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It is well established that spray characteristics from automotive injectors depend on, among other factors, whether cavitation arises in the injector nozzle. Bulk cavitation, which refers to the cavitation development distant from walls and thus far from the streamline curvature associated with salient points on a wall, has not been thoroughly investigated experimentally in injector nozzles. Consequently, it is not clear what is causing this phenomenon. The research objective of this study was to visualize cavitation in three different injector models (designated as Type A, Type B, and Type C) and quantify the liquid flow field in relation to the bulk cavitation phenomenon. In all models, bulk cavitation was present. We expected this bulk cavitation to be associated with a swirling flow with its axis parallel to that of the nozzle. However, liquid velocity measurements obtained through particle image velocimetry (PIV) demonstrated the absence of a swirling flow structure in the mean flow field just upstream of the nozzle exit, at a plane normal to the hypothetical axis of the injector. Consequently, we applied proper orthogonal decomposition (POD) to analyze the instantaneous liquid velocity data records in order to capture the dominant coherent structures potentially related to cavitation. It was found that the most energetic mode of the liquid flow field corresponded to the expected instantaneous swirling flow structure when bulk cavitation was present in the flow. [ABSTRACT FROM AUTHOR]

Spray characterization in a novel twin-jet airblast injector is reported in this paper with the focus on the study of the effect of injector air swirl on droplet characteristics and dispersion behavior. The operational principle of the injector is based on achieving atomization of two liquid jets, injected in a radially opposite direction from a central hub by high-speed annular swirling cross-stream flow of air. Liquid jet atomization within model atomizers and the resulting spray study have not gained much attention in spite of its practical importance, for example, in lean premixed prevaporized combustors. In the present work, droplet size and three-component velocity measurements are measured in the above injector using the phase Doppler particle analyzer technique. Air velocity without liquid injection is also obtained using the laser Doppler velocimetry technique. For given inlet air and liquid mass flow rates, experiments are conducted in the absence and presence of annular air swirl corresponding to swirl number, S = 0 and 0.74, respectively. The addition of air swirl is found to dramatically affect the spray topology and also the measured spray characteristics as the droplet size reduces significantly downstream of the injector exit, which is explained. Droplet dispersion is studied by evaluating droplet size velocity correlation and also droplet Stokes number. The results not only provide insight into the physics behind improved atomization due to air swirl, but also demonstrate the ability of the novel injector to achieve atomization quality and high spray dispersion over a wide operating range. [ABSTRACT FROM AUTHOR]

This paper intends to investigate the influence of unsteadiness in the liquid jet disintegration process on downstream fluctuations of spray characteristics in a coaxial twin-fluid injector. Time-resolved high-speed shadowgraphic imaging of the spray was obtained for different axial locations downstream of the injector exit at z = 0, 8Dl, and 30Dl, where Dl is the central liquid tube diameter. The primary jet breakup unsteadiness close to the injector exit was characterized by measuring both shear-driven Kelvin–Helmholtz (KH) instability and flapping instability in addition to jet breakup length fluctuations. Downstream of the liquid jet core region, the liquid shedding rate ( f s h e d ) was measured at z = 8Dl. The power spectrum of time series data of instantaneous volume mean diameter (VMD) measured at z = 30Dl indicated periodic variation of the droplet size. The corresponding frequency (fVMD) was obtained. It was found that for lower range of gas-to-liquid momentum flux ratio (M < 4), both f s h e d and fVMD are larger than the frequency of KH instability. Also, for such conditions, larger temporal variation of the droplet size is realized, and this leads to higher fluctuations of the local liquid mass flux. Proper orthogonal decomposition analysis of the shadowgraph images for different axial locations identified similar topology of the dominant mode that corresponds to flapping instability. The results suggest that even far downstream of the injector exit, some memory of the upstream unsteady jet breakup process is retained, which strongly influences spatio-temporal evolution of droplet characteristics, thereby contributing to local spray fluctuations. [ABSTRACT FROM AUTHOR]

