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This paper addresses the problem of primary dendrite spacing selection in directional solidification including melt flow. Natural convection is considered, driven by buoyancy forces due to local melt density variations caused by segregation. Within the interdendritic region convection modifies the solutal transport between dendrites and affects the spacing selection. Furthermore the flow pattern itself depends on the primary spacing and on the direction and strength of the gravity vector g. Phase field simulations for varying buoyancy conditions reveal the principle mechanisms, modification of the thermal length by depletion or enrichment of interdendritic impurities, leading to decreasing or increasing dendrite spacing for upward or downward buoyancy, respectively.