Graduate Thesis Or Dissertation
 

Flow characteristics of semi-confined impinging jet arrays

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gx41mm605

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  • The present study proposes to investigate the flow structure of different arrays of jets impinging on a flat surface enclosed by three walls, creating a channel that forces the air to leave in one preferential direction, generating a self sustain crossflow. Details of both the mean and some turbulence quantities for two 7x7 jet arrays are presented. The first array was a circular orifice array while the second was a "cusped" ellipse array. The non-symmetry of the cusped ellipse shape allowed two alignments with the crossflow, one where the crossflow was along the major axis (0°) and one where the crossflow was along the minor axis (90°). The Reynolds number ranged from 8,500 to 15,900. Surface flow visualization, jet orifice flow coefficient measurements and PIV measurements of the entire flow field were used to interpret the complex flow characteristics. The mean velocity, turbulent kinetic energy, mean vorticity and mean squared vorticity fields were calculated from the PIV data at three different locations along the downstream flow direction: close to the back, at the center of the jet array and close to the exit. The visual observation of the impinging flow pattern shows similar results for all of the different geometries and configurations. The jets generated cells defined by detachment-reattachment zones with a characteristic horseshoe shape around each jet. The cells become increasingly more oblong in shape with increasing crossflow due to an uneven shifting of the upstream and downstream cell boundaries, in the downstream direction. The flow coefficient of each jet decreases monotonically and quasi-linearly along the crossflow direction while the average value increases only a few percent for increasing Re. PIV measurements of the entire flow field in the vicinity of the jet exits reveals complex flow structures. A large rotating vortex created by the merging of the crossflow and the jet column return-flow, moves towards the jet column as the crossflow increases in strength. This interaction appears to generate multiple turbulent flow patterns that may have consequences in improved surface cooling. For the lower values of Re, the cusped ellipse jet array, placed in the (0°) position, appears to generate significantly more turbulence than the circular jet array. For higher values of Re, both configurations show similar results. But when placed in the (90°) position, there is a considerably smaller increase in the levels of turbulence when compared to the circular jets. Evidence of axis switching in the jet column for the cusped ellipse jets tends to prove that a transport in the lateral direction could be a secondary factor to take into account for surface cooling efficiency.
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file details DanoBertrandPE2001.pdf 2017-08-08 Público Descargar