NATURAL-CONVECTION IN ALUMINUM DIRECT CHILL CAST INGOT

Natural convection in a direct chill casting sump is modelled and the results are applied to centreline macrosegregation, The sump is treate d as a semicircular cavity: the upper straight and the lower curved si des represent the free surface and the solidification front, respectiv ely. The free surface is maintained at a higher temperature than the f ront to represent the superheat. Calculations show the flow field near the top of the sump to consist of a thin rapid wall jet travelling do wn the front adjacent to a strong, stable thermal stratification in th e upper region of the sump. The jet velocity is generally an older of magnitude greater than typical casting speeds. The core velocity is ty pically of the same oi der as the casting speed. The lower part of the sump is virtually isothermal, and will sustain growth ahead of the fr ont. Scaling laws for the jet velocity and width, the core velocity, a nd the depth of the strongly stratified upper region are derived from a simple dimensional analysis and are shown to agree well with computa tions. The predicted temperature profile con elates well with experime ntal measurements. From calculations and scaling equations, it is prop osed that the greater sensitivity of macrosegregation to ingot thickne ss than to casting speed is due to the greater sensitivity of sump dep th to ingot thickness, and not to a flow field effect.