Buildings.Fluids.Components

Package Content

Name	Description
MixingVolume	Mixing volume with inlet and outlet ports (flow reversal is allowed)

Buildings.Fluids.Components.MixingVolume

Information

This model represents the same physics as Modelica_Fluid.Volumes.MixingVolume, but it allows to have more than two fluid ports. This is convenient for modeling the room volume in a building energy simulation since rooms often have more than two fluid connections, such as an HVAC inlet, outlet and a leakage flow to other rooms or the outside. If a fluid port is connected twice, the model will terminate the simulation with an error message.

The thermal port need not be connected, but can have any number of connections.

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	V		Volume [m3]
Integer	nP	2	Number of ports
Initialization
Temp	initType	Types.Init.NoInit	Initialization option
AbsolutePressure	p_start	Medium.p_default	Start value of pressure [Pa]
Boolean	use_T_start	true	= true, use T_start, otherwise h_start
Temperature	T_start	if use_T_start then Medium.T...	Start value of temperature [K]
SpecificEnthalpy	h_start	if use_T_start then Medium.s...	Start value of specific enthalpy [J/kg]
MassFraction	X_start[Medium.nX]	Medium.X_default	Start value of mass fractions m_i/m [kg/kg]

Connectors

Type	Name	Description
FluidPort_a	port[nP]	Fluid port
HeatPort_a	thermalPort	Thermal port

Modelica definition

model MixingVolume 
  "Mixing volume with inlet and outlet ports (flow reversal is allowed)" 
  extends Modelica_Fluid.Interfaces.PartialInitializationParameters;
  replaceable package Medium = Modelica.Media.Interfaces.PartialMedium 
    "Medium in the component";
  parameter Modelica.SIunits.Volume V "Volume";
  parameter Integer nP(min=1) = 2 "Number of ports";
  Modelica_Fluid.Interfaces.FluidPort_a port[nP](redeclare each package Medium 
      =                                                                          Medium) 
    "Fluid port";
  Medium.BaseProperties medium(
    preferredMediumStates=true,
    p(start=p_start),
    h(start=h_start),
    T(start=T_start),
    Xi(start=X_start[1:Medium.nXi]));
  Modelica.SIunits.Energy U "Internal energy of fluid";
  Modelica.SIunits.Mass m "Mass of fluid";
  Modelica.SIunits.Mass mXi[Medium.nXi] 
    "Masses of independent components in the fluid";
  Modelica.SIunits.Volume V_lumped=V "Volume";
  
protected 
  Modelica.SIunits.HeatFlowRate Qs_flow 
    "Heat flow across boundaries or energy source/sink";
  Modelica.SIunits.Power Ws_flow=0 "Work flow across boundaries or source term";
  
public 
Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a thermalPort "Thermal port";
equation 
  thermalPort.T = medium.T;
  Qs_flow = thermalPort.Q_flow;
// boundary conditions  
  for i in 1:nP loop
    port[i].p = medium.p;
    port[i].H_flow = semiLinear(
      port[i].m_flow,
      port[i].h,
      medium.h);
    port[i].mXi_flow = semiLinear(
      port[i].m_flow,
      port[i].Xi,
      medium.Xi);
  end for;
// Total quantities
  m = V_lumped*medium.d;
  mXi = m*medium.Xi;
  U = m*medium.u;
  
// Mass and energy balance
  der(m) = sum(port[i].m_flow for i in 1:nP);
  der(mXi) = sum(port[i].mXi_flow for i in 1:nP);
  der(U) = sum(port[i].H_flow for i in 1:nP) + Qs_flow + Ws_flow;
  
initial equation 
// Initial conditions
  for i in 1:nP loop
    assert(cardinality(port[i]) == 1, "Only one port connection allowed. To fix, increase nP and use new port.");
  end for;
  
  if initType == Modelica_Fluid.Types.Init.NoInit then
  // no initial equations
  elseif initType == Modelica_Fluid.Types.Init.InitialValues then
    if not Medium.singleState then
      medium.p = p_start;
    end if;
    if use_T_start then
      medium.T = T_start;
    else
      medium.h = h_start;
    end if;
    medium.Xi = X_start[1:Medium.nXi];
  elseif initType == Modelica_Fluid.Types.Init.SteadyState then
    if not Medium.singleState then
      der(medium.p) = 0;
    end if;
    der(medium.h) = 0;
    der(medium.Xi) = zeros(Medium.nXi);
  elseif initType == Modelica_Fluid.Types.Init.SteadyStateHydraulic then
    if not Medium.singleState then
      der(medium.p) = 0;
    end if;
    if use_T_start then
      medium.T = T_start;
    else
      medium.h = h_start;
    end if;
    medium.Xi = X_start[1:Medium.nXi];
  else
    assert(false, "Unsupported initialization option");
  end if;
end MixingVolume;