Urgent-How to set Ref. temperature in cavity Flow
 

Hello I am attempting to evaluate nusselt for square cavity flow at the cavity bottom wall which has fixed temperature TH as illustrated in picture but I face a problem in setting Tref ? Is there a relation to predict it?

Thanks in advance

Any reply pls

I appreciate your reply thank you

What are the boundary conditions on the remaining three walls?

Hey thanks for the reply. There are a convection boundary of air stream with Tinfinity from left side while the bottom wall remained with hot temperature Th and the right wall with cold temperature Tc as illustrated in picture. I want to evaluate average Nusselt at the bottom wall?

Thanks
Mariam

The reference temperature is something that you have to define together with the heat transfer coefficient. You cannot discuss heat transfer coefficient or Nusselt number until you have defined this reference temperature.

But I see you have a Tinfinity, which is the reference temperature that goes with the h on the left wall. Wouldn't it make sense to use the same reference temperature also for all other walls? Otherwise, you have a different definition of heat transfer coefficient and a different definition of Nusselt number for each wall.

Thanks Lucky for the reply. I aware the Nusselt number is different in each wall. I attempted now to evaluate Nu just at the bottom wall of cavity I already used the Tinfinity as reference temperature for it but it give me wrong values of Nu some of it are negative?

Thanks

I'm afraid you still miss the point. Your idea is that the value of Nusselt number differs is due to the differences in heat flux. But the reference temperature that is used to define the heat transfer coefficient per Newton's Law of Cooling is what I meant by different definitions of heat transfer coefficient.

My previous point is that when you write down h as the boundary condition for the left wall, it already has an implied reference temperature. Now if you try to use a different reference temperature for the other walls, then you have two different types of heat transfer coefficients that are defined based on different reference temperatures. Maybe this is desired, maybe it is not. I don't know your preference.

For enclosures, one often takes some sort of mean temperatures e.g. (TH + TC)/2. You have a three temperature problem where it is not so clear which reference temperature to use, it depends on what story you want to tell and what picture you want to paint.