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Conjugate Heat Transfer Analysis

The analysis type Conjugate heat transfer (CHT) allows the simulation of the heat transfer between Solid and Fluid domains by exchanging thermal energy at the interfaces between them. It requires a multiregion mesh to have a clear definition of the interfaces in the computational domain. Such a mesh can be created with the Hex-dominant parametric operation in the mesh creator.

Typical applications of this analysis type is the simulation of heat exchangers, cooling of electronic equipment, and general-purpose cooling and heating systems.

Natural convection around a simplified heat sink

Natural convection around a simplified heat sink

In the following, conjugate heat transfer simulation setup is discussed.

Analysis properties

Under analysis properties the turbulence models and the time dependancy of the simulation are selected.

The various options available are detailed below.

Turbulence model

A turbulence model should be chosen in accordance to the flow regime. In a Laminar flow, associated with low Reynolds numbers, viscous effects dominate the flow and turbulence can be neglected. This flow regime is characterized by regular flow layers.

On the other hand, a Turbulent flow is characterized by chaotic and irregular patterns that are associated with high Reynolds numbers. In order to simulate a turbulent fluid flow an appropriate turbulence model should be chosen. Currently, these models are supported:

  • Laminar

  • Reynolds-Averaged Navier–Stokes (RANS)
    • k-Epsilon
    • kOmega-SST
  • Large eddy simulation (LES)
    • Smagorinsky

Time dependency

There are two variants of simulation: Steady-state and Transient flow.

Steady State If you are only interested in the time averaged or time independant solution, where the flow condition do not change over time, consider a Steady-State simulation. Steady-state simulations are computationally less demanding.

Transient In order to account for time-dependent effects, consider a Transient simulation. Transient simulations are computationally expensive and challenging.

Solver

The conjugate heat transfer simulations use the following OpenFOAM® solvers:

  • chtMultiRegionFoam.

For transient simulations with laminar or turbulent flow. This solver uses the PIMPLE method for iterative solution.

  • chtMultiRegionSimpleFoam.

For Steady State simulations with laminar or turbulent flow. This solver uses the SIMPLE method for iterative solution.

Domain

In order to perform a CHT simulation on a given domain you have to discretize your geometry by creating a mesh. Details of CAD handling and Meshing are described in the Pre-processing section.

Note

For CHT analysis, CAD must meet certain necessary requirements which can be seen in the Pre-processing section.

After a mesh domain is assigned to the simulation, it is possible to use domain-related entities associated with the mesh in setting up the simulation.

Additionally, one can view the mesh or define new entities, e.g. a Topological Entity Set, to facilitate the simulation setup process. Details of each step are described in the following sections:

Model

Under model the gravity and the thermal properties of the system (for all regions) must be defined. Moreover, if LES turbulence model is being used, the LES delta coefficient should be specified as well.

Material

For CHT, several fluid and solid materials are available from the Material Library.

Fluid material:

The Fluid material properties and behavior are defined by Thermal fluid model.

The user has various options to choose from which are described in detail under Thermal Fluid properties. - Thermal Fluid properties

Solid material:

The Solid material properties and behavior are defined by Thermal solid model.

The user has various options to choose from which are described in detail under Thermal Solid properties. - Thermal Solid properties

Initial conditions

In a conjugate heat transfer simulation, the computational domain will be solved for pressure (p), velocity (U), temperature (T), and etc. Depending on the choice of solver, additional turbulent transport quantities may be included. The initial conditions play a vital role in the stability and computing time of the simulation. Therefore, it is very important to define appropriate initial conditions for the simulation.

Important

It is recommended to set the initial conditions close to the expected solution to avoid potential convergence problems.

For CHT analysis type, the velocity and temperature variables could be initialized either uniformly or seperately via a ‘Subdomian’ for each region. the two methods are detailed below.

boundary conditions

Finally, the following boundary conditions are available for each variable:

Important

As a general practice, the boundary conditions must be specified for all surfaces, except the interfaces.

Numerics

Numerical settings play an important role in simulation configuration. Ideally, they could enhance stability and robustness of the simulation. Since the optimal combination is not always trivial to find, default values are tried to be as meaningful and relevant as possible.

However, all numerical setting are made available for users to have full control over the simulation. These settings are divided into three categories:

  • Properties

    All properties regarding the iterative solvers of velocity and pressure equations are set here. Relaxation factors, residual controls, and solver-specific tweaks are among these settings. However, depending on the solver (e.g. PIMPLE, PISO, ...), these settings will be adjusted. For each field, a Help message is provided on the platform.

  • Solver

    In this part, linear solvers used in computing each variable could be chosen separately. Upon choosing a solver, a set of preconditioners/smoothers and their tolerances become available. To assist with selecting the best solver, a Help message is provided for each field.

  • Numerical Schemes

    These schemes determine how each term in the governing equations should be discretized. Schemes categorized in the following groups:

    • Time differentiation
    • Gradient
    • Divergence
    • Laplacian
    • Interpolation
    • Surface-normal gradient

Important

Generally, for Numerical Schemes the default selections are the best choice and require no changes.

Simulation Control

The Simulation Control settings define the general controls over the simulation. Number of iterations, simulation interval, timestep size, and several other setting could be set. The following controls are available:

Result Control

Result Control allows users to define extra simulation result outputs. Each result control item provides data that requires additional calculation. The following result control items are available:

Disclaimer

Conjugate heat transfer analysis is performed using the OpenFOAM software. See our Third-party software section for further information.

This offering is not approved or endorsed by OpenCFD Limited, the producer of the OpenFOAM software and owner of the OPENFOAM® and OpenCFD® trade marks. OPENFOAM® is a registered trade mark of OpenCFD Limited, the producer of the OpenFOAM software