Buildings.Fluid.MixingVolumes

Information

This package contains models for completely mixed volumes. Optionally, heat can be added to the volume by setting the parameter use_HeatTransfer to true.

For most situations, the model Buildings.Fluid.MixingVolumes.MixingVolume should be used. The other models are only of interest if water should be added to or subtracted from the fluid volume, such as needed in a dynamic model of a coil with water vapor condensation.

Package Content

Name	Description
MixingVolume	Mixing volume with inlet and outlet ports (flow reversal is allowed)
MixingVolumeDryAir	Mixing volume with heat port for latent heat exchange, to be used with dry air
MixingVolumeMoistAir	Mixing volume with heat port for latent heat exchange, to be used with media that contain water
Examples	Collection of models that illustrate model use and test models
BaseClasses	Package with base classes for mixing volumes

Buildings.Fluid.MixingVolumes.MixingVolume

Information

Extends from Buildings.Fluid.Interfaces.PartialLumpedVolume (Lumped volume with mass and energy balance), Buildings.BaseClasses.BaseIcon (Base icon).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	fluidVolume	V	Volume [m3]
Volume	V		Volume [m3]
Assumptions
Dynamics
Dynamics	energyDynamics	system.energyDynamics	Formulation of energy balance
Dynamics	massDynamics	system.massDynamics	Formulation of mass balance
Dynamics	substanceDynamics	energyDynamics	Formulation of substance balance
Dynamics	traceDynamics	energyDynamics	Formulation of trace substance balance
Heat transfer
Boolean	use_HeatTransfer	false	= true to use the HeatTransfer model
Initialization
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 system.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]
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.)

Connectors

Type	Name	Description
VesselFluidPorts_b	ports[nPorts]	Fluid inlets and outlets
HeatPort_a	heatPort

Modelica definition

model MixingVolume 
  "Mixing volume with inlet and outlet ports (flow reversal is allowed)"
  extends Buildings.Fluid.Interfaces.PartialLumpedVolume(
      m(start=V*rho_nominal, fixed=false),
      final fluidVolume = V,
      mC(nominal=V*rho_nominal*C_nominal));
  extends Buildings.BaseClasses.BaseIcon;
    import Modelica.Constants.pi;

  // Port definitions
  parameter Integer nPorts=0 "Number of ports";
  Modelica.Fluid.Vessels.BaseClasses.VesselFluidPorts_b ports[nPorts](
      redeclare each package Medium = Medium) "Fluid inlets and outlets";

//  parameter Modelica.SIunits.MassFlowRate m_flow_small(min=0)=system.m_flow_small
//    "Regularization range at zero mass flow rate"
//    annotation(Dialog(tab="Advanced", group="Port properties", enable=stiffCharacteristicForEmptyPort));
  Medium.EnthalpyFlowRate ports_H_flow[nPorts];
  Medium.MassFlowRate ports_mXi_flow[nPorts,Medium.nXi];
  Medium.MassFlowRate[Medium.nXi] sum_ports_mXi_flow 
    "Substance mass flows through ports";
  Medium.ExtraPropertyFlowRate ports_mC_flow[nPorts,Medium.nC];
  Medium.ExtraPropertyFlowRate[Medium.nC] sum_ports_mC_flow 
    "Trace substance mass flows through ports";

  // Heat transfer through boundary
  parameter Boolean use_HeatTransfer = false 
    "= true to use the HeatTransfer model";
  replaceable model HeatTransfer =
      Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.IdealHeatTransfer
    constrainedby 
    Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.PartialVesselHeatTransfer 
    "Wall heat transfer";
  HeatTransfer heatTransfer(
    redeclare final package Medium = Medium,
    surfaceAreas={4*pi*(3/4*V/pi)^(2/3)},
    final n=1,
    final states = {medium.state},
    final use_k = use_HeatTransfer);
  Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_a heatPort if use_HeatTransfer;

  parameter Modelica.SIunits.Volume V "Volume";

protected 
   parameter Modelica.SIunits.Density rho_nominal=Medium.density(
     Medium.setState_pTX(
      T=T_start,
      p=p_start,
      X=X_start[1:Medium.nXi])) "Density, used to compute fluid mass";

equation 
// Only one connection allowed to a port to avoid unwanted ideal mixing
  for i in 1:nPorts loop
    assert(cardinality(ports[i]) <= 1,"
each ports[i] of volume can at most be connected to one component.
If two or more connections are present, ideal mixing takes
place with these connections, which is usually not the intention
of the modeller. Increase nPorts to add an additional port.
");
  end for;

