Buildings.Fluid.BaseClasses

Information

This package contains base classes that are used to construct the models in Buildings.Fluid.

Extends from Modelica.Icons.BasesPackage (Icon for packages containing base classes).

Package Content

Name	Description
FlowModels	Flow models for pressure drop calculations
PartialResistance	Partial model for a hydraulic resistance
PartialThreeWayResistance	Flow splitter with partial resistance model at each port

Buildings.Fluid.BaseClasses.PartialResistance

Information

Partial model for a flow resistance, possible with variable flow coefficient. The pressure drop is computed by an instance of Buildings.Fluid.BaseClasses.FlowModels.BasicFlowModel, i.e., using a regularized implementation of the equation

m = sign(Δp) k √ Δp  

Extends from Buildings.Fluid.Interfaces.PartialTwoPortInterface (Partial model transporting fluid between two ports without storing mass or energy).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Pressure	dp_nominal		Pressure drop at nominal mass flow rate [Pa]
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRate	m_flow_small	1E-4*abs(m_flow_nominal)	Small mass flow rate for regularization of zero flow [kg/s]
Boolean	homotopyInitialization	true	= true, use homotopy method
Boolean	from_dp	false	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	linearized	false	= true, use linear relation between m_flow and dp for any flow rate
Diagnostics
Boolean	show_V_flow	false	= true, if volume flow rate at inflowing port is computed
Boolean	show_T	false	= true, if actual temperature at port is computed (may lead to events)

Connectors

Type	Name	Description
FluidPort_a	port_a	Fluid connector a (positive design flow direction is from port_a to port_b)
FluidPort_b	port_b	Fluid connector b (positive design flow direction is from port_a to port_b)

Modelica definition

partial model PartialResistance 
  "Partial model for a hydraulic resistance"
    extends Buildings.Fluid.Interfaces.PartialTwoPortInterface(
     show_T=false, show_V_flow=false,
     m_flow(start=0, nominal=m_flow_nominal_pos),
     dp(start=0, nominal=dp_nominal_pos));

  parameter Boolean from_dp = false 
    "= true, use m_flow = f(dp) else dp = f(m_flow)";
  parameter Modelica.SIunits.MassFlowRate m_flow_nominal 
    "Nominal mass flow rate";
  parameter Modelica.SIunits.Pressure dp_nominal(displayUnit="Pa") 
    "Pressure drop at nominal mass flow rate";
  parameter Boolean homotopyInitialization = true "= true, use homotopy method";
  parameter Boolean linearized = false 
    "= true, use linear relation between m_flow and dp for any flow rate";

  Real k(unit="") "Flow coefficient, k=m_flow/sqrt(dp), with unit=(kg.m)^(1/2)";
  Modelica.SIunits.MassFlowRate m_flow_turbulent(min=0) 
    "Turbulent flow if |m_flow| >= m_flow_turbulent, not a parameter because k can be a function of time";

protected 
  parameter Medium.ThermodynamicState sta0=
     Medium.setState_pTX(T=Medium.T_default, p=Medium.p_default, X=Medium.X_default);
  parameter Modelica.SIunits.DynamicViscosity eta_nominal=Medium.dynamicViscosity(sta0) 
    "Dynamic viscosity, used to compute transition to turbulent flow regime";
  final parameter Boolean computeFlowResistance=(dp_nominal_pos > Modelica.Constants.eps) 
    "Flag to enable/disable computation of flow resistance";
protected 
  final parameter Modelica.SIunits.MassFlowRate m_flow_nominal_pos = abs(m_flow_nominal) 
    "Absolute value of nominal flow rate";
  final parameter Modelica.SIunits.Pressure dp_nominal_pos = abs(dp_nominal) 
    "Absolute value of nominal pressure";
initial equation 
  if computeFlowResistance then
    assert(m_flow_turbulent > 0, "m_flow_turbulent must be bigger than zero.");
  end if;
equation 
  // Pressure drop calculation
  if computeFlowResistance then
    if linearized then
      m_flow*m_flow_nominal_pos = k^2*dp;
    else
      if homotopyInitialization then
        if from_dp then
          m_flow=homotopy(actual=FlowModels.basicFlowFunction_dp(dp=dp, k=k,
                                   m_flow_turbulent=m_flow_turbulent),
                                   simplified=m_flow_nominal_pos*dp/dp_nominal_pos);
        else
          dp=homotopy(actual=FlowModels.basicFlowFunction_m_flow(m_flow=m_flow, k=k,
                                   m_flow_turbulent=m_flow_turbulent),
                    simplified=dp_nominal_pos*m_flow/m_flow_nominal_pos);
         end if;  // from_dp
      else // do not use homotopy
        if from_dp then
          m_flow=FlowModels.basicFlowFunction_dp(dp=dp, k=k, m_flow_turbulent=m_flow_turbulent);
        else
          dp=FlowModels.basicFlowFunction_m_flow(m_flow=m_flow, k=k, m_flow_turbulent=m_flow_turbulent);
        end if;  // from_dp
      end if; // homotopyInitialization
    end if; // linearized
  else // do not compute flow resistance
    dp = 0;
  end if;  // computeFlowResistance

