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Inconsistent boundary conditions
Posted Mar 4, 2010, 10:50 a.m. EST 3 Replies
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I have an 2D axisymmetric thermal model of a tube of water. I'm running a time dependent model over several seconds (I have confirmed that the output values stabilise to a steady state)
Heat is conducting into the side wall of the cylinder through continuity boundary with an insulating layer with a fixed external temperature.
The bottom face is thermally insulating.
The top face has a heat flux discontinuity boundary condition which is intended to draw out an amount of thermal energy to return the water to a chosen target temperature.
The value of this is calculated by using an integration coupling variable which finds the total heat flux in through the side wall (and I have confirmed from post-processing integrations that the value is correct) and then divides by the area of the top of the cylinder.
I have confirmed that this operates as a thermostat correctly, the temperature and pressure of the water are steady, so the energy balance is correct to thermostatically regulate the water.
However,
The rotated boundary integral shows that more than twice as much heat energy is being extracted as is conducting in, yet the internal temperature is not changing.
If anyone has any ideas why the model is destroying energy in this situation, and why that isn't having any effect on the temperature I'd be very grateful to hear from you, as I am completely baffled!
Heat is conducting into the side wall of the cylinder through continuity boundary with an insulating layer with a fixed external temperature.
The bottom face is thermally insulating.
The top face has a heat flux discontinuity boundary condition which is intended to draw out an amount of thermal energy to return the water to a chosen target temperature.
The value of this is calculated by using an integration coupling variable which finds the total heat flux in through the side wall (and I have confirmed from post-processing integrations that the value is correct) and then divides by the area of the top of the cylinder.
I have confirmed that this operates as a thermostat correctly, the temperature and pressure of the water are steady, so the energy balance is correct to thermostatically regulate the water.
However,
The rotated boundary integral shows that more than twice as much heat energy is being extracted as is conducting in, yet the internal temperature is not changing.
If anyone has any ideas why the model is destroying energy in this situation, and why that isn't having any effect on the temperature I'd be very grateful to hear from you, as I am completely baffled!
3 Replies Last Post Mar 5, 2010, 12:40 p.m. EST
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Posted:
1 decade ago
Hi
As such no idea, but I know that often I forget the "2*pi*r" multiplicant in 2D-axi, as these must be explicitely introduced in the integration coupling variables, but are considered by "click" settings in the postpro. I have burned myfinger a couple of times on that, but usually you catch it via the units check.
If not such a simple thing I would try "support"
Good luck
Ivar
As such no idea, but I know that often I forget the "2*pi*r" multiplicant in 2D-axi, as these must be explicitely introduced in the integration coupling variables, but are considered by "click" settings in the postpro. I have burned myfinger a couple of times on that, but usually you catch it via the units check.
If not such a simple thing I would try "support"
Good luck
Ivar
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Posted:
1 decade ago
Yes, that was my first thought too, but after a lot of careful checking I'm pretty sure I've got the right geometric coefficients in the right places. Thanks for the reply though.
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Posted:
1 decade ago
care to post your model so we can take a look?