If you set the global advection scheme (that is, if you do not specify equation-class advection schemes) to upwind or high resolution, then the volume fraction equation will also be solved using upwind or high resolution. This is differs from Ansys CFX-5.5.1 where upwind was always used for the volume fraction equations. However, if you set the global advection scheme to a specified blend, then the volume fraction equations will actually use the high resolution advection scheme but the maximum blend factor will be limited to the specified value instead of 1.