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Re: [EnergyPlus_Support] Re: Modelling interzone air exchange with AirflowNetwork
> I want to consider only the density difference between two zones as a driving force of the zone air mixing. Is there any other ways?
Both Lixing and Stuart have given the technical explanation: the
airflow network, which has models for density-driven flow, requires
that the network connects to a zone of constant pressure. It sounds
like the EnergyPlus implementation also requires that the
constant-pressure node be the outdoors. Therefore if you want to use
the airflow network, you need a connection to outdoors.
By the way, this requirement is akin to the need to define a "ground"
node in an electrical network. If you don't pin at least one node of
a circuit to a reference voltage, then the entire circuit could be
kicked up or down by 10,000 volts. This causes all kinds of problems
for numerical solvers (because it creates a system with N+1 variables
and only N defining equations). And in fact the airflow network is
even more dependent on a reference pressure (voltage), because it uses
a gas law to find density as a function of pressure.
So those are the technical explanations. And because this is a
technical problem, there are, of course, ways to do what you want.
However, I'm going to ask you to indulge a grumpy old man in a
philosophical aside for a moment--
You want to account for how density differences between two zones
affect the flows between them. This implies that you have some
devotion to "accuracy" or "fidelity to the physics", which is a good
thing. But let's take it a step further. In the real world, the
flows between zones depend not only on density differences between
them, but also on the relative amount of airflow from the ventilation
system, and the exit paths that the walls, doors, windows, and so on
provide for air. Not to mention temperature-driven flows from
adjacent zones, and wind-driven flows, and so on.
Because of these externalities, one can easily have two adjacent zones
that *would* have temperature-driven flow between them (say, from room
A to B at the floor, and back to A at the ceiling), except that some
external pressure forces a one-way flow (say, from A to B via all
paths).
In other words, in the airflow network, no path acts in isolation.
They are all solved as a network. Changing the flow in one path can
force adjustments in all the paths, as the airflow solver adjusts
pressures and flows in order to balance the net airflow in and out of
each zone (just like changing the resistance in one part of an
electrical circuit can change the current in every branch).
This is the purpose and design intent of the airflow network. It
provides a mechanism for all those paths to interact, more or less the
way they do in a real building ("more or less" because the models are
not perfect, or complete). In fact if it was up to me, every single
airflow model in EnergyPlus would be implemented through the airflow
network, so that it would be impossible to impose an airflow in one
part of the building without possibly affecting the rest of the flows.
Now with that as background, think about what you want to do. You
want to tap into the power of the airflow network to try to predict
density-driven airflow between two zones. But you also want to
cripple the airflow network, by shutting off its functionality in
every part of the building that doesn't interest you.
Once you do that, any idea of accuracy or devotion to the physics goes
right out the window. You won't have any better idea of how those
zones interact than if you just made it up-- because in the real
world, those zones only interact in the context of the entire network.
So you can think of that error message as the airflow network telling
you "You are asking me to solve the flows in the network, but you are
not giving me a meaningful network to work with." And thus the grumpy
old man in me says "If you want to model the building at the level of
detail of density-driven flows between two zones, then go ahead and
define realistic airflow paths. You can't isolate the two rooms from
everything around them, and then pretend that you get a meaningful
simulation."
That said-- there are ways to do what you want. For example:
+ You could connect one of those two zones to the outside, using a
series of very tight (not leaky) one-way flow paths. Then the airflow
network would likely not push too much air in or out of those paths,
and they would act as if their only meaningful interaction was with
each other.
+ If you are truly just interested in the density-driven flows, say
to get a ballpark estimate of the effect, then create a one-zone
building, and treat the outside as the second zone (because
mathematically, the outside is just a special zone with a known
pressure).
+ If you want a more modern, more capable airflow network solver,
then look at using CONTAM. You can use it stand-alone to explore the
effects that interest you, or you can integrate it with EnergyPlus via
an FMU capability.
-Dave
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Posted by: David Lorenzetti <dmlorenzetti@xxxxxxx>
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