Interferometric Laser Imaging For Droplet Sizing Research Articles

Coriolis-induced liquid breakup and spray evolution in a rotary slinger atomizer: Experiments and analysis

Understanding the physics of primary liquid breakup process and its correlation with the evolution of spray characteristics in a rotary slinger atomizer is the goal of the present research. Experiments were conducted in a high-speed slinger test rig that houses a static liquid delivery manifold to uniformly supply the liquid to the rotating slinger disc that contains a single row of orifices carved on its peripheral surface for liquid injection.The atomizer was operated for a wide range of conditions by varying the rotational speed and liquid feed rate. The liquid breakup structure at the exit of the slinger orifices was visualized using front light illumination technique, while the droplet size was measured at different radial stations away from the slinger surface by application of the Interferometric Laser Imaging for Droplet Sizing (ILIDS) technique. The visualization images highlighted strong influence of Coriolis force as the liquid tends to accumulate on one side of the channel (that is opposite to the rotational direction) for all cases. It was observed that while the liquid thickness is smaller for higher rotational speed, it does not vary much with liquid feed rate at the same speed and, instead, in such case the span of the liquid is wider. A theoretical analysis was developed to describe the in-channel liquid behaviour that accounts for the effect of Coriolis and surface tension forces. Interestingly, the theory could explain the above observations. The differences in the predictions in comparison to the analysis by Dahm et al. (2006a) was attributed to the assumption of annular film flow in the latter. The liquid breakup mode (stream, sheet or transition mode) could be described by Coriolis Bond number (Bo) that refers to the ratio of Coriolis to surface tension forces and proportional to the spread parameter (span to thickness ratio), while the liquid breakup regimes were identified on a Bo−q plot, where q is the liquid to air momentum flux ratio. The variation of characteristic droplet sizes with both rotational speed and feed rate were examined, and again some interesting trends are identified. The correlation between liquid breakup mode/regime with the measured droplet size was established using the above non-dimensional numbers.

EXPERIMENTAL STUDY OF THE MORPHOLOGY OF TWO-PHASE FLAME INSTABILITIES IN MICROGRAVITY

This paper reports experimental studies of cellular flames instabilities in the case of an expanding two-phase spherical flame under microgravity conditions. The presence of liquid fuel droplets leads to the triggering of those instabilities on the flame surface (cracks or cellular) which have an extensive impact on flame behaviour, such as propagation speed and morphology. The main focus is to figure out on the interaction of the droplets with the propagating flame, and specifically on the possible correlation between the aerosol properties (i.e. droplets size and inter-distance) and the flame morphology. A high-speed shadowgraph technique is used along with a specific segmentation post-treatment to obtain a quantitative access to the flame propagation and morphology. In a second time, CH* chemiluminescence is used concurrently with high-speed ILIDS (Interferometric Laser Imaging for Droplet Sizing) or laser tomography for a simultaneous characterization of the flame structure and aerosol properties. By a comparison with equivalent gaseous flames, the impact of the two-phase configuration is assessed. It is shown that it not only triggers the instability earlier, but that it also decreases the minimum size within the cell population. In addition to the minimum and maximum cell sizes, the distribution of cell size is provided, the extrema of which gives a reliable characteristic size of the cell population.

Interferometric laser imaging for in-flight cloud droplet sizing

A non-intrusive particle sizing method with a high spatial distribution is used to estimate cloud droplet spectra during flight test campaigns. The interferometric laser imaging for droplet sizing (ILIDS) method derives particle diameters of transparent spheres by evaluating the out-of-focus image patterns. This sizing approach requires a polarized monochromatic light source, a camera including an objective lens with a slit aperture, a synchronization unit and a processing tool for data evaluation. These components are adapted to a flight test environment to enable the microphysical investigation of different cloud genera. The present work addresses the design and specifications of ILIDS system, flight test preparation and selected results obtained in the lower and middle troposphere. The research platform was a Dornier Do228-101 commuter aircraft at the DLR Flight Operation Center in Braunschweig. It was equipped with the required instrumentation including a high-energy laser as the light source. A comprehensive data set of around 71 800 ILIDS images was acquired over the course of five flights. The data evaluation of the characteristic ILIDS fringe patterns relies, among other things, on a relationship between the fringe spacing and the diameter of the particle. The simplest way to extract this information from a pattern is by fringe counting, which is not viable for such an extensive number of data. A brief contrasting comparison of evaluation methods based on frequency analysis by means of fast Fourier transform and on correlation methods such as minimum quadratic difference is used to encompass the limits and accuracy of the ILIDS method for such applications.

