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

该项目开发并演示了一种独特的、门控的、皮秒的数字全息系统，用于成像密集液滴喷雾的详细结构。稠密喷雾常见于柴油和燃气轮机发动机，以及食品和药品生产的喷雾干燥过程中。了解喷雾系统中的支配行为在很大程度上取决于识别喷雾核心处发生的过程，在那里，散装液体正在分解为韧带和液滴。密集的喷雾使识别这些过程变得非常困难，因为它们的光学厚度很大，很难从喷雾的深处获取任何图像信息。该项目的创新之处在于将数字全息术和皮秒门控相结合，以充分限制光学噪声的数量，从而能够通过光学密度高的介质进行高分辨率的3D成像。这种方法有效地概括了现有的伪弹道成像系统，其中通过喷雾的光子被选择性地收集，而那些以广角多次散射的光子被选择性地收集。数字全息术通过相干滤波进一步增强了光子选择。为了将弹道光子和全息成像结合起来，我们使用了足够短的激光脉冲来增强弹道光子的检测，并且足够长以允许以可接受的空间分辨率进行全息记录。激光输出被分成三种不同的光束：1)控制克尔单元光开关的光束，2)物光，以及3)全息图的参考光束。克尔池提供了光子定时选择性。它被调整为恰好在全息脉冲到达之前打开。这两个脉冲的重叠时间决定了有效选通时间。当克尔门打开时，物体和参考波通过并被记录在数码相机上。这种形式的成像方法可以提供三维样本体积中所有颗粒的详细结构。结果将有助于识别诊断限制，并将产生有用的数据，用于与喷雾建模进行比较。剩下的工作将致力于通过应用设计规则进行系统改进，以量化权衡和优化未来的现场测量。这一新的诊断将允许对密集喷雾的三维成像，这是现有技术无法实现的。这一新的测量能力有可能产生关于喷雾形成和韧带断裂的迫切需要的数据，这些数据对于理解喷雾物理和建立喷雾物理模型至关重要。例如，重质燃料的喷雾行为是决定许多装置的燃烧效率的控制过程。虽然喷雾物理已经研究了几十年，但在准确模拟喷嘴附近的喷雾破碎和燃料分布方面仍然存在很大的局限性。近场成像数据非常少。这项工作将弹道成像的噪声抑制方面与数字全息术相结合，生产出一种演示仪器，能够满足在密集喷雾中对近场三维成像数据的需求。该项目包括了与Metroaser，Inc.的大学-行业合作，这将通过细化研究重点，同时扩大研究范围，使其超越纯粹的学术练习，从而极大地加强研究。这一结果将有助于利用杠杆作用开发喷雾和燃烧界所希望和需要的商业可行的诊断仪器。

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.