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atlan August 6, 2014 14:13

Energy balance
 
Hi all,
Can anyone advise me please how to check the energy balance in a simple straight tube?
I have tube with one inlet and one outlet and one wall. At the inlet I impose constant temperature and at the wall a heat flux.
When I use wallHeatFlux utility to calculate the power it shows at the inlet and outlet values in order lower than the heat loss at the wall.
However, if I roughly estimate the power from the mass flow rate and the temperature difference between inlet and outlet (cp*m*dt) it looks similar to the heat loss trough the wall. But the problem is that there is already temperature profile at the inlet and I cannot set the outlet temperature exactly.
The solver is buoyantSimpleFoam.
Thanks
Atlan

RodriguezFatz August 7, 2014 05:59

I have no experience with "wallHeatFlux" utility, but just from the wording: Maybe it just calculates the heat flux as a spatial derivative of temperature times the heat conductivity, because that is how you would calculate the heat flux at a wall:
q=lambda*dT/dy.
At the inlet and outlet you don't have a wall but the moving fluid. Thus, "q" gets an additional component by the heat that is transported by the moving fluid rho * u * E0.
This could be the reason the balance doesn't work how you do it, because heat conduction at inlet and outlet ist just way less than convective heat transport.

atlan August 7, 2014 10:54

Energy balance
 
You are right. I have realized, it is not possible to use the wallHeatFlux utility for inlet and outlet. It is applicable only for walls.


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