Buildings.Fluid.FixedResistances

Package with models for fixed flow resistances (pipes, diffusers etc.)

Package Content

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


Buildings.Fluid.FixedResistances.FixedResistanceDpM Buildings.Fluid.FixedResistances.FixedResistanceDpM

Fixed flow resistance with dp and m_flow as parameter

Buildings.Fluid.FixedResistances.FixedResistanceDpM

Information


This is a model of a resistance with a fixed flow coefficient k = m_flow/sqrt(dP).

Near the origin, the square root relation is regularized to ensure that the derivative is bounded.

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

Parameters

TypeNameDefaultDescription
replaceable package MediumPartialMediumMedium in the component
Booleanuse_dhfalseSet to true to specify hydraulic diameter
Lengthdh1Hydraulic diameter [m]
RealReC4000Reynolds number where transition to turbulent starts
RealdeltaM0.3Fraction of nominal mass flow rate where transition to turbulent occurs
Nominal condition
MassFlowRatem_flow_nominal Nominal mass flow rate [kg/s]
Pressuredp_nominal Pressure drop at nominal mass flow rate [Pa]
Initialization
MassFlowRatem_flow.start0Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressuredp.start0Pressure difference between port_a and port_b [Pa]
Assumptions
BooleanallowFlowReversalsystem.allowFlowReversal= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRatem_flow_small1E-4*m_flow_nominalSmall mass flow rate for regularization of zero flow [kg/s]
Booleanfrom_dpfalse= true, use m_flow = f(dp) else dp = f(m_flow)
Booleanlinearizedfalse= true, use linear relation between m_flow and dp for any flow rate
Diagnostics
Booleanshow_V_flowfalse= true, if volume flow rate at inflowing port is computed
Booleanshow_Tfalse= true, if actual temperature at port is computed (may lead to events)

Connectors

TypeNameDescription
FluidPort_aport_aFluid connector a (positive design flow direction is from port_a to port_b)
FluidPort_bport_bFluid 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=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 
 if ( m_flow_turbulent > m_flow_nominal) then
   Modelica.Utilities.Streams.print("Warning: In FixedResistanceDpM, m_flow_nominal is smaller than m_flow_turbulent."
           + "\n"
           + "  m_flow_nominal = " + realString(m_flow_nominal) + "\n"
           + "  dh      = " + realString(dh) + "\n"
           + "  To fix, set dh < " +
                realString(     4*m_flow_nominal/eta_nominal/Modelica.Constants.pi/ReC) + "\n"
           + "  Suggested value: dh = " +
                realString(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;
    if linearized then
     k = m_flow_nominal / dp_nominal / conv2;
    else
     k = m_flow_nominal / sqrt(dp_nominal);
    end if;
  else
      m_flow_turbulent = 0;
      k = 0;
   end if;
end FixedResistanceDpM;

Buildings.Fluid.FixedResistances.LosslessPipe Buildings.Fluid.FixedResistances.LosslessPipe

Pipe with no flow friction and no heat transfer

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.PartialStaticTwoPortInterface (Partial model transporting fluid between two ports without storing mass or energy), Buildings.BaseClasses.BaseIcon (Base icon).

Parameters

TypeNameDefaultDescription
replaceable package MediumPartialMediumMedium in the component
Nominal condition
MassFlowRatem_flow_nominal Nominal mass flow rate [kg/s]
Initialization
MassFlowRatem_flow.start0Mass flow rate from port_a to port_b (m_flow > 0 is design flow direction) [kg/s]
Pressuredp.start0Pressure difference between port_a and port_b [Pa]
Assumptions
BooleanallowFlowReversalsystem.allowFlowReversal= true to allow flow reversal, false restricts to design direction (port_a -> port_b)
Advanced
MassFlowRatem_flow_small1E-4*m_flow_nominalSmall mass flow rate for regularization of zero flow [kg/s]
Diagnostics
Booleanshow_V_flowfalse= true, if volume flow rate at inflowing port is computed
Booleanshow_Tfalse= true, if actual temperature at port is computed (may lead to events)

Connectors

TypeNameDescription
FluidPort_aport_aFluid connector a (positive design flow direction is from port_a to port_b)
FluidPort_bport_bFluid 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.PartialStaticTwoPortInterface(
     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 Buildings.Fluid.FixedResistances.SplitterFixedResistanceDpM

Flow splitter with fixed resistance at each port

Buildings.Fluid.FixedResistances.SplitterFixedResistanceDpM

Information


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

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

Parameters

TypeNameDefaultDescription
replaceable package MediumPartialMediumFluid medium model
Booleanuse_dhfalseSet to true to specify hydraulic diameter
RealdeltaM0.3Fraction of nominal mass flow rate where transition to turbulent occurs
Lengthdh[3]{1,1,1}Hydraulic diameter [m]
RealReC[3]{4000,4000,4000}Reynolds number where transition to turbulent starts
Nominal condition
MassFlowRatem_flow_nominal[3] Mass flow rate [kg/s]
Pressuredp_nominal[3] Pressure [Pa]
Advanced
Booleanfrom_dptrue= true, use m_flow = f(dp) else dp = f(m_flow)
Booleanlinearizedfalse= true, use linear relation between m_flow and dp for any flow rate
Assumptions
Dynamics
BooleandynamicBalancetrueSet to true to use a dynamic balance, which often leads to smaller systems of equations
Timetau10Time constant at nominal flow for dynamic energy and momentum balance [s]
MassFlowRatemDyn_flow_nominalsum(m_flow_nominal[:]/3)Nominal mass flow rate for dynamic momentum and energy balance [kg/s]
DynamicsenergyDynamicssystem.energyDynamicsFormulation of energy balance
DynamicsmassDynamicssystem.massDynamicsFormulation of mass balance
Initialization
AbsolutePressurep_startMedium.p_defaultStart value of pressure [Pa]
Booleanuse_T_starttrue= true, use T_start, otherwise h_start
TemperatureT_startif use_T_start then Medium.T...Start value of temperature [K]
SpecificEnthalpyh_startif use_T_start then Medium.s...Start value of specific enthalpy [J/kg]
MassFractionX_start[Medium.nX]Medium.X_defaultStart value of mass fractions m_i/m [kg/kg]
ExtraPropertyC_start[Medium.nC]fill(0, Medium.nC)Start value of trace substances

Connectors

TypeNameDescription
FluidPort_aport_1 
FluidPort_bport_2 
FluidPort_aport_3 

Modelica definition

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

  parameter Boolean use_dh = false "Set to true to specify hydraulic diameter";
  parameter Modelica.SIunits.MassFlowRate[3] m_flow_nominal(each min=0) 
    "Mass flow rate";
  parameter Modelica.SIunits.Pressure[3] dp_nominal(each min=0) "Pressure";
  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={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";
end SplitterFixedResistanceDpM;

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