# Modelling a gas-gas heat exchanger

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 January 30, 2023, 03:05 Modelling a gas-gas heat exchanger #1 New Member   Hossein Sheykh Join Date: Jan 2023 Posts: 6 Rep Power: 3 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

 January 30, 2023, 03:51 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,748 Rep Power: 143 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. Ho_Sh likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

 January 30, 2023, 10:32 #3 New Member   Hossein Sheykh Join Date: Jan 2023 Posts: 6 Rep Power: 3 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 Last edited by Ho_Sh; January 30, 2023 at 12:07.

 January 30, 2023, 15:45 #4 Senior Member   Join Date: Jun 2009 Posts: 1,825 Rep Power: 33 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. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.