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 lego November 13, 2003 07:49

Hi!

I ask for your advices. I try to calculate a radiation problem with the CFX-5.6 with Monte-Carlo model, but I think the MC model of the CFX-4.3,4 is the same. So I have heated rods in a vessel. There is dry steam of high temperature (t>1000 C) inside. The steel vessel is cooled from outside, so I think modeling it as opaque wall of 1.0 Emissivity and of some fixed temperature is OK (the heated rods have to be much hotter). But how should I model the heated rod's surfaces? Whit which type of boundary condition? I know the net heat flux from the rods to the domain. The main goal to state the rod surface temperatures in which they emit this heat flux. The problem is that there is quite close interaction between the rods because they are in a quite tight lattice, so the rods heat each other.

The geometry is something like this:

----------I*****O*****II**OOO**II*****O*****I----------

Where "-" and "I" are walls of the vessel and "O" : is one of the heated rods.

 Rami November 13, 2003 09:39

lego,

I am unfamiliar with CFX, so I can only suggest a method in general terms.

Basically, this is a two-phase in the system - solid and fluid. I assume that radiative interactions occur only between the solid surfaces (i.e., the fluid is transparent to radiation). If this is not the case, the problem is more complicated, and a participating medium should be considered. If all the surfaces optical properties are given, and assuming for the moment the solid surface temperature distribuion is also known - MC (Monte-Carlo ray-tracing) may be used to solve the absorption and emission distribution everywhere.

The fluid equations are rather standard, but some additions may be needed in the momentum and energy equations due to drag and convective HT, respectively. If there is flow, you should include a model of the drag exerted by the rods on the fluid, or use a very fine grid around each rod, with no-slip condition on the surface. You should have a convection HTC,h, so that the convection flux is given as q=h(Tsolid-Tfluid). The h should be calculated from the problem parameters (geometric arrangement, Re, etc) based on literature (e.g., Zukauskas).

Since the solid is stationary, there is no need to solve for its momentum and mass. The solid energy equation should include the same convection term in opposite sign, and may also allow for conduction within the rods if necessary. This equation may be used to solve for the solid temperature.

You should apply an iterative process for the solution of all variables and terms, then use the solid temperature for the MC to solve the radiative contribution to the solid energy equation, and continue to convergence.

I hope this helps, Rami

PS: Just of curiosity - is this problem simulating the heat transfer in a nuclear reactor rod bundle?

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