  // Isenthalpic state transformation (no storage and no loss of energy)
  port_a.h_outflow = inStream(port_b.h_outflow);
  port_b.h_outflow = inStream(port_a.h_outflow);

  // Mass balance (no storage)
  port_a.m_flow + port_b.m_flow = 0;

  // Transport of substances
  port_a.Xi_outflow = inStream(port_b.Xi_outflow);
  port_b.Xi_outflow = inStream(port_a.Xi_outflow);

  port_a.C_outflow = inStream(port_b.C_outflow);
  port_b.C_outflow = inStream(port_a.C_outflow);

end PartialResistance;

Buildings.Fluid.BaseClasses.PartialThreeWayResistance

Information

Partial model for flow resistances with three ports such as a flow mixer/splitter or a three way valve.

If dynamicBalance=true, then at the junction of the three flows, a mixing volume will be present. This will introduce a dynamic energy and momentum balance, which often breaks algebraic loops. The time constant of the mixing volume is determined by the parameter tau.

Extends from Buildings.Fluid.Interfaces.LumpedVolumeDeclarations (Declarations for lumped volumes).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
PartialTwoPortInterface	res1	redeclare Buildings.Fluid.In...	Partial model, to be replaced with a fluid component
PartialTwoPortInterface	res2	redeclare Buildings.Fluid.In...	Partial model, to be replaced with a fluid component
PartialTwoPortInterface	res3	redeclare Buildings.Fluid.In...	Partial model, to be replaced with a fluid component
Dynamics
Equations
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of energy balance
Dynamics	massDynamics	energyDynamics	Formulation of mass balance
Boolean	dynamicBalance	true	Set to true to use a dynamic balance, which often leads to smaller systems of equations
MassFlowRate	mDyn_flow_nominal		Nominal mass flow rate for dynamic momentum and energy balance [kg/s]
Nominal condition
Time	tau	10	Time constant at nominal flow for dynamic energy and momentum balance [s]
Initialization
AbsolutePressure	p_start	Medium.p_default	Start value of pressure [Pa]
Temperature	T_start	Medium.T_default	Start value of temperature [K]
MassFraction	X_start[Medium.nX]	Medium.X_default	Start value of mass fractions m_i/m [kg/kg]
ExtraProperty	C_start[Medium.nC]	fill(0, Medium.nC)	Start value of trace substances
ExtraProperty	C_nominal[Medium.nC]	fill(1E-2, Medium.nC)	Nominal value of trace substances. (Set to typical order of magnitude.)
Advanced
Boolean	from_dp	true	= true, use m_flow = f(dp) else dp = f(m_flow)
Boolean	homotopyInitialization	true	= true, use homotopy method

Connectors

Type	Name	Description
FluidPort_a	port_1
FluidPort_b	port_2
FluidPort_a	port_3

Modelica definition

partial model PartialThreeWayResistance 
  "Flow splitter with partial resistance model at each port"
  extends Buildings.Fluid.Interfaces.LumpedVolumeDeclarations;
  outer Modelica.Fluid.System system "System properties";

