DEMONSTRATION OF DIGITAL HOLOGRAPHIC DIAGNOSTICS FOR THE BREAKUP OF LIQUID JETS USING A COMMERCIAL-GRADE CCD SENSOR

摘要

Digital holography was used for drop size and velocity measurements during the breakup of liquid jets because it is unaffected by the nonspherical droplets that are encountered very close to the injector exit, which would otherwise cause problems for techniques such as the phase Doppler particle analyzer. It also works well for observing the very dense spray conditions that are encountered just downstream of the injector exit. In view of the recent advancement in digital holography, the objective of this study was to develop an inexpensive method that provides visualization and measurements of droplet sizes and velocities of a micro-liquid jet. In the present experimental setup, two Nd:YAG lasers are used to generate two independent laser pulses. A commercial CCD sensor (Nikon D-70, ～700) is used for recording holograms of micro-liquid jets. By using the commercial-grade CCD sensor instead of research-grade PW CCD camera for holographic diagnostics, the cost of recording holograms has been reduced from 20K to 0.5K. For the velocity measurement, unlike the case when using a PIV-CCD camera, two images were recorded on the same frame. The in-line holography arrangement reduces the spatial resolution requirements on the CCD sensor, which typically has a much lower resolution than traditional holographic plates. Digital microscopic holography is similar to standard digital in-line holography except that no lens is used to collimate the object beam. The laser beams are expanded with an objective lens (M 5x) and a spatial filter (15 μm pinhole). This eliminates two lenses from the typical optical path used for in-line holography, which results in a much cleaner hologram recording. Moreover, the expanding beam increases the resolution of the setup because the fringes that are needed to reconstruct the image are expanding with the expanding laser beam. As the recording distance is shortened, the resolution is increased.