Droplet size and morphology characterization for dense sprays by image processing: application to the Diesel spray

Up to now, measurement of drop size remains difficult in dense sprays such as those encountered in Diesel applications. Commonly used diagnostics are often limited due to multi-scattering effects, high drop velocity and concentration and also nonspherical shapes. The advantage of image-based techniques on the others is its ability to describe the shape of liquid particles that are not fully atomized or relaxed. In the present study, a model is developed to correct the main drawbacks of imaging. It permits to define criteria for the correction of the apparent size of an unfocused drop and to determine a measurement volume independent of the drop size. This considerably reduces the over-estimation of large drops in the drop size distribution. Drop shapes are also characterized by four morphological parameters. The image-based granulometer is satisfactorily compared to a PDPA and a diffraction-based granulometer for measurements on an ultrasonic spray. Then, the new granulometer is applied to a diesel spray. One of the results of the analysis is that even if mean drop size distributions are stable 30 mm downstream from the nozzle outlet, the shape of the drops is still evolving towards the spherical shape. The atomization process is thus not totally established at this position in opposition to what can be deduced from the drop size distribution alone.     

Simultaneous planar measurement of droplet velocity and size with gas phase velocities in a spray by combined ILIDS and PIV techniques

A new approach for simultaneous planar measurement of droplet velocity and size with gas phase velocities is reported, which combines the out-of-focus imaging technique ‘Interferometric Laser Imaging Droplet Sizing’ (ILIDS) for planar simultaneous droplet size and velocity measurements with the in-focus technique ‘Particle Image Velocimetry’ (PIV) for gas velocity measurements in the vicinity of individual droplets. Discrimination between the gas phase seeding and the droplets is achieved in the PIV images by removing the glare points of focused droplet images, using the droplet position obtained through ILIDS processing. Combination of the two optical arrangements can result in a discrepancy in the location of the centre of a droplet, when imaging through ILIDS and PIV techniques, of up to about 1 mm, which may lead to erroneous identification of the glare points from droplets on the PIV images. The magnitude of the discrepancy is a function of position of the droplet’s image on the CCD array and the degree of defocus, but almost independent of droplet size. Specifically, it varies approximately linearly across the image along the direction corresponding to the direction of propagation of the laser sheet for a given defocus setting in ILIDS. The experimental finding is supported by a theoretical analysis, which was based on geometrical optics for a simple optical configuration that replicates the essential features of the optical system. The discrepancy in the location was measured using a monodisperse droplet generator, and this was subtracted from the droplet centres identified in the ILIDS images of a polydisperse spray without ‘seeding’ particles. This reduced the discrepancy between PIV and ILIDS droplet centres from about 1 mm to about 0.1 mm and hence increased the probability of finding the corresponding fringe patterns on the ILIDS image and glare points on the PIV image. In conclusion, it is shown that the proposed combined method can discriminate between droplets and ‘seeding’ particles and is capable of two-phase measurements in polydisperse sprays.     

Disengagement of the gas phase in bubble columns

The technique of gas disengagement is popularly used to assess the bubble size distribution in bubble columns. The technique involves the dynamic measurement of dispersion height when the gas supply is stopped. In this paper a mathematical model has been proposed for the process of dynamic gas disengagement. It has been shown that the initial faster disengagement is due to the presence of internal liquid circulation and not due to the presence of very large bubbles. Further, slower disengagement has been attributed to the transition from heterogeneous dispersion to homogeneous dispersion. The new model also explains the effects of superficial gas velocity, column diameter, column height and liquid phase physical properties on the gas disengagement.     

Monofiber optical probes for gas detection and gas velocity measurements: conical probes

Optical probes are a very common tool for the investigation of gas–liquid flow dynamics. A single sensitive tip gives access to the phase indicator function, while using two tips some distance apart provides estimates of the gas velocity. Recently, it has been shown that the gas velocity could also be inferred from a monofiber probe provided that its latency length is known. To improve the capabilities of this new technique, an optimisation of the probe geometry, based on optical simulations and controlled piercing experiments, has been undertaken. In this first paper, conical probes (1C) produced using a new manufacturing technique are considered. Although they are effective for simultaneous gas velocity measurements, their actual response is sensitive to small geometrical defects occurring at their tips. Therefore, calibrations on well controlled interfaces appear necessary both to check the presence of pre-signals responsible for incorrect phase detections, and to establish a significant rise-time/velocity correlation.     

