Effect of velocity ratio on the turbulent mixing of confined, co-axial jets

The present work is an experimental investigation of turbulent mixing of two co-axial jets with low annular to core area ratio in a non-separating confinement. Mixing of jets with low area ratio finds application in the mixing chamber of low bypass turbofan engines used in military aircraft. Experiments were conducted in a newly designed set-up having a duct to nozzle diameter ratio of 1.19. The air flow was generated by two centrifugal blowers driven by motors of 30 kW power each. A static frequency inverter, capable of varying the speed of each motor, was used for changing the velocity ratio between the two streams. Effect of velocity ratio of the two streams is studied by changing the velocity ratio from 0.3 to 10.0. Mean velocity and the streamwise and transverse turbulence intensity distributions at different streamwise locations were obtained using laser Doppler velocimeter (LDV) for seven velocity ratios of 0.3, 0.6, 1.3, 1.5, 3.0, 6.0, 10.0 within a relatively short length of x/r(0) = 5.0 (where r(0) is the radius of the duct). The Reynold's number, based on mass averaged velocity and duct diameter (380 mm) was in the range of 4.2 x 10(5) to 7.0 x 10(5). A fiberoptics LDV system of Dantec make was used for carrying out the mean velocity and turbulence measurements. Total and static pressure measurements vr,cre made to obtain mean velocity distributions and total pressure loss due to mixing. The results show that mixing process in confined jets depends strongly not only on the velocity ratio, but also on the interaction between the boundary layer, mixing layer and the main flow, particularly when the area ratio is small. (C) 2000 Published by Elsevier Science Inc. All rights reserved.