Light multiple scattering correction of laser-diffraction spray drop-size distribution measurements

This paper reports an experimental investigation on the practical use of a laser-diffraction instrument, the Malvern Spraytec 2007 to characterize sprays produced by a high-pressure GDI injector. The sprays are highly transient, large, composed of very small drops, dense and heterogeneously distributed in space. These characteristics are at the origin of undesirable effects (beam steering, vignetting and light multiple scattering) whose manifestations are experimentally identified. Ignoring the diodes concerned by beam steering erases the effect of this phenomenon but vignetting and light multiple scattering effects combine to bias the measurements and both require to be corrected. A nth order empirical correction procedure, based on the analysis of the light intensity distribution, is developed. It is a generalization of a second order procedure presented in a previous investigation. The increase of the correction order is demonstrated to be necessary when the injection pressure is greater than 11 MPa. The application of this correction procedure reported that light multiple scattering affects laser-diffraction when the transmission is less than 40%. This limit is in agreement with investigations of the literature and gives credit to the empirical correction procedure. Despite the correction procedure is applicable for the present operating conditions only, this work defines an experimental protocol to apprehend laser-diffraction spray characterization in severe operating conditions and that can be reproduced with ease. Furthermore, it is emphasized that the combination of the Spraytec and the correction procedure performances allows cycle-to-cycle spray drop-size distribution variations to be determined. Such information is of paramount importance and the Spraytec is probably the sole instrument able to provide it.     

Droplet clustering in sprays

In this contribution, the spatial particle distribution in sprays of different atomizers is analyzed. Steady and unsteady particle structures are identified by evaluating the interparticle arrival time statistics at a certain position, which is the time increment between two succeeding particles. In addition to its characteristics of size and velocity, each particle exhibits an individual interparticle arrival time that is used to identify unsteady characteristics in the flow. Unsteadiness in sprays is thereby of interest for several reasons and in several applications, for example, in the combustion industry. A typical example of an unsteady spray behaviour is droplet clustering which can be caused, for example, by pulsating liquid disintegration procedures or particle interaction with large-scale eddy structures in the gas. The aim of the investigation is the analysis of such unsteady spray conditions. The evaluation of spray unsteadiness is done by means of point wise and time resolved PDA measurements in the spray of a pressure and twin-fluid atomizer, respectively.     

Study on imaging method for measuring droplet size in large sprays

Information of droplet size and size distribution lays the basis for investigations of atomization mechanisms and performance optimization.However,the laser diffraction and phase Doppler particle analyzers have difficulty in accurately characterizing sprays with a wide range of droplet sizes and very large droplets,especially if a large number of droplets are aspherical.A method to measure size in such largedroplet sprays based on digital imaging with backward illumination was developed,including an image acquisition system and image process programs.Calibration of the measurement system was performed using a dot calibration target with different dot sizes.An experimental setup was designed and established to characterize spray nozzles under different operation loads,as well as different nozzle arrangements.Results show that the droplet size of sprays ranges from dozens of microns to several millimeters.The superiority of wide load range for such nozzles was indicated by the size-measurement results under half-load to full-load operations.The present study revealed that the image processing technique can be effectively implemented for in-line size measurements of sprays with a wide distribution of droplet size and aspherical droplets,which would be difficult to characterize by other methods.     

Study on imaging method for measuring droplet size in large sprays

Information of droplet size and size distribution lays the basis for investigations of atomization mechanisms and performance optimization. However, the laser diffraction and phase Doppler particle analyzers have difficulty in accurately characterizing sprays with a wide range of droplet sizes and very large droplets, especially if a large number of droplets are aspherical. A method to measure size in such large-droplet sprays based on digital imaging with backward illumination was developed, including an image acquisition system and image process programs. Calibration of the measurement system was performed using a dot calibration target with different dot sizes. An experimental setup was designed and established to characterize spray nozzles under different operation loads, as well as different nozzle arrangements. Results show that the droplet size of sprays ranges from dozens of microns to several millimeters. The superiority of wide load range for such nozzles was indicated by the size-measurement results under half-load to full-load operations. The present study revealed that the image processing technique can be effectively implemented for in-line size measurements of sprays with a wide distribution of droplet size and aspherical droplets, which would be difficult to characterize by other methods.     

