Applying holographic particle image velocimetry to sprays (original) (raw)
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Nuclear Engineering and Design, 1998
The present paper deals with the analysis of the disintegration process of both subcooled and superheated sprays generated in a flat spray nozzle. Both the macroscopic structures of a spray, such as the breakup-length and the spray-angle, and its microscopic structures, such as the number, the size, the location, and the velocities of the generated droplets as a function of varying injection conditions have been investigated. The short time holography has been applied as the measuring technique. Two holograms of the generated spray are taken simultaneously which results in two three-dimensional (3-D) reconstructions of the spray, seen from different directions. Computer based algorithms have been developed to determine the quantities to be measured that are stored on the holograms. A stereo-matching module correlates both views and determines the position and/or the velocity of each droplet. The applicability of the employed holographic technique and of the filtering and correlating modules is proven by the presented results.
Focused-image holography as a dense-spray diagnostic
Applied Optics, 1994
The denser regions of sprays need to be probed for us to understand further the basic phenomena of the breakup of a bulk liquid into droplets and the subsequent drop dynamics. The instruments currently available to spray diagnosticians, the Malvern spray analyzer and the phase/Doppler particle analyzer, cannot be used in the denser regions of the spray and in regions where ligaments or nonspherical droplets exist. Holography, as applied in the past, has permitted the interrogation of nonspherical drops but in general has been applied to droplet-dominated dilute regions of sprays. Using a focused-image holographic system, with the advantages of an imaging lens and side lighting, we can probe highly complex regions of sprays, such as those that include bubble explosions and complex ligament formation at the nozzle exit. In this paper we present components of a focused-image holographic system for spray analysis, advantages and limitations of that system, and how the longitudinal magnification varies as a function of the length of the object along the optical path.
DIGITAL HOLOGRAPHY BASED PARTICLE TRACKING VELOCIMETRY
Digital holography appears to be the next-generation technology for holographic diagnostics of particle fields and has been applied to holographic particle tracking flow measurements. In the digital holography, the Fresnel or Fourier holograms are recorded directly by the CCD or CMOS cameras and stored digitally. No film material involving wet-chemical or other processing is necessary as in optical holography. The reconstruction of the wave-field, which is done optically by illumination of a hologram, is performed by numerical methods. Digital holography also enables the use of complex amplitude information inaccessible by optical reconstruction, thereby allowing flexible reconstruction algorithms to achieve optimization of specific information. However, due to the poor pixel resolution of CCD or CMOS cameras as compared to that of holographic films, the digital holography gives poor depth resolution for images. This problem severely impairs the usefulness of digital holography especially in densely populated particle fields. In this paper the authors present a simple digital in-line holography recording setup and numerical reconstruction process based on the Fourier transform and convolution schemes. The use of the complex amplitude, available from these reconstructed waves, in particle extraction significantly improves particle axial-location accuracy compared with other numerical reconstruction schemes that are merely traditional optical reconstruction. This method also alleviates the speckle noise problem intrinsic to in-line particle holography since speckles and particles can be clearly differentiated in complex wave-field.
Use of holography in particle image velocimetry measurements of a swirling flow
Experiments in Fluids, 1999
Particle Image Velocimetry (PIV) is now a well established experimental technique to measure two components of the velocity in a planar region of a flow field. This paper shows how its proven capabilities can be further extended by using holographic recording to register the particle displacements. Among other unique characteristics, holography enables the acquisition of multiple images on a single plate, and the recording of three dimensional images. These features are used to circumvent some of the limitations of conventional PIV. Some of these possibilities are demonstrated in this study by applying the technique to a high Reynolds number swirling flow using a lens-less off-axis orthogonal recording geometry.
Applied Optics, 2015
The 3D measurement of the particles in a gas-solid pipe flow is of great interest, but remains challenging due to curved pipe walls in various engineering applications. Because of the astigmatism induced by the pipe, concentric ellipse fringes in the hologram of spherical particles are observed in the experiments. With a theoretical analysis of the particle holography by an ABCD matrix, the in-focus particle image can be reconstructed by the modified convolution method and fractional Fourier transform. Thereafter, the particle size, 3D position, and velocity are simultaneously measured by digital holographic particle tracking velocimetry (DHPTV). The successful application of DHPTV to the particle size and 3D velocity measurement in a glass pipe's flow can facilitate its 3D diagnostics.
