Light field imaging of fuel droplets and sprays

Spray flows are typically highly unsteady, three-dimensional and often densely saturated with droplets that impact other droplets, coalesce and breakup. Droplets, fluid ligaments and bags can form from fluid streams and sheets being accelerated in air cross flows or along solid surfaces. This work shows the potential for imaging these complex features with novel three-dimensional imaging methods derived from the combination of light field imaging (LFI) and synthetic aperture (SA) refocusing. In particular, our imaging system is designed to measure and locate features such as bubbles, droplets and particles in three spatial dimensions over time. A multi-camera array is used to capture the light field and raw images are reparameterized to digitally refocus the flow field post-capture into a volumetric image. LFI with SA refocusing facilitate full 3D reconstructions and allow the camera array to effectively “see through” partial occlusions in the scene. Flow features, such as individual droplets, can be located in three-dimensions by refocusing throughout the volume and extracting features on each plane, making this method particularly attractive for use in multiphase flows that contain many fine features, such as droplets or bubbles, which limit visibility in the depth direction. Light field imaging involves the reparameterization of images captured using an array of cameras, or from a single senor and lenslet array (i.e. a plenoptic camera), to digitally refocus a flow field post-capture. All cameras record a volumetric scene in-focus, and by recombining images in a specific manner, individual focal planes can be isolated in software to form refocused images. Flow features, such as individual droplets, can be located in three-dimensions by refocusing throughout the volume and extracting features on each plane. Herein we present two distinct experiments, each with complex flow presentations such as dense spray droplets and unsteady flow features: break up of a turbulent sheet along a solid boundary and a liquid jet in air cross flow. Each of these experiments involved the use of a planar camera array with nine or ten cameras. The images were processed using the light field imaging and synthetic aperture refocusing algorithms as described above that were written in MATLAB. A refocused image for the jet in cross flow is shown in figure 1. The cameras used in all of the experiments presented herein were Flea 2 model FL2-08S2M/C from Point Grey Research, Inc. All ten cameras in the array were synced and simultaneously captured 1024 x 768 pixels, 8 bit, monochromatic images at 30 frames per second maximum. Although this frame rate was not high enough to achieve fine temporal resolution, it was effective for recording images that could be refocused and from which flow structures, such as droplets and ligaments, could be extracted and investigated for instantaneous time steps. Initial applications by Belden et al. (2010) also proved these techniques viable for quantitative flow measurements when combined with 3D particle imaging velocimetry algorithms and special particle identification thresholding. High-speed cameras were used by Belden et al. (2012) with similar success for bubbly flows but with significant cost increase. Ultimately these advances, along with others coming from the vision community hold great potential for opening doors to previously under-sampled unsteady, turbulent and multi-phase fluid flows. Whether for determining new fluid phenomena, evaluating new designs, or benchmarking computational codes, fully spatiallyand time-resolved experimental data is paramount. Given the recent advances in camera and imaging technologies, and the growing prevalence of commercially available light field imaging systems, the opportunities for obtaining such data is achievable at a lower cost and with greater resolution and computational savings. This new method using LFI extends measurement capabilities in complicated flows where knowledge is incomplete and allows for finer measurements of flow quantities and structures that would have been impossible with prior methods.