Heat transfer analysis and gamma heating with ABAQUS

[Narel]

Dear all,

I have some problems with heat transfer analysis, where the heat is generated with gamma heating. I have modelled the gamma heating in Abaqus using "Body heat flux" to an axisymmetric model (in 2D).The gamma heating is assumed to be instantaneous and the given volume of the gamma heating is for example 4 W/g (watts/gram). In Abaqus the body heat flux unit is W/m^3. I have converted the 4 W/g into units W/m^3 by multiplying the given volume (4 W/g) with density, when the grams will be reduced.

My problem is the heat transfer in the system itself. The radial structure of the system is quite complicated. I have a tube which has inside the gamma heated piece (steel component). The piece is sinked into the coolant. Out side of this tube is a helium layer, which works as a thermal barrier with the outer tube. In other words there is two tubes and between them is a helium layer. The helium is not flowing and is thus modelled as solid elements. As a boundary condition, I have set for the external surface of the outer tube a constant temperature of 45 celcius. This is the case for the temperature in the reactor pool water.

I have also taken into account the radiation in the helium layer. I have modelled it as "Cavity radiation" setting emissivity of 0.8 for the internal surfaces of the two tubes enclosing the helium.

The maximum diameter of the whole device is about 30mm, so the device with all the components is not that big and the reactor water outside should provide quite good cooling for the system. The problem is now too high temperatures in the device itself. In due to device geometry, which is very complicated, every part can not be modelled precisely and that is why many simplifications had to be made.

Can this be a reason for the too high temperatures, because the device (steel component) contains many slots and gaps (which are very small though), where the coolant can access? These slots and gaps can not be modelled into axisymmectric 2D-model. Another reason for the high temperatures can be the helium layer, which is too thick (t=2,8mm) and that is why the heat can not conduct from the system in due to very small conductivity of helium (0,2 W/mK). Any suggestion how to reduce the temperature in the steel component and improve the conduction in the system using ABAQUS?


Thank you to all for answers in advance

Best regards, Narel

 

[gwolf2]

Narel,

From what you say it sounds like you mostly know what you are doing and mostly understand why your results are not very good. I have done some small scale heat transfer models - very detailed - and have the following comments.

1) I think that your assumption regarding Helium conductivity needs to be checked. I am assuming that you do not have a forced flow - is this true. Assuming no flow I recommend that you perform a simple natural convection calculation. The resulting heat transfer coefficients can easily multiply your effective conductivity by 1.5 to 3x.

2) It sounds like you do not have all of the surfaces modelled and this will reduce the ammount of heat flow. Is it time for a 3D model?

3) As you are using an axisymmetric model, are you sure that you have got all of your boundary condition units corrrect? It is worth a ckeck.

4) Have you got your emissivites correct? The radiation heat flow between facing surfaces is very sensitive to just one of the surfaces having a low emissivity.

gwolf

 

[Narel]

Thank you for your answer gwolf. You are right, I have to check my assumption regarding helium conductivity. In this case there is no forced flow of helium. That´s why I might have to take into account a "cellural flow" between the two tubes enclosing the helium. A cellural flow occurs when fluid/gas ascends along the hot wall and descends along the cold wall (see link for picture). For this I have to determine the heat transfer coefficient using hand calculation.

I have remodified my model, now every surface should be taken into account, where the coolant can have access. This did not have significant effect on temperatures as I assumed. I have also rechecked the boyndary condition units. They should be correct. Also I have set for the emissivity of the tube surfaces e=1. This decreased the temperatures a bit, but not enough to the wanted level.

I have not switched to 3D-model yet, because the modelling of the coolant (fluid) could be too difficult in due to many un-axisymmetric parts in the device (e.g. support beams).

Link for the picture of the cellural flow:

http://files.engineering.com/getfil...435e-828c-5da57adcd95b&file=Cellular_flow.JPG

 

[gwolf2]

There should be a pretty simple hand calc for the cellural flow between two cylindrical surfaces. Give it a try.

Also, to try and debug models like this I sometimes do a parametric study on a few of the variables e.g. heat transfer coefficient x 1/10, x 1/2, x 2, x 10 etc. This helps to get a better feel for the model.

Lastly, I know that you say that there is no helium flow, but is it possible that there is an out of plane convection which you do not model , and which removes a significant amount of heat?

 

[IRstuff]

2.8 mm sounds to be too narrow for proper convective flow, but, typical, straight conductive flow is better than convective for short distances.

TTFN

FAQ731-376