Droplet imaging velocimeter and sizer: a two-dimensional technique to measure droplet size

This paper discusses the droplet imaging velocimeter and sizer (DIVAS), a two-dimensional technique to measure droplet size from very small droplets (a few microns) to very large drops (millimeters), as well as droplet velocity and concentration. DIVAS uses a particle imaging velocimetry (PIV) configuration; that is, pulsed lasers illuminate the droplets in the measurement plane and CCD cameras collect the off-axis scattered light, thus yielding a small measurement volume and correspondingly a high number density measurement capability. DIVAS is based on glare point imaging as described by van de Hulst and Wang (Appl Opt 30(33):4755–4763, 1991). By measuring the separation of cross-polarized glare points, DIVAS is capable of measuring droplet size down to a few microns with sub-pixel resolution. The measurement of large drops from the separation of glare points is straightforward. However, the paper discusses that large drops are often non-spherical and more advanced signal processing is required to retrieve their size. Both analytical studies and experiments are presented to demonstrate the feasibility of the technique.     

Simultaneous velocity field measurements in two-phase flows for turbulent mixing of sprays by means of two-phase PIV

This paper describes a novel derivative of the PIV method for measuring the velocity fields of droplets and gas phases simultaneously using fluorescence images rather than Mie scattering images. Two-phase PIV allows the simultaneous and independent velocity field measurement of the liquid phase droplets and ambient gas in the case of two-phase flow mixing. For phase discrimination, each phase is labelled by a different fluorescent dye: the gas phase is seeded with small liquid droplets, tagged by an efficient fluorescent dye while the droplets of the liquid phases are tagged by a different fluorescent dye. For each phase, the wavelength shift of fluorescence is used to separate fluorescence from Mie scattering and to distinguish between the fluorescence of each phase. With the use of two cross-correlation PIV cameras and adequate optical filters, we obtain two double frame images, one for each phase. Thus standard PIV or PTV algorithms are used to obtain the simultaneous and independent velocity fields of the two phases. Because the two-phase PIV technique relies on the ability to produce two simultaneous and independent images of the two phases, the choice of the labelling dyes and of the associated optical filter sets is relevant for the image acquisition. Thus a spectroscopic study has been carried out to choose the optimal fluorescent dyes and the associated optical filters. The method has been evaluated in a simple two-phase flow: droplets of 30–40 μm diameter, produced by an ultrasonic nozzle are injected into a gas coflow seeded with small particles. Some initial results have been obtained which demonstrate the potential of the method.     

Simultaneous density-field visualization and PIV of a shock-accelerated gas curtain

We describe a highly-detailed experimental characterization of the Richtmyer-Meshkov instability (the impulsively driven Rayleigh-Taylor instability) (Meshkov 1969; Richtmyer 1960). In our experiment, a vertical curtain of heavy gas (SF₆) flows into the test section of an air-filled, horizontal shock tube. The instability evolves after a Mach 1.2 shock passes through the curtain. For visualization, we pre-mix the SF₆ with a small (∼10⁻⁵) volume fraction of sub-micron-sized glycol/water droplets. A horizontal section of the flow is illuminated by a light sheet produced by a combination of a customized, burst-mode Nd:YAG laser and a commercial pulsed laser. Three CCD cameras are employed in visualization. The “dynamic imaging camera” images the entire test section, but does not detect the individual droplets. It produces a sequence of instantaneous images of local droplet concentration, which in the post-shock flow is proportional to density. The gas curtain is convected out of the test section about 1 ms after the shock passes through the curtain. A second camera images the initial conditions with high resolution, since the initial conditions vary from test to test. The third camera, “PIV camera,” has a spatial resolution sufficient to detect the individual droplets in the light sheet. Images from this camera are interrogated using Particle Image Velocimetry (PIV) to recover instantaneous snapshots of the velocity field in a small (19 × 14 mm) field of view. The fidelity of the flow-seeding technique for density-field acquisition and the reliability of the PIV technique are both quantified in this paper. In combination with wide-field density data, PIV measurements give us additional physical insight into the evolution of the Richtmyer-Meshkov instability in a problem which serves as an excellent test case for general transition-to-turbulence studies. Received: 26 June 1999/Accepted: 29 October 1999     

