Schlieren Image Velocimetry of Swirl Sprays

Abstract

Schlieren image velocimetry is based on light de ection through ow heterogeneities and image crosscorrelations. This is a low-cost and relatively low complexity technique that allows measurement of the droplet velocity eld 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 lters 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 eld.The digital images were obtained with 5×10 3 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 uctuations. Injection pressures varied from 0.05 to 7 bar and mass ow rates varied from 1.389 to 13.89 g/s, using water as test uid. The wideband warm white LED with Laplacian or high-pass lters 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 32.5 o to 69.5 o , for injection pressures from 0.05 to 7 bar, and results of triangulation with a blue LED were closer to semiempirical data.

Hollow Porous Structures for Improving the Sensitivity of Colorimetric Array Gas Sensors

“…Schlieren Imaging: Schlieren imaging is an optical imaging technique. [39,40] The principle is to convert the refractive index information of a gas medium into light irradiation, thereby enabling the visualization of fluid flow behavior that is invisible to the naked eye. This technique is widely used for the detection an analysis of gas flow field distribution.…”

Comparative study of droplet and liquid film velocities in sprays

“…The mean liquid film velocity for The instantaneous axial droplet velocity field was obtained from two consecutive images with a time interval of 0.1221 ms and shutter speed of 2 µs. Machado et al [20] conducted a study on the optimization of the OpenPIV software, using schlieren or shadowgraphy images and verified the optimum combination of interrogation windows and overlaps to crosscorrelate sequential images. These images were correlated by the OpenPIV software, using interrogation windows of 64 × 64 pixels and overlaps of 8 × 8 pixels.…”

“…Simultaneous measurements of droplet velocities can be performed by schlieren imaging velocimetry (SIV) using a high-speed camera. The instantaneous droplet velocity is determined from the correlation between two consecutive images [20]. SIV is a low-cost and low-complexity method, since it does not require the use of lasers and seeding of the flow [21].…”