Georgios Charalampous

Method to reduce errors of droplet sizing based on the ratio of fluorescent and scattered light intensities (laser-induced fluorescence/Mie technique)

The droplet sizing accuracy of the laser technique, based on the ratio of laser-induced fluorescence (LIF) and scattered light (Mie) intensities from droplets, is examined. We develop an analytical model of the ratio of fluorescent to scattered light intensities of droplets, which shows that the LIF/Mie technique is susceptible to sizing errors that depend on the mean droplet size and the spread of the droplet size distribution. The sizing uncertainty due to the oscillations of the scattered light intensity as a function of droplet size is first quantified. Then, a new data processing method is proposed that can improve the sizing uncertainty of the technique for the sprays that were examined in this study by more than 5% by accounting for the size spread of the measured droplets, while improvements of 25% are possible when accounting for the mean droplet size. The sizing accuracy of the technique is evaluated in terms of the refractive index of liquid, scattering angle, and dye concentration in the liquid. It is found that the proposed approach leads to sizing uncertainty of less than 14% when combined with light collection at forward scattering angles close to 60° and the lowest fluorescent dye concentration in the liquid for all refractive indices.

Numerical evaluation of droplet sizing based on the ratio of fluorescent and scattered light intensities (LIF/Mie technique)

The dependence of fluorescent and scattered light intensities from spherical droplets on droplet diameter was evaluated using Mie theory. The emphasis is on the evaluation of droplet sizing, based on the ratio of laser-induced fluorescence and scattered light intensities (LIF/Mie technique). A parametric study is presented, which includes the effects of scattering angle, the real part of the refractive index and the dye concentration in the liquid (determining the imaginary part of the refractive index). The assumption that the fluorescent and scattered light intensities are proportional to the volume and surface area of the droplets for accurate sizing measurements is not generally valid. More accurate sizing measurements can be performed with minimal dye concentration in the liquid and by collecting light at a scattering angle of 60° rather than the commonly used angle of 90°. Unfavorable to the sizing accuracy are oscillations of the scattered light intensity with droplet diameter that are profound at the sidescatter direction (90°) and for droplets with refractive indices around 1.4.

Structure of the Continuous Liquid Jet Core during Coaxial Air-Blast Atomisation

This paper investigates the structure of the continuous liquid jet of a coaxial air-blast atomiser over a range of Weber numbers 60-1040, Reynolds numbers of liquid jet 5400-21700 and air to liquid momentum ratios of the two streams of 1.7–335. A novel optical technique, based on internal illumination of the liquid jet through the jet nozzle by a laser pulse, which excites a fluorescing dye introduced in the atomizing liquid, was used to obtain instantaneous measurements of the breakup length and the three dimensional location of the liquid core of the continuous liquid jet. The latter was achieved by simultaneously imaging the liquid jet from two directions normal to each other. Such measurements are usually prevented by droplets surrounding the liquid jet at the dense spray near the nozzle exit. The measurements showed that the break-up length of the liquid jet scaled well with the air to liquid momentum ratio. The standard deviation of the temporal fluctuations of the break-up length was around 10% of the mean breakup length for each considered flow condition. The instantaneous jet surface does not develop axi-symmetric wave structures but the time-averaged liquid jet is axi-symmetric around the nozzle axis, while the maximum deflection of the liquid jet occurs close to the breaking point.

Estimation of the droplet size spread with the laser induced fluorescence/Mie technique

The laser measurement technique based on the ratio between the laser induced fluorescence (LIF) and the scattered light (Mie) intensities of droplets is presently limited to the evaluation of the Sauter mean diameter of the droplets. The important measurement of the droplet size spread is currently missing. An extension of the LIF/Mie technique for the measurement of droplet size spread is proposed here and is evaluated numerically. The method is based on the imperfect relationships between the scattered light intensity and the droplet surface area or the fluorescent light intensity and the droplet volume, which convey additional information that can be used to evaluate the droplet size spread.

How do liquid fuel physical properties affect liquid jet development in atomisers

The influence of liquid fuel properties on atomisation remains an open question. The droplet sizes in sprays from atomisers operated with different fuels may be modified despite the small changes of the liquid properties. This paper examines experimentally the development of a liquid jet injected from a plain orifice in order to evaluate changes in its behaviour due to modifications of the liquid properties, which may influence the final atomisation characteristics. Two aviation kerosenes with similar, but not identical physical properties are considered, namely, standard JP8 kerosene as the reference fuel and bio-derived hydro-processed renewable jet fuel as an alternative biofuel. The corresponding density, dynamic viscosity, kinematic viscosity, and surface tension change by about +5%, −5%, −10%, and +5%, respectively, which are typical for “drop-in” fuel substitution. Three aspects of the liquid jet behaviour are experimentally considered. The pressure losses of the liquid jet through the nozzle are e...

Application of Proper Orthogonal Decomposition to the morphological analysis of confined co-axial jets of immiscible liquids with comparable densities

The development of a round liquid jet under the influence of a confined coaxial flow of an immiscible liquid of comparable density (central to annular flow density ratio of 8:10) was investigated in the vicinity of the nozzle exit. Two flow regimes were considered; one where the annular flow is faster than the central jet, so the central liquid jet is accelerated and one where the annular flow is slower, so the central liquid jet is decelerated. The central jet was visualised by high speed photography. Three modes of jet development were identified and classified in terms of the Reynolds number, Re, of the central jet which was in the range of 525 < Re < 2725, a modified definition of the Weber number, We, which allows the distinction between accelerating and deceleration flows and was in the range of −22 < We < 67 and the annular to central Momentum Ratio, MR, of the two streams which was in the range of 3.6 < MR < 91. By processing the time resolved jet images using Proper Orthogonal Decomposition (POD), it was possible to reduce the description of jet morphology to a small number of spatial modes, which isolated the most significant morphologies of the jet development. In this way, the temporal and spatial characteristics of the instabilities on the interface were clearly identified which highlights the advantages of POD over direct observation of the images. Relationships between the flow parameters and the interfacial waves were established. The wavelength of the interfacial instability was found to depend on the velocity of the fastest moving stream, which is contrary to findings for fluids with large density differences.

