Influence of cavitation on near nozzle exit spray (original) (raw)
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Link between in-nozzle cavitation and jet spray in a gasoline multi-hole injector
The importance of cavitation inside multi-hole injectors has been addressed in many previous investigations where the cavitation formation and its development, fuel spray characteristics and atomisation have quantified. Different types of geometrical and vortex cavitations have been previously reported inside the nozzles of multi-hole injectors with good indication of their influences on the emerging spray. However, the effect of cavitation on jet spray, its stability and liquid breakup and atomisation is not yet fully understood. The current research work is aimed to address some of the above issues. As the initial phase, the current experimental work focuses on the initiation and development of different type of cavitation inside a 15-times enlarged model of a symmetric 6-hole SIDI injector and tries to quantify the effects of the cavitation on the near nozzle jet spray in terms of jet cone angle and its stability. To achieve this, a high speed camera has been used to visualise the innozzle flow and emerging spray simultaneously.
The effect of string cavitation in various transparent Diesel injector nozzles on near nozzle spray dispersion angle is examined. Additional PDA measurements on spray characteristics produced from real-size transparent nozzle tips are presented. Highspeed imaging has provided qualitative information on the existence of geometric and string cavitation, simultaneously with the temporal variation of the spray angle. Additional use of commercial and in-house developed CFD models has provided complimentary information on the local flow field. Results show that there is strong connection between string cavitation structures and spray instabilities. Moreover, elimination of string cavitation results in a stable spray shape that is only controlled by the extent of geometric-induced cavitation pockets. Finally, PDA measurements on real-size transparent nozzle tips have confirmed that such nozzles reproduce successfully the sprays generated by production metal nozzles.
International Conference on Liquid Atomization and Spray Systems (ICLASS), 2021
The accurate control of the the spray dynamics arising from the nozzle in GDI injectors is key in order to control the subsequent combustion process. In this work we present LES of in nozzle dynamics of gasoline direct injection and the resulting early development of spray plumes from an 8-hole injector (ECN Spray G). In order to do so a new three fluid solver within OpenFOAM has been developed. The objective is to evaluate the impact of the cavitation in a multihole gasoline injector, along with its influence on the flow field at the start of injection. It is shown how the interaction between the gaseous phases can influence the initial stages of the spray formation and the different pattern of cavitation across the injectors holes.
Cavitation in Fuel Injection Systems for Spray-Guided Direct Injection Gasoline Engines
SAE Technical …, 2007
Cavitation formation and development inside various types of nozzles for close-spacing spray-guided fuel injection systems is predicted using a computational fluid dynamics cavitation model. The fuel injection systems investigated include generic geometries of multi-hole nozzles and outwards opening pintle injectors. Model validation is performed against experimental data reported elsewhere in large-scale transparent nozzle replicas. The results confirm that cavitation strongly depends on the geometry of the nozzle and the operating conditions. For multi-hole nozzles, cavitation structures similar to those realised in Diesel injectors are formed. These include the needle seat cavitation realised at low needle lifts, the geometrically-induced hole entry cavitation and string cavitation developing inside the sac volume. A more chaotic and less understood cavitation pattern develops at the sealing area of inward seal band outwards opening nozzles. Vapour pockets have been found to develop around the circumferential area of the needle sealing area in a transient mode. Parametric studies obtained under realistic injection and back pressure conditions reveal the effect of nozzle design on the different nozzle flow patterns that may form during the injection timing.
2020
As energy is essential for human development, society faces a dual challenge: the rapid growth in energy demand and carbon emissions including the risks of climate change. One important method of reducing emissions in Diesel engines is to improve fuel injector spray breakup, producing smaller and more disperse droplets. The flow inside the fuel injector nozzle is known to have a significant effect on the spray. Recent investigations have suggested that the cavitation occurring within the fuel injector nozzle significantly affects spray breakup. However the hydrodynamic cavitation behavior of diesel flow inside the fuel injector nozzle is still need to explore. In this paper numerical simulations were performed with and extensive validation has been established with available experimental data. Geometry of two dimensional real size nozzles has been used to assess the effect of mixture and volume of fluid (VOF) multiphase model. Effect of back pressure (15 bar to 85 bar) with constant...
