modelPartialCompression

Partial model for compression that contains basic definitions used in various compressor models

Extends from AixLib.Fluid.Interfaces.PartialTwoPortTransport.

Information

This is a base model for compression processes that are used, for example, in rotary or scroll compressors.

Definition needed for completion

Seven equations need to be added by an extending class using this component:

  • Calculation of the mass flow rate ṁ.
  • Calculation of the power consumed by the refrigerant Q̇ref.
  • Calculation of the compressor's power consumption Pel.
  • Definition of input and output pressures pInl and pOut.
  • Definition of input and output specific enthalpies hInl and hOut.
  • port_b.h_outflow for flow in design direction.
  • port_a.h_outflow for flow in reverse direction.

Moreover, appropriate values shall be assigned to the following parameters:

  • dp_start for a guess of the pressure drop
  • m_flow_small for regularization of zero flow.
  • dp_nominal for nominal pressure drop.
  • m_flow_nominal for nominal mass flow rate.

Efficiency modeling approaches

Actually, three different efficiency models are suggested and the modelling approaches of these efficiencies are shortly characterised below:

"Efficiencies" border="1" cellspacing="0" cellpadding= "2" style="border-collapse:collapse;">
Efficiency Formula Comment
Engine ηeng = Q̇ref / Pel Used for calculation of compressor's power consumption
Isentropic ηise = (houtIse - hinl) / (hout - hinl) Used for calculation of thermodynamic change of state
Volumetric ηvol = V̇ide / V̇rea Used for calculation of mass flow rate

These efficiency models are stored in AixLib.Fluid.Movers.Compressors.Utilities.EngineEfficiency, AixLib.Fluid.Movers.Compressors.Utilities.IsentropicEfficiency and AixLib.Fluid.Movers.Compressors.Utilities.VolumetricEfficiency. Therefore, the calculation procedure of the efficiencies are introduced as replaceable models and must be defined by the User.

Transient behaviour

The base model has a parameter useInpFil that is used to model the compressors's transient behaviour while changing rotational speed. Generally, this approach uses the same modeling attempt as the stat-up and shut-down transients introtuced for flow machines (see AixLib.Fluid.Movers.UsersGuide). Therefore, just the parameter's affections are presented here:

  1. If useInpFil=false, then the input signal opeSet.y is equal to the compressor's rotational speed. Thus, a step change in the input signal causes a step change in the rotational speed.
  2. If useInpFil=true, which is the default, then the rotational speed is equal to the output of a filter. This filter is implemented as a 2nd order differential equation. Thus, a step change in the compressor's input signal will cause a gradual change in the rotational speed. The filter has a parameter risTim, which by default is set to 1 second. The rise time is the time required to reach 99.6% of the full rotational speed, or, if the compressor is shut-down, to reach a rotational speed of 0.4%.

References

In the following, some general references are given for information about modelling compressors. The modelling approach presented here is alligned to the modelling approaches presented in the literature:

W. Eifler, E. Schlücker, U. Spicher and G. Will (2009): Küttner Kolbenmaschinen: Kolbenpumpen, Kolbenverdichter, Brennkraftmaschinen (in German). Publisher: Vieweg + Teubner

H. Qiao, R. Radermacher and V. Aute (2010): A review for numerical simulation of vapor compression systems. In: International Refrigeration and Air Conditioning Conference

P.C. Hanlon (2011): Compressor Handbook. Publisher: McGraw-Hill

V. A. Cara Martin and R. Radermacher (2015): AHRI Project 8013: A Study of Methods to Represent Compressor Performance Data over an Operating Envelope Based on a Finite Set of Test Data. Publisher: Air-Conditioning, Heating, and Refrigeration Institute (AHRI)

Parameters

TypeNameDefaultDescription
General › Geometry
Modelica.Units.SI.VolumeVDis13e-6Displacement volume of the compressor
Modelica.Units.SI.EfficiencyepsRef0.04Ratio of the real and the ideal displacement volume
General › Compressor's characterisitcs
Modelica.Units.SI.FrequencyrotSpeMax120Maximal rotational speed executable by the compressor
RealpiPreMax15Maximal pressure ratio executable by the compressor
Transient behaviour
BooleanuseInpFiltrue= true, if transient behaviour of rotational speed is computed
Modelica.Units.SI.TimerisTim0.5Time until rotational speed reaches 99.6 % of its set value
Efficiencies and similitude theory › Engine efficiency
BooleanuseIseWoroveEngEff.useIseWor= true, if overal machanic efficiency is related to isentropic compressor work
Advanced
Modelica.Units.SI.MassFlowRatem_flow_nominal0.1Nominal mass flow rate
Advanced › Diagnostics
Booleanshow_staEfffalse= true, if thermodynamic states and efficiencies are computed
Booleanshow_quafalse= true, if vapour qualities are computed
Advanced › Initialisation
Modelica.Units.SI.FrequencyrotSpe060Compressor's rotational spped at initialisation
Modelica.Units.SI.AbsolutePressurepInl03e5Pressure at compressor's inlet at initialisation
Modelica.Units.SI.TemperatureTInl0283.15Temperature at compressor's inlet at initialisation
Modelica.Units.SI.DensitydInl0Medium.density(Medium.setState_pTX(p = pInl0, T = TInl0))Density at compressor's inlet at initialisation
Modelica.Units.SI.SpecificEnthalpyhInl0Medium.specificEnthalpy(Medium.setState_pTX(p = pInl0, T = TInl0))Specific enthalpy at compressor's inlet at initialisation

Components

TypeNameDefaultDescription
Modelica.Blocks.Interfaces.RealInputmanVarComPrescribed compressor's rotational speed
Modelica.Blocks.Interfaces.RealOutputcurManVarComCurrent compressor's rotational speed
Modelica.Blocks.Continuous.FilterfilRotSpeSecond order filter to approximate change of compressor's rotational speed
Modelica.Blocks.Routing.RealPassThroughrotSpeThrDummy passing through of compressor's rotational speed to allow usage of filter
Modelica.Thermal.HeatTransfer.Interfaces.HeatPort_bheatPortHeat port connector to calculate heat losses to ambient
EngineEfficiencyoveEngEffInstance of model 'engine efficiency'
VolumetricEfficiencyoveVolEffInstance of model 'volumetric efficiency'
IsentropicEfficiencyoveIseEffInstance of model 'isentropic efficiency'
CompressorStatescomStaRecord containing compressor's thermodynamic states and efficiencies
CompressorQualitiescomQuaRecord containing compressor's vapour qualities
Modelica.Units.SI.PowerPEleCompressor's current electrical power consumption
Modelica.Units.SI.PowerQ_flow_refCurrent power transferred to reffrigerant
Modelica.Units.SI.FrequencyrotSpeCompressor's current rotational speed
RealpiPreRatio of compressor's outlet and inlet pressure

Contents

NameDescription
EngineEfficiency
VolumetricEfficiency
IsentropicEfficiency
CompressorStatesRecord that contains compressor's thermodynamic states at inlet and outlet
CompressorQualitiesRecord that contains compressor's qualities at inlet and outlet

Revisions

  • October 20, 2017, by Mirko Engelpracht:
    First implementation (see issue 467).