[zoheir]

How to get the local surface Nusselt number and the average Nusselt number in the fluent? at a square cavity the top and the bottom are adiabatic,and the left wall is with low constant temperature and the right wall with high constant temperature.
the average Nusselt number on the vertical mid-plane of the cavity;???????

[Sufyan]

I think it would be better if you can post the schematic of your problem. As far I understand your question is, you are simulating a square cavity and you like to get Local and average Nusselt number at the center/mid-section of the caity.

It simple in Fluent.
Please go to plot results>Report>Surface integrals and then press help button to get step by step tutorial for plotting different results including Nusselt number.

For unsteady simulation, you should have sampled data for sufficient amount of time.

Best wishes