Heat transfer and nuclear 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

 

[corus]

If you have a lot of slots you haven't modelled then you're losing a lot of surface area from which heat can dissipate. I would presume that the reason for the slots is to help heat flow.

Another thought is that if the gap for the helium is large then you may get convection occuring. This may aid in the heat loss. Personally I wouldn't bother with including gas as a solid as the conduction across it is usually negligible and thus usually just apply cavity radiation. Make sure you've not got blocking on cavity radiation as the solid elements you've used for the gas may block all radiation between surfaces.

If the genoetry is such that axial symmetry isn't applicable then use a 3D model but allow for symmetry to reduce the size and compexity of it.

corus

 

[Narel]

Thank you for your reply Corus. I might have been a bit misleading with my definition about the slots. There is quite lot of slots in the device, but most of them (maybe 95%) are filled mostly with bolts to fix the supports between the flanges and grid plates. I have now modelled all possible gaps and slots, where the coolant can access, but I still haven´t got any significant decrease of temperatures. I also checked the blocking for radiation by suppressing the modelled helium layer, this had almost negligible effect on temperatures (actually it even increased temperatures a bit). So there should not be any blocking on cavity radiation. Also if I don´t use "cavity radiation", this increases the temperatures even higher, almost douples the temperatures. In due to this tests I presume the cavity radiation is determined correctly in my model.

I might have to take into account some how the convection in the helium layer, which occurs from cellural flow (see picture attached). For that I have to determine the heat transfer coefficient (in Abaqus "Film coefficient")with hand calculations to increase heat loss. I don´t know will the heat transfer coefficient be big enough to increase heat loss, it has to be more than 200 W/m2/°C to have any effect on the heat loss...

Picture link:

http://files.engineering.com/getfil...4cf9-a463-23c31e5d6acd&file=Cellular_flow.JPG