Abstract: A radially-extending arm is adapted to rotate within a stream of first fluid flowing thereacross. In one embodiment, the arm comprises a plurality of lands located at different radial distances from the axis of rotation, and a port located on each land is operatively coupled to a source of a second fluid. The second fluid is sprayed from the rotating arm into the stream of first fluid that flows across the lands, and is thereby atomized. In another embodiment, the arm comprises a land stepped into the trailing edge thereof, and a port for injecting the second fluid is located on the land. In another embodiment, the land comprises a groove located between the port and an associated riser surface stepped into the trailing edge. In another embodiment, the arm comprises a port in the trailing edge thereof from which the second fluid is injected, and a groove is located on the trailing edge in a radially increasing direction from the port.