This page provides information about the fluid material model used by the HEAT solver which applies to both liquids and gases. The fluid material in the HEAT solver is used to define boundary conditions to the simulation region. More specifically, it is used to define convective boundary conditions in the simulation region. A convective boundary condition between a solid and a fluid placed inside the simulation area / volume can be added in the boundary condition group. For convective models that use analytic formula to calculate the convective heat transfer coefficients h, the input parameters in the models include different material properties of the fluid. For more details on the available analytic model please refer to the Boundary Conditions page.

NOTE: The fluid material is designed to be used by the HEAT solver to model convective heat transfer into and out of the simulation structure. It can however be also included in the electrical simulation by the CHARGE solver. In such cases, the CHARGE solver treats the fluid material as an insulator and it is described by its relative dielectric permittivity. |

NOTE: The fluid material is not supported by the CHARGE solver when the 'coupled' mode is selected to perform self-consistent electrothermal simulation. The fluid object should be replaced by an insulator type object in such simulations. |

## Electronic properties:

The fluid material is designed to be used by the HEAT solver to model convective heat transfer into and out of the simulation structure. It can however be also included in the electrical simulation by the CHARGE solver. In such cases, the CHARGE solver treats the fluid material as an insulator and it is described by its (DC) relative dielectric permittivity.

## Thermal properties:

The thermal material model of fluids include,

- DENSITY: The mass density of the fluid is simply defined as the mass of the fluid per unit volume. The unit is kg/m3. In the case of gases, the mass density can be defined as a function of temperature as discussed here.
- SPECIFIC HEAT: The specific heat, also known as the heat capacity of thermal capacity of the fluid is defined as the ratio of the heat added to (or removed from) a fluid to the resulting temperature change. The unit is J/kg K. The specific heat of fluids can be defined as a function of temperature as discussed here.
- THERMAL CONDUCTIVITY: The thermal conductivity of a fluid is a property that quantifies its ability to conduct heat. The unit is W/m K. The thermal conductivity of fluids can be defined as a function of temperature as discussed here.
- DYNAMIC VISCOSITY: The dynamic viscosity of a fluid is defined as a resistance to the movement of the fluid when multiples layers in the fluid move at different velocities. The unit is Pa s. The dynamic viscosity can be defined as a function of temperature as discussed here.
- THERMAL EXPANSIVITY: The thermal expansivity of a fluid defines how the volume of the fluid changes with temperature. The unit is /K. For ideal gases, the thermal expansivity can be defined as a function of temperature as discussed here.

## Using fluids with the HEAT solver

The fluid type materials are used in a thermal simulation through the boundary conditions. The HEAT solver solves the heat transport in solid materials only and includes the effect of any surrounding fluids through convective boundary conditions defined at the material interfaces. For example, in order to model the heat loss into air in a silicon waveguide, one needs to set up a convection boundary condition at the material interface between silicon and air. Once the (fluid) material property and the interface boundary condition are defined, the solver solves the heat transport in the solid (neglecting the fluid) and applies the convective boundary condition at the solid-fluid interface.