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 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.

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

  • t f is the ambient (or bulk) fluid temperature (you provide)

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.

If you select multiple faces when defining Convection, the same ambient fluid temperature and film coefficient is applied to all selected faces.

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

  • 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 is available when the film coefficient is temperature dependent. Its value can be evaluated at the average film temperature (average of surface and bulk temperatures), the surface temperature, the bulk temperature, or the absolute value of the difference between surface and bulk temperatures.

    Note:  If you change the units from Celsius to Fahrenheit, or Fahrenheit to Celsius, when the convection coefficient type Difference between surface and bulk is in use, 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 is the temperature of the surrounding fluid. It is possible to have a time or spatially dependent ambient temperature.

  • Convection Matrix: 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. Not available in an LS-DYNA analysis.

  • Edit Data For: 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. Not available in an LS-DYNA analysis.

  • Fluid Flow (available 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): Supported for Film Coefficient. Not supported for LS-DYNA analyses.

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

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

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 Available in an LS-DYNA analysis.

  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: This property is a scoping feature used to apply a Convection to the shared faces of two bodies, one solid and one shell. When you have properly scoped the geometric entities, using 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. Not supported in an LS-DYNA analysis.

Shell Face: This property is supported for 3D shell bodies only and 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:

  • Program Controlled (default)

  • Diagonal

  • Consistent

Edit Data For: this property displays when the Film Coefficient and/or Ambient Temperature properties are set to Tabular or Function. Not Available in an LS-DYNA analysis.

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

Fluid Flow Controls

Fluid Flow (Steady-State Thermal and Transient Thermal analyses only): 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 desired edge/edges, vertex, or node or select a Named Selections from the available drop-down list.

Display Connection Lines: This property requires that the mesh be generated. Options for this property include Yes and 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

This category displays when the Film Coefficient and/or Ambient Temperature properties are set to Function. For additional information, see the Spatial Load and Displacement Function Data section of the Help. Not Available in an LS-DYNA analysis.

Tabular Data

This category displays when the Film Coefficient and/or Ambient Temperature properties are set to Tabular. For additional information, see the Spatial Load Tabular Data section of the Help. Not Available in an LS-DYNA analysis.

Graph Controls

This category displays based upon the specifications made in the Function and/or Tabular categories. For additional information, see the Spatial Load and Displacement Function Data section or the Spatial Load Tabular Data section of the Help. Not Available in an LS-DYNA analysis.

[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.

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.