# Periodic boundary conditions problem

 Register Blogs Members List Search Today's Posts Mark Forums Read

April 26, 2016, 18:40
Periodic boundary conditions problem
#1
New Member

hervis
Join Date: Apr 2016
Posts: 2
Rep Power: 0
Hello,
I am currently simulating a tube bank, so I read the anys tutorial in order to have some guidance, but I am having some trouble with the analysis of the results, at the begining I thought I was wrong, but after taking a closer look at the tutorial, they have the same results, in the figures attached can be seen that the minimun temperature of the system is 300 K in the boundary conditions, and even so the simulation gives a minimun temperature of 277 K.
By the way when checking the heat fluxes (Reports-Fluxes-Total Heat Transfer Rate), it shows like if there wre no fluxes in the periodic boundary.

If anyone has an explanation for this results I would appreciate any help.

Thanks
Attached Images
 periodic_2.JPG (50.0 KB, 17 views) periodic_1.JPG (109.5 KB, 21 views)

 April 27, 2016, 01:25 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,146 Rep Power: 61 The inlet bulk temperature is 300K, which means that the mass-weighted average static temperature at the inlet must average to 300K. This means you will find some local places where the static temperature will exceed 300K and also other places where the temperature will be less than 300K.

April 27, 2016, 03:10
#3
New Member

hervis
Join Date: Apr 2016
Posts: 2
Rep Power: 0
Dear LuckyTran,

Excuse me but I didn't understand you, because when I do the same simulation without periodic boundary condition, why it doesn't happen?

In the image attached, the inlet temperature is 583.15 K, and the wall temperature 473.15 K, and as you can see this problem is not observed(in this simulacion periodic boundary conditions were not used).
Attached Images
 Nonperiodic.JPG (39.0 KB, 17 views)

 April 27, 2016, 08:59 #4 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,146 Rep Power: 61 Because when you use any other inlet boundary condition, you specify a uniform inlet temperature distribution (and not the bulk temperature per se). Basically, when you use another type of inlet you specify a uniform temperature profile (and since it's uniform, no need to talk about the average). This is just the default though. Imaging if you had a temperature profile at the inlet, then you would have to think about what the average is. Using a periodic BC you have a a profile and a bulk average.

 Tags heat exchanger, heat transfer, periodic, temperature, tube bank