Study of the thermal mixing between two non-isothermal sprays using combined three-color LIF thermometry and phase Doppler analyzer

The aim of this experimental work was to demonstrate the ability of three-color laser-induced fluorescence (3cLIF) thermometry to study the thermal mixing of two non-isothermal water sprays. Combined 3cLIF-phase Doppler analyzer measurements were also implemented to derive correlations between droplet size and temperature. Both sprays had different characteristics in terms of flow rate and droplet size distribution. The liquid spray was successively pre-heated, and the other spray was maintained and injected at ambient temperature. The thermal mixing will be discussed in light of a wide set of experimental results obtained under various experimental conditions, including different liquid flow rates, droplet size distributions and droplet concentrations. To analyze the potential effect of droplet coalescence on the mean local liquid temperature, both sprays were alternatively seeded with fluorescent dye. Main results show that significant heating of cold spray is possible when the hot spray is injected with the higher flow rate. Moreover, this heating affects only the smallest droplets.     

Visualization of the evaporation of a diesel spray using combined Mie and Rayleigh scattering techniques

Evaporating Diesel sprays are studied by laser Rayleigh scattering measurements in an optically accessible high-pressure/high-temperature cell that reproduces the thermodynamic conditions which exist in the combustion chamber of a Diesel engine during injection. n-Decane is injected into the vessel using a state-of-the-art near-production three-hole nozzle. Global images of the distributions of the liquid and vapor phases of the injected fuel are obtained using a combined Schlieren and Mie scattering setup. More details about the evaporation are revealed when the spray is illuminated by a laser light sheet: laser light can be scattered by molecules in the gas phase (Rayleigh scattering) or comparably large fuel droplets (Mie scattering). The former is seen in regions where the fuel has completely evaporated, and the latter is dominant in regions with high droplet concentrations. Studying the polarization of the signal light allows the distinction of three different regions in the spray that are characterized by a moderate, low or negligible concentration of liquid fuel droplets. The characteristics of fuel evaporation are investigated for different observation times after the start of injection, chamber conditions and injection pressures. For the quantification of the fuel concentration measurements based on Rayleigh scattering, a calibration method that uses propane as a reference gas is presented and tested. At high ambient temperatures, the accuracy of the concentration measurements is limited by pyrolysis of the fuel molecules. This paper was originally presented at the 14th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, 2008.     

Gas-phase velocity field measurements in sprays without particle seeding

A laser-based technique is presented that can be used to measure the instantaneous velocity field of the continuous phase in sprays and aerosols. In contrast to most well established laser-based velocity measurement techniques, this method is independent of particle seeding and Mie scattering. Instead of that it is based on gaseous flow tracers and laser-induced fluorescence (LIF). Inhomogeneous tracer gas distributions, which are created by an incomplete, turbulent mixing process, are exploited for flow tracing. The velocity field can be measured close to the droplets, because frequency-shifted LIF is separated from Mie scattering by optical filters. Validation tests and results from a water spray in air are given. Accuracy and spatial resolution are discussed in detail. Received: 26 April 1999/Accepted: 16 October 1999     

Accurate analysis of multicomponent fuel spray evaporation in turbulent flow

The aim of this paper is to perform an accurate analysis of the evaporation of single component and binary mixture fuels sprays in a hot weakly turbulent pipe flow by means of experimental measurement and numerical simulation. This gives a deeper insight into the relationship between fuel composition and spray evaporation. The turbulence intensity in the test section is equal to 10%, and the integral length scale is three orders of magnitude larger than the droplet size while the turbulence microscale (Kolmogorov scales) is of same order as the droplet diameter. The spray produced by means of a calibrated droplet generator was injected in a gas flow electrically preheated. N-nonane, isopropanol, and their mixtures were used in the tests. The generalized scattering imaging technique was applied to simultaneously determine size, velocity, and spatial location of the droplets carried by the turbulent flow in the quartz tube. The spray evaporation was computed using a Lagrangian particle solver coupled to a gas-phase solver. Computations of spray mean diameter and droplet size distributions at different locations along the pipe compare very favorably with the measurement results. This combined research tool enabled further investigation concerning the influencing parameters upon the evaporation process such as the turbulence, droplet internal mixing, and liquid-phase thermophysical properties.     

