Effect of Lobes on the Mixing and Spreading Characteristics of Sonic Jets

The main objective of the present work is to enhance the mixing and spreading characteristics of subsonic jets by means of passive flow control techniques. Apart from a conventional convergent circular nozzle, three novel nozzles with non-circular cross-sections are used in the present study and they are four lobed, six lobed and tilt square nozzle. The centerline total pressure decay (axial) and the radial total pressure decay were measured experimentally at six different axial locations for all the nozzle models used. The experiments are conducted in three different operating conditions such as for at the inlet total pressures of 129, 154, and 192 kPa corresponding to the Mach numbers of 0.6, 0.8, and 1.0, respectively. The results obtained are then compared with a base nozzle (circular nozzle). From the experiments, significant understanding about the mixing rates, the potential core lengths and the radial jet decay are known. In addition, CFX simulations are performed to validate the experimental data under the same operating conditions, and it provides a reasonable agreement with the experimental data. Based on the experimental results, jets ejected out of six lobed nozzle shows the highest percentage of reduction in the core length of about 24% at M = 0.6 when compared with all other cases and the lowest percentage of reduction in core length is observed for the jet from the tilted square nozzle case at M = 0.6 of about 7%. Overall, it may be concluded that, amongst three nozzles with altered geometries, four lobed nozzle shows increased mixing and spreading rates as indicated by the axial and radial pressure decay followed by six lobed nozzle and tilted square nozzle cases.