Digital Holographic Spray Analyzer

Advanced spray diagnostics are needed for studying the formation of drops in a variety of natural and technological spray processes, e.g. water falls, bow waves of ships, and many types of commercial spray atomizers, among others. Of interest is the dense-spray near-injector region which is typically opaque for spray diagnostics such as phase Doppler particle analyzers (PDPA). This is unfortunate because primary breakup processes that control spray size and velocity distributions occur in this optically challenging region. The present setup; digital holographic spray analyzer, allows the probing of dense spray regions and provides the user with droplet sizes and velocities measurements in three dimensions. The setup is based on typical in-line holography except that the holographic film is replaced with a CCD sensor. The actual process of capturing the hologram is a relatively simple process only requiring a laser, optics to form a collimated beam, and a digital camera. The hologram is then stored digitally and reconstructed numerically with a reconstruction program. After reconstructing the hologram in many different planes, the droplet size distribution is measured. In addition droplet velocities are measured by means of double pulsed exposure configuration and PIV program. All these processes can be automated which is the strength of this technique. The output is a three dimensional map of droplets locations, sizes, and velocities. This digital holographic spray analyzer was tested by measuring droplet sizes inside the dense spray created by an aerated injector subjected to a subsonic crossflow typical of test condition encountered in ramjet engine.