Heat Transfer
It is often necessary to consider the effect of heat flow and temperature-dependent material properties (such as the viscosity or the surface tension) in microfluidic systems. The COMSOL Multiphysics base package includes physics interfaces for heat transfer in incompressible fluids and solids. The Heat Transfer Module is required in addition to this module to model heat flow in compressible fluids and to include viscous heating terms. This section describes how to couple heat transfer to microfluidics models using the functionality available in the COMSOL Multiphysics base package and the Microfluidics Module.
The Heat Transfer interface includes the equations for heat transfer in both fluid and solid domains. To model a system consisting of both solid and liquid domains add a single Heat Transfer interface and then add nodes for Heat Transfer in Solids and Heat Transfer in Fluids with selections corresponding to the solid and fluid domains respectively. For a fluid domain the convective flow is coupled into domain by selecting the velocity field from a corresponding Laminar Flow or Creeping Flow interface.
The fluid domain models the heat transfer including both convection and conduction according to the equation:
where,  ρ is the fluid density (SI unit: kg/m3),  Cp is the specific heat capacity at constant pressure (SI unit: J/(kg·K)),  T is absolute temperature (SI unit: K),  u is the velocity vector (SI unit: m/s),  q is the heat flux by conduction (SI unit: W/m2), and  Q contains heat sources other than viscous heating (SI unit: W/m3). For a solid domain the following equation applies:
When using the heat transfer in solids and in fluids domain properties the software automatically includes the heat transfer between the solid and the fluid. It is important to include the correct thermal boundary conditions on regions where fluid flows into or out of a domain, typically a Temperature condition for an inlet and an Outflow condition for an outlet.
For a detailed discussion of the fundamentals of heat transfer, see Ref. 4. More details of the heat transfer capabilities can be found in The Heat Transfer Interfaces in the COMSOL Multiphysics Reference Manual.