calculating bulk temperature and nusslet number

Hi,

If we have a vertical slit that a flow is coming in from the bottom and the walls are heated .how can we calculate the bulk temperture in the slit in comsol? more spesipically if we want to calculate the expression below in COMSOL how can we do that?
Tb=( ∬TVdA)/(∬VdA)
T is the tempreature and V is the velocity of the fluid
also if we want calculate the local nusselt number with the expression given below how can we do that in COMSOL?

Nu= 4QB/k(Tb-T0)
that Q, B and K are constant values.


Thank you very much,
Sara