Buildings.Fluid.FixedResistances

Information

k = m ⁄ √ΔP.

For models of valves and air dampers, see Buildings.Fluid.Actuators. For models of flow resistances as part of the building constructions, see Buildings.Airflow.Multizone.

The model Buildings.Fluid.FixedResistances.FixedResistanceDpM is a fixed flow resistance that takes as parameter a nominal flow rate and a nominal pressure drop. The actual resistance is scaled using the above equation.

The model Buildings.Fluid.FixedResistances.LosslessPipe is an ideal pipe segment with no pressure drop. It is primarily used in models in which the above pressure drop model need to be replaced by a model with no pressure drop.

The model Buildings.Fluid.FixedResistances.SplitterFixedResistanceDpM can be used to model flow splitters or flow merges.

Extends from Modelica.Icons.VariantsPackage (Icon for package containing variants).

Package Content

Name	Description
FixedResistanceDpM	Fixed flow resistance with dp and m_flow as parameter
LosslessPipe	Pipe with no flow friction and no heat transfer
SplitterFixedResistanceDpM	Flow splitter with fixed resistance at each port
Examples	Collection of models that illustrate model use and test models

Buildings.Fluid.FixedResistances.FixedResistanceDpM

Information

This is a model of a resistance with a fixed flow coefficient

k = m ⁄ √ΔP.

Extends from Buildings.Fluid.BaseClasses.PartialResistance (Partial model for a hydraulic resistance).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Boolean	use_dh	false	Set to true to specify hydraulic diameter
Length	dh	1	Hydraulic diameter [m]
Real	ReC	4000	Reynolds number where transition to turbulent starts
Real	deltaM	0.3	Fraction of nominal mass flow rate where transition to turbulent occurs
Nominal condition
MassFlowRate	m_flow_nominal		Nominal mass flow rate [kg/s]
Pressure	dp_nominal		Pressure drop at nominal mass flow rate [Pa]
Initialization
MassFlowRate	m_flow.start	0	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp.start	0	Pressure difference between port_a and port_b [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

model FixedResistanceDpM 
  "Fixed flow resistance with dp and m_flow as parameter"
  extends Buildings.Fluid.BaseClasses.PartialResistance;
  parameter Boolean use_dh = false "Set to true to specify hydraulic diameter";
  parameter Modelica.SIunits.Length dh=1 "Hydraulic diameter";
  parameter Real ReC(min=0)=4000 
    "Reynolds number where transition to turbulent starts";
  parameter Real deltaM(min=0.01) = 0.3 
    "Fraction of nominal mass flow rate where transition to turbulent occurs";
initial equation 
 assert(m_flow_nominal_pos > 0, "m_flow_nominal_pos must be non-zero. Check parameters.");
 if ( m_flow_turbulent > m_flow_nominal_pos) then
   Modelica.Utilities.Streams.print("Warning: In FixedResistanceDpM, m_flow_nominal is smaller than m_flow_turbulent."
           + "\n"
           + "  m_flow_nominal = " + String(m_flow_nominal) + "\n"
           + "  dh      = " + String(dh) + "\n"
           + "  To fix, set dh < " +
                String(     4*m_flow_nominal/eta_nominal/Modelica.Constants.pi/ReC) + "\n"
           + "  Suggested value: dh = " +
                String(1/10*4*m_flow_nominal/eta_nominal/Modelica.Constants.pi/ReC));
 end if;

equation 
 // if computeFlowResistance = false, then equations of this model are disabled.
 if computeFlowResistance then
   m_flow_turbulent = if use_dh then 
                      eta_nominal*dh/4*Modelica.Constants.pi*ReC else 
                      deltaM * m_flow_nominal_pos;
   k = m_flow_nominal_pos / sqrt(dp_nominal_pos);
 else
   m_flow_turbulent = 0;
   k = 0;
 end if;
end FixedResistanceDpM;

Buildings.Fluid.FixedResistances.LosslessPipe

Information

Model of a pipe with no flow resistance and no heat loss. This model can be used to replace a replaceable pipe model in flow legs in which no friction should be modeled, such as in the outlet port of a three way valve.

