Focused-image holography as a dense-spray diagnostic (original) (raw)
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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.
Applying holographic particle image velocimetry to sprays
Proquest Dissertations and Theses Thesis Oklahoma State University 2010 Publication Number Aat 1489425 Isbn 9781124511085 Source Masters Abstracts International Volume 49 04 Page 2693 72 P, 2010
Measuring the velocity field of dispersed flows is of major importance in many applications, such as sprays and particle-laden jets. Traditional particle image velocimetry (PIV) setup is common in laboratories but it is limited to planar measurements. The objective of the present study was to conduct velocity measurements for sprays using HPIV. The present study involved extension of present holographic PIV method to spray fields. The present optical setup is based on inline digital holographic microscopy. Double-pulsed holograms were recorded and reconstructed to yield many two-dimensional slices (planes) of the flow field at two time instances. A large number of focused and unfocused drops appeared in each slice resulting in a lot of noise. Thus it was required to develop new image processing subroutines which could bypass the noise and consider only the focused droplets present in each plane while rejecting the shadow of non focused droplets not present in plane. Threshold subroutines help user to decide the signal to noise ratio in final processed images from hologram. The PIV correlation was used to calculate velocity from images processed with subroutines. vi
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.
International Journal of Heat and Mass Transfer, 1992
An application of pulsed laser holography for direct measurement of the growth of subcooled spray droplets in a pure saturated vapour is presented. The spray flow corresponds to intermediate and high droplet Reynolds numbers (100 $ Re ,< 3500) while the vapour pressure is varied in a wide range of reduced pressures (0.03 < pr < 0.3). The holographic reconstructions reveal a strong relation between the vapour pressure and the spray geometry. At moderate liquid flow rate and relatively high vapour pressure, subsequent pressure increments produce dramatic changes in the spray geometry and considerably reduce the condensation rate.
Image Processing Algorithms for Digital Holographic Analysis of Near-Injector Sprays
2013
Three-dimensional spray diagnostics are important to understand a variety of atomization processes. Digital holography is capable of probing non-spherical droplets in the near-injector region since the technique is imagebased. The image processing algorithms used affect both the technique speed and its accuracy to support statistically meaningful, near injector investigations. This paper discusses the methodology used to implement such an algorithm. A FFT technique to filter noise that is falsely identified as a drop is presented. Details of the subroutines used for larger datasets are given, along with supporting error analysis. Classification error rates for true positive, false positive, and false negative detections of droplets are given so that global uncertainties of the program can be quantified. Overall uncertainty is comparable to previous studies using inline digital holography in the near injector field; analyzed without extensive supporting image processing algorithms. I...
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...
A digital image analysis technique for quantitative characterisation of high-speed sprays
Optics and Lasers in Engineering, 2007
A digital image analysis technique developed as a particle or droplet sizing tool and capable of measuring non-spherical objects has been examined in terms of its suitability for quantitative measurements in moderately dense sprays and in particular the potential capability for the characterisation of small diameter, high-speed two-phase flows by employing high-intensity pulsed lasers for illumination. In order to evaluate robustness of the image analysis technique (PDIA), measurement certainty and also to assess whether measurement performance is sensitive to the optical set-up, the technique was applied to data obtained from a hollow cone spray via two independent optical configurations which employed firstly a diode laser and secondly an Nd:YAG laser. The calibration response of the two optical set-ups revealed significant differences in terms of the depth-of-field characteristics and thus effective measurement volume dimensions. Despite these differences, a comparison of PDIA spray data revealed excellent agreement between the two datasets for measured diameters in the range 10-90 mm in the number distributions which not only confirmed robustness of the technique but also the potential of PDIA for the measurement of fast, small diameter objects. Subsequent comparisons of the PDIA data were made with PDA data obtained within the same spray in space and time and showed excellent agreement between the two techniques for droplets larger than approximately 25 mm in diameter. Discrepancies between PDIA and PDA were observed in the volume size distributions for the larger droplets measured whose diameters were greater than approximately 40 mm. This discrepancy is due to the ability of PDIA to measure the diameter of non-spherical droplets which were shown to exist in significant numbers at this measurement location within the spray. In contrast, the well-established technique PDA, which relies on the assumption of droplet sphericity clearly does not detect the presence of these larger deformed droplets. r
Quantitative optical techniques for dense sprays investigation: A survey
Optics and Lasers in Engineering, 2012
The experimental study of dense sprays by optical techniques poses many challenges and no methods have proven to be completely reliable when accurate quantitative data are required, for example to validate breakup models and CFD simulations. The present survey is aimed to a critical analysis of optical techniques capable to provide quantitative and reliable data in dense sprays and to point out the conditions necessary to safely obtain such measurements. A single parameter, the optical depth, is proposed to quantify the concept of dense spray and to indicate when multiple scattering becomes predominant and could make the experimental results questionable. Many available optical techniques are divided into two categories: the ''classical'' ones, like PDA, LDV, PIV, etc., that work well in dilute sprays but show many limitations in dense sprays, and the ''emerging'' ones more suitable for dense sprays. Among the last ones, those considered more promising are discussed in detail. A number of significant applications are also presented and discussed to better clarify the nature of such complex problem and the feasibility of the new proposed approaches.
Droplet Size Distribution and Sphericity Measurements of Low-Density Sprays Through Image Analysis
Particle & Particle Systems Characterization, 2000
An image analysis technique has been developed in order to determine the drop size distributions of sprays produced by low-velocity plain cylindrical jets. The particle sizing method is based on incoherent backlight images. Each drop is analyzed individually in the image. The two-dimensional image resulting from the projection of the three-dimensional object shape (the drop) on a screen (the video sensor surface) is modeled. The model, based on the point spread function formulation, has been developed to derive a relation between contrast and relative width of individual drops. This relation is used to extend the domain of validity of drop size in terms of size range, out of focus and image resolution. The shape parameter is determined for each drop image through morphological analysis. Spherical and non-spherical droplets are then sorted on the basis of this parameter. Non-spherical drops are regarded as non-fully atomized liquid bulks or coalesced drops. Finally, the droplet size distribution of true spherical droplets is established for a low-velocity plain cylindrical liquid jet.
Applied Optics, 1998
The size and the position of individual particles in a falling spray crossing a horizontal plane are measured simultaneously with a shadowgraph imaging setup. An original method for measurement of drop sizes based on a prescribed analytical form of the luminance distribution of the objects ͑i.e., the drops͒ is presented. The processing time is strongly reduced compared with that for the usual inversion techniques. The turbulent dispersion of drops in a grid-generated turbulence is studied. Evidence for the presence of coalescence in the spray is presented.