FILM BOILING AND TRANSITION BOILING REGIMES IN THE SECONDARY COOLING ZONE DURING THE DIRECT-CHILL CASTING OF ALUMINIUM ALLOYS

摘要

Accurate knowledge of the boundary conditions is essential when modelling the direct-chill casting process. Determining the surface heat flux in the secondary cooling zone, where the greater part of the heat removal takes place, is therefore of critical importance. Boiling water heat transfer phenomena are quantified with boiling curves that express the heat flux density as a function of the surface temperature. Compilations of boiling curves for the direct-chill casting of aluminium alloys present a good agreement at low surface temperatures but a very poor agreement at higher surface temperatures, in the transition boiling and film boiling modes. Secondary cooling was simulated by spraying instrumented samples with jets of cooling water. The ejection of the water film in quenching tests leads to an advanced cooling front effect, in which cooling occurs through axial conduction within the sample rather than through boiling water heat transfer at the surface. The heat flux density and surface temperature were evaluated using the measured thermal history data in conjunction with a two-dimensional inverse heat conduction model. The inverse heat conduction model developed at the University of British Columbia was able to take into account the advanced cooling front effect. The effect of various parameters (water flow rate, initial sample temperature, water jet velocity) on the rate of heat removal in the film boiling and transition boiling regimes was investigated. Taking into account the results of the inverse heat conduction analyses, idealised boiling curves were drawn for different water flow rates and initial temperatures.