Particle image velocimetry studies of an incipient spot in the Blasius boundary layer

Spatial evolution of a small amplitude localized disturbance introduced into the laminar boundary layer of a flat plate has been studied experimentally using the particle image velocimetry (PIV) technique. PIV data have been acquired in the spanwise and wall normal planes. Long and well defined high and low speed streaks are seen in the spanwise plane. The number of streaks are found to increase in the downstream direction. Breathing mode type oscillation of the boundary layer is observed. Associated with the streaks and akin to the bypass transition, ‘backward’ and ‘forward’ jet like structures of the fluctuating velocity components are observed.AS Banerjee: summer trainee, IIT Kharagpur, India     

Instantaneous and simultaneous planar velocity field measurements of two phases for turbulent mixing of high pressure sprays

This paper describes the tests of accuracy and the first application of a combined planar visualization technique. Its goal is two-phase flow discrimination, i.e. simultaneous measurements of velocity of droplets and ambient gas in the case of two-phase flow mixing, at the same location and with possible conditioning by “apparent diameter” (AD) of the droplets. It combines the mature techniques of particle image velocimetry (PIV), planar Mie scattering diffusion (PMSD), planar laser-induced fluorescence (PLIF), and it necessitates two synchronized cross-correlation systems, digital image treatment and analysis. This technique was developed with the objective of better describing the mixing between liquid and gaseous phases as in the case of high-pressure spray atomization in quiescent ambient gas. The basic principle of separation is to seed the ambient gas with micrometer particles and to tag the liquid with fluorescent dye. We use digital image treatment and analysis to discriminate between the phases. We use two cross-correlation PIV systems in order to obtain the velocity field of the droplets and gas simultaneously and separately at the same location. The digital image processing for separating the phases involves geometric measurement of droplet shapes. This leads to measurement of droplet parameters close to their real diameter, which could be used for analysis of actual mixing. A synchronized system composed of two CCD cameras is used for image recording, and two Nd:YAG lasers are used for generating pulsed light sheets at times t and t + δt. Tests were performed to check for different sources of errors. The combined technique was applied to measurements in high-pressure spray flow atomizing in a quiescent ambient gas, and first results are presented.     

Interaction of a polydisperse spray with vortices

The objective of the present work is to provide, through the association of optical diagnostics on a well-chosen experimental configuration, new insights into the coupling of a vortical gaseous flow with a polydisperse evaporating spray representative of practical injections. A cloud of droplets is injected in an inert laminar round jet, axisymmetric and pulsated, enabling the study of the interaction of strong-vorticity structures with a polydisperse spray. The experiment is a laboratory-scale representation of realistic injection configurations such as in engine combustion chambers or industrial burners. The chosen set-up leads to a well-controlled configuration and allows the coupling of two optical diagnostics, particle imaging velocimetry (PIV) and interferometric particle imaging (IPI), which leads to the study of both the flow dynamic and the droplet size distribution. The behaviour of droplets is analysed regarding their relaxing and evaporating properties. Size-conditioned preferential concentration of both weakly evaporating and strongly evaporating sprays is investigated. Droplet trajectories are also analysed by means of high-rate tomographic visualizations. The time history between their ejection from the nozzle and their interaction with the vortex is strongly related to the droplet preferential concentration and the observed heterogeneous repartition in the gas flow.     