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

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]
Initialization
MassFlowRate	m_flow.start	0	Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressure	dp.start	0	Pressure difference between port_a and port_b [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
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

model LosslessPipe "Pipe with no flow friction and no heat transfer"
  extends Buildings.Fluid.Interfaces.PartialTwoPortInterface(
     show_T=false, show_V_flow=false);
  extends Buildings.BaseClasses.BaseIcon;
  final parameter Boolean from_dp=true "Used to satisfy replaceable models";
equation 
  dp=0;
  // 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 LosslessPipe;

Buildings.Fluid.FixedResistances.SplitterFixedResistanceDpM

Information

Model of a flow splitter or mixer with a fixed resistance in each flow leg.

Extends from Buildings.BaseClasses.BaseIcon (Base icon), Buildings.Fluid.BaseClasses.PartialThreeWayResistance (Flow splitter with partial resistance model at each port).

Parameters

Type	Name	Default	Description
replaceable package Medium		PartialMedium	Medium in the component
Boolean	use_dh	false	Set to true to specify hydraulic diameter
Real	deltaM	0.3	Fraction of nominal mass flow rate where transition to turbulent occurs
Length	dh[3]	{1,1,1}	Hydraulic diameter [m]
Real	ReC[3]	{4000,4000,4000}	Reynolds number where transition to turbulent starts
Nominal condition
MassFlowRate	m_flow_nominal[3]		Mass flow rate. Set negative at outflowing ports. [kg/s]
Pressure	dp_nominal[3]		Pressure. Set negative at outflowing ports. [Pa]
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	sum(abs(m_flow_nominal[:])/3)	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
Boolean	linearized	false	= true, use linear relation between m_flow and dp for any flow rate

Connectors

Type	Name	Description
FluidPort_a	port_1
FluidPort_b	port_2
FluidPort_a	port_3

Modelica definition

model SplitterFixedResistanceDpM 
  "Flow splitter with fixed resistance at each port"
    extends Buildings.BaseClasses.BaseIcon;
    extends Buildings.Fluid.BaseClasses.PartialThreeWayResistance(
    mDyn_flow_nominal = sum(abs(m_flow_nominal[:])/3),
      redeclare Buildings.Fluid.FixedResistances.FixedResistanceDpM res1(
         redeclare package Medium=Medium,
            final allowFlowReversal=true,
            from_dp=from_dp,
            final m_flow_nominal=m_flow_nominal[1],
            final dp_nominal=dp_nominal[1],
            final ReC=ReC[1],
            final dh=dh[1],
            linearized=linearized,
            homotopyInitialization=homotopyInitialization,
            deltaM=deltaM),
      redeclare Buildings.Fluid.FixedResistances.FixedResistanceDpM res2(
         redeclare package Medium=Medium,
            final allowFlowReversal=true,
            from_dp=from_dp,
            final m_flow_nominal=m_flow_nominal[2],
            final dp_nominal=dp_nominal[2],
            final ReC=ReC[2],
            final dh=dh[2],
            linearized=linearized,
            homotopyInitialization=homotopyInitialization,
            deltaM=deltaM),
      redeclare Buildings.Fluid.FixedResistances.FixedResistanceDpM res3(
         redeclare package Medium=Medium,
            final allowFlowReversal=true,
            from_dp=from_dp,
            final m_flow_nominal=m_flow_nominal[3],
            final dp_nominal=dp_nominal[3],
            final ReC=ReC[3],
            final dh=dh[3],
            linearized=linearized,
            homotopyInitialization=homotopyInitialization,
            deltaM=deltaM));

  parameter Boolean use_dh = false "Set to true to specify hydraulic diameter";
  parameter Modelica.SIunits.MassFlowRate[3] m_flow_nominal 
    "Mass flow rate. Set negative at outflowing ports.";
  parameter Modelica.SIunits.Pressure[3] dp_nominal(each displayUnit = "Pa") 
    "Pressure. Set negative at outflowing ports.";
  parameter Real deltaM(min=0) = 0.3 
    "Fraction of nominal mass flow rate where transition to turbulent occurs";

  parameter Modelica.SIunits.Length[3] dh={1, 1, 1} "Hydraulic diameter";
  parameter Real[3] ReC(each min=0)={4000, 4000, 4000} 
    "Reynolds number where transition to turbulent starts";
  parameter Boolean linearized = false 
    "= true, use linear relation between m_flow and dp for any flow rate";
  parameter Boolean homotopyInitialization = true "= true, use homotopy method";

end SplitterFixedResistanceDpM;