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Heat Flux at wall in a conjugate heat transfer problem

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Old   June 16, 2011, 05:26
Default Heat Flux at wall in a conjugate heat transfer problem
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Hi,
I am simulating a simple laminar flow with conjugate heat transfer in a micro-channel. I want to find out the variation of heat transfer coefficient (htc) along the length of the channel. I define my htc as below:

htc(x)= Avg. Heat flux(x)/(avg. solid wall temperature (x) - fluid bulk temp. (x))

Here x is the channel axial location. The Avg. Heat flux and avg. solid wall temp. are calculated by averaging Heat Flux and Solid wall temperature on a polyline at axial location x.

I take Heat flux values at the wall (which is the interface between fluid and solid) from CFD post.

Now my htc(x) shows some sudden changes along the channel length. The reason for this is the sudden change in Heat flux(x) at the wall. How is this heat flux calculated in CFX? I have checked the documentation. The Ansys CFX solver modeling guide says it is calculated as

qw=hc(Tw-Tnw)

But where this hc comes from? Is this formulation equivalent to saying that
qw = -k (dT/dy) where k is thermal conductivity of fluid and y is the wall normal direction?

Please help as I need to explain why this heat flux changes so suddenly in some portion of the channel wall thus leading to sudden change in htc(x).
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Old   June 16, 2011, 08:06
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Your equation is not correct. I assume you want the heat flow at the interface between the solid and liquid domains - in this case for laminar flow the heat transfer is simply conduction across the interface. There is no "hc".

Your second equation is correct. The heat fluxes in the solid and liquid are balanced such that k dT/dy is equal (there is also a transient term if relevant).
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Old   July 9, 2011, 22:22
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Thank you Glenn for the explanation.
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