To find the Nusselt number for combined forced/natural convection, you must first find the Nusselt for forced convection as if it occured by iteslf, and then for natural convection as if it occured by itself. Those Nusselt numbers are then combined according to

Nu_combined = (Nu_forcedⁿ ± Nu_naturalⁿ)^1/n

The (+) is used if the modes "help" each other. The (-) is used if the modes are opposed to each other. For example, you would use the (+) if a fan were blowing air up a heated wall and the (-) if it blew air down the wall. The exponent n is determined by geometry, but usually has a vlaue ranging from 3 to 4.