recordGeneric_epsNTU

Generic data record for a CDU that uses the epsilon-NTU method to compute the thermal performance

Extends from Modelica.Icons.Record (Icon for records).

Information

Generic record for a CDU that models the heat transfer using the epsilon-NTU method.

This is the base record that is used to characterize the performance data of CDU. The data record is structured as follows.

Required Parameters

These parameters have no default values and must be provided by the user.

Parameter Description Type / Unit
Q_flow_nominal Nominal heat flow rate (negative as it is for cooling). [W]
TApp_nominal Approach temperature on IT loop size (to cooling minus from rack). [K]
TRacOut_nominal Outlet temperature to IT rack loop. [K]
mPla_flow_nominal Nominal mass flow rate on cooling plant side. [kg/s]
mRac_flow_nominal Nominal mass flow rate on rack side. [kg/s]
dpHexPla_nominal Heat exchanger design pressure drop at plant side. [Pa]
medRac Media for which performance data are specified. Types.Media
phiGlyRac Glycol volume fraction for which rack-side performance data are specified. Set to 0 if only water is used. [1]
dpPumpExt_nominal Pump head available for flow network that is connected to the CDU on the rack-side at m_flow_nominal. [Pa]
pumpExtHead Pump head available for flow network that is connected to the CDU on the rack-side. flowParameters

Defaulted Parameters (Overridable)

These parameters are pre-configured with default values or initial equations, but may be overridden by the user.

Parameter Description Type / Unit Default Value / Equation
deltaMPla Fraction of nominal flow rate where flow transitions to laminar on cooling plant side. [1] 0.1
dpHexRac_nominal Heat exchanger design pressure drop at rack side. [Pa] dpHexPla_nominal
dpFilRac_nominal Clean filter design pressure drop at rack-side. [Pa] 65000
nPla Flow exponent, n=1 for laminar, n=2 for turbulent (plant side). [1] 2
nRac Flow exponent, n=1 for laminar, n=2 for turbulent (rack side). [1] 2
nFilRac Flow exponent for filter on rack-side, n=1 for laminar, n=2 for turbulent. [1] 1.76
deltaMRac Fraction of nominal flow rate where flow transitions to laminar on rack side. [1] 0.1
medPla Media for which performance data are specified (plant side). Types.Media Water
phiGlyPla Glycol volume fraction for which performance data are specified (plant side). Set to 0 if only water is used. [1] 0
dpValve_nominal Nominal pressure drop of fully open valve on chiller plant side. [Pa] dpHexPla_nominal
strokeTime Time needed to fully open or close actuator. [s] 120
riseTime Time needed to change motor speed between zero and full speed. [s] 30
pumpEfficiency Total pump efficiency vs. volumetric flow rate. efficiencyParameters V_flow = mRac_flow_nominal/rhoRac_default*{1}, eta = {0.7}
pumpHead Actual head of the pump, composed of external pump head, and head used for heat exchanger and filter. flowParameters Computed from pumpExtHead, dpHexRac_nominal, dpFilRac_nominal
r_nominal Ratio between convective heat transfer coefficients at nominal conditions (hAPla_nominal/hARac_nominal). [1] (kPla_default * (mPla_flow_nominal/etaPla_default)^nConPla * PrPla_default^(1/3)) / (kRac_default * (mRac_flow_nominal/etaRac_default)^nConRac * PrRac_default^(1/3))
nConPla Exponent for convective heat transfer coefficient (h~m_flownCon) on plant side. [1] 0.8
nConRac Exponent for convective heat transfer coefficient (h~m_flownCon) on rack side. [1] nConPla
VExp Size of expansion vessel on IT loop side. [m3] -0.056/2E6*Q_flow_nominal

The filter pressure drop and flow exponent is obtained from a data fit using catalogue data from Vertiv Liebert XDU 1350.

For the parameter pumpExtHead, it is recommended to use a array such as of the form 

pumpExtHead(
      V_flow = mRac_flow_nominal/rhoRac_default * {0.000, 0.250, 0.500, 0.750, 1.0},
      dp     = dpPumpExt_nominal *                {11.90, 11.61, 9.810, 6.202, 1.0})

With this setting, at design flow rate (the last element of V_flow, which is set to 1), the pressure available to the flow network connected to the CDU is equal to dpPumpExt_nominal, and at lower volume flow rates, the available pressure will increase, as it is customary for pumps. The increase is steeper than is typical for a pump because the CDU-internal resistance for the heat exchanger and filter decrease as the volume flow rate decreases, and hence, the resulting curve for the pump head that is available for pressure drops that are outside the CDU is steep.

The size of the expansion vessel VExp is by default set to the size used for the CDU Buildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.GoogleProjectDeschutes_2MW, which is 0.056 m3 for a cooling capacity of 2 MW.

Derived Parameters

These parameters are calculated by the model based on the above parameters and cannot be changed by the user.

