Flow models for pressure drop calculations
Information
This package contains a basic flow model that is used by the
various models that compute pressure drop.
Because the density does not change signficantly in heating,
ventilation and air conditioning systems for buildings,
this model computes the pressure drop based on the mass flow
rate and not the volume flow rate. This typically leads to simpler
equations because it does not require
the mass density, which changes when the flow is reversed.
Although, for conceptual design of building energy system, there is
in general not enough information available that would warrant a more
detailed pressure drop calculation.
If a more detailed computation of the flow resistance is needed,
then a user can use models from the
Modelica.Fluid library.
Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).
Package Content
Basic class for flow models
Information
Function that computes the pressure drop of flow elements as
m = sign(Δp) k √ Δp
with regularization near the origin.
Therefore, the flow coefficient is
k = m ⁄ √ Δp
The input m_flow_turbulent determines the location of the regularization.
Inputs
| Type | Name | Default | Description |
|---|
| Pressure | dp | | Pressure difference between port_a and port_b (= port_a.p - port_b.p) [Pa] |
| Real | k | | Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2) |
| MassFlowRate | m_flow_turbulent | | Mass flow rate [kg/s] |
Outputs
| Type | Name | Description |
|---|
| MassFlowRate | m_flow | Mass flow rate in design flow direction [kg/s] |
Modelica definition
function basicFlowFunction_dp
"Basic class for flow models"
input Modelica.SIunits.Pressure dp(displayUnit="Pa")
"Pressure difference between port_a and port_b (= port_a.p - port_b.p)";
input Real k(min=0, unit="")
"Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)";
input Modelica.SIunits.MassFlowRate m_flow_turbulent(min=0)
"Mass flow rate";
output Modelica.SIunits.MassFlowRate m_flow
"Mass flow rate in design flow direction";
protected
Modelica.SIunits.Pressure dp_turbulent(displayUnit="Pa")
"Turbulent flow if |dp| >= dp_small, not a parameter because k can be a function of time";
protected
Real kSqu(unit="kg.m")
"Flow coefficient, kSqu=k^2=m_flow^2/|dp|";
algorithm
kSqu:=k*k;
dp_turbulent :=m_flow_turbulent^2/kSqu;
m_flow :=
Modelica.Fluid.Utilities.regRoot2(x=dp, x_small=dp_turbulent, k1=kSqu, k2=kSqu);
end basicFlowFunction_dp;
Basic class for flow models
Information
Function that computes the pressure drop of flow elements as
Δp = sign(m) (m ⁄ k)2
with regularization near the origin.
Therefore, the flow coefficient is
k = m ⁄ √ Δp
The input m_flow_turbulent determines the location of the regularization.
Inputs
| Type | Name | Default | Description |
|---|
| MassFlowRate | m_flow | | Mass flow rate in design flow direction [kg/s] |
| Real | k | | Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2) |
| MassFlowRate | m_flow_turbulent | | Mass flow rate [kg/s] |
Outputs
| Type | Name | Description |
|---|
| Pressure | dp | Pressure difference between port_a and port_b (= port_a.p - port_b.p) [Pa] |
Modelica definition
function basicFlowFunction_m_flow
"Basic class for flow models"
input Modelica.SIunits.MassFlowRate m_flow
"Mass flow rate in design flow direction";
input Real k(unit="")
"Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)";
input Modelica.SIunits.MassFlowRate m_flow_turbulent(min=0)
"Mass flow rate";
output Modelica.SIunits.Pressure dp(displayUnit="Pa")
"Pressure difference between port_a and port_b (= port_a.p - port_b.p)";
protected
Real kSquInv(unit="1/(kg.m)")
"Flow coefficient";
algorithm
kSquInv:=1/k^2;
dp :=
Modelica.Fluid.Utilities.regSquare2(x=m_flow, x_small=m_flow_turbulent, k1=kSquInv, k2=kSquInv);
end basicFlowFunction_m_flow;
Automatically generated Mon May 4 10:22:54 2015.
basicFlowFunction_dp
Examples
Buildings.Fluid.BaseClasses.FlowModels.basicFlowFunction_dp