Begell House Inc.
Atomization and Sprays
AAS
1044-5110
26
12
2016
PREFACE: FLASH−BOILING ATOMIZATION
v-vi
10.1615/AtomizSpr.v26.i12.10
Eran
Sher
Faculty of Aerospace Engineering, Technion-Israel Institute of Technology,
Haifa, Israel
AN APPROACH TO MODELING FLASH-BOILING FUEL SPRAYS FOR DIRECT-INJECTION SPARK-IGNITION ENGINES
1197-1239
10.1615/AtomizSpr.2016015807
Christopher
Price
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
Arash
Hamzehloo
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
Pavlos
Aleiferis
Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, United Kingdom
David
Richardson
Jaguar Land Rover, Coventry, CV3 4LF, United Kingdom
flash-boiling
spray droplet evaporation
direct-injection
spark- ignition engines
Flash-boiling is a phenomenon which occurs when a liquid is discharged into an environment with an ambient pressure below the saturation pressure of the liquid. The present computational work provides an approach to modeling flash-boiling fuel sprays using the Lagrangian particle tracking technique. An atomization model based on nucleation inside the nozzle is implemented as a boundary condition at the nozzle exit and alongside a superheat evaporation model for the emerging spray droplets. The near-nozzle dense spray region of flash-boiling sprays is also investigated by consideration to the initial spray plume cone angle. The model was able to predict important flash-boiling phenomena such as spray collapse and droplet recirculation automatically, validated against experimental data.
EFFECT OF THE PROPELLANT MASS FRACTION IN A BINARY MIXTURE ON THE SPRAY CHARACTERISTICS AS GENERATED BY HOMOGENEOUS FLASH BOILING
1241-1257
10.1615/AtomizSpr.2016014272
Moti
Levy
Faculty of Aerospace Engineering, Technion - Israel Institute of Technology, Haifa, Israel
Yeshayahou
Levy
Faculty of Aerospace Engineering, Technion - Israel Institute of Technology, Haifa, Israel
Eran
Sher
Faculty of Aerospace Engineering, Technion-Israel Institute of Technology,
Haifa, Israel
spray structure
flash boiling
atomization
homogeneous nucleation
droplet size distribution
When a binary mixture of a high vapor pressure propellant and a low vapor pressure component is injected through a simple atomizer at high enough pressure, flash boiling of the propellant may occur. Due to the sudden depressurization, propellant-vapor nuclei are developed to a rapid evaporation, resulting in break-up of the liquid mixture into a fine spray that is characterized by tiny and fairly uniform droplets. This method is presently used in quite a number of applications. Depending on the initial conditions we distinguish between heterogeneous and homogeneous nucleation. Flash-boiling process within the aperture leads to highly efficient atomization. Finer sprays are achieved when increasing the superheating degree; thus, homogeneous nucleation is apparently the preferred regime for atomization. Nevertheless, experimental studies involving atomization under homogeneous nucleation regime are rather scarce. In the present work we study the effect of the propellant mass fraction in a binary mixture on the spray characteristics as generated by a homogeneous flash-boiling process. We have selected Chlorodifluoromethane (CHClF2 or R-22), and PMX-200 silicon oil as the two components of the mixture and studied the effect of the mixing ratio on the droplets' velocity and size distribution and the radial distribution of the droplets' velocity and size. We used a TSI's Phase Doppler Particle Analyzer (PDPA) to characterize the spray, and a controlled 3D positioning system to measure the droplets characteristics at accurate and specific positions. We show that lowering the mass fraction of the propellant results in a progressively higher value of the mean velocity at any radial distance, a higher mean droplets' size, and a higher standard deviation. For the present homogeneous nucleation flash-boiling atomization system, we found that the break-up efficiency, as defined by Sher and Zeigerson-Katz [Atomization Sprays, vol. 6, no. 4 (1996)], is rather low, on the order of 10−6, while a higher propellant mass fraction yields a lower process efficiency. The lower efficiency is attributed to the different mechanisms of spray formation in homogeneous and heterogeneous nucleation.
