Abstract
Schlieren image velocimetry (SIV) is based on light deflection through flow heterogeneities and image cross-correlations. This is a low-cost and relatively low complexity technique that allows measurement of the droplet velocity field in a large region of a spray. A Z-type Toepler schlieren system with a high-speed camera was used to determine mean vertical and horizontal droplet velocities, as well as the cone angles of sprays produced by a pressure swirl injector with characteristic geometric constant K = 2. Different LEDs and digital filters were evaluated for edge detection and improvement of image contrast. Open software was adopted for digital image processing and velocimetry. Interrogation windows and overlaps of different sizes were tested to obtain an appropriate correlation for determination of the velocity field. The digital images were obtained with 5 × 103 fps and a resolution of 2.77 pixels/mm. Since the swirl sprays analysed presented instabilities, a number of 100 cross-correlations of images was required to reduce mean velocity fluctuations. Injection pressures varied from 0.05 to 7 bar and mass flow rates varied from 1.389 to 13.89 g/s, using water as test fluid. The wideband warm white LED with Laplacian or high-pass filters provided velocity data for a larger range of injection pressures. Mean axial velocities varied from 3.3 to 11.3 m/s, approximately, with mean horizontal velocities varying from around 0.17 to 3.3 m/s for pressures from 0.05 to 3.22 bar. The velocity data were compared to microscopic shadowgraphy results, showing a good agreement. Spray cone angles ranged from about 32.5o to 69.5°, for injection pressures from 0.05 to 7 bar, and results of triangulation with a blue LED were closer to semi-empirical data.
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Acknowledgements
The authors thank Prof. Alex Liberzon from Tel-Aviv University for his support in using OpenPIV.
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This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES).
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DAM developed the SIV system, carried out the tests, and wrote the manuscript. FSC coordinated experimental tests, and theoretical support for the spray study and wrote the manuscript. JCA assisted in test bench assembling and spray testing. GSD assisted spray testing. GAAF performed the design of experiments. All the authors reviewed the manuscript.
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Machado, D.A., de Souza Costa, F., de Andrade, J.C. et al. Schlieren Image Velocimetry of Swirl Sprays. Flow Turbulence Combust 110, 489–513 (2023). https://doi.org/10.1007/s10494-022-00385-z
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DOI: https://doi.org/10.1007/s10494-022-00385-z