NON-STATIONARY BOUNDARY CONDITIONS IN THERMAL MODELS OF CONTINUOUS CASTING

The article focuses on the issue of boundary conditions for simulations of non-steady states in continuous casting machines. Numerical models of the temperature field of the continuously cast strand based on the finite difference method were implemented in off-line and on-line versions. The correctness of the simulation depends on the accuracy of the boundary conditions. Their determination is a technically demanding and costly procedure. From operational data, only approximate boundary conditions can typically be obtained; often average values in time or in space rather than the detailed local and immediate values. Modelling of unsteady behaviour of the casting process is therefore characterized by uncertainty. The boundary condition in the mould can be determined by means of measuring temperatures directly in the mould wall using thermocouples. However, it is demanding to keep greater number of sensors installed permanently. The boundary condition in the secondary cooling area is based on the measurement of heat transfer coefficient by means of the laboratory physical model. Significant complication is caused by non-linearity of heat transfer coefficient in the secondary cooling area due to the Leidenfrost phenomenon. Two reasons of the boundary conditions time variations can be distinguished. The first are deterministic, induced by the operator's changes of casting and cooling parameters, the second are random or quasiperiodic changes, caused by spontaneous fluctuations of parameters and internal quantities.