Drop sizes in annular two-phase flow

Drop sizes in annular flow have been measured using a diffraction technique. Several series of experiments were carried out to determine the effect of gas velocity, drop concentration, film flow rate and tube diameter on drop size. Film flow rate and tube diameter have been found to have very little influence on the sizes of drops produced. An empirical equation which describes the drop sizes is presented.     

Monofiber optical probes for gas detection and gas velocity measurements: optimised sensing tips

Optical probes are a very common tool for the investigation of gas–liquid flow dynamics. A single sensitive tip gives access to the phase indicator function, while using two tips some distance apart provides estimates of the gas velocity. Recently, it has been shown that the gas velocity could also be inferred from a monofiber probe provided that its latency length is known. To improve the capabilities of this new technique, an optimisation of the probe geometry, based on optical simulations and controlled piercing experiments, has been undertaken. In this first paper, conical probes (1C) produced using a new manufacturing technique are considered. Although they are effective for simultaneous gas velocity measurements, their actual response is sensitive to small geometrical defects occurring at their tips. Therefore, calibrations on well controlled interfaces appear necessary both to check the presence of pre-signals responsible for incorrect phase detections, and to establish a significant rise-time/velocity correlation.     

Drop size spectra in sprays from pressure-swirl atomizers

A study on the characterization of sprays from Newtonian liquids produced by pressure-swirl atomizers is presented. The global drop size spectra of the sprays are measured with phase-Doppler anemometry, and global mean drop sizes are derived as moments of the spectra for varying atomizer geometry, liquid flow rate, and physical properties of the liquids. Dimensional analysis provides a correlation for the non-dimensional global Sauter mean diameter. A relationship between the global Sauter mean drop size and the global drop size RMS is established. A method is developed for predicting the global drop size spectra in the sprays, using easily accessible experimental input parameters. The basis for the function defining the spectrum is a gamma distribution, which is known from the literature as physically relevant for ligament-mediated sprays.     

Experimental analysis of the flow characteristics of a pressure-atomised spray

An experimental setup has been created to allow measurements of the properties of the gas phase, the liquid phase and the mixture in a pressure-atomised spray of water, in terms of both mean quantities and Reynolds stresses. This setup involves laser Doppler velocimetry for determining the velocity of either the gas or liquid phase, according to the parameters used, such as seeding or no-seeding of the ambient air, laser source power, or photo-multiplier gains, droplet tracking velocimetry for determining the velocity and characteristic size of the droplets, and a single optical probe for determining the mean volume fraction of the liquid, from which the liquid mean mass fraction and the mean density of the mixture are inferred. The experimental conditions, in particular in terms of liquid and gas Weber numbers, were chosen in a range for which the liquid phase turbulent kinetic energy should be mainly responsible for the liquid-jet primary break-up, these flow conditions lying within the second wind-induced atomization regime. Results reported herein are more specifically focused on the region ranging from 400 nozzle diameters to 800 nozzle diameters, where the liquid core is disrupted. They provide new information about the formation and properties of such pressure-atomised sprays, in particular in terms of the role played by the Reynolds stresses resulting from the slip velocity between the liquid and the gas. The mean slip velocity is directly related to the turbulent flux of liquid. Such information will be used in the future to develop new turbulence models since very limited experimental information is so far available for these terms.     

Characterization of the gas pulse frequency, amplitude and velocity in non-steady dense phase pneumatic conveying of powders

Current modelling techniques for the prediction of conveying line pressure drop in low velocity dense phase pneumatic conveying are largely based on steady state analyses.Work in this area has been on-going for many years with only marginal improvements in the accuracy of prediction being achieved.Experimental and theoretical investigations undertaken by the authors suggest that the flow mechanisms involved in dense phase conveying are dominated by transient effects rather than those of steady state and are possibly the principal reasons for the limited improvement in accuracy.This paper reports on investigations on the pressure fluctuation behaviour in dense phase pneumatic conveying of powders.The pressure behaviour of the gas flow in the top section of the pipeline was found to exhibit pulsatile oscillations.In particular,the pulse velocity showed variation in magnitude while the frequency of the oscillations rarely exceeded 5 Hz.A wavelet analysis using the Daubechie 4 wavelet found that the amplitude of the oscillations increased along the pipeline.Furthermore,there was significant variation in gas pulse amplitude for different types of particulate material.     

