Abstract: A rotor to establish a swirling flow in liquid(s) contained in a vessel. In one form the rotor includes two parallel spaced apart discs. An array of vanes is interposed between the discs. One of the discs is annular to define an inlet to the array of vanes. The inlet controls the size of an inner core region of the swirling flow. In another form the rotor has at least two hollow spokes directed radially outwardly from the central hub. The spokes each have a passage with an inlet directed toward the hub and a radially directed outlet. Rotation of the rotor produces a flow through the spokes that contributes to the rotational radially outward flow that establishes the swirling flow. In another form of the invention the rotor includes at least two blades directed radially outwardly from a central hub. The blades include at least one surface inclined to the direction of rotation of the rotor.