A kilohertz frame rate cinemagraphic PIV system for laboratory-scale turbulent and unsteady flows

A kilohertz frame rate cinemagraphic particle image velocimetry (PIV) system has been developed for acquiring time-resolved image sequences of laboratory-scale gas and liquid-phase turbulent flows. Up to 8000 instantaneous PIV images per second are obtained, with sequence lengths exceeding 4000 images. The two-frame cross-correlation method employed precludes directional ambiguity and has a higher signal-to-noise ratio than single-frame autocorrelation or cross-correlation methods, facilitating acquisition of long uninterrupted sequences of valid PIV images. Low and high velocities can be measured simultaneously with similar accuracy by adaptively cross-correlating images with the appropriate time delay. Seed particle illumination is provided by two frequency-doubled Nd:YAG lasers producing Q-switched pulses at the camera frame rate. PIV images are acquired using a 16 mm high-speed rotating prism camera. Frame-to-frame registration is accomplished by imaging two pairs of crossed lines onto each frame and aligning the digitized image sequence to these markers using image processing algorithms. No flow disturbance is created by the markers because only their image is projected to the PIV imaging plane, with the physical projection device residing outside the flow field. The frame-to-frame alignment uncertainty contributes 2% to the overall velocity measurement uncertainty, which is otherwise comparable to similar film-based PIV methods. Received: 11 July 2000 / Accepted: 21 June 2001 Published online: 29 November 2001     

Droplet formation in quickly stretched liquid filaments

Film splitting necessarily occurs in roll coating and unwanted droplets can occur at high machine speeds when the resulting filaments break up. To study this ‘misting’ problem, an apparatus was designed and built to simulate filament fluid mechanics. The device creates a filament by elongating a liquid bridge and stretches the filament at a high and constant rate of acceleration to mimic coating machine kinematics. Filament breakup was observed using a high-speed video camera and the images were analyzed to yield droplet size and number. With Newtonian fluids, a single droplet formed at Ohnesorge (Oh) numbers less than 0.1 and more and smaller droplets were produced at Oh numbers above 0.1. Associative polymer solutions, prepared as weakly elastic fluids to represent industrial coatings, produced even more and smaller droplets, but only for Ohnesorge numbers in the range of 0.01 to 0.1.     

Vertical, Bubbly, Cross-Flow Characteristics over Tube Bundles

Two-phase flow over tube bundles is commonly observed in shell and tube-type heat exchangers. However, only limited amount of data concerning flow pattern and void fraction exists due to the flow complexity and the difficulties in measurement. The detailed flow structure in tube bundles needs to be understood for reliable and effective design. Therefore, the objective of this study was to clarify the two-phase structure of cross-flow in tube bundles by PIV. Experiments were conducted using two types of models, namely in-line and staggered arrays with a pitch-to-diameter ratio of 1.5. Each test section contains 20 rows of five 15 mm O.D. tubes in each row. The experiment’s data were obtained under very low void fraction (α<0.02). Liquid and gas velocity data in the whole flow field were measured successfully by optical filtering and image processing. The structures of bubbly flow in the two different configurations of tube bundles were described in terms of the velocity vector field, turbulence intensity and void fraction.     

Gas temperature in the trace of water droplets streamlined by hot air flow

In order to obtain the knowledge necessary for developing new effective fire extinguishing technologies, we determined experimentally the gas temperature in the trace of water droplets streamlined by hot air flow. It was important to establish how much the temperature in the droplet trace decreases and how fast it recovery to the initial temperature field after the droplet evaporation. The following parameters were varied: droplet size from 1.3 mm to 1.7 mm, velocity from 1 m/s to 5 m/s, initial airflow temperature from 473 K to 773 K, number of droplets (one or two), and the arrangement of droplets relative to the hot inflow (serial or parallel). The study proves the theoretical hypothesis about a significant influence of evaporation on the temperature in the water droplet trace. When a temperature trace of water droplets is formed, irrespective of their arrangement, the role of the evaporation process strengthens with the gas flow temperature rising. Furthermore, the study specifies typical longitudinal dimensions of the aerodynamic and temperature traces of water droplets. It has been established that when droplets are located in series and in parallel, their combined impact on the temperature and velocity of the gas flow in the medium differs rather considerably.     

