17.6.2.18. Convection

This boundary condition causes convective heat transfer to occur through one or more flat or curved faces (in contact with a fluid).

The ambient (or bulk) fluid temperature is measured at a distance from the face outside of the thermal boundary layer. The face temperature refers to the temperature at the face of the simulation model.

Convective Heat Transfer

Convection is related to heat flux by use of Newton's law of cooling:

q/A = h(t s - t f)

where:

  • q/A is heat flux out of the face (calculated within the application)

  • h is the film coefficient (you provide). This coefficient is also known as the heat transfer coefficient or unit thermal conductance.

  • t s is the temperature on the face or face temperature (calculated within the application)

  • t f is the ambient fluid temperature or ambient temperature (you provide). It is also known as the bulk fluid temperature or bulk temperature, depending on the analysis context.

When the ambient fluid temperature exceeds the face temperature, energy flows into a part. When the face temperature exceeds the ambient fluid temperature, a part loses energy.

This page includes the following sections:

Analysis Types

Convection is available for the following analysis types:
Dimensional Types

The supported dimensional types for the Convection boundary condition include:

  • 3D Simulation

  • 2D Simulation: Supported for Plane Stress and Axisymmetric behaviors only.

Geometry Types

The supported geometry types for the Convection boundary condition include:

  • Solid

  • Surface/Shell

Topology Selection Options

The supported topology selection options for the Convection boundary condition include:

  • Body: Supported for 3D External Faces only.

  • Face: If you select multiple faces when defining a Convection boundary condition, the same ambient fluid temperature and film coefficient is applied to all selected faces.

  • Edge: Supported for 2D only.

  • Vertex: Not supported. This selection specifies the reference temperature.

  • Nodes: Not supported. This selection specifies the reference temperature.

  • Element Face

Define By Options

The supported Define By options for the Convection boundary condition include:

  • Film Coefficient: The film coefficient (also called the heat transfer coefficient or unit thermal conductance) is based on the composition of the fluid in contact with the face, the geometry of the face, and the hydrodynamics of the fluid flow past the face. It is possible to have a time, temperature or spatially dependent film coefficient. Refer to heat transfer handbooks or other references to obtain appropriate values for film coefficient.

  • Coefficient Type: This field appears when the film coefficient is temperature dependent. It is read-only when the film coefficient is specified via table.

    The Coefficient Type option displays the temperature at which a temperature-dependent film coefficient value is evaluated.

    Average film temperature

    The average of surface and ambient temperatures

    Surface temperature

    The temperature at the heat transfer surface

    Bulk fluid temperature

    The ambient temperature of the fluid

    Difference between surface and bulk

    The absolute value of the difference between surface and ambient temperatures

    Note:  If you change the units from Celsius to Fahrenheit, or Fahrenheit to Celsius, when the convection coefficient type is set to Difference between surface and bulk, the displayed temperature values indicate a temperature difference only. The addition or subtraction of 32o for each temperature in the conversion formula offset one another. In addition, switching to or from the Difference between surface and bulk Coefficient Type option from any other option clears the values in the Convection Coefficient table. This helps to ensure that you enter correct temperature values.

  • Ambient Temperature: The ambient temperature (also called the bulk temperature) is the temperature of the surrounding fluid. It is possible to have a time or spatially dependent ambient temperature.

  • Convection Matrix: Not supported for LS-DYNA analyses. Specifies whether to use a diagonal film coefficient matrix or a consistent film coefficient matrix. The default setting, Program Controlled, allows the solver to determine whether to use a diagonal or consistent film coefficient matrix.

  • Edit Data For: Not supported for LS-DYNA analyses. This field allows you to select and edit Film Coefficient or Ambient Temperature. The Tabular Data window, Details pane, graph, and graphics view will change based on the selection in the Edit Data For field.

    For example, when the Film Coefficient property is specified as Tabular or Function and Edit Data For is Film Coefficient, you will actively edit data for the Film Coefficient in the appropriate Details pane and Tabular Data fields.

  • Fluid Flow: Supported for Steady-State Thermal and Transient Thermal analyses only. Option include Yes and No (default). Setting this property to Yes activates convection on one or more line bodies whose Model Type property is set to Thermal Fluid. Also, when set to Yes, the Fluid Flow Scoping and Display Connection Lines properties display. You use the Fluid Flow Scoping property to specify a desired edge or edges, vertex, or node using either direct selection or using a Named Selection.

