[TRNSYS-users] Conceptual loop question

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Here's a general question about how TRNSYS operates:

Suppose 2 components are linked, for example flow from a tank to an
auxiliary heater and back to the tank. Both component's inputs has a
default flow value. How does TRNSYS determine which flow to use?

My only guess is ordering of the components in the dck file. First
component listed determines the value? Then what determines component
ordering in the dck file?


Brian W. Raichle
Professor and Assistant Chair
Department of Sustainable Technology & the Built Environment
Appalachian State University

Brian Raichle's picture
Joined: 2016-04-05
Reputation: 0

In your example of a tank and an auxiliary heater you are correct
that the first component appearing in the input file determines the flow
for both. The order of components in the input file is determined by the
order in which they were placed on the assembly panel in the Studio.
However, you can modify the order by opening the control cards (where
you set the simulation start and stop times) and then by clicking on the
"component order" tab. Click and drag components around the list to
change their order.

TRNSYS typically uses the "successive substitution" method to solve
the system of components. The outputs of one component are substituted
into the inputs for the components to which that output is connected.
Each component takes its current set of inputs and computes its outputs,
passing them back to the TRNSYS kernel (brains) to see whether they have
changed from the previous iteration. Once everything stops changing from
iteration to iteration the kernel decides that convergence has been
reached and advances to the next timestep.

Your example changes somewhat if you put a pump into the system.
Pumps and fans in TRNSYS control the downstream flow. The output flow
rate from a pump or a fan is the rated flow rate multiplied by the
control signal (usually; there are some components that do not conform
to this method). The flow rate from the pump or fan is then passed to
the downstream components and should get passed all the way around the
flow loop such that there is a mass balance. Most pump and fan models
issue a warning as to how much of the simulation their mass balance failed.

kind regards,

Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI 53703 USA

d.bradley at tess-inc.com


David BRADLEY2's picture
Joined: 2011-10-02
Reputation: 200