GOALI: Probing Dense Sprays with Gated, Picosecond, Digital Particle Field Holography

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This project develops and demonstrates a unique, gated, picosecond, digital holography system for imaging the detailed structure of dense sprays of liquid droplets. Dense sprays are commonly found in diesel and gas turbine engines, and in spray drying processes for food and pharmaceutical production. Understanding the governing behaviors in spray systems depends heavily on identifying the processes occurring at the core of the spray where the bulk liquid is breaking up into ligaments and droplets. Dense sprays make identifying those processes tremendously challenging because they are optically thick making it difficult to obtain any image information from deep within the spray. The innovation in this project is to employ a combination of digital holography and picosecond gating to limit the amount of optical noise sufficiently to enable high resolution, 3D imaging through an optically dense medium. The approach effectively generalizes existing pseudo-ballistic imaging systems, where photons that pass through the spray with relatively few near-forward scattering interactions are selectively collected while those scattered multiple times at wide angles are rejected. Digital holography further enhances the photon selection by coherence filtering. To combine ballistic photon and holographic imaging we utilize a laser pulse short enough to enhance the ballistic photon detection and long enough to allow holographic recording with acceptable spatial resolution. The laser output is split into three different beams: 1) a beam that controls a Kerr-cell optical switch, 2) an object beam, and 3) a reference beam for the hologram. The Kerr cell provides the photon timing selectivity. It is adjusted to open just before the holography pulse arrives. The overlap time of these two pulses determines the effective gating time. When the Kerr gate is open, the object and reference waves pass through and are recorded on the digital camera. An imaging approach of this form can provide a detailed look at the structure of all of the particles in a three-dimensional sample volume. Results will aid in identification of the diagnostic limitations and will produce data useful for comparisons to spray modeling. The remainder of the effort will be dedicated to system refinement via the application of design rules to quantify tradeoffs and optimization for future field measurements.This new diagnostic will allow three-dimensional imaging of dense sprays unachievable with existing techniques. This new measurement capability has the potential to produce much needed data on spray formation and ligament breakup essential for understanding and modeling of spray physics. For example, spray behavior of heavy fuels is a controlling process determining combustion efficiencies of many devices. While spray physics has been researched for decades, significant limitations still exist in accurate modeling of spray breakup and fuel distribution near the nozzle. Very little near field imaging data exists. This work combines the noise rejection aspects of ballistic imaging with digital holography to produce a demonstrated instrument capable of addressing the need for near field, three-dimensional imaging data in dense sprays. The university?industry collaboration with Metrolaser, Inc. that is included in this project will greatly enhance the research by refining its focus while simultaneously broadening it beyond a purely academic exercise. The result will be useful for leveraging towards the development of a commercially viable diagnostic instrument that is desired and needed by the spray and combustion communities.