Experimental investigation of three-phase flow in a vertical pipe: Local characteristics of the gas phase for gas-lift conditions

Laboratory experiments have been performed on the flow of oil, water and air through a vertical pipe in order to study the gas-lift technique for oil–water flows. Special attention was paid to the phase inversion phenomenon, by which the continuous phase switches to the dispersed phase and vice versa. By using different types of gas injectors the influence of the bubble size of the injected air on the efficiency of the gas-lift technique (in particular at the point of phase inversion) was studied. Also the gas and liquid mixture velocities were varied. The air bubbles were detected by means of optical fibre probes. Local measurements of the time-averaged gas volume fraction, bubble size and bubble velocity were carried out, as well as pressure measurements.     

Characterization of the gas pulse frequency,amplitude and velocity in non-steady dense phase pneumatic conveying of powders

Current modelling techniques for the prediction of conveying line pressure drop in low velocity dense phase pneumatic conveying are largely based on steady state analyses. Work in this area has been on-going for many years with only marginal improvements in the accuracy of prediction being achieved. Experimental and theoretical investigations undertaken by the authors suggest that the flow mechanisms involved in dense phase conveying are dominated by transient effects rather than those of steady state and are possibly the principal reasons for the limited improvement in accuracy. This paper reports on investigations on the pressure fluctuation behaviour in dense phase pneumatic conveying of powders. The pressure behaviour of the gas flow in the top section of the pipeline was found to exhibit pulsatile oscillations. In particular, the pulse velocity showed variation in magnitude while the frequency of the oscillations rarely exceeded 5 Hz. A wavelet analysis using the Daubechie 4 wavelet found that the amplitude of the oscillations increased along the pipeline. Furthermore, there was significant variation in gas pulse amplitude for different types of particulate material.     

Influence of gas injection on phase inversion in an oil–water flow through a vertical tube

An experimental study has been made of the influence of gas injection on the phase inversion between oil and water flowing through a vertical tube. Particular attention was paid to the influence on the critical concentration of oil and water where phase inversion occurs and on the pressure drop increase over the tube during phase inversion. By using different types of gas injectors also the influence of the bubble size of the injected gas on the phase inversion was studied. It was found that gas injection does not significantly change the critical concentration, but the influence on the pressure drop is considerable. For mixture velocities larger than 1 m/s, the pressure drop over the tube increases with decreasing bubble size and at inversion can become even larger than the pressure drop during the flow of oil and water without gas injection.     

Aerodynamic behavior of liquid sprays

An analysis is presented for the effect of entrained gas flows on drop trajectories and spray distributions from liquid atomizing nozzles. In particular, the effect of the pressure (or density) of the environment into which the liquid is sprayed is examined. The contraction of atomized sprays at elevated pressure which has been observed by various workers is explained, and the analysis is substantially confirmed by their data and by new data presented here. Both the data and the theory show that the amount of spray contraction increases with increasing ambient pressure and nozzle pressure drop, and decreases with increasing nozzle diameter and drop size. The theory examines the entrained gas flow around and into a spray and its subsequent effect on the trajectories of the liquid droplets comprising the spray.     

A backlighted imaging technique for particle size measurements in two-phase flows

 An imaging technique that uses backlighting has been developed to measure drop sizes in annular two-phase flows with small concentrations of drops in the gas phase. Advantages over conventional photography are realized in that data collection and analysis times are shortened considerably, and consistent unbiased results can be expected. A magnification of 1.9 was used to measure drops above 50 μm. A drop size distribution was obtained for an air–water system as a superficial gas velocity of 30 m/s and a liquid flow of 20 g/s. The data are used to substantiate a theory for the rate of deposition. Received: 6 February 1997/Accepted: 3 February 1998     

Particle concentration and particle size measurements in a particle laden turbulent free jet

Particle concentration and particle size distribution curves have been measured for particle-laden jets of silica gel powder for different loading ratios and air velocities using a Laser Diffraction Method (LDM) and a tomography data transform technique. It was found that the mean particle size at the outer edge of the jet decreases with increasing gas velocity, and that the jet widens with decreasing particle concentration and increasing gas velocity.     

