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vidyaraja May 25, 2009 15:24

Source Terms in Momentum Balance

I am trying to model a packed bed (a cylinder containing 1 mm diamter particles). I am also trying to compare the values for velocity as calculated by Fluent to that calculated by hand using the Ergun's Equation for packed beds (porous media).

I first ran a trial case wherein a packed bed containing the 1mm dia particles was subject to a 2 psi pressure drop. I calculated the velocity using the Ergun's equations and this matched perfectly well to that calculated by Fluent (I asked Fluent to compute the velocity over the inlet, outlet, and porous zones and these values matched up with those
calculated by hand).

Now, I am trying to modify the Ergun Equation by introducing a source term and seeing if Fluent's calculations match up to the hand calculations for this case. I wrote up a user-defined code to describe this and hooked it up to Fluent and find that the
calculations are not the same. I am not sure if I am correctly interpreting the way that Fluent uses the Ergun's equation and so if my source code would be correctly written to
describe this. Does Fluent calculate the deltaP/L value directly from the Ergun's equation and display this value when asked or does it calculate a velocity field and return this value? Say I have a source term in the form av + bv^2 -c where a,b,c are known constants and we need Fluent to calculate the values of the velocity v at various points (inlet, outlet, etc). Do we write the whole Ergun Equation in the code and then subtract out the source terms and the "c" term? I believe the Fluent documentation says that +ve terms are source terms and -ve terms are sinks.

How does Fluent calculate the velocity field? Does it return a value for either the velocity or the source term at any point/instant?

I can send the sample code that I have written if anyone needs to take a look. Any help will be appreciated.

Thank you


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