Re: Wrong IISibat Project?

["David Bradley [trnsys]" <trnsys@xxxxxxxxxxxxx>]

Catalin,
  The error in your input file is I think a fairly common one and
one that probably goes undetected often. In your simulation, you have a
pump and a loads component but no piping. Consequently there is not
enough capacitance in the loop and the system is not converging on a
solution whenever the pump is on. If you look at the list file, you will
see a number of the following error:

 ***** ERROR
*****        TRNSYS ERROR # 206
 THE SOLUTION CAN NOT BE FOUND AT TIME   
6.00

having one or two of these errors is not a problem so TRNSYS does
not stop the simulation until a certain number of these errors have been
encountered (controlled by the LIMITS statement). When you shrink your
timestep to a quarter hour, you are probably going over the limit.

To explain the problem a little better, imagine that the effect of the
load component is to remove 10 C from the inlet fluid temperature. On the
first iteration, the fluid coming from the pump is at 30 C. It enters the
load component and leaves at 20 C and goes back to the pump. Now (at the
same timestep) the pump inlet fluid is at 20 C. It leaves the pump at 20
C (pretend that the pump doesn't add any thermal energy to the fluid). It
enters the load component again and comes out at 10 C. The loop is
repeated and repeated and no converged state is ever reached. 

To solve the problem, you need to add a pipe to your system whose volume
is large enough to contain at least one timestep's worth of fluid. This
will add enough capacitance to the system so that it becomes stable and
doesn't have a runaway solution. 

In effect, EVERY closed loop simulation in TRNSYS should employ a device
(storage tank, insulated pipe etc.) capable of holding one timestep's
worth of fluid or iteration problems will occur.  In many cases,
such as in a solar collector loop, these problems go unnoticed,  as the
inlet temperature to the collector rises the losses will increase until
the iterations converge - but not at the correct solution.....


Cheers,
  David



At 05:25 AM 9/14/2003 -0700, you wrote:
> Hello all TRNSYS/IISibat
> users.
>
> Could anyone explain me, please, what's wrong
> with my project? Why the temperature of the fluid
> doesn't change in time? If the simulation step
> time becomes shorter, 0.25 for example, the
> simulation terminates with one error. Where is
> the fault?
>
> Thank you in advance for your help.
>
> Best regards, Catalin LUNGU.
>
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