Buildings.Fluid.MixingVolumes

Information

This package contains models for completely mixed volumes.

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 in a coil with water vapor condensation.

Package Content

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

Buildings.Fluid.MixingVolumes.MixingVolume

Information

This model represents an instantaneously mixed volume. Potential and kinetic energy at the port are neglected, and there is no pressure drop at the ports. The volume can exchange heat through its heatPort.

The volume can be parameterized as a steady-state model or as dynamic model.

To increase the numerical robustness of the model, the parameter prescribedHeatFlowRate can be set by the user. This parameter only has an effect if the model has exactly two fluid ports connected, and if it is used as a steady-state model. Use the following settings:

• Set prescribedHeatFlowRate=true if there is a model connected to heatPort that computes the heat flow rate not as a function of the temperature difference between the medium and an ambient temperature. Examples include an ideal electrical heater, a pump that rejects heat into the fluid stream, or a chiller that removes heat based on a performance curve.
• Set prescribedHeatFlowRate=true if the only means of heat flow at the heatPort is computed as K * (T-heatPort.T), for some temperature T and some conductance K, which may itself be a function of temperature or mass flow rate.

Implementation

If the model is operated in steady-state and has two fluid ports connected, then the same energy and mass balance implementation is used as in steady-state component models, i.e., the use of actualStream is not used for the properties at the port.

The implementation of these balance equations is done in the instances dynBal for the dynamic balance and steBal for the steady-state balance. Both models use the same input variables:

• The variable Q_flow is used to add sensible and latent heat to the fluid. For example, Q_flow participates in the steady-state energy balance

      port_b.h_outflow = inStream(port_a.h_outflow) + Q_flow * m_flowInv;
  

  where m_flowInv approximates the expression 1/m_flow.
• The variable mXi_flow is used to add a species mass flow rate to the fluid.

For simple models that uses this model, see Buildings.Fluid.HeatExchangers.HeaterCoolerPrescribed and Buildings.Fluid.MassExchangers.HumidifierPrescribed.

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	V		Volume [m3]
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Dynamics
Equations
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of energy balance
Dynamics	massDynamics	energyDynamics	Formulation of mass balance
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
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
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal in medium, false restricts to design direction (ports[1] -> ports[2]). Used only if model has two ports.
Heat transfer
Boolean	prescribedHeatFlowRate	false	Set to true if the model has a prescribed heat flow at its heatPort

Connectors

Type	Name	Description
VesselFluidPorts_b	ports[nPorts]	Fluid inlets and outlets
HeatPort_a	heatPort	Heat port connected to outflowing medium

Modelica definition

model MixingVolume 
  "Mixing volume with inlet and outlet ports (flow reversal is allowed)"
  extends Buildings.Fluid.MixingVolumes.BaseClasses.PartialMixingVolume;
protected 
  Modelica.Blocks.Sources.Constant masExc(k=0) 
    "Block to set mass exchange in volume";
  Modelica.Blocks.Sources.RealExpression heaInp(y=heatPort.Q_flow) 
    "Block to set heat input into volume";
equation 
  connect(heaInp.y, steBal.Q_flow);
  connect(heaInp.y, dynBal.Q_flow);
  connect(masExc.y, dynBal.mWat_flow);
  connect(masExc.y, steBal.mWat_flow);
end MixingVolume;

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. 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.MixingVolume.

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Volume	V		Volume [m3]
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Dynamics
Equations
Dynamics	energyDynamics	Modelica.Fluid.Types.Dynamic...	Formulation of energy balance
Dynamics	massDynamics	energyDynamics	Formulation of mass balance
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
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
Assumptions
Boolean	allowFlowReversal	system.allowFlowReversal	= true to allow flow reversal in medium, false restricts to design direction (ports[1] -> ports[2]). Used only if model has two ports.
Heat transfer
Boolean	prescribedHeatFlowRate	false	Set to true if the model has a prescribed heat flow at its heatPort

Connectors

Type	Name	Description
replaceable package Medium		Medium in the component
VesselFluidPorts_b	ports[nPorts]	Fluid inlets and outlets
HeatPort_a	heatPort	Heat port connected to outflowing medium
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.PartialMixingVolume(
    redeclare replaceable package Medium =
        Modelica.Media.Interfaces.PartialCondensingGases,
    steBal(final sensibleOnly = false));

  Modelica.Blocks.Interfaces.RealInput mWat_flow(final quantity="MassFlowRate",
                                                 final unit = "kg/s") 
    "Water flow rate added into the medium";
  Modelica.Blocks.Interfaces.RealInput TWat(final quantity="ThermodynamicTemperature",
                                            final unit = "K", displayUnit = "degC", min=260) 
    "Temperature of liquid that is drained from or injected into volume";
  Modelica.Blocks.Interfaces.RealOutput X_w "Species composition of medium";

  Modelica.SIunits.HeatFlowRate HWat_flow = mWat_flow * Medium.enthalpyOfLiquid(TWat) 
    "Enthalpy flow rate of extracted water";
protected 
  parameter Integer i_w(fixed=false) "Index for water substance";
  parameter Real s[Medium.nXi](each fixed=false) 
    "Vector with zero everywhere except where species is";

  Modelica.Blocks.Sources.RealExpression heaInp(final y=
     heatPort.Q_flow + HWat_flow) "Block to set heat input into volume";
initial algorithm 
  i_w := 0;
  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.");

equation 
// Medium species concentration
  X_w = s * Xi;

  connect(heaInp.y, steBal.Q_flow);
  connect(heaInp.y, dynBal.Q_flow);
  connect(mWat_flow, steBal.mWat_flow);
  connect(mWat_flow, dynBal.mWat_flow);
end MixingVolumeMoistAir;