AbstractNatural convection in a direct chill casting sump is modelled and the results are applied to centreline macrosegregation. The sump is treated as a semicircular cavity: the upper straight and the lower curved sides represent the free surface and the solidification front, respectively. The free surface is maintained at a higher temperature than the front to represent the superheat. Calculations show the flow field near the top of the sump to consist of a thin rapid wall jet travelling down the front adjacent to a strong, stable thermal stratification in the upper region of the sump. The jet velocity is generally an order of magnitude greater than typical casting speeds. The core velocity is typically of the same order as the casting speed. The lower part of the sump is virtually isothermal, and will sustain growth ahead of the front. Scaling laws for the jet velocity and width, the core velocity, and the depth of the strongly stratified upper region are derived from a simple dimensional analysis and are shown to agree well with computations. The predicted temperature profile correlates well with experimental measurements. From calculations and scaling equations, it is proposed that the greater sensitivity of macrosegregation to ingot thickness than to casting speed is due to the greater sensitivity of sump depth to ingot thickness, and not to a flow field effect.MST/1970