Flow physics of indeterminate origin nozzle jets and passive control of sprays
Abstract
The transfer efficiency of small droplets in spray painting is influenced by local turbulence characteristics, which can be passively controlled by using nozzles of Indeterminate Origin (IO). To achieve better understanding of the physics underlying the IO nozzle jet, an experimental investigation in a large-scale water jet was performed using Planner Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV) techniques. Flow visualizations show that IO nozzles introduce streamwise vortex pairs into the jet shear layer. "Incursion" streamwise vortex pairs are generated at the 10 nozzle valley locations as each braid region is shed from the nozzle, i.e. they are formed periodically during the K-H shedding processes. They reorganize to form "excursion" streamwise-vortex pairs at the peak locations. Streamwise vortices greatly influence the evolution of the IO nozzle jet. Interaction between streamwise and spanwise vortices promotes earlier turbulence transition. Due to "excursion" streamwise vortex pairs, IO nozzle jets have higher spreading rates in the peak plane at near-nozzle region, but the differences vanish when the jets propagate downstream. Streamwise vortices introduce radial shear flow into the jet, which facilitates extra turbulence production and enhances mixing between the jet and ambient fluids. These vortices also dissipate TKE in the near-nozzle region, so that the turbulence intensity and Reynolds stresses are lower compared to the conventional round nozzle jet downstream of x/D > 7. Although IO nozzles greatly influence the jet at the near-nozzle region, the influence fades as the jet propagates downstream. In an impinging jet in the near wall region (impingement region located at x/D ≅ 11), mean velocity fields are almost identical for the IO nozzle and a conventional nozzle. Reynolds stresses are lower by a factor of 20--50% for the IO nozzle. In order to discover the influence of IO nozzles in an actual spray, the performance of an effervescent atomizer controlled with IO nozzles was investigated. The results were compared with the traditional round nozzle case. Although IO nozzles influenced the spreading angle of the spray, they did not increase the wall-normal velocity fluctuation in the downstream region, therefore, transfer efficiency was not increased when using IO nozzles.
Degree
Ph.D.
Advisors
Plesniak, Purdue University.
Subject Area
Mechanical engineering
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