Modelling a gas-gas heat exchanger
 

Hello,

I am modelling a gas-gas heat exchanger to find the outlet pressure and temperature for hot and cold fluids. The known parameters are:

Hot and cold inlet mass flow rate
Hot and cold inlet temperature
Hot and Cold Inlet Pressure

As the outlet pressure is unknown, I want to use the pressure at the inlet BC and the mass flow rate of the fluids at the outlet BC to find the total pressure of the outlets through the simulation. In this way, I can calculate the pressure drop of the fluids.

Also, I leave the outlet temperature blank to get overwritten by the software.

I wonder if this setting is sensible in CFX?

Thanks
Hossein

Please read the section in the CFX documentation on well posed boundary conditions.

If the flows can be considered incompressible then you can model the inlet and outlets as mass flow inlets and a zero pressure outlet. This will give the pressure drop over the system at that flow rate.

If the flows are considered compressible then things get a bit more complex. We would need to know what Mach numbers it is operating at.

Thanks, Glenn; I have run the simulation with mass flow rate to get the pressure drop but I was getting non-zero exit code: 25 and non-zero exit code: 2 and no result was achieved.

Changed the setting to the following and got some results eventually:

Hot Inlet:
Mass flow rate = 0.007
Total temperature 1040K

Hot outlet:
Opening pressure and direction
Relative pressure = 101325 pa
Opening temperature (In heat transfer section) 445K

Cold inlet:
total pressure(stable) = 286000 pa
Total temperature (in heat transfer section)= 455K

Cold outlet:
Opening pressure and direction (relative pressure = 1 atm (101325 pa))

Opening temperature = 970K

This boundary condition gave me the Maximum Mach number of 1.66 and Y+ of 649.

Can I ask if this setting sounds right? as the Mach is above 1, shouldn't I change it to supersonic?


Many thanks
Hossein

In general, static pressure inlet conditions are not stable; therefore, I would not use it.

My advice is:
1 - Use mass flow outlet for both streams
2 - with the mass flow you can estimate the dynamic pressure at the inlet since you already know; therefore, an estimate for your total pressure inlets
3 - Similar to the total temperature condition, and I think you already estimated them in your previous simulations.
4 - compute those solutions, and verify how far the static pressures at the inlet are with respect to your known values. Once you feel comfortable with the solutions, your pressure drop is there.