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title: Physical Properties Package description: Calculate density, viscosity, thermal conductivity, diffusivity, and interfacial properties with NeqSim. —

NeqSim calculates phase-specific physical properties after a thermodynamic state has been established. This package provides model sets for common fluids, individual transport-property models, and interfacial-property calculations.

Contents

• Overview (this page) - Package architecture and basic usage
• Viscosity Models - Dynamic viscosity calculation methods
• Thermal Conductivity Models - Thermal conductivity methods
• Diffusivity Models - Binary and multicomponent diffusion coefficients
• Interfacial Properties - Surface tension and related calculations
• Density Models - Liquid density correlations

Calculation workflow

Use the physical-property API in this order:

1. Define the fluid and thermodynamic model.
2. Establish the phase equilibrium with a flash calculation.
3. Select a physical-property model set or override an individual phase model.
4. Initialize the affected physical properties.
5. Read properties from a named phase.

The following complete example calculates gas density, dynamic viscosity, kinematic viscosity, and thermal conductivity. Temperature is in kelvin and pressure is absolute in bara.

import neqsim.physicalproperties.system.PhysicalPropertyModel;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public class PhysicalPropertiesOverview {
  public static void main(String[] args) {
    SystemInterface fluid = new SystemSrkEos(298.15, 50.0);
    fluid.addComponent("methane", 0.90);
    fluid.addComponent("ethane", 0.10);
    fluid.setMixingRule("classic");

    new ThermodynamicOperations(fluid).TPflash();
    fluid.setPhysicalPropertyModel(PhysicalPropertyModel.DEFAULT);
    fluid.initPhysicalProperties();

    double viscosityPas = fluid.getPhase("gas").getViscosity("kg/msec");
    double conductivityWPerMeterK =
        fluid.getPhase("gas").getThermalConductivity("W/mK");
    double densityKgPerM3 = fluid.getPhase("gas").getDensity("kg/m3");
    double kinematicViscosityM2PerS =
        fluid.getPhase("gas").getPhysicalProperties().getKinematicViscosity();

    if (viscosityPas <= 0.0
        || conductivityWPerMeterK <= 0.0
        || densityKgPerM3 <= 0.0
        || kinematicViscosityM2PerS <= 0.0) {
      throw new IllegalStateException("Expected positive gas physical properties");
    }
  }
}

initPhysicalProperties() is separate from thermodynamic initialization. Re-run it after a material change in temperature, pressure, composition, phase equilibrium, or selected physical-property model.

Preconfigured model sets

PhysicalPropertyModel selects a consistent set of phase-specific implementations:

Model	Intended use
DEFAULT	General hydrocarbon systems
WATER	Aqueous systems
SALT_WATER	Brines
GLYCOL	Glycol dehydration systems
AMINE	Amine gas-treating systems
CO2WATER	CO₂-water systems
BASIC	Minimal property calculations

Select the model set before initializing the properties. For example, call fluid.setPhysicalPropertyModel(PhysicalPropertyModel.GLYCOL) and then fluid.initPhysicalProperties().

The string overload of initPhysicalProperties selects a PhysicalPropertyType such as DYNAMIC_VISCOSITY; it does not select a PhysicalPropertyModel. Therefore, initPhysicalProperties("GLYCOL") is not a model-set selection call. For compatibility, the legacy keys DENSITY, VISCOSITY, and CONDUCTIVITY map to MASS_DENSITY, DYNAMIC_VISCOSITY, and THERMAL_CONDUCTIVITY, respectively.

Overriding individual phase models

Individual models are selected on a phase’s PhysicalProperties object. Model keys are case-sensitive. Reinitialize each changed phase after selecting a model.

import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public class PhaseSpecificPhysicalProperties {
  public static void main(String[] args) {
    SystemInterface fluid = new SystemSrkEos(280.0, 30.0);
    fluid.addComponent("methane", 0.50);
    fluid.addComponent("n-pentane", 0.50);
    fluid.setMixingRule("classic");
    fluid.setMultiPhaseCheck(true);

    new ThermodynamicOperations(fluid).TPflash();
    fluid.initPhysicalProperties();

    if (!fluid.hasPhaseType("gas") || !fluid.hasPhaseType("oil")) {
      throw new IllegalStateException("Expected gas and oil phases");
    }