Internal flow and cavitation in a multi-hole injector for gasoline direct-injection engines
SAE paper, 2007
A transparent enlarged model of a six-hole injector used in the development of emerging gasoline directinjection engines was manufactured with full optical access. The working fluid was water circulating through the injector nozzle under steady-state flow conditions at different flow rates, pressures and needle positions. Simultaneous matching of the Reynolds and cavitation numbers has allowed direct comparison between the cavitation regimes present in real-size and enlarged nozzles. The experimental results from the model injector, as part of a research programme into second-generation direct-injection spark-ignition engines, are presented and discussed. The main objective of this investigation was to characterise the cavitation process in the sac volume and nozzle holes under different operating conditions. This has been achieved by visualizing the nozzle cavitation structures in two planes simultaneously using two synchronised high-speed cameras. Imaging of the flow inside the injector nozzle identified the formation of three different types of cavitation as a function of the cavitation number, C N. The first is needle cavitation, formed randomly at low C N (0.5-0.7) in the vicinity of the needle, which penetrates into the opposite hole when it is fully developed. The second is the well known geometric cavitation originating at the entrance of the nozzle hole due to the local pressure drop induced by the nozzle inlet hole geometry with its onset at around C N =0.75. Finally, and at the same time as the onset of geometric cavitation, string type cavitation can be formed inside the nozzle sac and hole volume having a strong swirl component as a result of the large vortical flow structures present there; these become stronger with increasing C N. Its link with geometric cavitation creates a very complex two-phase flow structure in the nozzle holes which seems to be responsible for holeto-hole and cycle-to-cycle spray variations.
Modelling of cavitation in diesel injector nozzles
A computational fluid dynamics cavitation model based on the Eulerian–Lagrangian approach and suitable for hole-type diesel injector nozzles is presented and discussed. The model accounts for a number of primary physical processes pertinent to cavitation bubbles, which are integrated into the stochastic framework of the model. Its predictive capability has been assessed through comparison of the calculated onset and development of cavitation inside diesel nozzle holes against experimental data obtained in real-size and enlarged models of single-and multi-hole nozzles. For the real-size nozzle geometry, high-speed cavitation images obtained under realistic injection pressures are compared against model predictions, whereas for the large-scale nozzle, validation data include images from a charge-coupled device (CCD) camera, computed tomography (CT) measurements of the liquid volume fraction and laser Doppler velocimetry (LDV) measurements of the liquid mean and root mean square (r.m.s.) velocities at different cavitation numbers (CN) and two needle lifts, corresponding to different cavitation regimes inside the injection hole. Overall, and on the basis of this validation exercise, it can be argued that cavitation modelling has reached a stage of maturity, where it can usefully identify many of the cavitation structures present in internal nozzle flows and their dependence on nozzle design and flow conditions. 1. Introduction Current common-rail fuel injection systems for direct injection diesel engines operate at very high pressures, up to 1800 bar, while the whole injection process lasts for very short time intervals – of just a few milliseconds. The injection rate is controlled through the fast opening and closing of the needle valve, whereas the typical diameter of nozzle holes is 0.1–0.2 mm. As the flow from the injector enters into the nozzle discharge holes, it has to turn sharply from the needle seat area, which leads to the static pressure of the liquid at the entrance of the holes falling below its vapour pressure and initiation of cavitation. The occurrence of cavitation in orifices and its significant effect on spray formation have been known for quite some time. From the early experiments of Bergwerk (1959), using simplified large-scale and real-size single-hole acrylic nozzles, it was found that the discharge coefficient of the nozzle is mainly dependent on the cavitation number, which is a non-dimensional parameter indicating the expected cavitation intensity (see (1)), and is independent of the Reynolds number, i.e.
