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-   -   Catalyst Modelling Boundary Problem (Surface reaction) (http://www.cfd-online.com/Forums/star-ccm/121836-catalyst-modelling-boundary-problem-surface-reaction.html)

Fritz August 6, 2013 09:10

Catalyst Modelling Boundary Problem (Surface reaction)
 
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Hey guys,


i have experimental Data and I now want to simulate the process with star-ccm+. I am modelling 8 partially sliced and one whole channel of a Rh-honeycomb monolith Catalyst. length: 11 mm, channel length 795E-6m. Inlet is on the down left site of the attached pictures. To avoid turbulances before the entry of the channel there is 5mm space bevor catalyst. (on the pictures of the temperature distribution, a plane section through the middle of catalyst, from front to back was chosen)
(Partial catalytic oxidation of methan is studied)



I imported Chemkin files which include 38 surface reactions. Gas-phase reactions are neglected.
Results go into the right direction but still don't agree satisfyingly with the data i have.

An here is my problem: Analysing my Simulation I found something really weird:

Though I fixed the Catalyst surface Temperature (with the thermal specification "Temperature") to 973K (flow-inlet temperature is fixed to 873 K) the simulation gives me temperature profiles where temperatures ABOVE 973 K are accuring (depending on the Mesh i used between 20 and 10K). As u see on the attached pictures the rising temperatures start right on the top of the Catalyst and then spread flow downwards.
I am using the Trimmer and the surface remesher.



Started with 350 000 Cells (temperatures of 20K above boundary fixed temp. of 973 occur)

then tried 700 000 cells (finer mesh in the first 3mm of the catalyst - up to 10K above 973 occur )

Then tried 2.4 Mio Cells - here I put a couple of much finer layers on every Catalyst - Fluid boundary.... still 10K over.

(Within the first 3mm is the exothermic oxidation zone, Methan and Oxygen react. More downwards a couple of endothermic processes take place, which then explains why there is no rise above temp. of 973K)

I am wondering why star-ccm+ says temperatures above 973K occur, since no gas-phase reactions are taking place, boundary is fixed to be 973 and pressure differs by max. 20Pa globally.

What one can see, is that the temperature above 973K seems to be spreading more slowly towards the middle of the channels with the increasing fineness of the mesh. BUT in every case it did not matter how fine the Mesh was right on the boundary form Fluid to Catalyst, the temperature rises above 973, at least within the first 3mm, the exothermic oxidation zone.

Is my mesh the problem or do u guys have any other helpful advices??

I would be really thankful for ur help.

Best Wishes

F.Roehnert


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