The following boundary conditions can be specified at outward and inner boundaries of the region.
Known temperature boundary condition specifies a known value of temperature T0 at the vertex or at the edge of the model (for example on a liquid-cooled surface). T0 value at the edge can be specified as a linear function of coordinates. The function parameters can vary from one edge to another, but have to be adjusted to avoid discontinuities at edges' junction points.
This boundary condition sometimes is called the boundary condition of the first kind.
Heat flux boundary condition is defined by the following equations:
Fn = -qs |
- at outward boundaries, |
Fn+ - Fn- = -qs |
- at inner boundaries, |
where Fn is a normal component of heat flux density, "+" and "–" superscripts denote quantities to the left and to the right side of the boundary. For inner boundary qs, denotes the generated power per unit area, for outward boundary it specifies the known value of the heat flux density across the boundary. If qs, value is zero, the boundary condition is called homogeneous. The homogeneous condition at the outward boundary indicates vanishing of the heat flux across the surface. This type of boundary condition is the natural one, it is defaulted at all outward boundary parts where no explicit boundary condition is specified. This kind of boundary condition is used at an outward boundary of the region, which is formed by the symmetry plane of the problem.
If the surface heat source is to be specified at the plane of problem symmetry and this plane constitutes the outward boundary of the region, the surface power has to be halved.
This boundary condition sometimes is called the boundary condition of the second kind.
Convection boundary condition can be specified at outward boundary of the region. It describes convective heat transfer and is defined by the following equation:
Fn = α(T - T0),
where α is a film coefficient, and T0 - temperature of contacting fluid medium. Parameters α and T0 may differ from part to part of the boundary.
This boundary condition sometimes is called the boundary condition of the third kind.
Radiation boundary condition can be specified at outward boundary of the region. It describes radiative heat transfer and is defined by the following equation:
Fn = β·kSB·(T4 - T04),
where kSB is a Stephan-Boltsman constant (5.67032·10-8 W/m2/K4), β is an emissivity coefficient, and T0 - ambient radiation temperature. Parameters β and T0 may differ from part to part of the boundary.
Note. For heat transfer problem to be defined correctly the known temperature boundary condition, or the convection, or the radiation has to be specified at least at some parts of the boundary.
Constant temperature boundary condition may be used to describe bodies with very high heat conductivity. You can exclude interior of these bodies from the consideration and describe their surface as the constant temperature boundary.
Note. The edge described as possessing constant temperature cannot have common points with any edge where the temperature is specified explicitly. In that case the constant temperature edge has to be described by the boundary condition of the first kind with an appropriate temperature value.