## Fluids

#### Printed Circuit Heat Exchanger simulation of water-steam system

• anassohail
Subscriber

I am validating a research paper on the Printed circuit heat exchanger in which the model is reduced to one hot and one cold channel. The hot channel is at a pressure of 15.3 Mpa and the cold channel is at 6Mpa. The hot side remains in a single-phase whereas the cold side undergoes a phase change. The boundary conditions are top-bottom and left-right periodic conditions. The author of the paper simulated this model on some other CFD software which uses wall boiling model and Rohsenow correlation of boiling. I want to do this problem in FLUENT.

I have three questions :

How can I enter the properties of water-liquid at two different pressures with varying temperatures? FLUENT asks properties in terms of temperature using linear piecewise because there are two streams at different pressures.

When I applied the periodic boundary condition, in the periodic conditions tab, FLUENT asks for upstream bulk temperature, flow direction and pressure gradient. What do they mean when I am applying the condition on the wall.

How to implement the Rohsenow correlation in the FLUENT RPI wall boiling model.

The attached Picture depicts the model.

• YasserSelima
Subscriber
The problem is not easy, and you have some work to do.nFirst, I think you meant periodic temperature or periodic heat flux on the walls, not periodic flow. Is this right? If yes, you can use expression to set this. Don't choose periodic boundarynSecond, For Rohsenow correlation, Fluent allows you to enter UDF for the convection coefficient of heat transfer. So, you need to divide the equation by (A *DELTA_T) and write user defined function to enter the rest .. this will not be easy because you will use many macros to get the properties and the flow conditions. Good Luck!nn
• anassohail
Subscriber
Thanks for the reply.nI have to set the wall as periodic. I used the text user interface command as mentioned in the FLUENT user guiden/mesh/modify-zones/make-periodic/nand then specified the ids of the two boundaries.nIs this the procedure to set the wall as periodic? or is it wrong?n
• YasserSelima
Subscriber
This is the procedure to create conformal periodic zones. But I am not sure if this is what you need. Can you explain how these walls are periodic in your case?n
• anassohail
Subscriber
I am trying to say that the same geometry repeats itself i.e. the arrangement of the PCHE channels is periodically repeated.nTo quote one research paper on PCHE:n Since the hot and cold channels are alternated vertically, the heat conduction in this direction can be characterized as periodic.nSo, how can I apply this boundary condition in FLUENT? The procedure I followed to create periodic zones simulate the same physics or not? I have also attached the screenshot of the periodic conditions tab in FLUENT in my simulation.n I am curious about our first comment where you mentioned periodic temperature and periodic heat flux. Can you tell me more about it or refer me to the appropriate documentation on applying it FLUENT?nn
• anassohail
Subscriber
How can we set periodic temperature or periodic heat flux on the walls using an expression? like you said in the first commentn
• Rob
Ansys Employee
Periodic conditions have a few limitations, look in the manual as they're in the flow and heat transfer chapters of the User's Guide. If you're using an older version of the code the documentation may be less complete, I think the update was around R19 or 2019Rx. The ones that'll cause you a problem concern solids, wall thermal boundary settings and only having one pair of boundaries. nWhen we used to run gas turbine sectors to look at the burner interaction we used 3 or 5 nozzles to avoid the edge effects. You may find that is needed here. Note, in Fluent having disconnected fluid zones can cause stability issues.n