Thermal

Introduction

Thermal fields, which are an extension of the scalar field, are used to describe the time-evolution of a temperature field across the flow domain. When evoking the thermal field, users specify the initial temperature of the fluid, the thermal diffusivity, the specific heat and, if desired, the fluid expansion coefficient and viscosity activation energy. The thermal diffusivity and specific heat influence the rate of thermal transport through the fluid, and temperature increase associated with thermal viscous dissipation. The fluid expansion coefficient, which informs natural convection, describes the link between temperature and density. The viscosity activation energy links the local viscosity to the local temperature field.

Tank walls can be either insulating or assume a constant surface temperature. Likewise, tank internals can be either insulating or assume a constant surface temperature. The temperature at each inlet can likewise be specified independently. The time-evolution of the fluid temperature is recorded in the output file thermodynamics.dat.

For tank internals defined with a constant surface temperature, the time-evolution of the surface heat flux into the fluid is printed to the internalsN.dat file. From this heat flux data, users can predict convective heat transfer coefficients using a representative temperature difference and the surface area. The average temperature across each inlet and outlet is recorded in the inletOutletDataN.dat files. With the temperature field activated, the solver automatically accounts for the effects of viscous dissipation on temperature rise, thereby satisfying the first law of thermodynamics.

Property Grid

Thermal Properties

Thermal Properties

Definition.

Constant

Definition

Initial Temperature

K | Fluid Initial Temperature.

Diffusivity

m ² /s | Definition.

Heat Capacity

J/m ³ K | Definition.

Expansion Coefficient

1/K | Definition.

UDF

Definition

Initial Temperature UDF

K | This UDF defines the initial temperature field across the fluid. One output must be defined within the UDF: a floating-point variable named T. This output variable defines the local initial temperature across the fluid. This is a voxel-based local UDF, calculated on a voxel-by-voxel basis. However, only the voxel position, lattice spacing, and simulation time step are available as input variables.

Download Sample File: Initial Temperature

Thermal Properties UDF

m ² /s and J/m ³ K | This UDF defines the local thermal diffusivity and local volumetric heat capacity of the fluid. Two outputs must be defined within the UDF: floating point variables alpha and cpv. These output variables define the local thermal diffusivity and local volumetric specific heat. This is a voxel-based local UDF, calculated on a voxel-by-voxel basis using the local fluid properties.

Download Sample File: Thermal Properties

Expansion Coefficient UDF

1/K | This UDF defines the fluid’s local thermal expansion coefficient. It requires a single output: a floating-point variable named beta, which represents the local thermal expansion coefficient. This is a voxel-based local UDF, calculated on a voxel-by-voxel basis using the local fluid properties.

Download Sample File: Expansion Coefficient

General

Minimum Temperature

Maximum Temperature

Global Fluid Heating UDF

W/m ³ | This UDF defines the heat flux applied uniformly over the entire fluid. This is a System UDF.

Download Sample File: Global Fluid Heating

Dissipation Heating

Limiter

Advection Algorithm

If there is a static inlet outlet boundary condition, the following section will launch:

{Static Inlet Outlet} Boundary Condition

Thermal Boundary Condition

Definition.

Specified Temperature

Zero Gradient

Recirc Inlet Temperature Expression

• Recirculation Return Temperature UDF - K | This UDF defines the recirculation return temperature. It can be a function of the recirculation intake temperature, T. This is a System UDF.

Download Sample File: Recirculation Return Temperature

**If there is a moving inlet outlet boundary condition, the following section will launch:*

{Moving Inlet Outlet} Boundary Condition

Thermal Boundary Condition

Definition

Zero Flux -

Specified Value -

• Fluid Temperature UDF - K | This UDF defines the temperature of the fluid entering or leaving along the boundary condition surface. This is a System UDF.

Download Sample File: Fluid Temperature UDF

{Static Body} Fluid

Thermal Boundary Condition

Definition

Fixed Temperature

Definition.

Surface Temperature UDF

K | This UDF defines the temperature of the thermal boundary condition. This is the temperature of the fluid adjacent to the solid. This is a System UDF.

Download Sample File: Surface Temperature

Adiabatic

Heat Flux

Definition

Thermal Flux Specification

Definition

Constant

UDF

Definition

Solid Surface Thermal Flux UDF

W or W/m ² | This UDF defines the heat flux along the surface of a static body. One output must be defined within the UDF: a floating point variable named flux. This output variable defines the heat flux at the solid/liquid interface. This value can be specified as either a total flux (flow) or aeral flux (per unit surface area).

If the Custom Flux Specification is set to Areal, this value will be used directly in the code to define a local flux field boundary condition along the solid surface. If the Flux Specification is set to Total, this value will be divided by the local solid surface area to define a local flux field boundary condition along the solid surface. This is a local UDF, calculated on a voxel-by-voxel basis using the local fluid properties. The UDF is only applied to cells adjacent to the solid surface.

Download Sample File: Solid Surface Thermal Flux

Nusselt Flat Plate

Nusselt Number

Advanced

Update Frequency

Legacy Text:

Boundary Conditions

This section lists out all available boundary conditions that can be configured with the thermal field. Each boundary may be configured with a boundary condition type.

Other conditions are defined separately with their respective families, including fluid, scalar field, miscible scalar, free surface volume fraction, immiscible two fluid volume fraction, and particle boundary conditions.

• Constant Temperature: Holds temperature of surface constant at specified temperature [K]

• Adiabatic: Enforces a zero gradient boundary condition on the surface

• Heat Flux: Specify the heat flux at a boundary condition with a constant, CPU, or GPU code.