Heat flux


I am trying to calculate the heat transfer coefficients in a circular pipe but i have had some problems with the energy balance. My case is this:

Circular pipe 10 m long and 1 m diameter.
Inlet: velocity 0.01 m/s, Temperature 150 deg
Outlet: default zero gradient
Cylinder wall: Temperature fixed value 100 deg, nut Spalding Wall-function

My y+ values are between 20 and 40.

Energy balance:

In: (Inlet velocity)*(Inlet temperature)*(Inlet area)*(specific heat*density)
Out: (heat flux at wall)*(wall area) + (Outlet velocity)*(Outlet temperature)*(Outlet area)*(specific heat*density)

This does not add up to zero unless the heat flux at the wall is multiplied with the density.
Also if I use a heat flux boundary condition on the wall face instead of a fixed temperature there is no difference in outlet temperature when i change the density.

Heat Flux definition fixed in Caedium v5

This Heat Flux definition bug for incompressible flow is fixed in Caedium v5.

Heat Flux definition bug for incompressible flow

Thanks for your explanation of the problem.

I can confirm your findings. This is a bug, where the Heat Flux boundary specification and the Heat Flux scalar field result are represented as 'Heat Flux / rho' for incompressible flow. This will be fixed in the next version of Caedium. As a workaround you will need to:

  • Specify the Heat Flux boundary condition as 'Heat Flux / rho'
  • Create a new Scalar Field result 'Heat Flux * rho', where rho is a constant set to your density