what is the diffrence between volume of fraction in patch section and porosity in boundary condition(fluid zone for mixture)?it means what should i do for defining of a solid bed with voidage=0.5? my case is fluidized bed. regards

I think you will find that porosity is for a fixed bed. To simulate a fluidised bed, patch the volume fraction - however depending on the conditions you may find that it settles out before becoming fluidised. Unless you are modelling startup of the settled bed it may not be wise to patch the bed with solid at 0.5 fraction.

I have been modelling a fluidised catalytic cracker which may be similar to your fluidised bed. I found better convergance by starting with an empty vessel and allowing the solid to enter with the gas through the inlet. It takes some time to reach a 'steady state' but diverged less than when I patched the bed initially.

Good luck,

Owen

Dear Owen! i dont know yet that,if i patch vof of solid and determine partocle diameter,is it need to define porosity in fluid zone for solid or no?i want to simulate batch fluidized bed(not continious) ,then i cant allow particles to enter with gas! regards

Woah, long question Masood. I'll attempt to answer it for you.

For Eulerian multiphase model you can specify your catalyst properties under Define_Phases. Make phase 2 your catalyst, check the 'Granular' box, then you can set the particle diameter here .

Now you can patch a region with solid particles to the solid fraction that you wish - depending on how much the particles settle or are fluidised in operation, will determine the porosity of your bed.

The maximum packing factor for the solid will determine the maximum solid fraction you can patch initially. This is set under Define_phases_phase 2. It has a default value of 0.63.

One last point if the bulk of your bed is stationary there is an option for Fixed Bed under phase options which you should read about in the documentation and decide if it's suitable for your problem.

Cheers,

Owen