    When scoping to:

    • Edge(s) on a fluid line body: Convection is accounted for with surface elements SURF151 and SURF152. The application maps the elements to the fluid elements using a distance-based algorithm. This option uses the two nodes from the mapped fluid element to specify the reference temperature. Note that, following mesh generation, you can use the Display Connection Lines property to display the connection lines between the centroid of each element face/edge of the Convection surface(s)/edge(s) to the corresponding closest node on the Fluid Flow scoping. The closest node is computed using a distance-based algorithm.

    • Vertex/Node on a fluid line body. This option uses the specified vertex or node to specify the reference temperature. You can specify a single node or vertex only.

Magnitude Options for Film Coefficient and Ambient Temperature

The following magnitude options are supported for Film Coefficient and Ambient Temperature. These parameters are defined independently, allowing you to mix and match different magnitude options. (For example, you can define a constant value for Film Coefficient and a time-varying function for Ambient Temperature.)

  • Constant

  • Tabular (Time Varying)

  • Tabular (Spatially Varying): Not supported for LS-DYNA analyses.

  • Tabular (Temperature Varying): Not supported for LS-DYNA analyses. Supported for Film Coefficient.

  • Function (Time Varying): Not supported for LS-DYNA analyses.

  • Function (Spatially Varying): Not supported for LS-DYNA analyses.

  • Table name: Supported for the Mechanical APDL solver only. Select the name of a multi-variable table that defines the film coefficient or temperature for the selection. Depending on which parameter you are setting, the Magnitude field automatically lists the names of all tables that contain film coefficient or temperature as dependent variables.

    For additional instructions on how to use tables with Convection boundaries, see Specify Ambient Temperature and Film Coefficient with Tables.

  • New Table. Supported for the Mechanical APDL solver only. Select this menu option to create a new multivariable table of film coefficient or temperature values.

You can vary the magnitude of film coefficient and ambient temperature in a single coordinate direction using tabular data or a function. For the specific steps to apply tabular and/or function loads, see the Specifying Boundary Condition Magnitude section.

Note:

  • Scaling based on time is not supported for Convection.

  • The activate/deactivate option is only available when the Independent Variable property of the Tabular Data category is set to Time.

Applying a Convection Boundary Condition

To apply a Convection boundary condition:

  1. Select Convection from the Environment Context tab. Alternatively, right-click the Environment object, or in the Geometry window, and select Insert>Convection.

  2. Define the Scoping Method as either Geometry Selection or Named Selection and then specify the geometry.

  3. Enter the magnitude of the Film Coefficient.

  4. Enter the magnitude of the Ambient Temperature.

  5. Define the Convection Matrix as Program Controlled (default), Diagonal, or Consistent. Not supported for LS-DYNA analyses.

  6. Set the Fluid Flow property to Yes if applicable. Define your Scoping Method and select the desired edge/vertex/node through direct selection or by selecting an appropriately defined Named Selection from the Fluid Flow Scoping property.

Details Pane Properties

The selections available in the Details pane are described below.

CategoryProperty/Option/Description

Scope

Scoping Method: This property specifies how you perform geometric entity selection. Options include Geometry Selection (default) and Named Selection.

Geometry: Visible when the Scoping Method is set to Geometry Selection. Displays the type of geometry (Body, Face, etc.) and the number of geometric entities (for example: 1 Body, 2 Edges) to which the boundary has been applied using the selection tools.

Note:  The Body filter selects 3D external faces only.

Named Selection: Visible when the Scoping Method is set to Named Selection. This field provides a drop-down list of available user–defined Named Selections.

Shared Reference Body: Not supported for LS-DYNA analyses. This property is a scoping feature used to apply a Convection boundary condition to the shared faces of two bodies, one solid and one shell. After you scope the geometric entities (either Geometry Selection or an appropriate Named Selection), the property provides a drop-down list of the names of the bodies that share the scoped features. Select the desired body from the list. Once selected, the Geometry or Named Selection property displays a parenthetical of the shared face, “(1 Shared Face),” to indicate the condition.

Shell Face: Supported for 3D shell bodies only. Requires that you activate beta options as well as a beta feature selection for thermal variation. This property enables you to apply the Convection load to the temperature degree of freedom associated with the selected face. Assuming you have specified a Convection load, to display the Shell Face property:

  1. Activate beta options and beta feature selection.[a].

  2. Select the shell body under the Geometry object and set the Thermal Variation (Beta) property to an option other than No Variation Along Thickness.

  3. Select the Convection object and define the Shell Face property. Options include Top (default) or Bottom.

Important:  If you specify a shell body as a Shared Reference Body, the solver does not support the Linear Variation and Quadratic Variation options of the Thermal Variation (Beta) property. In addition, you need to set the Offset Type property to Bottom.