STATE OF THE ART REVIEW OF FLASH-BOILING ATOMIZATION
1259-1305
10.1615/AtomizSpr.2016015626
Tali
Bar-Kohany
School of Mechanical Engineering The Iby and Aladar Fleischman Faculty of Engineering Tel-Aviv University Israel; Department of Mechanical Engineering, nrcn, Beer-Sheva, Israel
Moti
Levy
Faculty of Aerospace Engineering, Technion - Israel Institute of Technology, Haifa, Israel
flash boiling
cavitation
homogeneous nucleation
heterogeneous nucleation
effervescent
spray
Flash boiling atomization is now a widespread practice for creating fine sprays. The present paper aims to review and analyze our current knowledge on flash atomization processes and applications. First, the fundamental physical processes of flash-boiling atomization, i.e., nucleation and bubble growth. Then, their role in creating optimal spray (small droplet diameters and short breakup length) is analyzed. Special attention is given to reviewing and comparing different transition criteria. The conclusions can be used by those who aim to avoid accidental scenarios, or, to minimize hazardous scenarios. New surfaces are created thanks to boiling (or cavitation). The relative magnitude of the new surfaces increases as the bubbles grow, and once the two-phase fluid discharges, due to the relative kinetic energies of the liquid and the gas. Better jet atomization is observed when flashing occurs within the aperture. Higher superheat degrees lead to finer spray; thereby homogeneous nucleation should be aspired to when designing an optimal injector. Twin orifice injector with an expansion chamber is preferable for multi-component liquid, especially since it enables ones to achieve the desired sprays for lower pressures and superheat degrees. While in a single orifice injector, the highest superheat degree should be aspired to, the desired superheat degree has an upper limit for injectors with an expansion chamber. Too high superheat degrees can lead to extensive bubble coalescence and to flow stratification, thereby damaging the spray quality. The optimal degree is the one that will lead to high nucleation rate, accompanied with the highest slip between the phases. There is a lack in data of depressurization rates and their relation to spray characteristics for different liquids. In addition, experiments and data are lacking with regard to multi-component liquids.
TOPOLOGY AND DISTINCT FEATURES OF FLASHING FLOW IN AN INJECTOR NOZZLE
1307-1336
10.1615/AtomizSpr.2016016510
Ioannis K.
Karathanassis
School of Mathematics, Computer Science, and Engineering, City University London, Northampton Square, EC1V 0HB London, United Kingdom
P.
Koukouvinis
School of Mathematics, Computer Science, and Engineering, City University London, Northampton Square, EC1V 0HB London, United Kingdom
Manolis
Gavaises
School of Mathematics, Computer Science, and Engineering, City University London, Northampton Square, EC1V 0HB London, UK
two-phase flow
numerical models
thermodynamic non-equilibrium
cavitation
mass-transfer rate
The effect of thermodynamic non-equilibrium conditions (liquid superheat) on the two-phase flow field developing inside an axisymmetric, single-orifice nozzle is numerically investigated by means of different variations of a two-phase mixture model. A number of "hybrid" mass-transfer models that take into account both the effect of inertial forces (cavitation) and liquid superheat have been proposed and evaluated against widely used, pure-cavitation models, in order to pinpoint the flow conditions necessary for flash boiling to occur and to elucidate the distinct features of the phase and velocity fields that characterize flashing flows. The effect of the number of nucleation sites, required as an input by the models, on the developing two-phase flow has also been looked into. The numerical results have shown that incorporation of an additional term corresponding to liquid superheat into the mass-transfer rate leads to increased evaporation rate, compared to pure-cavitation models with liquid vaporization taking place within the entire nozzle cross section. The cavitation nucleation sites have been confirmed to act as the necessary flow perturbations required for flash boiling to occur. In addition, the developing velocity field has been found to be in close correlation to the mass-transfer rate imposed. It has been established that increased liquid evaporation leads to choked-flow conditions prevailing in a larger part of the nozzle and accompanied by a more significant expansion of the two-phase mixture downstream of the injector exit that results to increased jet cone angle. Finally, the results demonstrated that liquid cooling due to the increased mass-transfer rate is not significant within the nozzle and thus consider that a constant liquid temperature produces adequately accurate results with a decreased computational cost.
