This is a model of a flow resistance with a fixed flow coefficient. The mass flow rate is computed as
ṁ = k √Δp,
where
k is a constant and
Δp is the pressure drop.
The constant k is equal to
k=m_flow_nominal/sqrt(dp_nominal),
where m_flow_nominal is a parameter.
In the region
abs(m_flow) < m_flow_turbulent,
the square root is replaced by a differentiable function
with finite slope.
The value of m_flow_turbulent is
computed as
m_flow_turbulent = eta_nominal*dh/4*π*ReC,
where
eta_nominal is the dynamic viscosity, obtained from
the medium model. The parameter
dh is the hydraulic diameter and
ReC=4000 is the critical Reynolds number, which both
can be set by the user.
By default, the pressure drop at nominal flow rate is computed as
dp_nominal = fac * dpStraightPipe_nominal,
where dpStraightPipe_nominal is a parameter that is automatically computed
based on the
nominal mass flow rate, hydraulic diameter, pipe roughness and medium properties.
The hydraulic diameter dh is by default
computed based on the flow velocity v_nominal and the nominal
mass flow rate m_flow_nominal. Hence, users should change the
default values of dh or v_nominal
if they are not applicable for their model.
The factor fac takes into account additional resistances such as
for bends. The default value of 2 can be changed by the user.
The parameter from_dp is used to determine
whether the mass flow rate is computed as a function of the
pressure drop (if from_dp=true), or vice versa.
This setting can affect the size of the nonlinear system of equations.
If the parameter linearized is set to true,
then the pressure drop is computed as a linear function of the
mass flow rate.
Setting allowFlowReversal=false can lead to simpler
equations. However, this should only be set to false
if one can guarantee that the flow never reverses its direction.
This can be difficult to guarantee, as pressure imbalance after
the initialization, or due to medium expansion and contraction,
can lead to reverse flow.
If the parameter
show_T is set to true,
then the model will compute the
temperature at its ports. Note that this can lead to state events
when the mass flow rate approaches zero,
which can increase computing time.
For more detailed models that compute the actual flow friction,
models from the package
Modelica.Fluid
can be used and combined with models from the
Buildings library.
For a model that uses dp_nominal as a parameter rather than
geoemetric data, use
Buildings.Fluid.FixedResistances.PressureDrop.
The pressure drop is computed by calling a function in the package Buildings.Fluid.BaseClasses.FlowModels, This package contains regularized implementations of the equation
ṁ = sign(Δp) k √ Δp
and its inverse function.
To decouple the energy equation from the mass equations, the pressure drop is a function of the mass flow rate, and not the volume flow rate. This leads to simpler equations.