Device for mixing two fluids and use thereof for cooling a very high temperature fluid

A device for mixing a first fluid at a first temperature with a second fluid at a second temperature, the fluids being mixed in the form of coaxial streams flowing in the same direction, the device comprising a generally cylindrical tubular casing having a substantially rectilinear axis defining a cylindrical mixer chamber coaxially inside the casing and comprising, at a first axial end, a first coupling element for coupling to first fluid feed means, and at a second axial end, opposite from the first, a second coupling element for coupling to means for exhausting the mixture of the first and second fluids, and at least one guide duct for guiding at least one of the first and second fluids, the at least one duct being substantially rectilinear and disposed coaxially inside the mixer chamber, the device including a third coupling element for coupling the mixer chamber to second fluid feed means, the third coupling element being in a position in the axial direction that is intermediate between the first and second coupling elements and extending in a transverse direction that is substantially perpendicular to the axial direction, and the guide duct extends axially in the mixer chamber between the first coupling element and a zone of the mixer casing downstream from the third coupling element in the direction going from the first coupling element to the second, and comprises a tubular wall including at least one coaxial insulating internal annular space in communication with a zone of the mixer chamber, said space extending substantially over the entire length of the guide duct, the third coupling element opening out into the mixer chamber so as to face an outside surface of the wall of the guide duct.