Droplet–turbulence interaction in a confined polydispersed spray: effect of droplet size and flow length scales on spatial droplet–gas velocity correlations

AbstractThis paper discusses the interaction between droplets and entrained turbulent air flow in the far-downstream locations of a confined polydispersed isothermal spray. Simultaneous and planar measurements of droplet and gas velocities in the spray along with droplet size are obtained with the application of a novel experimental technique, developed by Hardalupaset al. (Exp. Fluids, vol. 49, 2010, pp. 417–434), which combines interferometric laser imaging for droplet sizing (ILIDS) with particle image velocimetry (PIV). These measurements quantified the spatial correlation coefficients of droplet–gas velocity fluctuations ($R_{dg}$) and droplet–droplet velocity fluctuations ($R_{dd}$) conditional on droplet size classes, for various separation distances, and for axial and cross-stream velocity components. At the measurement location close to the spray edge, with increasing droplet size,$R_{dg}$was found to increase in axial direction and decrease in cross-stream direction. This suggests that as the gas-phase turbulence becomes more anisotropic away from the spray axis, the gravitational influence on droplet–gas correlated motion tends to increase. The effective length scales of the correlated droplet–gas motion were evaluated and compared with that for gas and droplet motion. The role of different turbulent eddies of the gas flow on the droplet–gas interaction was examined. The flow structures were extracted using proper orthogonal decomposition (POD) of the instantaneous gas velocity data, and their contribution on the spatial droplet–gas velocity correlation was evaluated, which quantified the momentum transfer between the two phases at different length scales of the gas flow. The droplets were observed to augment turbulence for the first three POD modes (larger scales) and attenuate it for the rest of the modes (smaller scales). It has been realized that apart from droplet Stokes number and mass loading, the dynamic range of length scales of the gas flow and the relative turbulent kinetic energy content of the flow structures (POD modes) must be considered in order to conclude if the droplets enhance or reduce the carrier-phase turbulence especially at the lower wavenumbers.

Interferometric laser imaging for droplet sizing revisited: elaboration of transfer matrix models for the description of complete systems

We report the development of an interferometric laser imaging for droplet sizing (ILIDS) numerical simulator. It is based on the use of generalized Huygens-Fresnel integrals associated to transfer matrices that describe the whole imaging setup. This simulator allows easy simulation of any kind of ILIDS setup. Simulations are shown to be in good agreement with experimental results. This simulator offers important perspectives in the design, realization, and calibration of ILIDS instruments, as airborne instruments, or in situ measurements in flows.

Measurement of Droplet Size, Velocity and Spatial Distribution of Mass Flux in Spray by Combining Focus and Defocus Imaging Technique

The novel imaging technique combining Glare-Points technique (GPT) with interferometric laser imaging for droplet sizing (ILIDS) has been developed to measure droplet size, velocity and spatial distribution of mass flux in a spray. This technique enables to advance the droplet positioning error by ILIDS and expand the dynamic range of droplet sizing, because the large and small sized droplets were obtained by using GPT and ILIDS, respectively. The novel receiving optical system called as Double-planes Particle Imaging (DPI) was constructed in order to capture the focus plane (Glare-Points) and the defocus plane (interferometric fringe) with only one imaging sensor. Firstly, the captured image is processed for calculating of the droplet position, size and velocity by GPT and ILIDS, respectively. The droplet position by ILIDS is corrected by the mapping function which is obtained from the ray tracing simulation. Secondly, the same droplet image is matched with the aid of velocity information. In addition, the droplet position is defined as the one of GPT, the droplet size is employed the ILIDS' for smaller and the GPT's for larger than threshold value. Finally, the depthwise positions of droplet are estimated from the optical path difference between the focus and defocus images. From the result of validation experiment using a mono-disperse generator, the DPI enables to estimate the three-dimensional droplet position within the measurement error of 8.4% and expand the dynamic range of the accurate droplet sizing up to approximately 200 μm. Furthermore, this developed technique was applied to a spray flow, and the spatial distribution of mass flux was evaluated from the obtained droplet position, size and velocity information.

