Effects of Density on Mixing of Low Reynolds Number Vertical Jets
Authors: Nowicki Natalie L. 
Published: April 2008

Laser sheet smoke visualization experiments were performed on vertical air/helium jets to quantify the effects of low density driven bursts on the jet structure and entrainment. The parameters of relative jet density, S, and jet exit Reynolds number, Re, are of most importance in determining the bursting. Previous research has shown that vertical jets of S ≤ 0.5, in a range of Rej = 1300 – 2500, display strong side ejections due to the baroclinic instability in the strained vorticity sheet between the primary torroidal vortices. The objective of this work was to determine if this phenomenon resulted in a significant increase in the mixing and jet entrainment compared to standard jets. The present study demonstrated that the strong and clearly visible burst phenomenon had a very minor impact on the time averaged spreading and mixing in the shear layer surrounding the potential core. Experiments were performed using laser sheet illumination with a YAG pulse laser and cylindrical lens with oil smoke droplet seeding. The images were acquired using a 12 bit CCD camera with a 1024 × 1280 pixel array. All images were acquired at a low enough frequency to ensure their statistical independence. The laser sheet was estimated to be 0.5 mm thick with a pulse duration of 6 ns. Planar instantaneous images both coplanar and normal to the jet centerline were obtained. The jet emerged into room air from an 11 mm diameter bicubic nozzle with a contraction ratio of 5.5. Mixed flows of air and helium were fed into a settling chamber and then passed through a flow straightening honeycomb upstream of the jet. Flow rates and Reynolds numbers were controlled using choked flow nozzles that fed the settling chamber. Oil droplet smoke was added to the air flow with an adiabatic venturi-jet oil atomizer. In the instantaneous images of the jets, the bursts were clearly visible in individual frames and qualitatively appeared to play a significant role in the downstream mixing of the jet. However, quantitative analysis of time averages of many sequential images revealed that the bursts are much less significant to the mixing and entrainment of the jet than they appear. Longitudinal images were acquired in sets of 100 or 200 and used to obtain averaged images of the plume from the source out to approximately 10 jet diameters. The pixel noise floor was subtracted from the mean images. These mean images were interpreted as an analogue for scalar concentration, and thus used to quantitatively estimate the plume spread. From these mean images, concentration profiles were obtained and plotted. The bursting phenomenon was shown to be insignificant on an engineering scale after analyzing the mean images. In fact, the mass in the region where the bursts occurred was only visible when a function which showed very small gradient differences was applied to the images. While the baroclinic instability bursting is interesting from a scientific point of view, it has been shown through the quantitative analysis of the means of instantaneous images that there is only a slight effect on the overall jet entrainment compared with regular jets.