A moment method for splashing and evaporation processes of polydisperse sprays

A new moment method for the modelling of polydisperse sprays is proposed that simultaneously takes into account the dispersion in droplet size and droplet velocity. For the derivation of this Eulerian method the kinetic spray equation is used which constitutes a partial differential equation for the probability density function of droplets. To reduce the complex kinetic spray equation to a form that can be managed with the available numerical procedures, moment transforms with respect to the droplet velocity and the droplet size are conducted. The resulting moment equations are closed by choosing an approximate probability density function which applies to polydisperse sprays. The method is successfully tested for configurations in which a polydisperse spray is either splashed, evaporated or effected by a Stokes drag force. The tests are organised in such a way that crossing of two spray distributions is always included. The new method is able to capture the polydisperse nature of sprays as well as the bi-(or multi-) modal character of the droplet velocity distribution function, for example, when droplets cross each other.     

High-speed laser diagnostics for the study of flame dynamics in a lean premixed gas turbine model combustor

A series of measurements was taken on two technically premixed, swirl-stabilized methane-air flames (at overall equivalence ratios of ϕ = 0.73 and 0.83) in an optically accessible gas turbine model combustor. The primary diagnostics used were combined planar laser-induced fluorescence of the OH radical and stereoscopic particle image velocimetry (PIV) with simultaneous repetition rates of 10 kHz and a measurement duration of 0.8 s. Also measured were acoustic pulsations and OH chemiluminescence. Analysis revealed strong local periodicity in the thermoacoustically self-excited (or ‘noisy’) flame (ϕ = 0.73) in the regions of the flow corresponding to the inner shear layer and the jet-inflow. This periodicity appears to be the result of a helical precessing vortex core (PVC) present in that region of the combustor. The PVC has a precession frequency double (at 570 Hz) that of the thermo-acoustic pulsation (at 288 Hz). A comparison of the various data sets and analysis techniques applied to each flame suggests a strong coupling between the PVC and the thermo-acoustic pulsation in the noisy flame. Measurements of the stable (‘quiet’) flame (ϕ = 0.83) revealed a global fluctuation in both velocity and heat-release around 364 Hz, but no clear evidence of a PVC.     

Impulse generated during unsteady maneuvering of swimming fish

The relationship between the maneuvering kinematics of a Giant Danio (Danio aequipinnatus) and the resulting vortical wake is investigated for a rapid, ‘C’-start maneuver using fully time-resolved (500 Hz) particle image velocimetry (PIV). PIV illuminates the two distinct vortices formed during the turn. The fish body rotation is facilitated by the initial, or “maneuvering” vortex formation, and the final fish velocity is augmented by the strength of the second, “propulsive” vortex. Results confirm that the axisymmetric vortex ring model is reasonable to use in calculating the hydrodynamic impulse acting on the fish. The total linear momentum change of the fish from its initial swimming trajectory to its final swimming trajectory is balanced by the vector sum of the impulses of both vortex rings. The timing of vortex formation is uniquely synchronized with the fish motion, and the choreography of the maneuver is addressed in the context of the resulting hydrodynamic forces.     

Particle tracking for polydisperse sedimenting droplets in phase separation

When a binary fluid demixes under a slow temperature ramp, nucleation, coarsening and sedimentation of droplets lead to an oscillatory evolution of the phase-separating system. The advection of the sedimenting droplets is found to be chaotic. The flow is driven by density differences between two phases. Here, we show how image processing can be combined with particle tracking to resolve droplet size and velocity simultaneously. Droplets are used as tracer particles, and the sedimentation velocity is determined. Taking these effects into account, droplets with radii in the range of 4−40 μm are detected and tracked. Based on these data, we resolve the oscillations in the droplet size distribution that are coupled to the convective flow.     

