Experimental study of the flow in circular swirling jets

Abstract

The investigation has been carried out on swirling jets having four different swirl numbers. Swirling jets are produced by flowing air through circular straight nozzles having triangular shaped internal helical thread of four different (\ .25.2.5. 3.75 and 5) pitches. All measurements are taken at Reynolds number Red = 5.3 x 10•. For each nozzle. axial mean velocity and static pressure are measured by a \.6 mm outside diameter pitot static tube and for flow angle corresponding pressures are measured by a 5-tube yaw meter. All data are taken by a capacitive type pressure transducer with a data logger and are recorded and analyzed by a computer. The mixing region starts approximately after the discharge and ends at x/d values between I and 2 depending on swirl and after that no such prominent mixing activities are found. Velocity profiles beyond x/d = 2 are nearly flat indicating completion of mlxmg process but with some small variations showing the memories of upstream characteristics. The jet tmder investigation is found to be more like a twin jet formed by the interaction of two flow streams created at the two sides of thread boundary having interconnection between them at the centre. These flows have similar characteristics and they interact with each other. The mixing region may broadly be classified into three zones; (i) the zone with water fall shaped profile (600 ~ e :?: 0°) (ii) the zone with humped shaped profile (120° :?: e :?: 60°) and (iii) the zone with saddle shaped profile (180° ~ e ~ 120°). In the axial direction the velocity maxima occur away from the centre and at different radial angles depending on x/d. This is due to the centrifugal force created by the swirl. The fixing of the extension tubes at the discharge of the nozzle constrains the mixing at the boundary and between the flow streams of the jet. At the discharge of the extension tube the swirl is fOlmd to be milder and less random than that without it. The flow profiles decay and spread gradually in the axial direction. TIle pitch and yaw of the flow indicate the flow direction in the axial vertical plane and horizontal plane respectively.