Heat Transfer Module Updates

For users of the Heat Transfer Module, COMSOL Multiphysics® version 5.5 includes a new Lumped Thermal System interface, multiple spectral bands for the Radiation in Participating Media and Radiation in Absorbing-Scattering Media interfaces, and open boundary condition improvements. Read about these heat transfer features and more below.

Lumped Thermal System Interface

The new Lumped Thermal System interface extends heat transfer modeling to discrete thermal systems. This is used for reducing a model's complexity by replacing portions of it with equivalent circuit elements, such as to represent the thermal interaction between parts in large assemblies, for example. Several types of circuit elements are available, as well as advanced devices and user-defined subsystems.

The following equivalent circuit elements are available:

  • Two-node
    • Conductive Thermal Resistor
    • Thermal Capacitor
    • Heat Rate Source
    • Convective Thermal Resistor
    • Radiative Thermal Resistor
    • Thermoelectric Module
    • Heat Pipe
  • One-node
    • Temperature
    • Heat Rate
    • Radiative Heat Rate
    • Thermal Mass
    • External Terminal

The external terminal feature connects a lumped thermal system to a finite element model in any dimension.

COMSOL Multiphysics results for a thermoelectric module comparing a lumped model to the FEM model. Using the Lumped Thermal System interface Comparing a thermal network (lumped system) representation of a thermoelectric module to the FEM model.

This feature is demonstrated in the following models:

Multiple Spectral Bands for RPM and RASM

The Radiation in Participating Media and Radiation in Absorbing-Scattering Media interfaces now support an arbitrary number of spectral bands for modeling wavelength-dependent material properties alongside general improved usability when using multiple spectral bands. You can define material properties such as the absorption coefficient, scattering coefficient, or surface emissivity from a wavelength-dependent function or from a table, with one value per spectral band. Having wavelength-dependent material properties provides a more accurate representation of the materials, in particular when they are exposed to radiation from a wide wavelength ranging from infrared (emitted by the material) to visible light (from an external source like a laser or the Sun). This functionality is demonstrated in the new Radiative Cooling of a Glass Plate with Wavelength-Dependent Radiative Properties model.

A glass plate modeled in COMSOL Multiphysics with incident radiation visualized and the participating medium settings shown. Multiple spectral bands example Definition of the absorption coefficient using a four-spectral-bands model and the resulting incident radiation.

Improved Open Boundary Feature

The Open Boundary feature now has an option to choose between two conditions for inflow: Flux (new default) or Temperature (previous implementation). The implementation is unchanged for outflow. The new Flux option corresponds to a Danckwerts condition that applies an inflow of heat coming from a virtual domain with known upstream conditions. This flux is proportional to the temperature difference between the boundary and the upstream, and it is also proportional to the flow rate. It tends to prescribe the upstream temperature at the boundary for a large flow rate, while the boundary temperature is influenced by the sources and sinks of the adjacent regions for small flow rates. This improvement leads to more accurate and realistic physical models compared to a temperature condition that is usually not possible to prescribe at the boundary. In addition, the flux condition induces a smoother numerical formulation, which results in a more robust model that can be faster to solve.

The Open Boundary feature in the Heat Transfer Module is used and the resulting temperature field and convergence are plotted for two different formulations. Using the Open Boundary feature Comparison of the temperature field and of the solver convergence rate with the temperature formulation on the top and the flux formulation on the bottom. In this example, the flux formulation leads to a more realistic temperature profile close to the vertical wall on the right and is more than 20% faster to solve.

This feature is demonstrated in the following models:

Wavelength-Dependent Emissivity for Ambient Radiation

The Surface-to-Surface Radiation interface now includes a ready-made option for specifying ambient emissivity. This is important when combined with the multiple spectral band capabilities of the Surface-to-Surface Radiation interface for modeling radiative cooling. Radiative cooling occurs due to the high transitivity, in clear-sky condition, of the atmosphere in the 8–12 µm wavelength range that can by modeled using an atmosphere emissivity εamb = 1-τamb. The best cooling performances are obtained with surfaces that both reflect and emit radiation in the range of 8–12 µm. This feature is demonstrated in the Radiative Cooling model.

A plot of atmospheric transmissivity for different wavelengths, up to 20 micrometers. Atmospheric transmissivity
A plot of atmospheric irradiation for different wavelengths, up to 26 micrometers. Atmospheric irradiation
Model for atmosphere transmissivity (left) and for irradiation coming from the atmosphere (right).

Open Boundary and Inflow Conditions for Moisture Transport

For users of the Moisture Transport interfaces, the Open Boundary and Inflow conditions have been introduced similarly to what is available in the heat transfer interfaces. The Inflow feature enables you to specify upstream moisture conditions to define the moisture inflow that would be obtained from adding a virtual domain upstream of the inlet. A Danckwerts condition is used to estimate the flux across the boundary. The Open Boundary feature behaves identically to the Inflow feature for an incoming flow, and automatically switches to an outflow condition for an outgoing flow. Similarly to the open boundary feature for heat transfer, it also provides a temperature option for the inflow condition. These boundary conditions are expected to be used when the moisture transport is coupled with a flow that has corresponding boundary conditions.

