Abstract: A passive injector including, a flow generator having a first inlet for gas or vapor at a first pressure and an outlet for gas or vapor, a mixing region having an inlet for the gas or vapor leaving the flow generator at supersonic velocity and a second inlet for a liquid at a second pressure, wherein liquid is incorporated into the flow of gas or vapor while maintaining supersonic velocity and a primary flow tube section aligned with an outlet of the mixing region and having an inlet and an injector outlet, the inlet of the flow tube being preceded by a gap surrounded by a cavity, wherein the flow velocity of the mixture is supersonic at the inlet of the primary flow tube section and the supersonic flow changes to subsonic flow within the primary flow tube section.