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Sören December 12, 2010 14:03

Molecular diffusion
Hi forum

I am a rookie in CFD modelling, currently a bit confused about physics of molecular diffusion and its implementation in Phoenics. I would like to study the distribution of species concentrations and I am unsure about the setup.

In the transport equations for momentum and energy solved with the CFD code the exchange coefficients are (laminar) viscosity and (laminar) thermal conductivity, respectively (as long as the flow is laminar). Both values can be changed by the user. In my opinion the exchange coefficient for the transport equation of species concentration should be the (molecular) diffusion coefficient depending on the species' considered, total pressure and some more. However, I cannot find a possibility to enter this quantity.

Although I believe that my results wouldn't change significantly (since the flow is turbulent), the question arises if and how molecular diffusion is considered in the program...

Many thanks

phoenics_cfd January 6, 2011 10:21

Molecular diffusion
The default in PHOENICS is to model the molecular diffusion of chemical species as a simple Fickian diffusion process by means of Fick's Law. More complex diffusion models are used in PHOENICS modules suitable for multicomponent species transport in laminar-flame and CVD applications. PHOENICS's users can always introduce more complex models themselves by means of the InForm facility in the Q1 input file, or by means of FORTRAN coding in Subroutine GROUND. The latter approach was adopted in the for the laminar-flame module ( see the open source file gxchki.for ) and in the PHOENICS-CVD software.

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