For any surface, regardless to the geometry, go to report and then surface integral and then select area weighted average and pick on the required surface and and lastly select heat transfer coefficient. To find nusselt number, you can follow same procedure but check the characteristic length that you applied.

I have gone through with that. But fluent calculates the same based on reference values, hence the results are not correct.

You can take mass weighted temperature at outlet surface and with known fluid properties (or calculated) and known mass flow and inlet temperature it is possible to calculate Alpha, Re number and Nusselt number.

If the issue is only related to Nusselt you can get heat transfer coefficient and then calculate Nusselt from; Nu = hL/k.

the solution is to use the heat flux calculated in fluent to find the value of 'h' and then use the relation mentioned above by waleed. ie; Nu=hL/k.