Measurements of droplet size in shear-driven atomization using ultra-small angle x-ray scattering

Measurements of droplet size in optically-thick, non-evaporating, shear-driven sprays have been made using ultra-small angle x-ray scattering (USAXS). The sprays are produced by orifice-type nozzles coupled to diesel injectors, with measurements conducted from 1 – 24 mm from the orifice, spanning from the optically-dense near-nozzle region to more dilute regions where optical diagnostics are feasible. The influence of nozzle diameter, liquid injection pressure, and ambient density were examined. The USAXS measurements reveal few if any nanoscale droplets, in conflict with a popular computational model of diesel spray breakup. The average droplet diameter rapidly decreases with downstream distance from the nozzle until a plateau value is reached, after which only small changes are seen in droplet diameter. This plateau droplet size is consistent with the droplets being small enough to be stable with respect to further breakup. Liquid injection pressure and nozzle diameter have the biggest impact on droplet size, while ambient density has a smaller effect.     

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.     

Numerical calculation of multiple scattering with the layer model

The optical measurement technique based on Mie scattering has been applied to various areas, in which single scattering at low particle concentration is assumed. Nevertheless, since multiple scattering is usually unavoidable in online measurements, we present in this work a multiple scattering calculation method, in which a layer model is employed. The three-dimensional particle system is divided into a pile of layers the number of which is automatically determined, depending on the obscuration of the particle system. The calculation is found to be fast, reasonable and reliable.     

Dynamic characteristics of pulsed supersonic fuel sprays

This paper describes the dynamic characteristics of pulsed, supersonic liquid fuel sprays or jets injected into ambient air. Simple, single hole nozzles were employed with the nozzle sac geometries being varied. Different fuel types, diesel fuel, bio-diesel, kerosene, and gasoline were used to determine the effects of fuel properties on the spray characteristics. A vertical two-stage light gas gun was employed as a projectile launcher to provide a high velocity impact to produce the liquid jet. The injection pressure was around 0.88–1.24 GPa in all cases. The pulsed, supersonic fuel sprays were visualized by using a high-speed video camera and shadowgraph method. The spray tip penetration and velocity attenuation and other characteristics were examined and are described here. An instantaneous spray tip velocity of 1,542 m/s (Mach number 4.52) was obtained. However, this spray tip velocity can be sustained for only a very short period (a few microseconds). It then attenuates very quickly. The phenomenon of multiple high frequency spray pulses generated by a single shot impact and the changed in the angle of the shock structure during the spray flight, which had already been observed in previous studies, is again noted. Multiple shock waves from the conical nozzle spray were also clearly captured.      

Light scattering: A review of particle characterization applications

This review covers the progress of light scattering applications in the field of particle characterization in the past decade. The review addresses static light scattering (the measurement of scattering intensities due to light–particle interaction at various spatial locations), dynamic light scattering (the measurement of scattering due to light–particle interaction as a function of time), and scattering tracking analysis (the tracking of particle movement through scattering measurement).     

Large area impingement spray cooling from multiple normal and inclined spray nozzles

An inclined spray chamber with four multiple nozzles to cool a 1 kW 6U electronic test card has been designed and tested in this study. The multiple inclined sprays can cover the same heated surface area as that with the multiple normal sprays but halve the volume of the spray chamber. The spray cooling system used R134a as a working fluid in a modified refrigeration cycle. It is observed that increasing mass flow rate and pressure drop across the nozzles improved the heat transfer coefficient with a maximum enhancement of 117 %, and reduced the maximum temperature difference at the heated surface from 13.8 to 8.4 °C in the inclined spray chamber with a heat flux of 5.25 W/cm², while the heat transfer coefficient of the normal spray increased with a maximum enhancement of 215 % and the maximum temperature difference decreased from 10.8 to 5.4 °C under similar operating conditions. We conclude that the multiple inclined sprays could produce a higher heat transfer coefficient but with an increase in non-uniformity of the surface temperature compared with the multiple normal sprays.     

