Experimental investigation of circular collared jets with inclined and non-inclined exit geometries

Abstract

An experimental study was conducted to investigate the general characteristics of circular collared nozzles with inclined and non-inclined collar exit geometries. In particular, the jet mixing response to a variation in collar length was also examined. The Reynolds number based on nozzle diameter and nozzle exit centerline velocity was approximately 30 000 and a collar to nozzle diameter ratio of 3 was chosen for all experiments. The results show that both an increase in collar length, L/d and inclination angle, θ lead to an improvement in the jet mixing behaviour. For both the non-inclined and inclined geometries, an increase in L/d enhances the centerline velocity decay, increases the near field fluctuating intensity and yields larger jet spreads. This is probably due to the higher shear imparted by the presence of the collar wall as compared to the unbounded flow. Similar results are observed with an increase in the inclination angle but this augmentation levels out at θ = 40 °. The effect of collar inclination upsets the axisymmetry of the flow field and causes the flow to deviate towards the azimuthal location where the collar lip protrudes out the furthest. This asymmetry becomes increasingly pronounced as the inclination angle is increased. Finally, the effects of flow reattachment are largely decoupled from any flow instability mechanism and do not lead to any strong form of self-excitation. Instead, there is evidence to show that flow reattachment accelerates the core flow and leads to a poorer jetmixing response.