[TRNSYS-users] Type 31 with high mass flows

[Diego A. Arias]

Dear Jens:

 From your message, I understand that you are trying to assess the 
sensitivity of the solution with respect to the discretization of the pipe.
If you look at the Fortran code of TYPE31, you will see that it 
calculates the fraction  FLOW*DELT / MASS, where FLOW is the mass flow 
rate entering the pipe, DELT the time step, and MASS the total mass that 
the pipe can contain (density*volume).

There are two limits for this fraction:
- If this ratio is very small (less than a tolerance TOL = 0.001), then 
the pipe is not updated and only the thermal losses are calculated.
- if  this fraction is greater than 1, you will get the error message 
'flow is bigger than the capacity of the pipe during one time step'.

As the TYPE31 is not a regular finite differences method, but a plug 
model,  you have to keep in mind these limits if you want to run a 
sensitivity analysis of the solution with respect to the pipe 
discretization.  I would recommend looking at section 5.6.4 of  the 
05-MathematicalReference.pdf, where you will find an explanation of the 
plug model.

I hope this explanation helps.

Diego


>does the Type 31 pipe have got any problems with
>segments (mass flow times the timestep) that are
>bigger than the pipe itself.?
>I study heat losses of pipes that are through-flown by
>different mass flows from a DHW load profile (always
>the same).
>I tested 1, 7 and 14 interconected pipes having all
>the same total volume and length with different time
>steps (60s, 20s, 6s). That leads to a variety of
>maximum segment masses and I get differences in heat
>losses of up to 7 per cent.
>I know that TRNSYS 16 writes a warning when the mass
>flow is bigger than the capacity of the pipe during
>one timestep but I have no closed loop, just a type 6
>and the pipe.
>
>Jens Apel
>FHTW Berlin, Germany
>  
>

-- 

Diego A. Arias
TRNSYS Coordinator
Solar Energy Laboratory
University of Wisconsin - Madison