The Effect of an Entraining Diffuser on the Performance of Circular-to-Slot Exhaust Ducts With a 90 Degree Bend

An experimental study has been undertaken on a 3-stage entraining diffuser with a distorted inlet flow. Two different circular-to-slot transition ducts were used as driving nozzles. Both transition ducts included a 90 degree bend. Varying degrees of inlet swirl were also considered. A 7-hole pressure probe was used to traverse the diffuser outlets. With a longitudinal duct the measured flows showed severe impingement of the primary flow on the walls of the entraining diffuser. Similar outlet flow distributions were observed for all cases of inlet swirl considered. In contrast, only minimal primary flow impingement was observed with a transverse duct. The use of the transverse duct also resulted in significant secondary flows including multiple large-scale vortices. These secondary flows intensified when swirl was added. Four performance parameters were calculated: the entrainment ratio, back pressure coefficient, pressure recovery coefficient and the entraining diffuser efficiency. The results showed that peak performance was attained at 20° of inlet swirl for both configurations. The addition of an entraining diffuser was found to result in minimal improvements of performance for the longitudinal duct configuration and significant improvements for the transverse orientation.