opening temperature for the opening BC
 

How to define the opening temperature for opening BC as temperature is unknown at the defined location.

If you are doing a thermal model and don't know the inlet temperature then you can't use an opening (or inlet) boundary. You have to either specify a temperature or move the boundary to somewhere you do know the temperature, or move the boundary somewhere you can use a boundary which does not require temperature to be specified (such as a wall or symmetry).

The domain has reverse flow at the outlet so needs to define as a opening. The inlet tempearure is specified at the inlet, the fluxes are assigned.

At the outlet tempearure is required output....but as it defined as a opening ask for opening or static temperature....guide to guess the temperature or any other choice.

I remember reading in one of the tutorials that it is suggested to use a CEL expression and take the average temperature of the opening

ave(T)@REGION:Outlet

maybe "areaAve" would be even better.

But keep in mind that this is not the best solution.
A better way to do it would be to put the outlet further downstream, so that no backflow occurs.

Can you quote the reference of the tutorial where CEL is used.

It's the conjugate heat transfer in a heating coil.
And they use

areaAve(T)@outflow