How to compute nusselt number in LBM?

I’m new to LBM and i don’t know how to compute Nusselt number in this method . Can anyone help ?

Dear Dhouha,
You just see the temperature difference between the wall and the grids nearest to it. Also multiply the characteristic length (Here number of nodes in the characteristic direction). This is the procedure for Nu calculation.
Suppose if a left heated wall is there then it will be,

do j=0,m
end do

leftnu - left Nusselt number
th(i,j) - Temperature value at i,j
n - Here characteristic length.(assume uniform mesh)

If you sum all the leftnu then you will get the total Nu. From that you divide it by float(m). This will give the value of average Nu of that wall.