A fluid flow inversion chamber comprises a spray nozzle (3) which has a coaxial configuration of feed flow (12) and steam flow (11) streams capable of providing increased contact of the two streams within the reaction zone, and, as a consequence of the inversion of the coaxial configuration of feed flow (12) and steam flow (11) streams, avoids the formation of coke, which facilitates inspection and replacement of nozzles worn by erosion.