgas/* realgas/lib: ammonia.fld ethane.fld Oxygen.fld R114.fld R123.fld R134a.fld R143a.fld R227ea.fld R23.fld R41.fld argon.fld isobutan.fld propane.fld R115.fld R124.fld R13.fld R14.fld R22.fld R245ca.fld RC318.fld butane.fld methane.fld propylen.fld R116.fld R125.fld R141b.fld R152a.fld R236EA.fld R245fa.fld WATER.fld CO2.fld Nitrogen.fld R113.fld R11.fld R12.fld R142b.fld R218.fld R236fa.fld R32.fld realgas/lnx86: librealgas.so realgas/src: Makefile realgas_fcns.c

 November 30, 2005, 07:15 Re: Equation of state #7 Ali Guest   Posts: n/a Thanks alot for the help.

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