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daveleo August 11, 2000 08:05

impingement cooling
 
i need to analyze the impingement cooling flow over a surface. that is, the surface is being cooled by jets of gas that impinge on it, then the gas flows over the surface to the exit. i am looking to map the streamlines of the post impingement flow, and (if possible) calculate the heat transfer that occurs.

PHOENICS looks like a great affordable code..... but can it model this type of problem accurately??

PS... i would be looking at the level 5 shareware version (the commercial 3.3 is out of my reach at this point)

regards,

daveleo

John C. Chien August 13, 2000 20:11

Re: impingement cooling
 
(1). You have two choices, one is yes, the other is no. (2). Why would people develop a code which can not solve your problem accurately? (3). I would give it a try, if you have the time to do so. (4). By the way, I think, "accurately" is easy to understand, but it is not very scientific. You need to define the standard solution first, then define the accuracy next. Without these two steps, it is hard to measure the accuracy. Even if such standard test data exist, the accuracy of the solution can not exceed the accuracy of the test data. Then I guess, you also need the accuracy of the test data also. (5). I would say that the geometry is very simple, and the solution is not complicated. If you have the right turbulence model, you can do it.(I am assuming that jet impingement is always turbulent.) good luck.

Mike Malin August 15, 2000 07:53

Re: impingement cooling
 
PHOENICS can simulate this type of problem, and as John Chien advises, the quality of the solution depends to a large extent on the turbulence model.

The commercial releases of PHOENICS contain ready-made input files which use several turbulence models to simulate the normal impingement of a turbulent round jet on a heated plate, as studied experimentally and numerically by many workers. These files use various high- and low-Reynolds-number forms of the k-e model including: the Yap k-e model, the 2-layer Rodi model, the Lam-Bremhorst k-e, and the Wilcox-Kolmogorov model. I can't recall which of these fares best, but convergence of the low-Re models is much more demanding. The main predictive problem with this case is to obtain quantitative agreement with measured heat transfer rates in the impingement zone. Away from this region the agreement is rather good.

However, the shareware version does not include these input files, nor a large range of turbulence models, these being restricted to the standard high-Reynolds-number forms of k-Lm and k-e models. The flexibility of the computer code is such that the user can add his own modifications and/or alternatives through the facilities provided by PHOENICS for user-generated FORTRAN coding sequences, namely: the GROUND & PLANT modules.

If you use high-Re turbulence models, then non-equilibrium wall functions (Launder & Spalding) should be used rather than equilibrium ones which presume that turbulent production is balanced by dissipation in the near wall cell. I cannot recall if these are in the Shareware version, but in PHOENICS jargon they are called GRND3 wall functions rather than GRND2 (equilibrium)wall functions.

CHAM and some other CFD users and vendors participated in a turbulence validation exercise at the CFD95 Conference in Canada. The results were reported by Pollard et al (1996), CFDSC/V/95-3, Queens University(1996). Jet impingement with heat transfer has had a lot of attention in the CFD literature so it should be possible to find lots of information on relevant turbulence modelling, e.g. Imperial College, UMIST, McGill University have all published lots of papers. The 2nd ERCOFTAC-IAHR Workshop on Refined-Flow modelling also considered impinging jets.

Finally note that the mesh will have to concentrated towards the plate in a careful manner so as to capture the wall jet emanating from the impingement region.

A.Hassaneen August 27, 2000 09:09

Re: impingement cooling
 
Yes, the meshing of the impingement area is very important. The other problem is that the velocity of the impinging gas reach zero or almost as it impinge into the surface and then start to bulid-up again. That will raise so many questions about the turbulence model in such complicated conditions (because of the low Re in this area).

pk mukhopadhyay November 9, 2000 22:22

Re: impingement cooling
 
viewed the items on impingement modeling only today ! will someone initiate detailed discussion on: (1)flow separation leading to stagnancy zone - Falkner's proposition ? (2)loss of eddies - hence energies (3)validity of turbulence models particularly k-epsilon model in such region : for heat transfer - what special wall treatment then ? especially if it is a low Pr fluid ?

Ahmed Hassaneen November 11, 2000 03:35

Re: impingement cooling
 
I think it's a good idea to post your questions as a new message in the main forum because normally users read recent messages.

pk mukhopadhyay November 20, 2000 00:25

Re: impingement cooling
 
Ahmed Hassaneen . Thanks. Since done.


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