Learning Forum

[Tazin Ahmed Adri]

Hello,

I have a model which consists of a cylinder, and a pellet inside a cylinder, with velocity prescribed in the inlet and pressure described in the outlet. In this model, I am describing a function on the pellet. In the result, I see there is no water generated. But, the moler concentration of h2o shows it is around 40 kg/(m^2*s). Later, I also tried using 40 kg/(m^2*s) on the pellet.  Then I get the following result. Why is that?

 

Regards,

Tazin

 

 

 

 

[jcooper]

Hi Tazin:

 

I can't see your overall setup, but since the expression didn't work and setting  40 [kg m^-2 s^-1] directly did, I conclude that there is something wrong with the molConH2 expression (Why is this called molConH2 and not MolConH2o?  Maybe you have one for H2 and applied it to the water by mistake?  This could produce the sort of inconsistent results you are seeing.)

 

Best Regards,

 

Judy

[Tazin Ahmed Adri]

Hello,

This is the expression that I used on the pellet. This is the only expression I have in my model.

 

Regards,

Tazin

[jcooper]

Your expression sets the incoming h2o molconc to twice the value of h2 molconc in the presence of a negative h2 molconc flux. This can run into problems, since there is no check on conservation before the value is set.  These text widgets are intended for simple fluxes only.  If you have conversion of h2 to h2o occurring, you should add a chemical reaction or source terms, so that things get linearized and accounted properly.

 

[oscar.banos_garcia]

Dear Ms. Cooper,

Ms. Adri left the project. I hope you do not mind, if I make a few questions.

Firstly, we have already tried source terms. But this option is available for the volume, if I am not mistaken. I thought that the equivalent on the surface would be to define a fixed flux. As you mention, we found this option confusing because the mass conservation is not verified, i.e. there is no field for the source or sink of total mass, like the one found in the volume.

I will try to learn about implementing the reaction, but I find the definition of sources and sinks more flexible. Hence, I would like to understand what do you mean by "simple fluxes".

Thank you four your help.

[jcooper]

Hi Oscar:

A  simple flux is something with a numerical value.  Fluxes that introduce dependencies on other species are unsupported because linearisation is required to keep these kinds of specifications from resulting in overconsumption/overshoot of species.

 

You are right that Fluent sources are volumetric only.  A surface source can be emulated by adding inflation to that surface and then splitting one or more of the inflation layers off to create to a new cell zone in Fluent.

To assign an inflation layer to a new zone in Fluent, you can follow the steps outlined below:

 If you are dealing with a constant number of elements in the inflation layer, you can use the following method:

1. Go to Adapt > Boundary > Option = Cell Distance > Number of Cells = inflation layers > Boundary Zone = [wall boundary with inflation] > Mark
2.  Then proceed to Mesh > Separate > Cells > Mark > Registers = Marked Zone > Separate.

 

 

 

 

You should get a message like:

 

And possibly others, depending on what that wall is connected to.

This process will allow you to separate the inflation layer into a separate cell zone, which will allow you to apply a volumetric source/sink.

Note that for a source of H2o based on H2 concentration (or mass fraction), ie: 0.001*molconH2, there MUST be a corresponding equal and opposite "source" of H2 -0.001*molconH2. (This was not the case in your implementation.)  Note that species source terms may have to be linearized if the source depends on the mass fraction of the specie it is being applied to.  Linearization will require the source term to be described via UDF as in this example for momentum source terms:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v241/en/flu_udf/flu_udf_DetailedExampleUDFs.html%23flu_udf_udf_mom_source

The source linearization coefficient is typically the derivative of the source term w.r.t. the solved specie.

 

Best Regards,

Judy