(also described in the theory for Laminar Flow), the part that corresponds to sound wave propagation is neglected. The reason is the energy in the sound waves are almost always negligible compared to the contribution from Equation 13-2. The software computes the pressure work using the absolute pressure.

D O M A I N S E L E C T I O N

From the Selection list, choose the domains to add pressure work. By default, the selection is the same as for the Heat Transfer in Solids or Heat Transfer in Fluids node it is attached to.

M O D E L I N P U T

Enter a value or expression for the Elastic contribution to entropy Ent (SI unit: J

m3·K)). The default is 0.

Heat Transfer in Fluids

The Heat Transfer in Fluids model uses the following version of the heat equation as the mathematical model for heat transfer in fluids:

•Heat Transfer by Free Convection: Model Library path

COMSOL_Multiphysics>Multiphysics>free_convection

Model

For a steady-state problem the temperature does not change with time and the first term disappears. It has these material properties:

•The density ( )

•The fluid heat capacity at constant pressure (Cp)—describes the amount of heat energy required to produce a unit temperature change in a unit mass

•The fluid thermal conductivity (k)—a scalar or a tensor if the thermal conductivity is anisotropic

•The fluid velocity field (u)—can be an analytic expression or a velocity field from a fluid-flow interface

•The heat source (or sink) (Q)—one or more heat sources can be added separately

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Right-click to add Viscous Heating (for heat generated by viscous friction) or Pressure Work nodes to the Heat Transfer in Fluids feature. Also, the ratio of specific heats is defined on this page. It is the ratio of heat capacity at constant pressure, Cp, to heat capacity at constant volume, Cv. When using the ideal gas law to describe a fluid, specifying is enough to evaluate Cp. For common diatomic gases such as air, 1.4 is the standard value. Most liquids have 1.1 while water has 1.0. 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. See Thermodynamics.

D O M A I N S E L E C T I O N

From the Selection list, choose the domains to define the heat transfer.

M O D E L I N P U T S

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.

There are also two standard model inputs—Absolute pressure and Velocity field. The absolute pressure is used in some predefined quantities that include the enthalpy (the energy flux, for example).

Absolute pressure is also used if the ideal gas law is applied. See

Thermodynamics.

Note

Absolute Pressure

Enter the Absolute pressure pA (SI unit: Pa). The default is atmosphere pressure, 1 atm (101,325 Pa).

This section controls both the variable as well as any property value (reference pressures) used when solving for pressure. There are usually two ways of calculating the pressure when describing fluid flow, and mass and heat transfer. Solve for the absolute pressure or a pressure (often denoted gauge pressure) that relates back to the absolute pressure through a reference pressure.

Using one or the other usually depends on the system and the equations being solved for. For example, in a straight incompressible flow problem, the pressure drop over the modeled domain is probably many orders of magnitude less than atmospheric pressure, which, if included, reduces the chances for stability and convergence during the solving

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process for this variable. In other cases, the absolute pressure may be required to be solved for, such as where pressure is a part of an expression for gas volume or diffusion coefficients.

The absolute pressure model input is controlled by both a drop-down list and a check box within this section. Use the User defined option to manually define the absolute pressure in a system. This is the default setting.

The pressure variables solved for by a fluid-flow interface can also be used, which is selected from the list as, for example, Pressure spf/fp. Selecting a pressure variable also activates a check box for defining the reference pressure, where 1[atm] (1 atmosphere) is the default value. This makes it possible to use a system-based (gauge) pressure as the pressure variable while automatically including the reference pressure in places where it is required, such as for gas flow governed by the gas law. While this check box maintains control over the pressure variable and instances where absolute pressure is required within this respective physics interface, it may not with physics interfaces that being coupled to. In such models, check the coupling between any interfaces using the same variable.

Velocity Field

From the Velocity field list, select an existing velocity field in the model (for example, Velocity field (spf/fp1) from a Laminar Flow interface) or select User defined to enter values or expressions for the components of the Velocity field (SI unit: m/s).

C O O R D I N A T E S Y S T E M S E L E C T I O N

The Global coordinate system is selected by default. The Coordinate system list contains any additional coordinate systems that the model includes (except boundary coordinate systems). The coordinate system is used for interpreting directions of orthotropic and anisotropic thermal conductivity.

H E A T C O N D U C T I O N

The default Thermal conductivity k (SI unit: W/(m·K)) is taken From material. If User defined is selected, choose Isotropic, Diagonal, Symmetric, or Anisotropic based on the characteristics of the thermal conductivity, and enter another value or expression.

The thermal conductivity describes the relationship between the heat flux vector q and the temperature gradient T as in q = k T which is Fourier’s law of heat conduction. Enter this quantity as power per length and temperature.

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