In this work, we investigate the atomization and spray characteristics of a novel rotary atomizer with micro-channel grooves, using optical diagnostics. Micro-jets emanate from the grooves provided in the rotating injector. The spiraling liquid jets interact with the surrounding high-speed swirling air flow, resulting in the creation of a well-dispersed fine spray. Attention is focused on the effects of air swirler orientation with respect to that of injector rotation. Accordingly, the co- and counter-swirl cases are investigated. The primary jet breakup process is visualized using back-lit illumination technique, which also provides the measurement of jet breakup length. The jet break-up modes are identified for different air swirl strength and orientation and injector rotational speed. The size, number density and velocity components of the spray droplets are measured using PDPA technique at different axial and radial locations. Air velocity is also measured (in the absence of liquid injection) using the LDV technique. In addition, Laser Sheet Imaging (LSI) is employed to investigate the overall spray structure. Results show that the droplet size significantly reduces when air swirl strength or injector rotational speed is increased. Though not much difference is observed in the characteristic droplet size between the co- and counter-swirl cases, the droplet size distribution is narrower and droplet dispersion is improved for the latter. The current findings demonstrate the potential of the rotary injector to dynamically control the spray characteristics as per requirement. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the mechanism of breakup of a transversely injected liquid jet due to swirling cross-stream flow of air in a model airblast injector. The focus is especially on examining the differences in the jet atomization in the absence and presence of air swirl within the injector. Multi-directional imaging of the liquid jet core is achieved by application of the Optical Connectivity (OC) technique for simultaneous visualization of the jet core from the side- and front-view. In addition, visualization of the jet core from the bottom view is also possible. Apart from the jet penetration and breakup length (obtained from the side-view image), the present technique facilitates measurement of jet width and spread rate (from the corresponding front-view image) at the same time. The airblast injector is operated for a wide range of operating conditions. The results highlight significant influence of swirling the air flow on the jet breakup morphology as well as primary breakup parameters. The measurements provided some new insight into the physics of the primary atomization within an atomizer, where the air flow is confined, in contrast to the widely studied jet-in-crossflow in wind-tunnels. In the present case, prior to its breakup into ligaments and droplets, the liquid jet undergoes jet-to-sheet transformation. Also, the liquid sheet deflects in the direction of air swirl and disintegrates in a complex manner. The addition of swirl to the air flow leads to reduction in the mean jet breakup parameters, while the fluctuations are higher. • The influence of swirling crossflow air on liquid jet atomization within an injector is studied. • A multi-directional fluorescence imaging technique is employed to visualize the jet core. • The jet penetration/breakup length and jet width/spreading angle are measured. • Significant influence of air swirl on the primary jet breakup parameters is identified. • Jet-to-sheet transformation occurs prior to jet breakup into ligaments and droplets. [ABSTRACT FROM AUTHOR]

• Effect of liquid preheating on primary jet breakup in twin-fluid injector is studied. • Jet breakup length, KH and flapping instabilities are experimentally characterized. • Measured parameters show considerable variation with T l beyond M ≈ 6. • L ¯ / D l , when scaled appropriately using Oh , depends solely on St KH , though for M < 6. • Jet flapping governs breakup length fluctuations without/with liquid preheating. This paper aims to investigate the influence of liquid preheating on primary jet breakup in a coaxial twin-fluid injector. The injector was operated over a broad range of conditions. Three different liquid injection temperatures were realized. Time-resolved shadowgraphic images of the liquid jet breakup were recorded using a high-speed camera. The images were analysed to measure various parameters including effective jet diameter at the nozzle exit, the mean and fluctuations of the jet breakup length, as well as time-scale of Kelvin–Helmholtz (KH) instability. The jet flapping instability was characterized by POD analysis. Even moderate liquid preheating is found to strongly influence the jet breakup behaviour. Different trends of the measured jet breakup parameters with respect to liquid injection temperature are identified depending on gas-to-liquid momentum flux rati M , being larger or smaller than about 6. In this context, the roles of KH and flapping instabilities are explained. [ABSTRACT FROM AUTHOR]

