Learning Forum

[Ric_Rin_DTU]

Good morning,

Once again, I will disturb you with my CFD model for a packed bed reactor.

 

I am implementing a CFD model for a packed bed reactor (PBR) for methanol production and comparing it with the performances predicted by a simpler 2D model.

When I switch the chemical reactions off, the two systems perform basically in an identical way. In this case, the chemical reactor behaves as a sort of heat exchanger, where the inlet mixture is progressively heated up by the medium that surrounds the reactor tube.

 Then, when I try to add the chemical reactions to the CFD model, the behaviour of the system is completely different from what is normally expected from a chemical reactor (and from my 2D model). I start by saying that the chemical reactions implemented in the CFD model are correct and identical to the ones I have used in my simplified 2D model (I have checked them many times with the "Evaluate" tab), so the error should not lie there.

More in detail, even if I switch the chemical reactions on but I do not introduce any Energy "Source" in the model, the system undergoes strong increases in the temperature profiles, as if heat were produced by the chemical reactions. Moreover, such heat production is not compatible with the involved reactions, as it is definitely too high (3 / 4 or even more times bigger than what expected). Two questions arise at this point:

1) Is CFX capable of predicting the reaction heat once chemical reactions are provided and hence there is no need to specify it? In other words, when I select the material "Sources" option in my subdomain (i.e., species formation / consumption due to the reaction), there is no further need to tell the program the corresponding variation of heat?

2) Even if the answer to the first question is positive, why is CFX predicting such a high value for the reaction heat? 

Have a nice day,

Riccardo

[rfblumen]

An example of a reacting flow setup is found in CFX Tutorial 15 (Reacting Flow in a Mixing Tube). In this tutorial, the energy source is specified through a subdomain which comprises the entire domain (this approach is typically how volumetric sources are modeled in CFX). This energy source involves the heat of reaction and the mass fraction of the species. If the energy source within this subdomain is turned off, there is no temperature increase in the model due to the reaction.
In your model, based on the information provided, it isn't clear why you're seeing a temperature increase if the energy source is turned off. In your out file, I would look at the "Boundary Flow and Total Source Term Summary" found at the end of the CFX out file. Under "H-Energy", you should see the source for the energy increase that's resulting in the temperature increase.