4.3.2    Defining of the boundary conditions

The user can also define boundary conditions that will be imposed on walls between objects made of specified media. For example, the QW-HFM module assumes that the *.pm2 file of name meat_air.pm2 contains the parameters of the boundary condition which will be applied on all the walls between objects made of medium named “meat” and objects made of medium “air”.

Such a situation is common in scenarios containing an oven cavity. Typically the cavity contains a load surrounded by air or other media, which are assumed lossless, do not have their *.pmo files and are ignored in the heat flow analysis. They can be safely neglected without any loss to the accuracy of computations as long as the walls of the load have been defined as boundaries of the computational domain and the right boundary conditions have been applied on them.

The boundary condition definition files let the user provide all the necessary information on boundaries to the QW-HFM module without the need to manually select all the walls and define their thermal properties. Walls without explicitly defined boundary conditions will be treated as boundaries with default conditions. The default boundary condition can be chosen by the user by modifying the DefaultBC option in the Preferences dialogue of QW-Simulator or in the initialisation file. An example file describing a boundary condition has been presented below.

Fig.  4.3.2-1 An example meat_air.pm2 file describing the boundary condition between objects made of meat and air.

The boundary condition definition file contains the Enthalpy and Temperature columns known already. What is new, is additional column Ha containing the values of the convective heat transfer coefficient (the unit is Wcm^-2K^-1) defined as a function of temperature and/or enthalpy. The heat transfer coefficient can assume values ranging from 0 (adiabatic boundary condition) to very large, tending to +INF (explicit boundary condition).

The *.pm2 file can also be used to define thermal resistance on selected internal boundaries of the project. A good example of such situation would be a project containing a microwave oven with a heated sample of meat located on a plate made of lossy glass. In such case the boundary between the plate and the sample is not external (provided that both media have valid *.pmo files). By properly defining the meat_glass.pm2 file it is possible to model the contact resistance that can be observed on this boundary. This requires that Ha coefficient no longer describes a convective boundary condition. In this scenario it should be rather treated as a reciprocity of the heat insulation coefficient that is directly related to thermal resistance. This feature is not available in the Fluent mode.

The order of media names in the *.pm2 filename is arbitrary. The file meat_air.pm2 can be replaced with the file of name air_meat.pm2. If the two files are found in the project folder an error is reported and the analysis is stopped.

The information and errors are reported in the the Simulator Log: