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natural convection problem with radiation

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Old   October 14, 2011, 09:26
Default natural convection problem with radiation
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I am not an experienced ANSYS user but I try to model a temperature sensor with extrusion/housing under laboratory conditions (no environmental impact) which has a constant temperature of 333K at the tip ( is placed on a hot plate). I model the sensor as axisymmetric in the y direction (to avoid working in 3D) and transient (to display temperature change in time). I split up the final model in three sub-models:
- Only conduction
- Conduction and radiation
- Only natural convection
- The final model combines the three sub-models and takes therefore conduction, radiation and natural convection into account
The green sub-model is completed and works fine. I try to solve the other models as well but I do have some questions about this.

Only natural convection:

I modelled only the extrusion (FLUID141 element) with a start temperature of the whole extrusion of 333K, in steady state. I did this to get a feeling with natural convection and verify my results easily. After I solved the model I made a temperature contour plot of the extrusion, it lost a certain amount of heat as I expected but I have questions about this topic:
(In the first place I put on the gravity and set the fluid prop to AIR-SI)
1) What exactly does ANSYS mean with global iterations, is this the number of iterations for the solution calculation. There are some other options as well in this interface, shall I let these on the default value?
2) Which solver should I use> I tried the pres solver but I do not think that this is the right one (You can find the solver options under FLOTRAN SETUP>CFD Solver cntls>....)
3) How can I display the flow lines of natural convection around and over my extrusion?
4) I saw a help file of ANSYS to perform a transient analysis and use radiation in FLOTRAN. I tried to use the transient functions but they were harder than those I used before in the thermal application. I first thought about to combine everything in FLOTRAN but I probably prefer to finish all the models in the thermal mode. The GUI (interface) for convection is namely the same, you have to give in the film coefficient (I calculated this one on 5.7166) and the bulk temperature (297K). Do you think this is acceptable for the natural convection or will this decrease the accuracy of the model too much?

Besides this I have a few question about radiation: (I used the radiosity solver)
1) What is the exact influence of the relaxation factor in the RADOPT solver on my simulation (modelled as transient)? What is the best value for my situation.
2) After solving the model I saw two arrows from the extrusion/housing, one at the inside of the extrusion pointing to the sensor and one at the outside of the extrusion pointing to the environment. My conclusion after seeing this is that the extrusion radiates to the environment as well as to the sensor. I am actually afraid that my sensor does not radiate (because there was no arrow). I modelled the radiation in the radiosity solver; I gave the nodes at the outside of the sensor and the nodes at the inside of the extrusion enclosure number 1. The nodes at the outside of the extrusion are marked with enclosure number 2 (I gave in the emission values as well). In the same popup menu as RADOPT, I gave the environment temperature in for both enclosures (297 as TEMPSPEC). There is also another option NODESPEC but I thought that TEMPSPEC would fit better. I am wondering if I have done this right? (SPECTEMP>297>define>1>apply>define>2>ok)
3) I do also have a question about the surrounding. I could not find information on this, is the default of the surrounding AIR-SI? I found this in FLOTRAN for natural convection but I could not find a tab like this for radiation with this solver.
4) What is the best way to verify my radiation model? I saved my form/shape factors in a file with ASCII format, I am not able to open it. I thought about to plot the heat loss in time, I know how to plot the temperature over the sensor and extrusion in time (I have done this for the conduction model) but I want to be sure that the whole assembly loses heat as well due to the radiation.

I hope that someone can help me. Many thank in advance.

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