Volumetric Three-Component Measurements of Air Jet Flows of Different Diffuser Designs

Abstract

The effects of three diffuser configurations on the near-field region have been experimentally evaluated with the goal to reduce the losses and noise of an existing large subsonic wind tunnel. Preliminary analysis were performed on over thirty diffuser designs and the most promising designs were selected for the experimental investigation. The two most promising diffusers are a 3.5°half-apex angle diffuser with a constant-area at the end, and a modified version of this diffuser with the addition of an annular conical diffuser after the flat section. The annular conical section consists of three concentric truncated cones. The air jet flow was measured with a volumetric three-component velocimetry. This experimental investigation is considered one of the first flow measurements using the volumetric three-component velocimetry in air flow. The measurement technique was assessed with a round free jet flow. The results of the round free jet show that the potential core region is shorter relative to results in literature. The results also show that in the nearfield region, the variations in the mean axial velocity decrease before the variations in the streamwise component of the Reynolds normal stresses, whereas the variations in the other components of the Reynolds normal stresses persist for a longer distance. Comparison between the existing diffuser and the modified diffusers shows that the modified diffusers reduce the velocity in the jet which supports the preliminary computational results. Also, the Reynolds shear stresses in the shear layer of these diffusers are suppressed. The modified diffuser with the an annular conical diffuser produces more reduction in the jet velocity suggesting its effectiveness in reducing the head losses. Also, this diffuser produces less turbulence, hence less noise. NomenclatureD = pipe diameter, D = 40.9 mm ReD = Reynolds number based on the pipe diameter and velocity in the pipe U∞ = jet exit velocity, U∞ = 27.6 m/sec u = streamwise component of the mean velocity v = normal component of the mean velocity w = transverse component of the mean velocity x = streamwise coordinate y = normal coordinate z = transverse coordinate 2 z = z component of the vorticity vector