Near field round jet flow downstream from an abrupt contraction nozzle with tube extension

摘要

Round air jet development downstream from an abrupt contraction coupled to a uniform circular tube extension with length to diameter ratio L/D = 1.2 and L/D = 53.2 is studied experimentally. Smoke visualisation and systematic hot film velocity measurements are performed for low to moderate Reynolds numbers 1130 < Re _b < 11320. Mean and turbulent velocity profiles are quantified at the tube exit and along the centerline from the tube exit down to 20 times the diameter D. Flow development is seen to be determined by the underlying jet structure at the tube exit which depends on Reynolds number, initial velocity statistics at the tube exit and the presence/absence of coherent structures. It is shown that the tube extension ratio L/D as well as the sharp edged abrupt contraction influence the initial jet structure at the tube exit. For both L/D ratios, the presence of the abrupt contraction results in transitional jet flow in the range 2000 < Re _b < 4000 and in flow features associated with forced jets and high Reynolds numbers Re _b > 10~4. The tube extension ratio L/D downstream from the abrupt contraction determines the shear layer roll up so that for L/D = 1.2 flow visualisation suggests the occurrence of toroidal vortices for Re b < 4000 whereas helical vortices are associated with the transitional regime for L/D = 53.2. Found flow features are compared to features reported in literature for smooth contraction nozzles and long pipe flow.