Parameter Description Type / Unit
TPlaIn_nominal Inlet temperature from cooling plant loop. [K]
TPlaOut_nominal Outlet temperature to cooling plant loop. [K]
TRacIn_nominal Inlet temperature from IT rack loop. [K]
XGlyPla Glycol mass fraction for performance data (plant side). [1]
XGlyRac Glycol mass fraction for performance data (rack side). [1]
etaPla_default Dynamic viscosity for plant-side performance data. [Pa.s]
cpPla_default Specific heat capacity at constant pressure for plant-side performance data. [J/(kg.K)]
kPla_default Thermal conductivity for plant-side performance data. [W/(m.K)]
PrPla_default Prandtl number for plant-side performance data. [1]
etaRac_default Dynamic viscosity for rack-side performance data. [Pa.s]
cpRac_default Specific heat capacity at constant pressure for rack-side performance data. [J/(kg.K)]
kRac_default Thermal conductivity for rack-side performance data. [W/(m.K)]
PrRac_default Prandtl number for rack-side performance data. [1]
rhoRac_default Mass density for rack-side performance data. [kg/m3]
CPla_flow_nominal Capacity flow rate at nominal conditions on cooling plant side. [W/K]
CRac_flow_nominal Capacity flow rate at nominal conditions on rack side. [W/K]
dTPla_nominal Fluid temperature difference at nominal conditions on cooling plant side. [K]
dTRac_nominal Fluid temperature difference at nominal conditions on rack side. [K]

