Mixing Processes in a Coaxial Geometry with a Central Lobed Mixer-Nozzle

An experimental investigation was undertaken to study the mixing processes in a coaxial jet geometry where the inner nozzle is a lobed mixer.Seven different inner jet nozzle geometries were explored: a baseline circular jet, three different six-lobed nozzles and three different four-lobed nozzles.In addition, three different velocity ratios o f 3:1, 1:1 and 1:3 (inner:outer) were examined at two velocity levels.Flow visualizations o f the inner stream using a passive scalar (tobacco smoke) were performed using a laser sheet from a Nd.YAG laser as the light source.The laser has a 9 nsec pulse duration, which effectively freezes the flow.Instantaneous cross-sectional images were taken at various distances downstream; streamwise views were also taken.Quantitative information on the mixing was obtained by processing sets of images.All three components o f velocity for one o f the nozzles were also obtained using a two component, coincident LDV system.The effects o f various parameters such as the increase in interfacial area due to the lobed nozzle geometry, the strength o f streamwise vortices, and the large-scale structures on the mixing process is evaluated and discussed.The results showed enhanced mixing as the strength o f the streamwise vortices increased.The results also showed that the presence of large-scale structures, due to the Kelvin-Helmholtz instabilities and their interaction with the streamwise vortices generated by the lobes, is crucial for enhanced mixing.There was generally a wider spread for the 3:1 velocity ratio, followed by the 1:1 and then the 1:3.There were also different mixing processes for each velocity ratio.The 3:1 velocity ratio behaved similarly to a single jet, except that the Kelvin-Helmholtz instability and the streamwise vorticity interacted together to enhance the mixing.The LDV results showed that the streamwise vortices spread radially with the jet for the 3:1 velocity ratio case.The 1:1 velocity ratio, with the Kelvin-Helmholtz instability minimized, relied almost exclusively on the streamwise vorticity to mix the two streams.For this case, the LDV data showed the streamwise vortices much more confined and not spreading as much as for the 3:1 case.The 1:3 velocity ratio flow condition, on the other hand, behaved more like a wake flow because o f the inward roll-up direction o f the interface.This condition produced the lowest jet spread in general, although it was sometimes comparable to the 1:1 case.The LDV results for this velocity ratio showed that the streamwise vorticity that did develop dissipated very quickly within a few diameters.The inner jet was also greatly influenced by the outer shear layer, i.e., between the outer flow and the ambient air, and it was this shear layer that had the greatest effect on the spread o f the inner jet.The fraction o f mixing enhancement due to streamwise vorticity (relative to mixing enhancement due to increased interfacial contact area) was found to increase as velocity ratio increased.In addition, this fraction increased with downstream distance.This dissertation is dedicated to my wife, Janet.