# Heat Transfer Module Glossary

This glossary contains application-specific terms used in the Heat Transfer Module software and documentation. For mathematical terms as well as geometry and CAD terms specific to the COMSOL Multiphysics software, please see the Multiphysics Glossary.

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### anisotropy

The condition of exhibiting properties with different values when measured in different directions.

### bioheat equation

An alternative form of the heat equation that incorporates the effects of blood perfusion, metabolism, and external heating. The equation describes heat transfer in tissue.

### blackbody

A blackbody is a surface that absorbs all incoming radiation; that is, it does not reflect radiation. The blackbody also emits the maximum possible radiation.

### conduction

Heat conduction takes place through different mechanisms in different media. Theoretically, conduction takes place through collisions of molecules in a gas, through oscillations of each molecule in a “cage” formed by its nearest neighbors in a fluid, and by the electrons carrying heat in metals or by molecular motion in other solids. Typical for heat conduction is that the heat flux is proportional to the temperature gradient.

Heat advection takes place through the net displacement of a fluid, which translates the heat content in a fluid through the fluid's own velocity.

### convection

The term convection is used for the heat dissipation from a solid surface to a fluid, where the heat transfer coefficient and the temperature difference across a fictitious film describes the flux.

### emissivity

A dimensionless factor between 0 and 1 that specifies the ability of a surface to emit radiative energy. The value 1 corresponds to an ideal surface, which emits the maximum possible radiative energy.

### heat capacity

See specific heat.

### highly conductive layer

A highly conductive layer is a thin layer on a boundary. It has much higher thermal conductivity than the material in the adjacent subdomain. This allows you to assume that the temperature is constant across the layer’s thickness. The General Heat Transfer physics interface supports heat transfer in highly conductive layers.

The total radiation that arrives at a surface.

### Navier-Stokes equations

The equations for the momentum balances coupled to the equation of continuity for a Newtonian incompressible fluid are often referred to as the Navier-Stokes equations. The most general versions of Navier-Stokes equations do however describe fully compressible flows.

### opaque material

An opaque body does not transmit any radiative heat flux, that is, the surface of an opaque body has a transmissivity equal to 0.

Heat transfer by radiation takes place through the transport of photons, which can be absorbed or reflected on solid surfaces. The Heat Transfer Module includes surface-to-surface radiation, which accounts for effects of shading and reflections between radiating surfaces. It also includes surface-to-ambient radiation where the ambient radiation can be fixed or given by an arbitrary function.

### participating media

A media that can absorb, emit, and scatter thermal radiation.

The total radiation that leaves a surface, that is, both the emitted and the reflected radiation.

### specific heat

Refers to the quantity that represents the amount of heat required to change one unit of mass of a substance by one degree. It has units of energy per mass per degree. This quantity is also called specific heat or specific heat capacity.

### specific heat capacity

See specific heat.

### thin conductive shell

An physics interface for modeling heat transfer in a thin shell. “Thin” means that the shell is thin enough, or has high enough thermal conductivity, for you to assume that the temperature is constant across the shell’s thickness. See also highly conductive layer.

### transparent material

A transparent body transmits radiative heat flux, that is, the surface of a transparent body has a transmissivity greater than 0.

### thermal conductivity

The definition of thermal conductivity is given by Fourier’s law, which relates the heat flux to the temperature gradient. In this equation, the thermal conductivity is the proportional constant.