Hi,

In your case there are two parameters that make the Nusselt number tends to infinity:

- The forced convection and therefore the turbulence => increasing the convection coefficient and therefore Reynolds and Nusselt numbers.

-Having microchannels which reduces the flow section and thereby increases the fluid velocity (Venturi effect). Secondly that the porosity decreases pure thermal conductivity.

To answer your question, you can see an example here a study on turbine vane heat transfer in which the authors use comparable to yours for the Nusselt number values:

http://www.grc.nasa.gov/WWW/RTT/docs/Boyle_GT69133.pdf

your question is not clear to me, why is not possible; especially, what problem you have encountered.