MEASUREMENTS OF ENTRAINMENT CHARACTERISTICS OF SWIRLING JETS

Entrainment characteristics of free swirling jets were investigated by the schlieren flow visualization and the newly contrived technique for measuring the entrainment velocity, which is demonstrated to be reasonably accurate and reliable. The visualization studies reveal that the entrainment rate increases with the swirl intensity. Schlieren photographs, which were synchronized with the fluctuation velocity signal, display that the precessing vortex core (PVC) induces large scale periodic motion in the jet boundary regions near the nozzle exit. The PVC effect on entrainment is scrutinized by employing two swirling jets, one with a PVC and the other with a substantially suppressed PVC. The results clearly indicate that the entrainment is enhanced considerably due to the PVC. For swirl number S < 0.6, the entrainment rate is fairly independent of the Reynolds number (Re) and increases more or less nonlinearly with the downstream region, while, for S > 0.6, it becomes higher with Re; this is due to the fact that the high-frequency PVC is induced at higher Reynolds number.