  // actual definition of port variables
  for i in 1:nPorts loop
    // fluid flow through ports
      // regular operation: fluidLevel is above ports[i]
      // Note: >= covers default values of zero as well
    ports[i].p          = medium.p;
    ports[i].h_outflow  = medium.h;
    ports[i].Xi_outflow = medium.Xi;
    ports[i].C_outflow  = C;

    ports_H_flow[i]     = ports[i].m_flow * actualStream(ports[i].h_outflow) 
      "Enthalpy flow";
    ports_mXi_flow[i,:] = ports[i].m_flow * actualStream(ports[i].Xi_outflow) 
      "Component mass flow";
    ports_mC_flow[i,:]  = ports[i].m_flow * actualStream(ports[i].C_outflow) 
      "Trace substance mass flow";
  end for;

  for i in 1:Medium.nXi loop
    sum_ports_mXi_flow[i] = sum(ports_mXi_flow[:,i]);
  end for;

  for i in 1:Medium.nC loop
    sum_ports_mC_flow[i]  = sum(ports_mC_flow[:,i]);
  end for;
  mb_flow = sum(ports.m_flow);
  mbXi_flow = sum_ports_mXi_flow;
  mbC_flow  = sum_ports_mC_flow;
  Hb_flow = sum(ports_H_flow);
  Qb_flow = heatTransfer.Q_flows[1];

  connect(heatPort, heatTransfer.heatPorts[1]);

end MixingVolume;

Buildings.Fluid.MixingVolumes.MixingVolumeDryAir

Information

This model has the same ports as Buildings.Fluid.MixingVolumes.MixingVolumeMoistAir. However, there is no mass exchange with the medium other than through the port ports.

For media that do provide water as a species, use the model Buildings.Fluid.MixingVolumes.MixingVolumeMoistAir to add or subtract moisture using a signal that is connected to the port mWat_flow and TWat.

Extends from BaseClasses.PartialMixingVolumeWaterPort (Partial mixing volume that allows adding or subtracting water vapor).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	V		Volume [m3]
Assumptions
Dynamics
Dynamics	energyDynamics	system.energyDynamics	Formulation of energy balance
Dynamics	massDynamics	system.massDynamics	Formulation of mass balance
Dynamics	substanceDynamics	energyDynamics	Formulation of substance balance
Dynamics	traceDynamics	energyDynamics	Formulation of trace substance balance
Heat transfer
Boolean	use_HeatTransfer	false	= true to use the HeatTransfer model
replaceable model HeatTransfer		Modelica.Fluid.Vessels.BaseC...	Wall heat transfer
Initialization
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 system.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]
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.)

Connectors

Type	Name	Description
FluidPorts_b	ports[nPorts]	Fluid outlets
HeatPort_a	heatPort
input RealInput	mWat_flow	Water flow rate added into the medium [kg/s]
input RealInput	TWat	Temperature of liquid that is drained from or injected into volume [K]
output RealOutput	X_w	Species composition of medium

Modelica definition

model MixingVolumeDryAir 
  "Mixing volume with heat port for latent heat exchange, to be used with dry air"
  extends BaseClasses.PartialMixingVolumeWaterPort;

equation 
  if cardinality(mWat_flow) == 0 then
    mWat_flow = 0;
  end if;
  if cardinality(TWat) == 0 then
    TWat = Medium.T_default;
  end if;
  HWat_flow = 0;
  mXi_flow  = zeros(Medium.nXi);
// Assign output port
  X_w = 0;
end MixingVolumeDryAir;

Buildings.Fluid.MixingVolumes.MixingVolumeMoistAir

Information

This model represents the same physics as Buildings.Fluid.MixingVolumes.MixingVolume, but in addition, it allows adding or subtracting water in liquid phase. The mass flow rate of the added or subtracted water is specified at the port mWat_flow. The water flow rate is assumed to be added or subtracted at the temperature of the input port TWat, or if this port is not connected, at the medium default temperature as defined by Medium.T_default. Adding water causes a change in enthalpy and species concentration in the volume.