• Atomization of a liquid jet within an injector due to high-speed cross-stream air is experimentally studied. • Front- and side-view of the liquid jet core are simultaneously visualized using the Optical Connectivity technique. • This facilitates measurement of jet penetration/breakup length as well as the jet width and spreading angle. • Large difference in jet trajectory is observed in comparison to the wind-tunnel based studies. • The results highlight' sheet breakup mode' of the liquid jet for all injector operating conditions. Detail characterization of the liquid jet core in a model cross-stream airblast atomizer by application of the Optical Connectivity (OC) technique is the goal of the present paper. The unique ability of the technique in imaging the fluorescence signal exclusively from the jet core region facilitates simultaneous visualization of the liquid jet from the side- and the front-view with high background contrast. This allows measurement of some important primary breakup characteristics. The jet penetration and breakup length are obtained from the side-view images, while the front view images provides the jet width and the spreading angle. The above parameters are measured for a wide range of injector operating conditions. The measurement of mean jet trajectory and mean penetration/breakup length indicated large difference in comparison to the past studies based on experiments conducted in wind-tunnels. This is attributed to air flow confinement within the atomizer in the present case and also accurate measurement of breakup location in the OC technique. The range of the measured mean width of the jet and spreading angle indicated 'sheet breakup mode' of the liquid jet for all injector operating conditions. Correlations are obtained to describe the evolution of different mean quantities with respect to relevant non-dimensional numbers. The fluctuations of jet breakup parameters are also measured. Some interesting correlations are obtained between fluctuations of different primary breakup characteristics which are discussed in the paper. [ABSTRACT FROM AUTHOR]

• Influence of nozzle exit geometry on jet instabilities in coaxial injectors is studied. • Nozzles with different fluid area ratio (AR) and lip-thickness-ratio (LTR) are considered. • Kelvin-Helmholtz and flapping instabilities are characterized using high speed imaging. • The fluctuations of jet breakup length are considerably affected by nozzle geometry. • Strouhal number of jet instability are found to bear strong correlations with AR and LTR. • Injector exit geometry plays an important role in triggering the interfacial disturbances. Detailed knowledge on liquid jet instabilities is important to improve the performance of coaxial injectors, which find wide range of industrial applications. The present paper aims to investigate the influence of nozzle exit geometry on jet instabilities during the unsteady primary jet breakup process in liquid-centered coaxial injectors. Time resolved images of the liquid jet were processed to characterize Kelvin–Helmholtz instability close to the nozzle exit and flapping instability near the jet breakup location for three different nozzles with different liquid-to-gas cross-sectional area ratio (AR) and lip thickness to diameter ratio (LTR) of the central tube. The results showed considerable effect of AR and LTR on both primary and large scale instabilities as well as jet breakup length and morphology. It is found that the injector exit geometry plays a crucial role in triggering the interfacial disturbances as it conditions the liquid and air flow prior to their interaction. [ABSTRACT FROM AUTHOR]

The evolution of pulsatile and split injection spray from a multihole, Gasoline Direct Injection (GDI) atomizer at different ambient conditions is examined experimentally and computationally. The fuel is injected at 80 bar for a duration of 1.2 ms (for single injection) and also for different split proportions (ranging from 0.3 ms to 0.9 ms for the first injection and 0.9 ms to 0.3 ms for the second injection). The influence of different split proportions on the spray characteristics such as spray penetration length, cone angle and droplet size is studied in detail, and they are compared against those of single injection case to highlight the differences. Experiments were conducted in a constant volume chamber for elevated ambient pressures. The fuel spray was visualized using high-speed shadowgraph technique. Three-dimensional spray simulations were carried out using ANSYS FLUENT 16.1. The RANS-DPM coupled simulations were performed for higher ambient pressure and temperature conditions. Good agreement between the experiments and simulations is achieved with respect to spray penetration length and cone angles for both single and split injection strategies. For larger proportion of the first injection, the interaction of the second spray with the rear of the first spray was prominent at higher chamber pressure. The spray penetration length was found to reduce for split injection strategy for all ambient pressure conditions. The spatio-temporal evolution of vapour plumes in terms of equivalence ratio highlights the possibility of attaining near homogeneous, rich mixture or a stratified mixture depending on the split proportion and dwell time. [ABSTRACT FROM AUTHOR]