Parameters

TypeNameDefaultDescription
Modelica.Units.SI.TemperatureTPlaIn_nominalTRacOut_nominal - TApp_nominalInlet temperature from cooling plant loop
Modelica.Units.SI.TemperatureTPlaOut_nominalTPlaIn_nominal - Q_flow_nominal/CPla_flow_nominalOutlet temperature to cooling plant loop
Modelica.Units.SI.TemperatureTRacIn_nominalTRacOut_nominal - Q_flow_nominal/CRac_flow_nominalInlet temperature from IT rack loop
Modelica.Units.SI.DynamicViscosityetaPla_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.dynamicViscosity_TX_a(medium = medPla, X_a = XGlyPla, T = TRacOut_nominal)Dynamic viscosity for plant-side performance data
Modelica.Units.SI.SpecificHeatCapacitycpPla_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.specificHeatCapacityCp_TX_a(medium = medPla, X_a = XGlyPla, T = TRacOut_nominal)Specific heat capacity at constant pressure for plant-side performance data
Modelica.Units.SI.ThermalConductivitykPla_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.thermalConductivity_TX_a(medium = medPla, X_a = XGlyPla, T = TRacOut_nominal)Thermal conductivity for plant-side performance data
Modelica.Units.SI.PrandtlNumberPrPla_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.prandtlNumber_TX_a(medium = medPla, X_a = XGlyPla, T = TRacOut_nominal)Prandtl number for plant-side performance data
Modelica.Units.SI.DynamicViscosityetaRac_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.dynamicViscosity_TX_a(medium = medRac, X_a = XGlyRac, T = TRacOut_nominal)Dynamic viscosity for rack-side performance data
Modelica.Units.SI.SpecificHeatCapacitycpRac_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.specificHeatCapacityCp_TX_a(medium = medRac, X_a = XGlyRac, T = TRacOut_nominal)Specific heat capacity at constant pressure for rack-side performance data
Modelica.Units.SI.ThermalConductivitykRac_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.thermalConductivity_TX_a(medium = medRac, X_a = XGlyRac, T = TRacOut_nominal)Thermal conductivity for rack-side performance data
Modelica.Units.SI.PrandtlNumberPrRac_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.prandtlNumber_TX_a(medium = medRac, X_a = XGlyRac, T = TRacOut_nominal)Prandtl number for rack-side performance data
Modelica.Units.SI.DensityrhoRac_defaultBuildings.Fluid.DataCenterEquipment.CDUs.LiquidToLiquid.Data.BaseClasses.density_TX_a(medium = medRac, X_a = XGlyRac, T = TRacOut_nominal)Mass density for rack-side performance data
Buildings.Fluid.Movers.BaseClasses.Characteristics.efficiencyParameterspumpEfficiencyTotal pump efficiency vs. volumetric flow rate
Modelica.Units.SI.ThermalConductanceCPla_flow_nominalmPla_flow_nominal*cpPla_defaultCapacity flow rate at nominal conditions on cooling plant side
Modelica.Units.SI.ThermalConductanceCRac_flow_nominalmRac_flow_nominal*cpRac_defaultCapacity flow rate at nominal conditions on rack side
Modelica.Units.SI.TemperatureDifferencedTPla_nominal-Q_flow_nominal/CPla_flow_nominalFluid temperature difference at nominal conditions on cooling plant side
Modelica.Units.SI.TemperatureDifferencedTRac_nominalQ_flow_nominal/CRac_flow_nominalFluid temperature difference at nominal conditions on rack side
Nominal thermal performance
Modelica.Units.SI.HeatFlowRateQ_flow_nominalNominal heat flow rate (negative as it is for cooling)
Modelica.Units.SI.TemperatureDifferenceTApp_nominalApproach temperature on IT loop size (to cooling minus from rack)
Modelica.Units.SI.TemperatureTRacOut_nominalOutlet temperature to IT rack loop
Nominal condition
Modelica.Units.SI.MassFlowRatemPla_flow_nominalNominal mass flow rate on cooling plant side
Modelica.Units.SI.MassFlowRatemRac_flow_nominalNominal mass flow rate on rack side
Modelica.Units.SI.PressureDifferencedpHexPla_nominalHeat exchanger design pressure drop at plant side
Modelica.Units.SI.PressureDifferencedpHexRac_nominaldpHexPla_nominalHeat exchanger design pressure drop at rack side
Modelica.Units.SI.PressureDifferencedpFilRac_nominal65000Clean filter design pressure drop at rack-side
Flow resistance › Medium 1
RealdeltaMPla0.1Fraction of nominal flow rate where flow transitions to laminar on cooling plant side
RealnPla2Flow exponent, n=1 for laminar, n=2 for turbulent
Flow resistance › Medium 2
RealnRac2Flow exponent, n=1 for laminar, n=2 for turbulent
RealnFilRac1.76Flow exponent for filter on rack-side, n=1 for laminar, n=2 for turbulent
RealdeltaMRac0.1Fraction of nominal flow rate where flow transitions to laminar on rack side
Plant-side medium for performance data
Fluid.DataCenterEquipment.CDUs.Types.MediamedPlaBuildings.Fluid.DataCenterEquipment.CDUs.Types.Media.WaterMedia for which performance data are specified
RealphiGlyPla0Glycol volume fraction for which performance data are specified
Modelica.Units.SI.MassFractionXGlyPlaif medPla == Buildings.Fluid.DataCenterEquipment.CDUs.Types.Media.Water then 0 elseif medPla == Buildings.Fluid.DataCenterEquipment.CDUs.Types.Media.EthyleneGlycol then Buildings.Media.Antifreeze.Functions.EthyleneGlycolWater.volumeToMassFraction(phi = phiGlyPla, T = TRacOut_nominal) else Buildings.Media.Antifreeze.Functions.PropyleneGlycolWater.volumeToMassFraction(phi = phiGlyPla, T = TRacOut_nominal)Glycol mass fraction for which performance data are specified. Set to 0 if only water is used.
Rack-side medium for performance data
Fluid.DataCenterEquipment.CDUs.Types.MediamedRacMedia for which performance data are specified
RealphiGlyRacGlycol volume fraction for which performance data are specified. Set to 0 if only water is used.
Modelica.Units.SI.MassFractionXGlyRacif medRac == Buildings.Fluid.DataCenterEquipment.CDUs.Types.Media.Water then 0 elseif medRac == Buildings.Fluid.DataCenterEquipment.CDUs.Types.Media.EthyleneGlycol then Buildings.Media.Antifreeze.Functions.EthyleneGlycolWater.volumeToMassFraction(phi = phiGlyRac, T = TRacOut_nominal) else Buildings.Media.Antifreeze.Functions.PropyleneGlycolWater.volumeToMassFraction(phi = phiGlyRac, T = TRacOut_nominal)Glycol mass fraction for which performance data are specified
Valve
Modelica.Units.SI.PressureDifferencedpValve_nominaldpHexPla_nominalNominal pressure drop of fully open valve on chiller plant side
Dynamics › Valve
Modelica.Units.SI.TimestrokeTime120Time needed to fully open or close actuator
Dynamics › Pump
Modelica.Units.SI.TimeriseTime30Time needed to change motor speed between zero and full speed
Pump
Modelica.Units.SI.PressureDifferencedpPumpExt_nominalPump head available for flow network that is connected to the CDU on the rack-side at m_flow_nominal
Buildings.Fluid.Movers.BaseClasses.Characteristics.flowParameterspumpExtHeadPump head available for flow network that is connected to the CDU on the rack-side
Buildings.Fluid.Movers.BaseClasses.Characteristics.flowParameterspumpHeadActual head of the pump, composed of external pump head, and head used for heat exchanger and filter
Advanced › Heat transfer coefficients
Realr_nominal(kPla_default*(mPla_flow_nominal/etaPla_default)^nConPla*PrPla_default^(1/3))/(kRac_default*(mRac_flow_nominal/etaRac_default)^nConRac*PrRac_default^(1/3))Ratio between convective heat transfer coefficients at nominal conditions, r_nominal = hAPla_nominal/hARac_nominal
RealnConPla0.8Exponent for convective heat transfer coefficient, h~m_flow^nCon
RealnConRacnConPlaExponent for convective heat transfer coefficient, h~m_flow^nCon
Advanced › Expansion vessel
Modelica.Units.SI.VolumeVExp-0.056/2E6*Q_flow_nominalSize of expansion vessel on IT loop side

Revisions

  • April 13, 2026, by Michael Wetter:
    First implementation.