Development and validation of echo PIV

The combination of ultrasound echo images with digital particle image velocimetry (DPIV) methods has resulted in a two-dimensional, two-component velocity field measurement technique appropriate for opaque flow conditions including blood flow in clinical applications. Advanced PIV processing algorithms including an iterative scheme and window offsetting were used to increase the spatial resolution of the velocity measurement to a maximum of 1.8 mm×3.1 mm. Velocity validation tests in fully developed laminar pipe flow showed good agreement with both optical PIV measurements and the expected parabolic profile. A dynamic range of 1 to 60 cm/s has been obtained to date.     

Two-color digital PIV employing a single CCD camera

 An extension of two color particle image velocimetry (PIV) is described where the color images are recorded onto a single high-resolution (3060×2036 pixel) color CCD sensor. Unlike mono-color CCD sensors, this system not only eliminates the processing time and the subsequent digitization time of film-based PIV but also resolves the directional ambiguity of the velocity vector without using conventional image-shifting techniques. For comparing the spatial resolutions of film and CCD data, a calibration experiment is conducted by recording the speckle pattern onto 35 mm color film and using a CCD sensor under identical conditions. This technique has been successfully implemented for simulated turbine film-cooling flows in order to obtain a more detailed characterization of the coolant-injection phenomenon and its interaction with freestream disturbances. Received: 20 November 1996/Accepted: 29 January 1998     

Limits and accuracy of the Stereo-LFC PIV technique and its application to flows of industrial interest

Particle image velocimetry with local field correction (LFC PIV) has been tested in the past to obtain two components of velocity in a two dimensional domain (2D2C). When compared to conventional correlation based algorithms, this advanced technique has shown improvements in three important aspects: robustness, resolution and ability to cope with large displacements gradients. A further step in the development of PIV algorithms consists in the combination of LFC with the stereo technique, which is able to obtain three components of velocity in a plane (2D3C PIV). In this work this combination is implemented and its performance is evaluated carrying out the following two different tasks:

PIV for granular flows

 Particle image velocimetry (PIV) has been adapted for use in measuring particle displacement and velocity fields in granular flows. “Seeding” is achieved by using light and dark particles. The granular flow adjacent to a clear bounding wall is illuminated with a strobe, and the recorded images are analyzed using standard PIV techniques. The application is demonstrated by measuring convection rolls in a granular bed undergoing vertical oscillations. The PIV measured displacement is consistent with displacement of a marked layer of particles. Received: 29 January 1998/Accepted: 8 April 1999     

Measuring evaporation of micro-fuel droplets using magnified DIH and DPIV

This paper details the use of magnified digital in-line holography (MDIH) and digital particle image velocimetry (DPIV) to measure the evaporation rates of fuel micro-droplets undergoing heating. The technique can be used to measure instantaneous evaporation along an individual droplet trajectory, or if applied to a series of droplets, the average evaporation over a number of successive measurement locations. The advantage of this technique over traditional optical techniques is greater spatial resolution and depth of field for the high magnification factors used. An application of the technique to the evaporation measurement of diesel fuel droplets ranging from 10 to 90 μm is presented. Results reveal that similar to larger droplets, temperature plays the dominant role in evaporation processes, with little sensitivity to initial droplet size found for a peak reactor temperature of 660 K.     

Dynamics of ripple bed vortices

 This paper presents the results of an experimental investigation into the dynamics of vortices formed by and shed from ripples under oscillatory motion. Particle Image Velocimetry (PIV) was used to acquire detailed velocity measurements at several phases of oscillation for each of four flow conditions: three purely horizontal oscillations and a ‘real’ wave with a vertical velocity component. For each of the flows, the vortices were located in the flow maps and defined at each time-step by their circulations and sizes. These calculated characteristics were compared with the predictions from a discrete-vortex model (DVM) developed at the University of Wales, Bangor. Received: 22 May 1997/Accepted: 29 January 1998