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The extended continuum mixture model with considering the floating grains motion was applied to calculate the macrosegregation of large-size billet of 2024 aluminum alloy during direct-chill (DC) casting. Transport equations of mass, momentum, species and heat were solved simultaneously with the Scheil-Gulliver micro-model. The effects of casting parameters (e.g. billet size, casting speed, casting temperature and intensity of secondary cooling zone) were calculated in nine cases. Influence of the transport mechanisms for macrosegregation formation dependent on casting parameters were discussed in detail. Our study demonstrated that the processing parameters directly impacted the shape and dimension of the sump, thereby affecting the final segregation patterns. The billet size and casting speed played the mostly important roles. A larger billet size usually means a slower cooling rate, a deeper and wider sump, which results in the severer segregation in the billet center. The sump depth dramatically increases with the increase of the casting speed. Therefore, higher casting speed promotes greater macrosegrgation. Nevertheless, the sump depth increases slightly with increasing of casting temperature.