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Twophase air water flow problems by activating Wall Lubrication Force 

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November 4, 2009, 04:28 
Twophase air water flow problems by activating Wall Lubrication Force

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Torsten Thomas Betschart
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I would like to ask some questions about a airwater twophase flow problem which I have to treat for a dimploma thesis. For a short description: the task is about the simulation of the flow in a test rig of a fuel rod bundle in a nuclear power plant with rectangular rod array. For the simulations, a quarter section of one subchannel is used, means it contains a quarter rod as a wall and four straight planes as symmetry boundary conditions. Several simulatinos was carried out so far. The approach is a RANS simulation with a two fluid model. For turbulence, a Reynolds Stress Transport Model is used (this was found to be necessary for a flow containing many walls). We started to investigate the infulence of the different nondrag forces for our case. For this purpose, we set a low void fraction of 0.05 (air volume fraction of course), to simulate a dispersed bubbly flow. You can imagine, that the non wall forces are quite important for this case. The results without any nondrag forces and only lift force activated caused no problems (at least in the simulation point of view). Now, to avoid a steep void fraction concentration in the near wall regions, we tried to activate Franks Wall Lubrication force. Now, the solution divereges away just after a few iterations. For a better understanding: The following models were used for the forces:  Schiller Naumann Drag (this will be changed to Grace Drag model, which was said to be better for airwater two phase flow)  Tomiyama Lift force  Frank Wall lubrication force The initial bubble diameter was choosen 2.5mm I looked for this force in the ANSYS CFX manual, because I somehow expected problems of this nature for this flow. My question is now:  What is the correct way to avoid these convergence problems? My problem is, that I have almost no experience on this type of simulation at all. Now, three parameter can be chosen for the Frank wall lubrication model. For the first run, the default values for these coefficient were used. Do you have some experience on this issue? Do you think, that just the wall lubrication force is the reason for this behavior, or is it possible, that there are other effects, which are just amplified by activating the wall lubrication force? Do you need any further information on the problem itself? 

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November 4, 2009, 06:17 

#2  
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Glenn Horrocks
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Also the "number of walls" is no impediment to a 2equation model. And finally RSM models are of dubious benefit in multiphase models. Don't kid yourself that the extra equation gives you any extra accuracy over a normal 2equation model. Can you explain why you think you need RSM? I think you will find things much easier and probably more accurate if you go to a ke or SST turbulence model. 

November 4, 2009, 08:08 

#3  
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Torsten Thomas Betschart
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Thanks first for the reply.
For the turbulence choice, I have to cite two articles out of the Nuclear Engineering and Design journal, namely: Technical note on turbulence models for rod bundle flow computations, Gabor Hazi, Annals of Nuclear Energy 32 (2005) and CFD analysis of flow field in a triangular rod bundle, S. Toth, A Aszodi, Nuclear Engineering and Design 2008 You have to know first about the geometry and flow specification of rod bundles I think. Measurements on this topics has shown, that near the rods, swirling secondary flow motions are likely to arise, which could be responsible for mixing processes inside such a bundle (one is not certain about this topic up to now). Mixing processes are a key issue of fuel rod bundle flows. Now, Toth and Aszodi mentioned in their paper, that "the kepsilon and the SST models could not predict the secondary flow, whereas the Reynolds stress models were able to calculate them.". This has also a reason, which was treated by Hazi: about these circulating motions near the rod wall, he wrote: "There is no sign of such circulation using the kepsilon model. The question arises: Why does the computation using the kepsilon model fail to predict this phenomenon?". Furthermore, he mentioned the following: "Based on (this) hypothesis, secondary flow develops in channels when the normal Reynolds stresses are anisotropic and there is a strong gradient in the lateral mean velocity due to, e.g., wall effects. Since measurements show that both criteria are satisfied in case of rod bundles, consequently a suitable turbulence model should respect these observations". Furthermore, he stated that: "... Indeed, this model (means the standard kepsilon) is based on the turbulentviscosity hypothesis..." I will not repeat the equations here. So, furthermore: "The introduction of this term (in the equation for the Reynolds stresses using the turbulentviscosity hypothesis) implies an isotropic assumption for the normal stresses and accordingly, this model should not be applied for the problem in question." Of course, also the SST Model will fail, because it is finally based on the same hypothesis. Also previous simulation I did showed this fact. My explanations about the "many walls" was not that accurat. But, i thought that this will support people which did not need a correct physical and geometric background, but had the same problems as I have right now. Therefore: I need convergence for this problem, with the BSL (or any other Reynolds stress transport model) Reynold stress model and activated Wall Lubrication Force by Frank. OK, briefly, these are the reasons why I think I have to choose a Reynolds stress transport model. Thanks again for the answer Best regards Torsten Betschart P.S.: I am not that good in english language, therefore you have to apologize my question... You said, that Quote:
Last edited by challenger85; November 4, 2009 at 11:00. 

November 4, 2009, 20:59 

#4  
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Glenn Horrocks
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Thanks for the additional detail. I am no expert in nuclear modelling but am I correct in saying both the references you cite are for single phase flows? Yes, in single phase flows RSM models will capture flow features 2eqn models cannot.
But in turbulence modelling in multiphase flows there are far more significant influences on turbulence in the multiple phases compared to whether you use a RSM or 2eqn model. So I still think you are making things hard for yourself by using RSM models in a multiphase flow. I will be very surprised if the RSM gives you a more accurate result than a 2eqn model, and certainly the additional difficulty in convergence of a RSM model will cause problems. Quote:


November 5, 2009, 02:57 

#5 
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Torsten Thomas Betschart
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Hmm. It is true, that these papers are for single phase flows. The problem (at least I found that) is, that there were not many simulations carried out on these fuel rod bundle using two phase flow, and therefore literature is sparse. But you made a very important point to me.
I will check other sources in literature (or at least search for some more, what will be not that easy). And parallel, one could run a simulation similar to the described by using a 2eqn turbulence model. Which of these two do you think would be the better choice here, the SST komega turbulence model by menter or the kepsilon model? Thanks a lot for your explanations and your help. This was fast. I tried to set up a simulation using the SST model. The problem is now, that the simulation divereged away after approximately the same number of iterations. Therefore, the problem should be found in the wall lubrication force model more than in the turbulence model. Do you have any idea? Could the grid resolution have an influence on the behavior of the simulation? Best regards Last edited by challenger85; November 5, 2009 at 03:15. 

November 5, 2009, 06:44 

#6 
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Glenn Horrocks
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To resolve the convergence issue I would start with a simple single phase flow and hopefully that should be easy to converge. Then, using the previous simulation as an initial condition add the physics one bit at a time. First add the multiphase, then in the next simulation the wall lubrication then the lift force. Finally, if everything is behaving itself then add the RSM turbulence model and see if that improves accuracy.
I would definitely add the RSM model last as it is going to be hard to converge and you need everything else sorted before it even has a hope of converging. 

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