Influences of geometric parameters on serpentine nozzles for turbofan

Serpentine nozzles possess excellent advantages in improving the stealth ability of aero-engine exhaust. For a real turbofan, the flow characteristics of the serpentine nozzle are evidently affected by the geometric parameters and the complex exhaust mixer. This is an issue that needs to be seriously considered. The aim of this paper is to investigate the influences of the critical geometric parameters on the serpentine nozzles for turbofan. The detailed flow field of the serpentine nozzle for a small-scale model is precisely measured. The influence of the ratio of axial length to inlet diameter on the flow field and on the aerodynamic performance of the serpentine nozzle is numerically explored. Results show that the pressure on the upper wall first drops rapidly and then ascends with the local minimum occurring at the first inflection. The relative error between the numerical prediction and the experimental data is less than 2%. The calculated distributions of the expansion-shock waves and the jet shear layers are highly consistent with the schlieren flow visualization data. Thus, the reliability of the numerical method is effectively confirmed. The internal flow and the external jet features of the serpentine nozzle are extremely non-uniform. Such poor uniformity is reflected in the phenomena including the existence of the local high-shear-stress regions and the high-velocity regions, the multiple bending of the limiting streamlines, as well as the velocity fluctuation of plume core. The aerodynamic performance of the serpentine nozzle first enhances greatly and then stays unchanged as the value of L/D rises. The vortex loss and the shock loss both reduce evidently owing to the flow separation and the disappearance of the shock wave during the rise of L/D from 2.2 to 2.4. The value of the discharge coefficient and the thrust coefficient increases by 1.5% and 2.1%, respectively. The length of the plume core is effectively shortened due to the disappearance of the shock wave inside the nozzle.