    fluid.getPhase("gas").getPhysicalProperties()
        .setViscosityModel("friction theory");
    fluid.getPhase("gas").getPhysicalProperties()
        .setConductivityModel("Chung");
    fluid.getPhase("oil").getPhysicalProperties()
        .setViscosityModel("LBC");
    fluid.getPhase("oil").getPhysicalProperties()
        .setConductivityModel("PFCT");

    fluid.getPhase("gas").initPhysicalProperties();
    fluid.getPhase("oil").initPhysicalProperties();

    double gasViscosityCp = fluid.getPhase("gas").getViscosity("cP");
    double oilViscosityCp = fluid.getPhase("oil").getViscosity("cP");
    if (gasViscosityCp <= 0.0 || oilViscosityCp <= 0.0) {
      throw new IllegalStateException("Expected positive phase viscosities");
    }
  }
}

The dedicated model pages list the implemented keys, applicability, equations, and tuning interfaces. Unsupported keys do not have one uniform fallback policy across all property types, so validate model selection and results in application tests.

Accessing calculated properties

After initialization, the most common phase accessors are:

Property	Accessor	Default unit
Dynamic viscosity	getViscosity()	Pa·s
Dynamic viscosity with unit	getViscosity("cP")	requested unit
Thermal conductivity	getThermalConductivity()	W/(m·K)
Thermal conductivity with unit	getThermalConductivity("W/mK")	requested unit
Mass density	getDensity()	kg/m³
Mass density with unit	getDensity("kg/m3")	requested unit
Kinematic viscosity	getPhysicalProperties().getKinematicViscosity()	m²/s

Diffusion coefficients are available from the phase’s PhysicalProperties object. See the diffusivity guide for Maxwell-Stefan, Fick, and effective diffusion coefficient access.

Surface tension requires two existing phases and an initialized interfacial model. Use phase guards and follow the complete examples in the interfacial-properties guide; do not assume that phase indexes 0 and 1 identify a valid interface.

Model tuning

Viscosity tuning parameters are model-specific. The LBC, PFCT/CSP, and advanced friction-theory interfaces are documented and tested in the viscosity guide. Use parameter sets from independent measurements or validated literature, preserve their units and validity range, and reinitialize the affected phase after changing them.

Sensitivity calculations

Create a ThermodynamicOperations instance for the fluid that is actually being flashed. This is especially important when cloning a base fluid:

import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;
import neqsim.thermodynamicoperations.ThermodynamicOperations;

public class PhysicalPropertyTemperatureSweep {
  public static void main(String[] args) {
    SystemInterface baseFluid = new SystemSrkEos(298.15, 50.0);
    baseFluid.addComponent("methane", 0.90);
    baseFluid.addComponent("ethane", 0.10);
    baseFluid.setMixingRule("classic");

    double[] temperaturesK = {280.0, 300.0, 320.0};
    for (double temperatureK : temperaturesK) {
      SystemInterface fluid = baseFluid.clone();
      fluid.setTemperature(temperatureK, "K");

      new ThermodynamicOperations(fluid).TPflash();
      fluid.initPhysicalProperties();

      double viscosityPas = fluid.getPhase("gas").getViscosity("kg/msec");
      if (viscosityPas <= 0.0 || !Double.isFinite(viscosityPas)) {
        throw new IllegalStateException("Invalid gas viscosity");
      }
    }
  }
}

Extending the package

A new physical-property model should:

1. Extend the property-method base class for the correct phase family.
2. Document equations, units, parameter provenance, and validity limits.
3. Register a stable, case-sensitive key in the relevant setter.
4. Add focused tests for selection, calculation, bounds, and unsupported inputs.
5. Add an executable documentation example and update the applicable model guide.

Avoid placeholder implementations that return zero: they can appear numerically valid while silently corrupting transport or equipment calculations.

Package architecture

The package separates these responsibilities:

1. PhysicalPropertyHandler maps phase types and model sets to property containers.
2. PhysicalProperties stores the selected density, viscosity, conductivity, and diffusivity methods for one phase.
3. Method packages implement individual correlations and corresponding-states models.
4. Physical-property mixing rules combine component contributions.
5. Interfacial-property classes calculate surface tension and adsorption behavior.

See also

• Fluid Creation Guide - Creating thermodynamic systems
• Flash Calculations Guide - Phase-equilibrium calculations
• Thermodynamic Operations - Thermodynamic calculation workflow