Note that this model can only be used with medium models that include water as a substance. In particular, the medium model needs to implement the function enthalpyOfLiquid(T) and the integer variable Water that contains the index to the water substance. For medium that do not provide this functionality, use Buildings.Fluid.MixingVolumes.MixingVolumeDryAir.

Extends from BaseClasses.PartialMixingVolumeWaterPort (Partial mixing volume that allows adding or subtracting water vapor).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	V		Volume [m3]
Assumptions
Dynamics
Dynamics	energyDynamics	system.energyDynamics	Formulation of energy balance
Dynamics	massDynamics	system.massDynamics	Formulation of mass balance
Dynamics	substanceDynamics	energyDynamics	Formulation of substance balance
Dynamics	traceDynamics	energyDynamics	Formulation of trace substance balance
Heat transfer
Boolean	use_HeatTransfer	false	= true to use the HeatTransfer model
replaceable model HeatTransfer		Modelica.Fluid.Vessels.BaseC...	Wall heat transfer
Initialization
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 system.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]
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.)

Connectors

Type	Name	Description
FluidPorts_b	ports[nPorts]	Fluid outlets
HeatPort_a	heatPort
input RealInput	mWat_flow	Water flow rate added into the medium [kg/s]
input RealInput	TWat	Temperature of liquid that is drained from or injected into volume [K]
output RealOutput	X_w	Species composition of medium

Modelica definition

model MixingVolumeMoistAir 
  "Mixing volume with heat port for latent heat exchange, to be used with media that contain water"
  extends BaseClasses.PartialMixingVolumeWaterPort;
  // redeclare Medium with a more restricting base class. This improves the error
  // message if a user selects a medium that does not contain the function
  // enthalpyOfLiquid(.)
  replaceable package Medium = Modelica.Media.Interfaces.PartialCondensingGases;

protected 
  parameter Integer i_w(min=1, fixed=false) "Index for water substance";
  parameter Real s[Medium.nXi](fixed=false) 
    "Vector with zero everywhere except where species is";
initial algorithm 
  i_w:= -1;
  if cardinality(mWat_flow) > 0 then
  for i in 1:Medium.nXi loop
      if Modelica.Utilities.Strings.isEqual(string1=Medium.substanceNames[i],
                                            string2="Water",
                                            caseSensitive=false) then
      i_w := i;
      s[i] :=1;
    else
      s[i] :=0;
    end if;
   end for;
    assert(i_w > 0, "Substance 'water' is not present in medium '"
         + Medium.mediumName + "'.\n"
         + "Check medium model.");
    end if;

equation 
  if cardinality(mWat_flow) == 0 then
    mWat_flow = 0;
    HWat_flow = 0;
    mXi_flow  = zeros(Medium.nXi);
  else
    if cardinality(TWat) == 0 then
       HWat_flow = mWat_flow * Medium.enthalpyOfLiquid(Medium.T_default);
    else
       HWat_flow = mWat_flow * Medium.enthalpyOfLiquid(TWat);
    end if;
  // We obtain the substance concentration with a vector multiplication
  // because Dymola 7.4 cannot find the derivative in the model
  // Buildings.Fluid.HeatExchangers.Examples.WetCoilDiscretizedPControl
  // if we set mXi_flow[i] = if ( i == i_w) then mWat_flow else 0;
    mXi_flow = mWat_flow * s;
  end if;
// Medium species concentration
  X_w = s*medium.Xi;
end MixingVolumeMoistAir;

Buildings.Fluid.MixingVolumes.MixingVolume.HeatTransfer

Parameters

Type	Name	Default	Description
Ambient
CoefficientOfHeatTransfer	k	0	Heat transfer coefficient to ambient [W/(m2.K)]
Temperature	T_ambient	system.T_ambient	Ambient temperature [K]
Internal Interface
replaceable package Medium		PartialMedium	Medium in the component
Integer	n	1	Number of heat transfer segments
Boolean	use_k	false	= true to use k value for thermal isolation

Connectors

Type	Name	Description
HeatPorts_a	heatPorts[n]	Heat port to component boundary

Modelica definition

replaceable model HeatTransfer =
    Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.IdealHeatTransfer
  constrainedby 
  Modelica.Fluid.Vessels.BaseClasses.HeatTransfer.PartialVesselHeatTransfer 
  "Wall heat transfer";