Hi I am modelling counter-flow vertical heat exchanger. I know the inlet temperatures and inlet velocities of hot and cold streams. There will be gravity effect.

1. What outlet conditions should I choose, Pressure outlet or outflow? 2. Where should I specify the reference pressure top or bottom? and 3. What should I choose as temp outlet boundary conditions?

Hi!

I think you should use pressure outlet bc.As outflow bc can not be used ( ref: fluent documentation ). Can u tell me what wall bc are u using.

Thanks. I had taken outer walls as adiabatic and i am not able to decide boundary condition for the wall which is in between hot and cold fluid.

The bc's for walls between hot and cold fluids should be "temperature" for both sides. Use average temperature on both walls.