I have a solar chimney that air move in channel with natural.convection.i have source term on absorber valume that heat air.and now i want to calculate nusselt number for finding indepence mesh.how can calculate?
Heat transfer coefficient is a measurement of fictitious resistance to heat transfer at the wall. So, if the source term for natural convection is not incorporated in the wall, then probably Nusselt number has no meaning.For mesh independence test, mesh size near the wall could be made finer to check the Nusselt number which is readily available in FLUENT.
For simulating natural convection using volumetric source term, probably defining Rayleigh number as a custom function makes more sense.