Collisions of droplets on spherical particles

Head-on collisions between droplets and spherical particles are examined for water droplets in the diameter range between 170 μm and 280 μm and spherical particles in the diameter range between 500 μm and 2000 μm. The droplet velocities range between 6 m/s and 11 m/s, while the spherical particles are fixed in space. The Weber and Ohnesorge numbers and ratio of droplet to particle diameter were between 92 < We < 1015, 0.0070 < Oh < 0.0089, and 0.09 < Ω < 0.55, respectively. The droplet-particle collisions are first quantified in terms of the outcome. In addition to the conventional deposition and splashing regimes, a regime is observed in the intermediate region, where the droplet forms a stable crown, which does not breakup but propagates along the particle surface and passes around the particle. This regime is prevalent when the droplets collide on small particles. The characteristics of the collision at the onset of rim instability are also described in terms of the location of the film on the particle...

Numerical and experimental evaluation of the optical connectivity technique for measurement of liquid breakup length in atomizers

During the first stages of atomization in an air-blast atomizer, the liquid stream is destabilized under the influence of a coaxial stream of air, until its continuity is broken. A novel technique [Charalampous et al. 2007 (1)] has been proposed to measure the length of the continuous liquid jet, based on the internal illumination of the liquid jet through the spray nozzle. The liquid jet acts as a light guide, which propagates along the length of the jet, in the same way as light travels along the length of an optical fiber. The light excites a fluorescent dye that is dissolved in the liquid jet, making the volume of the liquid jet luminous. Then, the connectivity of the liquid jet is linked to the optical connectivity of the fluorescent jet. However, since the surface of the liquid jet is not smooth as that of an optical fiber due to the development of waves on the jet surface, there are losses of light intensity due to refraction through the jet surface and absorption by the fluorescence dye, as it propagates along the liquid jet. The optical connectivity technique is examined numerically and experimentally. First, numerical simulations of the light propagation within liquid columns of various geometries are presented. The effects of the morphological characteristics of the interfacial waves on the gas-liquid interface (wavelength and amplitude) and the type of perturbation (sinus or varicose) as well as the absorption of laser light within the liquid jet and the characteristics of the laser beam (divergence and intensity profile) were considered. Next, a comparison of the continuous length of the liquid jet of an airblast atomizer is presented as measured with the optical connectivity technique and an electrical connectivity technique. In the latter approach, the continuity of the jet is determined by the electrical conductivity between the spray nozzle and a probe placed in the electrically charged liquid downstream of the liquid nozzle exit. The comparison shows that the optical connectivity technique performs better than the electrical connectivity technique in measuring the mean breakup length of the liquid jet.

Ray tracing analysis of realistic atomizing jet geometries for optical connectivity applications

The optical connectivity technique has been proposed for the visualization of the continuous core of atomizing jets with little optical interference from the surrounding droplets. By introducing a laser beam through the nozzle, the high intensity laser light propagates along the liquid jet in the same manner as light propagates in an optical fiber. Beyond the breaking point the laser light diffuses and its intensity diminishes. The addition of a fluorescent dye in the atomizing liquid makes the intensely illuminated continuous jet volume luminous, while the poorly illuminated detached droplets remain dark. The technique has been demonstrated to perform well experimentally. However, there are losses of the laser intensity as the beam travels downstream the liquid jet, due to refraction through the liquid interface. As such, the maximum length of an atomizing jet that can be visualized is limited. For this reason the propagation of a laser beam inside a liquid jet and the subsequent fluorescent emission has been examined numerically in the past by means of ray tracing [1]. The investigation considered a number of parameters including the gas and the liquid refractive indices, the laser beam divergence at the nozzle exit, the concentration of the dye in the liquid jet and the geometry of the liquid jet as a sinusoidal wave. While considerable insight was obtained on the applicability of the optical connectivity technique and the importance of the initial laser beam divergence highlighted, the sinusoidal interface does not represent faithfully the geometry of an atomizing jet under intense atomization conditions. Here we extend the previous investigation to computationally derived atomizing liquid jet geometries for We=1040 and MR=336, in order to evaluate the propagation of the laser illumination under more realistic liquid jet geometries and assess the applicability of the optical connectivity technique.

Investigation of injection characteristics of alternative aviation fuels by Laser-Induced Fluorescence imaging

The increasing supply demands for fuel have created the need for new sources to supplement limited fossil supplies. While alternative fuels are becoming available from various sources, due to their different chemical and physical characteristics, engine performance and emissions are not guaranteed to be identical with those of conventional fuels. Therefore there is a need to quantify the behavior of these fuels in order to assure acceptable engine performance and emissions. Here, an investigation is performed that focuses on the injection characteristics of three fuels injected from a plain orifice atomizer into a quiescent environment. The baseline fuel is JP8, which is widely used for aviation applications. Two alternative fuels, a hydro-processed renewable jet (HRJ) fuel and a Hightemperature Fischer–Tropsch (HTFT) derived Iso-Paraffinic Kerosene (IPK) are also studied. The morphological characteristics of the fuel jet are investigated in the near nozzle region by Laser-Induced Fluorescence (LIF) imaging while the injection performance of the injector is evaluated in terms of the discharge coefficient calculated from the pressure drop across the injector nozzle.