Fluid
This node uses the following version of the heat equation to model heat transfer in fluids:
(6-4)
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 several heat sources as separate physics features. See Heat Source node, and 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 has fields and values that are inputs to expressions that define material properties. If such user-defined property groups are added, the model inputs appear here.
Temperature
This section is available when temperature-dependent material properties are used. 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 Common Model Inputs node under Global Definitions, set a value for the Temperature in the Expression for remaining selection section.
Absolute Pressure
The absolute pressure is used in some predefined quantities that include the 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 User defined. 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 Common model input option corresponds to the minput.pA variable, set to 1 [atm] by default. To edit it, click the Go to Source button (), and in the Common Model Inputs node under Global Definitions, set a value for the Pressure in the Expression for remaining selection section.
Velocity Field
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 Common Model Inputs node under Global Definitions, set values for the Velocity components in the Expression for remaining selection section.
Concentration
This section can be edited anytime a material property is concentration dependent; for example, when the Fluid type is set to Moist air with Input quantity set to Concentration.
From the Concentration c (SI unit: mol/m3 or kg/m3) list, select an existing concentration variable from another physics interface, if any concentration variables exist, User defined to enter a value or expression for the concentration, or Common model input which corresponds to the minput.c variable.
Fluid Material
This section is available only when the Local Thermal Non-Equilibrium multiphysics coupling is included in the component to model porous media. It makes it possible to define different material properties for the fluid phase when the domain material corresponds to the solid phase (porous matrix) material.
Select any material from the list to define the Fluid material. The default uses the Domain material.
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 Anisotropic 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 sets 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 of the heat capacity at constant pressure, Cp, to 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 variables for heat fluxes and total energy fluxes. It is also used if the ideal gas law is applied.
The available Fluid type options are Gas/Liquid (default), Moist air, and Ideal gas. After selecting a Fluid type from the list, further settings display underneath.
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.
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 compute 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.
Moist Air
If Moist air is selected, the thermodynamics properties are defined as a function of the quantity of vapor in the moist air. The available Input quantity options to define the amount of vapor in the moist air are the following:
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Vapor mass fraction to define the ratio of the vapor mass to the total mass. Enter a value or expression for the Vapor mass fraction ω.
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Concentration to define the amount of water vapor in the total volume. If selected, a Concentration model input is added in the Model Inputs section.
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Moisture content (the default), also called mixing ratio or humidity ratio, to define the ratio of the water vapor mass to the dry air mass. For User defined, enter a value or expression for the Moisture Content xvap. Else, select an Ambient moisture content defined from an Ambient Thermal Properties node under Definitions.
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Relative humidity , a quantity defined between 0 and 1, where 0 corresponds to dry air and 1 to a water vapor-saturated air. The Relative humidity, temperature condition and Relative humidity, absolute pressure condition must be specified.
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Moist Air Fluid Type
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Local Thermal Non-Equilibrium
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Theory for Heat Transfer in Fluids
With certain COMSOL products, the Viscous Dissipation (for heat generated by viscous friction), Pressure Work, and Convectively Enhanced Conductivity subnodes are available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu.
When Surface-to-surface radiation is activated, the Opacity (Surface-to-Surface Radiation interface) subnode is automatically added to the entire selection, with Transparent option selected. The domain selection can’t be edited. To set some part of the domain as opaque, add a new Opacity (Surface-to-Surface Radiation interface) subnode 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 http://www.comsol.com/products/specifications/
Location in User Interface
Context menus
 Heat Transfer>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, 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