A reboiling jet apparatus includes at least two nozzles in series, configured to cause boiling of a hot liquid in the first nozzle, deceleration and reduction of the gas phase in the second nozzle, followed by acceleration and reboiling in the second nozzle. A second deceleration and reduction of the gas phase occurs at the outlet of the second nozzle. Each deceleration causes heating of the liquid by reduction of the gas phase; thus, energy of a pressurized input fluid is efficiently converted into heat by action of the nozzles. A convergent-divergent nozzle for steam injection with a mixing chamber may be used instead of the first nozzle to cause the first boiling. Another nozzle may be used to introduce a cold fluid at the outlet of the second nozzle for mixing with the hot flow prior to completion of the second deceleration.