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Old   March 16, 2009, 05:20
Default Multiphase radiation in CFX
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Lukasz
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Dear All,
I perform simulation of the coal burning boiler that heats the water flowing through the pipe that is situated across the fire (simplifying). Since the fire temperature is substancially high, the radiation has a big influence on the heat transfer to water. The water temp is about 70C so it has little influence on the heat flow in the fire volume.
The CFX manual states that multiphase radiation is a beta feature and in my edition of CFX it does not work (a notation is displayed that multiphase radiation is not permitted). Here is my question: how to perform this simulation:
a) To perform the full simulation with combustion, radiation and water flow but to avoid limitation of the radiation to single phase flows. Is there any idea how to do that? I mean for example swithing to variable composition mixture or something??? I would be very gratefull for any suggestions because I have a lot of other simulations with combustion with this problem.
b) To perform simulation of the combustion with radiation, with empty space of the wall temperatures say 120C in the place of water pipe. Export the heat transfer from output file. Perform second simulation of the water flow in the pipe only with the heat transfer to water initiated on the boundary from previous calculation.
c) To perform simulation of the combustion with radiation, with empty space of the wall temperatures say 120C in the place of water pipe. Export only RADIATION (not general) heat transfer from output file. Perform second simulation with combustion and water flow without radiation and imply the radiation heat flow to the water pipe. It has an influence on the fire volume (no radiation) and is unpredictive but has interesting feature with I dont know how to perform: how to add an "additional" flow to the boundary aside of the calculated one, during solving stage?. I mean is it possible to set up calculation of for example heat flow with an additional heat added "from nowhere" (for example constant value calculated from other factors taken in the other way). It would be very interesting.

Which option would You choose? I would be very grateful for help.

Lukasz
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Old   March 17, 2009, 00:29
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Harshad
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I would suggest first calculate heat absorbed by water considereing simple lumped calculations. Model only for flue gas flow, temp, radiation, combustion.... Radiation is allowed for coal particles in CFX. Apply heat trasfer coefficient at the pipe walls. This will take both convective and radiative heat trasfer through wall into account.
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Old   March 17, 2009, 03:34
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Lukasz
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You mean option b)?
Thanks for Your answer.

Luk
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Old   March 17, 2009, 04:20
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I would suggest heat transfer coefficient instead of specifying temperature because you can tune the heat transfer coefficient so as to get heat flux through pipe walls as calculated by lumped calculations.
I have tried similar things.
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Old   March 17, 2009, 04:41
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Ok, I understand almost everything:
1) In first calculation I start fire and radiation. I assume a heat transfer coefficient in the pipe wall (I suppose it can be taken for example from literature more or less) and anticipated water temperature.
2) In second calculation only water flows through a pipe. The heat flux (both radiative and conductive) is taken from the previous calculation. I can check if the outlet water temperature has reasonable value with assumed heat flux (from first calculation). If no, I get back to 1) and assume different water temp and/or heat transfer coeff?

Thanks You very much,
Lukasz
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Old   March 17, 2009, 04:51
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I don't know what kind of analysis you are doing. You can tune heat transfer coefficient. Please note that it does't reflect any physical situation like wall deposition.... Just use it as a tuning parameter to get necessary heat flux through wall.
If necessary,only then model for water flow. If you are designing for tube length, dia....
For combustion calculations, the wall heat flux is the only necessary part.
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