Definition

Type: Read-only field that displays the boundary condition type: Convection.

Film Coefficient

Coefficient Type (visible only for temperature dependent Film Coefficient)

Ambient Temperature (not visible if the Fluid Flow property is set to Yes.)

Convection Matrix options include:

  • Program Controlled (default)

  • Diagonal

  • Consistent

Parameterization: Check the box next to Film Coefficient or Ambient Temperature to parameterize these properties.

Element APDL Name: This property enables you to assign an APDL parameter name to the surface element type number. Using this name, you can programmatically identify the convection element type and real constant set that you can reference and use in a Commands (APDL) object.

Suppressed: Include (No - default) or exclude (Yes) the boundary condition.

Edit Data For: Not supported for LS-DYNA analyses. This property displays when the Film Coefficient and/or Ambient Temperature properties are set to Tabular or Function.

Fluid Flow Controls

Fluid Flow: Steady-State Thermal and Transient Thermal analyses only. Options for this property are Yes or No (default).

Scoping Method: This property displays when you set the Fluid Flow property to Yes. This scoping applies to edge, vertex, or node scoping of the fluid line body. Options include Geometry Selection and Named Selection.

Fluid Flow Scoping: Based on the above Scoping Method, directly select the desired edge/edges, vertex, or node. Alternatively, select a Named Selection from the available drop-down list.

Display Connection Lines: This property requires that the mesh be generated. Options for this property are Yes or No (default). Selecting Yes displays connection lines between the centroid of each element face/edge of the convection surface(s)/edge(s) to the corresponding closest node on the fluid flow scoping. The closest node is computed using a distance-based algorithm.

Note:  The display of the connection lines may not be an accurate representation of the connection in the solver. The solver, by default and when applicable, uses the closes two nodes from the scoped fluid elements to specify the reference temperature.

Function

Not supported for LS-DYNA analyses. This category displays when the Film Coefficient and/or Ambient Temperature properties are set to Function. For additional information, see Spatial Load and Displacement Function Data.

Tabular Data

Not supported for LS-DYNA analyses. This category displays when the Film Coefficient and/or Ambient Temperature properties are set to Tabular. For additional information, see Spatial Load Tabular Data.

Graph Controls

Not supported for LS-DYNA analyses. This category displays based upon the specifications made in the Function, table assignment, and/or Tabular categories. For additional information, see Spatial Load and Displacement Function Data or Spatial Load Tabular Data.

Film Coefficient and Ambient Temperature table options

Mechanical APDL solver only. If you selected the name of a table under Film Coefficient or Ambient Temperature, the following options are available.

  • Spatial Coordinate System (read only) displays the spatial coordinate system of the table.

  • Graphics Controls appears when you select a table that contains time as an independent variable.

  • Coefficient Type: (read only) appears when the film coefficient is temperature dependent and shows how the temperature of the fluid is measured.

For additional information on how to use tables with Convection boundary conditions, see Specify Ambient Temperature and Film Coefficient with Tables.

[a] Activate beta options and beta feature selection:

  1. On the Workbench Project page, select the Tools drop-down menu and then Options.

  2. In the dialog, select the Appearance group and then the Beta Options check box.

  3. Click OK.

  4. Again, select the Tools drop-down menu and then Options.

  5. In the dialog, select the Mechanical group.

  6. Under the heading "MAPDL Solver," select the Allow thermal variation along shell thickness check box.

  7. Click OK.

  8. The application prompts you to restart. Do so, and then open your shell model in Mechanical for a supported analysis type.

Mechanical APDL References and Notes

The following Mechanical APDL commands, element types, and considerations are applicable for this boundary condition.

  • Convection loading is applied using the element types SURF152 (3D thermal analyses) and SURF151 (2D thermal analyses).

  • Film Coefficient and Ambient Temperature are applied using the SF command.

  • Film Coefficient and Ambient Temperature (constant, tabular, and function) are always represented as tables in the input file.

  • Fluid Flow activates the Mechanical APDL element FLUID116 and specifics TEMP (Temperature) as the degree of freedom.

  • A table is a special type of numeric array that enables the Mechanical APDL solver to calculate (through linear interpolation) the values between entries in a multi-dimensional table of numeric data. The solver applies these interpolated values across the selected geometry when it computes the solution. See the discussion in Array Parameters.

LS-DYNA References and Notes

The following LS-DYNA keywords and considerations are applicable for this boundary condition.

API Reference

For specific scripting information, see the Convection section of the ACT API Reference Guide.