Planar velocity measurements of a two-dimensional compressible wake

The present study describes the application of particle image velocimetry (PIV) to investigate the compressible flow in the wake of a two-dimensional blunt base at a freestream Mach number M_X=2. The first part of the study addresses specific issues related to the application of PIV to supersonic wind tunnel flows, such as the seeding particle flow-tracing fidelity and the measurement spatial resolution. The seeding particle response is assessed through a planar oblique shock wave experiment. The measurement spatial resolution is enhanced by means of an advanced image-interrogation algorithm. In the second part, the experimental results are presented. The PIV measurements yield the spatial distribution of mean velocity and turbulence. The mean velocity distribution clearly reveals the main flow features such as expansion fans, separated shear layers, flow recirculation, reattachment, recompression and wake development. The turbulence distribution shows the growth of turbulent fluctuations in the separated shear layers up to the reattachment location. Increased velocity fluctuations are also present downstream of reattachment outside of the wake due to unsteady flow reattachment and recompression. The instantaneous velocity field is analyzed seeking coherent flow structures in the redeveloping wake. The instantaneous planar velocity and vorticity measurements return evidence of large-scale turbulent structures detected as spatially coherent vorticity fluctuations. The velocity pattern consistently shows large masses of fluid in vortical motion. The overall instantaneous wake flow is organized as a double row of counter-rotating structures. The single structures show vorticity contours of roughly elliptical shape in agreement with previous studies based on spatial correlation of planar light scattering. Peak vorticity is found to be five times higher than the mean vorticity value, suggesting that wake turbulence is dominated by the activity of large-scale structures. The unsteady behavior of the reattachment phenomenon is studied. Based on the instantaneous flow topology, the reattachment is observed to fluctuate mostly in the streamwise direction suggesting that the unsteady separation is dominated by a pumping-like motion.     

Estimation of the diameter–charge distribution in polydisperse electrically charged sprays of electrically insulating liquids

The majority of scientific and industrial electrical spray applications make use of sprays that contain a range of drop diameters. Indirect evidence suggests the mean drop diameter and the mean drop charge level are usually correlated. In addition, within each drop diameter class there is every reason to suspect a distribution of charge levels exist for a particular drop diameter class. This paper presents an experimental method that uses the joint PDF of drop velocity and diameter, obtained from phase Doppler anemometry measurements, and directly obtained spatially resolved distributions of the mass and charge flux to obtain a drop diameter and charge frequency distribution. The method is demonstrated using several data-sets obtained from experimental measurements of steady poly-disperse sprays of an electrically insulating liquid produced with the charge injection technique. The space charge repulsion in the spray plume produces a hollow cone spray structure. In addition an approximate self-similarity is observed, with the maximum radial mass and charge flow occurring at r/d ~ 200. The charge flux profile is slightly offset from the mass flux profile, and this gives direct evidence that the spray specific charge increases from approximately 20% of the bulk mean spray specific charge on the spray axis to approximately 200% of the bulk mean specific charge in the periphery of the spray. The results from the drop charge estimation model suggest a complex picture of the correlation between drop charge and drop diameter, with spray specific charge, injection velocity and orifice diameter all contributing to the shape of the drop diameter–charge distribution. Mean drop charge as a function of the Rayleigh limit is approximately 0.2, and is invariant with drop diameter and also across the spray cases tested.     

In-Cylinder Flow and Fuel Spray Interactions in a Stratified Spray-Guided Gasoline Engine Investigated by High-Speed Laser Imaging Techniques

Spray-guided direct injection spark-ignition engines operated in stratified charge mode have a high potential for improved fuel economy. As fuel is injected late in the compression stroke mixture preparation is crucial for reliable ignition. Multiple injections per cycle have proven to increase the overall combustion stability. Nevertheless cycle-to-cycle variations (ccv) are observed whose origin is not well understood. Strong impact of in-cylinder flows and spray-induced turbulence of preceding injections upon subsequent spray development and mixture formation is one possible reason for ccv. In this work mutual interactions of in-cylinder charge motion and sprays from multiple injections were investigated. Time resolved particle image velocimetry (PIV) and Mie scattering of fuel droplets at 16 kHz was used to simultaneously measure the temporal evolution of in-cylinder flow fields and spray formation. The data revealed significant spray-induced vortices perturbing the tumble flow. Sprays from subsequent injections were disturbed and showed greatly enhanced ccv compared to the first injection. A distinct upwards fluid flow impinging the cylinder head at the injector’s location (termed funnel flow) was identified as primary origin of spray deformation for second and third injections.     

Measurements of the vaporization dynamics in the development zone of a burning spray by planar laser induced fluorescence and Raman scattering

A linear measurement technique based on simultaneous planar imaging of laser induced dye fluorescence and Raman scattering in the liquid phase is reported. Calibrations in a stream of monosized droplets doped with weak concentrations of rhodamin show that the intensities on the droplet images are proportional to the actual droplet volume for Raman scattering and to the initial volume of the droplet for fluorescence, as the mass of dissolved dye does not vaporize. Thus, the mass fraction of liquid fuel that has vaporized before the probing event can be derived from these simultaneous measurements. Experiments are performed in the early development of a burning spray to derive cumulative information on the vaporization dynamics in terms of mass fraction or evaporation constant. Size distribution from conjoined phase-Doppler measurements are also used to derive the rate of droplet consumption along the axis of the burning spray.