Abstract: A fluid of lower density is dispersed in a jet of a fluid of higher density, by imposing on the jet of higher density a curved trajectory. Where the pressure is the highest, that is, where the wall that establishes the curved trajectory has its largest radius of curvature, the fluid of lower density is introduced, preferably under pressure. The apparatus can be embodied as a tuyere which is a figure of rotation, when only a relatively small proportion of lower density fluid is to be introduced. When the proportion of lower density fluid is greater, however, then the device can be embodied as a duct of quadrangular section wrapped in a helix with narrow slots for the introduction of the fluid of lower density formed in its outer wall.