Fluid
This node uses the following version of the heat equation to model heat transfer in fluids:
(6-5)
with the following material properties, fields, and sources:
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ρ (SI unit: kg/m3) is the fluid density.
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Cp (SI unit: J/(kg·K)) is the fluid heat capacity at constant pressure.
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k (SI unit: W/(m·K)) is the fluid thermal conductivity (a scalar or a tensor if the thermal conductivity is anisotropic).
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u (SI unit: m/s) is the fluid velocity field, either an analytic expression or a velocity field from a Fluid Flow interface.
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Q (SI unit: W/m3) is the heat source (or sink). Add one or more heat sources as separate physics features. See the Heat Source node and the Viscous Dissipation and Pressure Work subnodes, for example.
For a steady-state problem the temperature does not change with time, and the first term disappears.
Model Input
This section contains fields and values that are inputs for expressions defining material properties. If such user-defined property groups are added, the model inputs appear here.
Temperature
This section is available when material properties are temperature-dependent. By default, the temperature of the parent interface is used and the section is not editable. To edit the Temperature field, click Make All Model Inputs Editable (). The available options are User defined (default), Common model input (the minput.T variable, set to 293.15 K by default) and all temperature variables from the physics interfaces included in the model. To edit the minput.T variable, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set a value for the Temperature in the Expression for remaining selection section.
Absolute Pressure
Absolute pressure is used in certain predefined quantities that include enthalpy (the energy flux, for example).
It is also used if the ideal gas law is applied. See Thermodynamics, Fluid.
The default Absolute pressure pA is taken from Common model input. It corresponds to the minput.pA variable, set to 1 atm by default. To edit it, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set a value for the Pressure in the Expression for remaining selection section. When additional physics interfaces are added to the model, the absolute pressure variables defined by these physics interfaces can also be selected from the list. For example, if a Laminar Flow interface is added you can select Absolute pressure (spf) from the list. The last option is User defined.
Heat Convection
The default Velocity field u is User defined. For User defined enter values or expressions for the components based on space dimensions. Or select an existing velocity field in the component (for example, Velocity field (spf) from a Laminar Flow interface). The Common model input option corresponds to the minput.u variable. To edit it, click the Go to Source button (), and in the Default Model Inputs node under Global Definitions, set values for the Velocity components in the Expression for remaining selection section.
Heat Conduction, Fluid
The thermal conductivity k describes the relationship between the heat flux vector q and the temperature gradient ∇T in q = −k∇T, which is Fourier’s law of heat conduction. Enter this quantity as power per length and temperature.
The default Thermal conductivity k is taken From material. For User defined select Isotropic, Diagonal, Symmetric, or Full based on the characteristics of the thermal conductivity, and enter values or expressions for the thermal conductivity or its components. For Isotropic enter a scalar which will be used to define a diagonal tensor. For the other options, enter values or expressions into the editable fields of the tensor.
Thermodynamics, Fluid
This section defines the thermodynamics properties of the fluid.
The heat capacity at constant pressure Cp describes the amount of heat energy required to produce a unit temperature change in a unit mass.
The ratio of specific heats γ is the ratio between the heat capacity at constant pressure, Cp, and the heat capacity at constant volume, Cv. When using the ideal gas law to describe a fluid, specifying γ is sufficient to evaluate Cp. For common diatomic gases such as air, γ = 1.4 is the standard value. Most liquids have γ = 1.1 while water has γ = 1.0. γ is used in the streamline stabilization and in the ideal gas law when Ideal gas is selected.
When the density is not taken from a Nonisothermal Flow or a Nonisothermal Mixture Model coupling node, you should select a Fluid type option for the specification of the material properties. The available Fluid type options are From material, Ideal gas, and Gas/Liquid (default). After selecting a Fluid type from the list, further settings display underneath. See Nonisothermal Mixture Model in the CFD Module User’s Guide for details.
From Material
This option automatically detects whether the material applied on each domain selection is an ideal gas or not, and uses the relevant properties from the Material node for either case.
By using the following definition of the density for an ideal gas:
with Rs the specific gas constant, pA the absolute pressure, and T the temperature, the evaluation of the isobaric compressibility coefficient, αp, and of the isothermal Joule-Thomson coefficient, μJT, is simplified. This may improve efficiency, when computing pressure work in compressible nonisothermal flows for example, or when modeling inflow conditions.
The Air material, from the Built-in materials database, has an Ideal gas property group, and is thus detected as an ideal gas by the Fluid node. The Liquids and Gases Material Library, available with some COMSOL products, also provides such materials.
The Density and the Heat capacity at constant pressure are automatically taken From material and no further setting is required.
Automatic calculation of the ratio of specific heats
 
The Ratio of specific heats is calculated automatically using Mayer’s relation:
considering that:
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αp is the coefficient of thermal expansion (SI unit: 1/K):
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χt is the isothermal compressibility (SI unit: 1/Pa):
For the specification of user defined material properties, the Ideal Gas or Gas/Liquid options should be used instead.
Ideal Gas
This option uses the ideal gas law to describe the fluid. Only two properties are needed to define the thermodynamics of the fluid:
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The gas constant, with two options for the Gas constant type: Specific gas constant Rs or Mean molar mass Mn. If Mean molar mass is selected the software uses the universal gas constant R = 8.314 J/(mol·K), which is a built-in physical constant, to calculate the specific gas constant.
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Either the Heat capacity at constant pressure Cp or Ratio of specific heats γ by selecting the option from the Specify Cp or γ list. For an ideal gas, it is sufficient to specify either Cp or the ratio of specific heats, γ, as these properties are interdependent.
Gas/Liquid
This option specifies the Density, the Heat capacity at constant pressure, and the Ratio of specific heats for a general gas or liquid. By default, the Ratio of specific heats is set to Automatic, see Automatic calculation of the ratio of specific heats.
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Theory for Heat Transfer in Porous Media
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Theory for Heat Transfer in Fluids
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When the density is taken from a Nonisothermal Flow coupling node, the Heat capacity at constant pressure remain to be specified for a general gas or liquid. Select Ideal gas: ratio of specific heats or Ideal gas: heat capacity at constant pressure to specify only one property in the case of an ideal gas.
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When the Heat Transfer interface is coupled to a Phase Transport interface and to a Fluid Flow interface via the Nonisothermal Flow and Nonisothermal Mixture Model multiphysics couplings, all the material properties are taken from the coupling nodes. See Nonisothermal Flow and Nonisothermal Mixture Model in the CFD Module User’s Guide for details.
The Viscous Dissipation (for heat generated by viscous friction), Pressure Work (available with some COMSOL products), and Convectively Enhanced Conductivity subnodes are available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu.
Heat Sink: Application Library path Heat_Transfer_Module/Tutorials,_Forced_and_Natural_Convection/heat_sink
For a detailed overview of the functionality available in each product, visit https://www.comsol.com/products/specifications/
Location in User Interface
Context Menus
 Heat Transfer in Solids and Fluids>Fluid
Heat Transfer in Solids>Fluid
Heat Transfer in Fluids>Fluid
Heat Transfer in Porous Media>Fluid
Heat Transfer in Building Materials>Fluid
Bioheat Transfer>Fluid
Ribbon
Physics tab with interface as Heat Transfer in Solids and Fluids, Heat Transfer in Solids, Heat Transfer in Fluids, Heat Transfer in Porous Media, Heat Transfer in Building Materials, or Bioheat Transfer selected:
Domains>interface>Fluid