Benchmarking and improvements of measurement techniques for local-time-averaged two-phase flow parameters

Accurate measurements of local time-averaged two-phase flow parameters is of fundamental importance for the evaluation of two-phase flow models as well as for the development of closure relations used in the two-fluid model. For cross-calibration purposes, we compared the measurements of local-time-averaged two-phase flow parameters using a four-sensor conductivity probe developed by the TRSL (Thermal-hydraulics and Reactor Safety Laboratory) at Purdue University, and a four-sensor optical probe developed by the CEA (French Atomic Energy Commission). A system coupling methodology was also performed in order to isolate the effects of each instrumentation's components, i.e., probe, acquisition, thresholding, and processing effects. Experimental results have shown a reasonable agreement in the local void fraction, local bubble velocity, and local interfacial area concentration measurements comparisons. However, it is shown that particular attention must be given to the sensor probe design in order to get accurate measurements.     

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     

Monofiber optical probes for gas detection and gas velocity measurements: optimised sensing tips

In order to perform gas velocity measurements using a single optical probe, an optimisation of the probe geometry has been undertaken. The responses of conical probes, analysed in a previous article, were found to be strongly sensitive to deviations from an ideal geometry. To render the technique much sounder, two new shapes, namely, cone+cylinder (2C) and cone+cylinder+cone (3C) are considered. They are both effective for simultaneous gas detection and gas velocity measurements, but the latter, free of proximity detection, appears to be the most promising due to a calibration curve, i.e. the relationship between signal rise time and interface velocity, weakly sensitive to uncontrollable parameters such as the interface impact angle. In addition, its latency length can be controlled during the manufacturing process, allowing good reproducibility of probe tips. Analysis of the signal transients is used to provide guidelines for effective signal processing. Finally, possible extensions of the monofiber technique are discussed as well as remaining limitations.     

Experimental comparison of measurement techniques for drop size distributions in liquid/liquid dispersions

An online measurement technique for drop size distribution in stirred tank reactors is needed but has not yet been developed. Different approaches and different techniques have been published as the new standard during the last decade. Three of them (focus beam reflectance measurement, two-dimensional optical reflectance measurement techniques and a fiber optical FBR sensor) are tested, and their results are compared with trustful image analysis results from an in situ microscope. The measurement of drop sizes in liquid/liquid distribution is a major challenge for all tested measurement probes, and none provides exact results for the tested system of pure toluene/water compared to an endoscope. Not only the size analysis but also the change of the size over time gives unreasonable results. The influence of the power input on the drop size distribution was the only reasonable observation in this study. The FBR sensor was not applicable at all to the used system. While all three probes are based on laser back scattering, the general question of the usability of this principle for measuring evolving drop size distributions in liquid/liquid system is asked. The exterior smooth surface of droplets in such systems is leading to strong errors in the measurement of the size of the drops. That leads to widely divergent results. A different measurement principle should be used for online measurements of drop size distributions than laser back scattering.     

Grouping and trapping of evaporating droplets in an oscillating gas flow

A new approach to the analysis of droplet grouping in an oscillating gas flow is suggested. This is based on the investigation of droplet trajectories in the frame of reference moving with the phase velocity of the wave. Although the equations involved are relatively simple, the analysis shows distinctive characteristics of grouping and non-grouping cases. In the case of grouping, droplet trajectories converge to the points for which the ratio of flow velocity in this frame of reference and the amplitude of flow oscillations is less than 1, and the cosine of the arc sine of this ratio is positive. In the case of non-grouping, droplet trajectories in this frame of reference oscillate around the translational velocity close to the velocity of flow in the same frame of reference. The effect of droplet size on the grouping pattern is investigated. It has been pointed out that for the smaller droplets much more stable grouping is observed. The effect of droplet evaporation is studied in the limiting case when the contribution of the heat-up period can be ignored. It is shown that evaporation can lead to droplet grouping even in the case when the non-evaporating droplets are not grouped. This is related to the reduction in droplet diameter during the evaporation process. Coupling between gas and droplets is shown to decrease the grouping tendency. A qualitative agreement between predictions of the model and in-house experimental observations referring to Diesel engine sprays has been demonstrated.