These features are demonstrated in the following models:

General Moisture Transport Interface Improvements

A new Concentrated species formulation is available in the Moisture Transport in Air interface to model convection and diffusion of vapor in air when the vapor content is high. This is often the case when there is moderate or high relative humidity at a high temperature. Under these conditions, the moist air density may vary significantly in space and time due to vapor concentration gradients, and the default formulation should be replaced by the new Concentrated species formulation. You can see this demonstrated in the Condensation Detection in an Electronic Device with Transport and Diffusion and Evaporative Cooling of Water models.

The physics interface has also been improved to support supersaturation conditions that correspond to a relative humidity greater than one. This occurs when a hot stream saturated with water vapor is rapidly cooled down. Finally, the default solver settings for the various moisture transport, heat and moisture, and moisture flow interfaces have been set in a way that results in more robust and faster computation.

Surface-to-Surface Radiation with Ray Shooting Radiation Method

The Symmetry for Surface-to-Surface Radiation and External Radiation Source features are now available when the ray shooting radiation method is used. The symmetry condition can be used to define up to three symmetry planes or to define sector symmetry in order to reduce the computational effort for the view factor computation. You can see these features demonstrated in the How to Improve the Performance of View Factor Computation for Surface-to-Surface Radiation Modeling model. The External Radiation Source feature can be used to define radiation coming from an object that is not represented in the model geometry, like the Sun. For time-dependent analyses, the external radiation source position or direction may vary during the simulation. Additionally, new options are available to control the view factor update for time-dependent simulations, making it possible to define a good balance between accuracy (frequent view factor update) and computational time (minimal view factor update).

A sphere is modeled in COMSOL Multiphysics taking advantage of symmetry in the geometry via the Symmetry for Surface-to-Surface Radiation feature. Using the Symmetry for Surface-to-Surface Radiation feature The Symmetry for Surface-to-Surface Radiation settings in a model using the ray shooting radiation method.

Radiative Source Feature for Radiation in Media Interfaces

The Radiative Source feature is now available in the Radiation in Participating Media and Radiation in Absorbing and Scattering Media interfaces. It can be used to define a radiation source, like fluorescence, which is not described by the media absorptivity or scattering properties. The radiative source can be isotropic or, when the discrete ordinate method is used, directional.

Improved Layered Material Features

For the heat transfer interfaces used for layered shells, computations are faster in models with a very large number of boundaries. In addition, the user interface has been optimized to simplify the model setup for single-layer materials, which can now be defined by referencing a material in the Material node in the Model Builder. Additionally, you can scale the layer thickness for a single-layer material. The scale can be defined as an arbitrary expression, in particular stemming from other physics quantities. For example, you can define a film thickness computed by a Thin Film Flow interface.

An electronic enclosure modeled using layered material features in COMSOL Multiphysics version 5.5. Using layered material features Single-layer materials can be defined using a classic material. In this case, the classical material defines the layer thickness scale from the corresponding user input.

You can see these improvements demonstrated in the following models:

Nonisothermal Flow Multiphysics Coupling

The Nonisothermal Flow multiphysics coupling, available for laminar flow in COMSOL Multiphysics®, now has automatic settings in order to provide more physical results without the need of user action. In particular, the pressure work contribution to the energy equation is automatically determined from the flow compressibility settings. Also, the viscous dissipation is now activated by default, ensuring energy conservation with the default settings of the multiphysics coupling node. This functionality has previously only been available with certain add-on modules, but is now available in the core COMSOL Multiphysics® software.

Multiphysics Coupling for Heat Transfer in Thin Structures

Effects of thermal expansion can be important in solids as well as thin structures like shells and membranes. Multiphysics couplings have been added so that temperatures computed in a heat transfer analysis of a thin structure can be automatically transferred to corresponding structural mechanics interfaces.

To this end, three new multiphysics interfaces have been added:

  • Thermal Stress, Shell, combining the Heat Transfer in Shells and Shell interfaces
  • Thermal Stress, Layered Shell, combining the Heat Transfer in Shells and Layered Shell interfaces
  • Thermal Stress, Membrane, combining the Heat Transfer in Shells and Membrane interfaces

You can also connect to the structural mechanics interfaces from the Thin Layer feature within the Heat Transfer in Solids interface.

View this functionality in the updated Heating Circuit model.

Two side-by-side heating circuit models visualizing the temperature distribution and stress. Multiphysics model of a heating circuit Temperature distribution (left) and stress (right) in a heating circuit. The thin conducting layer is modeled using a Membrane interface and the substrate is modeled using the Solid Mechanics interface. Both interfaces receive the temperature from the heat transfer analysis through multiphysics couplings.

New Tutorial Models and Applications

Version 5.5 brings several new tutorial models and applications.