  Modelica.Fluid.Interfaces.FluidPort_a port_1(redeclare package Medium =
        Medium, m_flow(min=if (portFlowDirection_1 == Modelica.Fluid.Types.PortFlowDirection.Entering) then 
                0.0 else -Modelica.Constants.inf, max=if (portFlowDirection_1
           == Modelica.Fluid.Types.PortFlowDirection.Leaving) then 0.0 else Modelica.Constants.inf));
  Modelica.Fluid.Interfaces.FluidPort_b port_2(redeclare package Medium =
        Medium, m_flow(min=if (portFlowDirection_2 == Modelica.Fluid.Types.PortFlowDirection.Entering) then 
                0.0 else -Modelica.Constants.inf, max=if (portFlowDirection_2
           == Modelica.Fluid.Types.PortFlowDirection.Leaving) then 0.0 else Modelica.Constants.inf));
  Modelica.Fluid.Interfaces.FluidPort_a port_3(
    redeclare package Medium=Medium,
    m_flow(min=if (portFlowDirection_3==Modelica.Fluid.Types.PortFlowDirection.Entering) then 0.0 else -Modelica.Constants.inf,
    max=if (portFlowDirection_3==Modelica.Fluid.Types.PortFlowDirection.Leaving) then 0.0 else Modelica.Constants.inf));

  parameter Boolean from_dp = true 
    "= true, use m_flow = f(dp) else dp = f(m_flow)";
  parameter Boolean homotopyInitialization = true "= true, use homotopy method";

  replaceable Buildings.Fluid.Interfaces.PartialTwoPortInterface res1(redeclare 
      package Medium = Medium, allowFlowReversal=true, homotopyInitialization=homotopyInitialization) 
    "Partial model, to be replaced with a fluid component";
  replaceable Buildings.Fluid.Interfaces.PartialTwoPortInterface res2(redeclare 
      package Medium = Medium, allowFlowReversal=true, homotopyInitialization=homotopyInitialization) 
    "Partial model, to be replaced with a fluid component";
  replaceable Buildings.Fluid.Interfaces.PartialTwoPortInterface res3(redeclare 
      package Medium = Medium, allowFlowReversal=true, homotopyInitialization=homotopyInitialization) 
    "Partial model, to be replaced with a fluid component";

protected 
  parameter Modelica.Fluid.Types.PortFlowDirection portFlowDirection_1=Modelica.Fluid.Types.PortFlowDirection.Bidirectional 
    "Flow direction for port_1";
  parameter Modelica.Fluid.Types.PortFlowDirection portFlowDirection_2=Modelica.Fluid.Types.PortFlowDirection.Bidirectional 
    "Flow direction for port_2";
  parameter Modelica.Fluid.Types.PortFlowDirection portFlowDirection_3=Modelica.Fluid.Types.PortFlowDirection.Bidirectional 
    "Flow direction for port_3";

public 
  Buildings.Fluid.Delays.DelayFirstOrder vol(
    redeclare final package Medium = Medium,
    final nPorts=3,
    final tau=tau,
    final m_flow_nominal=mDyn_flow_nominal,
    final energyDynamics=energyDynamics,
    final massDynamics=massDynamics,
    final p_start=p_start,
    final T_start=T_start,
    final X_start=X_start,
    final C_start=C_start,
    final allowFlowReversal=true,
    final prescribedHeatFlowRate=false) if 
       dynamicBalance "Fluid volume to break algebraic loop";
  parameter Boolean dynamicBalance = true 
    "Set to true to use a dynamic balance, which often leads to smaller systems of equations";

  parameter Modelica.SIunits.Time tau=10 
    "Time constant at nominal flow for dynamic energy and momentum balance";
  parameter Modelica.SIunits.MassFlowRate mDyn_flow_nominal 
    "Nominal mass flow rate for dynamic momentum and energy balance";

equation 
  connect(port_1, res1.port_a);
  connect(res2.port_a, port_2);
  connect(res3.port_a, port_3);
  connect(res1.port_b,vol. ports[1]);
  connect(res2.port_b,vol. ports[2]);
  connect(res3.port_b,vol. ports[3]);
  if not dynamicBalance then
    connect(res1.port_b, res3.port_b);
    connect(res1.port_b, res2.port_b);
  end if;
end PartialThreeWayResistance;