The invention relates to a nozzle for the projection on to an object of a fluid such as a gaseous flow containing solid particles, comprising a body 1 through which there passes a longitudinal tubular passage, one end of which constitutes an inlet 2 that has to be connected to the intake of a fluid supply conduit and the other end of which constitutes an outlet 3 for the fluid that has passed through the nozzle. The nozzle is characterized in that the section of the tubular passage is variable between the inlet 2 and the outlet 3, and the passage has three successive portions, which are: an inlet chamber 11 with a constant section, an intermediate conduit 12 with a variable section, the walls of which are convergent from the chamber 11 to an oblong neck, which has a major axis and a minor axis and the area of which is equal to that of the circular section of the chamber 11, and an outlet tube 14 with a variable oblong section, the walls of which are divergent from the neck to an outlet orifice of oblong section having a major axis and a minor axis.