Applications of Holography in Fluid Mechanics and Particle Dynamics
Annual Review of Fluid Mechanics, 2010
The quantification of three-dimensional (3D) flow structures and particle dynamics is crucial for unveiling complex interactions in turbulent flows. This review summarizes recent advances in volumetric particle detection and 3D flow velocimetry involving holography. We introduce the fundamental principle of holography and discuss the debilitating depth-of-focus problem, along with methods that have been implemented to circumvent it. The focus of this review is on recent advances in the development of in-line digital holography in general, and digital holographic microscopy in particular. A mathematical background for the numerical reconstruction of digital holograms is followed by a summary of recently introduced 3D particle tracking and velocity measurement techniques. The review concludes with sample applications, including 3D velocity measurements that fully resolve the flow in the inner part of a turbulent boundary layer, the diffusion of oil droplets in high–Reynolds number tur...
Probing Dense Sprays with Gated, Picosecond, Digital Particle Field Holography
International Journal of Spray and Combustion Dynamics, 2011
This paper describes work that demonstrated the feasibility of producing a gated digital holography system that is capable of producing high-resolution images of three-dimensional particle and structure details deep within dense particle fields of a spray. We developed a gated picosecond digital holocamera, using optical Kerr cell gating, to demonstrate features of gated digital holography that make it an exceptional candidate for this application. The Kerr cell gate shuttered the camera after the initial burst of ballistic and snake photons had been recorded, suppressing longer path, multiple scattered illumination. By starting with a CW laser without gating and then incorporating a picosecond laser and an optical Kerr gate, we were able to assess the imaging quality of the gated holograms, and determine improvement gained by gating. We produced high quality images of 50–200 μm diameter particles, hairs and USAF resolution charts from digital holograms recorded through turbid media...
Single Beam Two-Views Holographic Particle Image Velocimetry
Applied Optics, 2003
Holographic Particle Image Velocimetry (HPIV) is presently the only method that can measure at high resolution all three components of the velocity in a finite volume. In systems that are based on recording one hologram, velocity components parallel to the hologram can be measured throughout the sample volume , but elongation of the particle traces in the depth direction, severely limits the accuracy of the velocity component that is perpendicular to the hologram. Previous studies overcame this limitation by simultaneously recording two orthogonal holograms, which inherently required four windows and two recording systems. This paper introduces a technique that maintains the advantages of recording two orthogonal views, but requires only one window and one recording system. Furthermore, it enables a four times increase in the spatial resolution.
Ultrashort Pulse Off-Axis Digital Holography for Imaging the Core Structure of Transient Sprays
Atomization and Sprays, 2018
A single-shot ultrashort pulse off-axis digital holography (UPODH) system successfully images microscopic details of fuel injection phenomena that are hidden from normal optical view by a dense cloud of droplets surrounding the near-nozzle region. The experiment approximates the optically dense conditions typical of fuel injection in modern diesel engines. Under these conditions an outer layer of small droplets can hide a core of larger droplets or liquid ligaments; this core is inaccessible to most imaging techniques due to multiple scattering in the outer layer. These conditions are mimicked by intentionally surrounding a core spray with a fine mist. The mist has a Sauter mean diameter of 4.28 micrometers. The core spray comes from driving water, with pressure ranging from 1 to 13.8 bar, through single orifices of 0.1 and 0.3 mm diameter. The spray field shows nearly opaque transmissivities as low as 6 × 10 −6. Transient phenomena, such as sheets of liquid becoming ligaments and their further breakup into small particles, are easily visible even when surrounded by the opaque mist with an optical density (OD) of 12. Holographic reconstruction allows these phenomena to be clearly observable in 3D, and single droplets as small as 25 µm are identifiable. The 3D capability allows UPODH to bring into focus small particles and ligaments at different depth planes, even centimeters apart.