A process for the preparation of lower olefins from a heavy hydrocarbon feedstock by vaporizing said heavy hydrocarbon feedstock with steam in a nozzle wherein a two-phase mixture is passed through a device having a frusto-conically-shaped element which is capable of inducing a gentle swirl pattern to a liquid-containing stream, which device is connected to the feed inlet pipe of the mixing nozzle. This swirl-inducing device effects a 90.degree. bending of the flow direction of the hydrocarbon feedstock while simultaneously effecting a swirl pattern of the liquid part thereof, thereby forcing the liquid against the wall of the feed inlet pipe extending downwardly to the mixing nozzle with such velocity component perpendicular to the tube's longitudinal axis to create an annular flow pattern as the mixture enters the nozzle and the vaporized hydrocarbon feedstock is subjected to a cracking treatment.