Cavitation in Injector Nozzle Holes - A Parametric Study
Cavitation in Injector Nozzle Holes - A Parametric Study, 2014
The fuel injection system in diesel engines has a consequential effect on the fuel consumption, combustion process and formation of emissions. Cavitation and turbulence inside a diesel injector play a critical role in primary spray breakup and development processes. Thus understanding the phenomenon of cavitation is significant in capturing the injection process with accuracy. In this study, the cavitating flow inside an injector nozzle hole was numerically investigated. The two-phase mixture model by Schnerr and Sauer (2001) was adopted along with k-ε turbulence model and Fluent CFD package was used to solve the governing equations numerically. The validation of the model was done by comparing the numerical results with the experimental results of Winklhofer et al. (2001) for U-throttle geometry and a good agreement was found. In this paper, a detailed parametric study on the effects of injection pressure, transient analysis for diesel and SME (Soy Methyl Esther) bio-diesel and different geometries on cavitation phenomenon inside the injector nozzle hole was done. The results show that the bio-diesel inhibits the cavitation phenomenon compared to diesel fuel, and positive Kfactor nozzles have higher mass flow rate and higher exit velocity. Keywords: cavitation, injector nozzle, diesel, SME bio-diesel
Modelling of cavitation in nozzles for diesel injection applications
2014
Extreme low pressure regions develop in the high pressure direct injection fuel flow inside the fuel injector holes, compelling the liquid fuel to transform to vapour phase in the form of vapour cavities or bubbles, a phenomenon known as cavitation. The cavitation phenomenon determines the quality of primary atomization and hence affects the performance of direct injection diesel or gasoline engines. A cavitation model, coupled with the mixture multiphase approach and RNG k − turbulence model, has been developed and implemented in this study for analysing cavitation. The cavitation model has been implemented in ANSYS Fluent platform. The model predictions have been compared with results from experimental works available in the literature. A good agreement of the model predictions has been observed. Comparisons of the model with other cavitation models (Schnerr & Sauer and Zwart-Gerber-Belamri) available in ANSYS Fluent have been carried out with both mixture and Eulerian-Eulerian mu...
MODELLING OF CAVITATION FLOW IN A NOZZLE AND ITS EFFECT ON SPRAY DEVELOPMENT
Jets, 2006
The experimental observations and theoretical models of spray formation due to the effect of cavitation development in injection nozzles are reviewed. Particular attention is focused on the effects of cavitation disturbances on jet and spray break-up. Models of jet and spray break-up which take into account the stochastic character and non-equilibrium spectrum of product droplets are essential when modelling the primary and secondary stages of break-up. Single-fluid models of cavitation are shown to be robust, but contain empirical rate parameters which require adjustments for specific flows. This study addresses the liquid quality and viscous shear stress effects on cavitation flow. In order to account for the liquid quality effect on cavitation a model derived from bubble dynamics theory is developed. The model for the concentration of cavitation nuclei in a liquid is derived by assuming hydrodynamic similarity of cavitation flows. The model accounts for the variation in the number density of cavitation bubbles as a function of liquid tension in the cavitation region. This model was developed using an analogy with the effect of liquid superheat on the number of nuclei in models for nucleate boiling. The model contains a parameter, which describes the liquid quality. This can be adjusted using one set of measurements for a given liquid. The influence of viscous shear stress on the cavitation threshold in high-speed flows, such as those observed in the nozzles of a direct-injection diesel engine, has been clarified. In order to describe this effect on hydrodynamic cavitation in high-speed turbulent flows a model that takes into account the critical vapour pressure was developed. The model was adjusted to describe sub-cavitation and super-cavitation flows in real-scale models of diesel injectors. Nomenclature t C -constant in equation (12); D -diameter of nozzle; H -height of nozzle; ∞ l -hydrodynamic length scale of the flow; n -number density of cavitation bubbles; * n -parameter in equation (11); p 1 -pressure at the inlet of nozzle; p 2 -pressure at the outlet of nozzle; p v -vapour pressure; p min -minimum pressure in cavitation region; R -radius of cavitation bubble; ij S -strain rate tensor; u -velocity; W -width of nozzle, m; α -volume fraction of the vapour phase;