Recirculation zones of unconfined and confined annular swirling jets

Abstract

An experimental study is presented for flowfields behind an axially mounted cylindrical bluff body of annular swirling jet flows. Both the confined and the unconfined cases are examined. The controlled parameters included the Reynolds number Re and the swirl number S. Smoke streaks were used to observe the dynamic flow structures of the recirculation zone behind the bluff body. Laser-Doppler anemometry was used for velocity measurements at the jet exit and downstream of the annular jet flows. The results indicate that the recirculation zones for both the unconfined and the confined cases can be classified into seven typical flow patterns based on the Reynolds number and swirl number : stable flow, vortex shedding, transition, prepenetration, penetration, vortex breakdown, and attachment. The flow patterns and their domains in (Re, S) space for unconfined and confined cases were basically the same. In the attachment regime, the central recirculation zone for the confined case was larger than that for the unconfined case. The boundaries of the recirculation zone for various flow conditions were investigated. Scaling analysis was used to correlate the lengths of the recirculation zone with the swirl number.