Effect of Aspect Ratio on Flow Through Serpentine Nozzles

The flowfields for two rectangular cross-sectional nozzles with two in-plane 90 deg bends were investigated both experimentally and computationally. The cross-sectional planform of the two nozzles were of opposite aspect ratios (2:1 and 1:2). The Reynolds number (based upon the hydraulic diameter and mean velocity within the nozzle) was 1:4 x 10^4, providing a Dean number of 8400 (based upon radius of curvature at the centerline of the nozzle). The experimental portion of the study was conducted by three-component laser Doppler velocimetry using a five-beam probe. A computational study was performed on the interior of the nozzle using a commercial code that employed a Reynolds stress model. With noted exceptions, good correlation was observed between the computational and experimental solutions. Secondary-flow velocities at the nozzle exit revealed evidence of a complex pattern of streamwise vortices generated by the two bends. It was found that despite the common Dean number, the difference in aspect ratio led to substantially different velocity fields. The flow was dominated by a single pair of counter-rotating streamwise vortices in one case, whereas its counterpart yielded a complex pattern of multiple smaller vortices. [PUBLICATION ABSTRACT]