TEMPERATURE CHARACTERISTICS IN A FLASH ATOMIZATION PROCESS
1337-1359
10.1615/AtomizSpr.2016013961
Astrid
Guenther
Institute of Particle Technology, Friedrich-Alexander-Universitat Erlangen-Nurnberg, D-91058 Erlangen, Germany
Andreas
Braeuer
Lehrstuhl fur Technische Thermodynamik, Friedrich-Alexander-Universitat Erlangen-Nurnberg, D-91058 Erlangen, Germany
Philipp
Siegler
Lehrstuhl fur Technische Thermodynamik, Friedrich-Alexander-Universitat Erlangen-Nurnberg, D-91058 Erlangen, Germany; Erlangen Graduate School in Advanced Optical Technologies, Friedrich-Alexander-Universitat Erlangen-Nurnberg, D-91058 Erlangen, Germany
Benedikt
Kninger
Institute of Particle Technology, Friedrich-Alexander-Universitat Erlangen-Nurnberg, D-91058 Erlangen, Germany
Karl-Ernst
Wirth
Institute of Particle Technology, Friedrich-Alexander-Universitat Erlangen-Nurnberg, D-91058 Erlangen, Germany
flash atomization
laser diffraction
linear Raman scattering
spectroscopy
shadowgraphy
temperature measurement
Superheated, or flash, atomization is a highly complicated process. Important factors characterizing the effectiveness of the spray breakup and the quality of the resulting spray are the mean droplet diameter and spray temperature. The smaller both quantities get, the more intense is the disintegration of the liquid. The determination of the spray liquid temperature is often conducted with thermocouples. This measurement method is disadvantageous owing to its invasive character that induces disturbances of the liquid flow. To improve the temperature measurement, linear Raman scattering is used. This laser-based, noninvasive measurement method allows the determination of the spray liquid phase without considering the gas phase. In this work, it is for the first time applied to characterize the development of liquid temperature in a spray generated by flash atomization. Shadowgraphy is used to assign the predominating break up mechanism to the corresponding spray temperature evolution. Furthermore, droplet diameters are measured with laser diffraction. The evaluation of linear Raman spectroscopy for the analysis of the spray temperature in combination with the description of spray characteristics are the outlines of the article. The results show that droplet temperatures can be determined with linear Raman scattering, yielding a lower spray temperature for higher applied superheat.
INVESTIGATION OF RAPID ATOMIZATION AND COLLAPSE OF SUPERHEATED LIQUID FUEL SPRAY UNDER SUPERHEATED CONDITIONS
1361-1384
10.1615/AtomizSpr.2016014231
Shengqi
Wu
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, 48109, USA; Powertrain Research and Advanced Engineering, Ford Motor Company, Dearborn, Michigan, 48124, USA
Hujie
Pan
School of Mechanical Engineering, Shanghai Jiao Tong University, National Engineering Laboratory of Electronic Control Technology, Shanghai, 200240, China
Min
Xu
School of Mechanical Engineering, Shanghai Jiao Tong University, Dongchuan Road 800 Shanghai, 200240, China; Hunan Huayan Laboratory Co., Ltd, Tanzhou Road 100 Hunan, 411100, China
David L. S.
Hung
University of Michigan-Shanghai Jiao Tong University Joint Institute,
Shanghai Jiao Tong University, 800 Dongchuan Rd., Shanghai, 200240, China
Tianyun
Li
School of Mechanical Engineering, Shanghai Jiao Tong University, National Engineering Laboratory of Electronic Control Technology, Shanghai, 200240, China
superheated GDI spray
rapid atomization
spray collapse
2-D transparent nozzle
Flash boiling sprays have demonstrated great potential in improving fuel atomization and evaporation even at very low injection pressure. In this study, fuel spray characteristics of three gasoline direct-injection injectors, namely, a one-hole injector, a six-hole injector, and a one-slot injector, were investigated via high-speed Schlieren technique. Experimental results reveal that different transformations of spray geometry were identified under various test conditions. Under flare flash boiling conditions, an expanded spray structure was observed of the one-hole injector, but collapse sprays were found of the six-hole injector and one-slot injector. The spray collapse led to longer spray penetration of the six-hole injector, but for the one-slot injector, the spray penetration decreased. To unveil this phenomenon, a two-dimensional transparent nozzle was designed to investigate the inner nozzle flow using high-speed microscopic backlit imaging technique. It showed that the vapor bubble was initiated inside the nozzle along the nozzle wall under transition and flare flash boiling conditions. Both vapor bubble size and volume fraction increased with increasing superheat degrees. Much bigger vapor bubbles and larger volume fraction were found under the flare flash boiling conditions. Both in-nozzle vapor bubbles and fuel boiling effect at the nozzle exit contributed to rapid fuel atomization and evaporation process. Contraction or expansion of the spray geometry was dependent on whether in-nozzle bubble or fuel boiling played the dominant role. In summary, more physical insights were revealed into the collapse phenomena and rapid atomization process of flash boiling sprays.
CONTENTS, VOLUME 26, 2016
1385-1394
10.1615/AtomizSpr.v26.i12.80