Multifrequency interferometric particle imaging for gas bubble sizing

A new configuration for measuring bubbles with interferometric laser imaging for droplet sizing (ILIDS) is proposed. In this configuration, perpendicularly polarized light scattered by a bubble is gathered at 90°. In this signal, two dominant frequencies are present, whose origin is explained and experimentally verified. Numerical simulations demonstrate how this frequency signature can be used to improve the robustness of the standard ILIDS technique and reject non-spherical bubbles.

Designing a maximum precision interferometric particle imaging set-up

In this article, a comprehensive uncertainty analysis will be given of a popular 2D particle measuring technique called Interferometric Laser Imaging for Droplet Sizing (ILIDS) for measuring droplets and bubbles. This includes the first estimates on the influence of droplet non-sphericity. Next to this, the uncertainty on the frequency determination and the calibration procedures were also investigated. To this end, the two most common calibration procedures were experimentally compared. This showed that it often dominates the total uncertainty. Design guidelines are then derived concerning observation angle, out-focus disk size and calibration procedure which could lead to a combined uncertainty below 1.5% for droplets and 2% for bubbles.

2D Simultaneous Measurements of Droplets Diameter and Velocity in a Diesel Spray by Using Improved ILIDS Method

The characteristics of a diesel spray, such as the diameter, the velocity and the droplet distribution on a 2D plane were simultaneously measured by using an improved ILIDS (Interferometric Laser Imaging for Droplet Sizing) method. The experiments were performed with an accumulator type unit injector system and a constant-volume high-pressure vessel under the condition of quiescent ambient gas. The injection pressure and ambient gas pressure were set at 60/100 MPa and 0.1/1 MPa, respectively. The injection duration was set at 2 ms, and measurements were made at 1.5 ms after the end of injection. The orifice diameter of the injector used was 0.24 mm with a single hole. The measurement was made at 40-60 mm from the nozzle-tip. As a result, the correlations between the droplet size, the velocity and the spatial distribution were found under different injection and ambient gas pressures.

A219 異なる二種類の光学像を用いた噴霧液滴の径および速度の同時計測法

In general, spray field is a complicated and three-dimensional two-phase flow. The structure of it indicates strong correlation between size and velocity of droplets. Thus simultaneous measurement of size and velocity of the individual droplet in the spray is needed. In the recent years, the techniques based on Interferometric laser imaging for droplet sizing (ILIDS) for the simultaneous measurement of droplet size and velocity were proposed as a field measurement technique, and the most of them are basically 2D measurement. However, multi-dimensional field measurement is indispensable for understanding of spray flow.In this report, a technique of simultaneous measurement of size and 3D velocity field of droplets is based on the characteristic of light scattered from droplets. In this technique, droplet size was estimated by number of interference fringes, and 3D velocity field was reconstructed with 3D-PTV. Moreover, holography technique has been applied in order to separate the overlapping interferometric image, and its feasibility was verified.

Improvements of the interferometric technique for simultaneous measurement of droplet size and velocity vector field and its application to a transient spray

The paper describes a planar measurement technique for droplet size and velocity vector for applications to transient flow fields. Interferometric laser imaging for droplet sizing (ILIDS) is a novel measurement technique that gives an instantaneous spatial size distribution of droplets. The inter-fringe spacing of the ILIDS interferogram is correlated with individual particle sizes and the measurement accuracy depends upon the image processing technique for estimating the fringe spacing. The relationship between the diameter and fringe spacing was found by considering the angular phase difference of the external reflection and the direct refraction. Conventional ILIDS, which observes a circular image with fringes on a film, has difficulties in evaluating the fringe spacing accurately due to overlapping of the circular image at high concentration. The objective of the present study is to develop a simultaneous measurement technique of size and velocity vector of individual spherical particles by using anamorphic image acquisition optics that have different focal lengths along mutually perpendicular radii and accelerated data processing on a fully automated computing system. In the present technique, the circular image with fringes is optically compressed into linear images that are horizontally defocused and vertically focused keeping the information of the location and the size of droplets. The present "squeezing" technique can reduce the fringe overlapping and avoid complexity for the evaluation of fringe spacing. In the case of 100 droplets in a 1 M-pixel image plane, the particle validation ratio of the present technique is more than 95% in contrast with the validation ratio of the conventional technique that is less than 20%. Moreover, the vertical integration of the image can eliminate noise and consequently the signal-to-noise ratio of the interference signals becomes much higher than that of the conventional technique.