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In this study, computational fluid dynamics simulations of the Engine Combustion Network Spray C injector were performed to investigate the effect of the internal nozzle flow on the development of the ensuing spray in automotive fuel injectors. A set of simulation best practices was first developed to achieve an accurate representation of the internal nozzle flow. Detailed, spatiotemporally defined information was extracted at the injector's orifice exit and stored in maps that were subsequently employed to initialize a Lagrangian spray by means of a static, one-way coupling (OWC) approach. To highlight the impact of the in-nozzle flow on the spray, the OWC approach was compared with the conventional rate-of-injection (ROI) approach, for which the spray parcels are commonly initialized using a blob injection model. The results indicated that, using the ROI approach, considerable evaporation was observed within the plume in the near-nozzle region. On the other hand, the OWC approach showed evidence of evaporation farther downstream and at the plume periphery. The different behavior observed with the two approaches was mostly due to substantial differences in fuel mass distribution. The liquid fuel concentration predicted by the OWC approach matched well with X-ray tomography measurements of projected fuel mass per unit volume, in terms of both magnitude and distribution. In contrast, an unphysical distribution of fuel mass characterized by an extremely low liquid concentration was predicted by the ROI approach. In the far-field, the effect of the in-nozzle flow was found to be weaker as the spray penetrated and interacted with the ambient gas. In this region, the OWC and ROI approaches provided similar predictions of vapor penetration, spray morphology, and mixing-related characteristics. • Effect of internal nozzle flow on the development of ensuing spray was studied via CFD. • A Eulerian-Lagrangian one-way coupling (OWC) approach was employed and evaluated. • Predictions via OWC matched well with X-ray measurements in the near-nozzle region. • The effect of in-nozzle flow became weaker as the spray interacted with ambient gas. [ABSTRACT FROM AUTHOR]

• Interaction between under-expanded flashing jets was studied in near-nozzle area. • Unique shock structure similar to that of under-expanded gaseous jets was formed. • Spray collapse caused by interaction of under-expanded flashing jets was revealed. • Spray collapse was enhanced with higher superheat level or lower inter-hole angle. In this research, n-hexane flashing jets discharged from two-hole GDI injectors were studied numerically with different superheat levels and inter-hole angles. The mutual interaction between under-expanded flashing jets was discussed in terms of its effect and mechanism. It was found that under certain conditions, the jets deflected towards each other and merged into one jet which moved along the central axis, indicating the occurrence of spray collapse. The spray collapse is ascribed to the pressure drop in the central area between the two jets, which is an effect of the low-pressure cores within individual jets. The pressure drop in the central area is mainly determined by two effects, i.e. the intensity and volume of the low-pressure cores in individual jets, and the formation of the secondary cell which protects the central area from being affected by the ambience. In the transitional stage from non-collapse to fully-collapse, the pressure drop was enhanced with the rise of superheat level or the decrease of inter-hole angle, and the extent of spray collapse increased. Besides, unique shock structures similar to those of under-expanded gaseous twin-jets were formed at high superheat levels, which consist of two primary cells and a secondary cell. [ABSTRACT FROM AUTHOR]

Highlights • Flashing jet behaviors of a single-hole GDI injector were studied numerically. • Drastic vaporization leads to an under-expanded two-phase flow near nozzle exit. • Shock-wave structures like those in under-expanded gaseous jets were identified. • Low-pressure region caused by expansion is believed as origin of spray collapse. Abstract In this paper, n-hexane flashing jets discharged from a single-hole gasoline direct injector (GDI) were studied numerically with the adoption of diffuse Eulerian framework and the homogeneous relaxation model (HRM). The fuel temperature ranged from 30 to 130 °C, and the ambient pressure varied from 20 to 101 kPa. The results showed that considerable vaporization started at the counter bore and a liquid core existed near the nozzle exit. Due to drastic vaporization, the pressure within the liquid core increased so the two-phase flow became under-expanded. Violent expansion then occurred and a low-pressure region was formed, which is believed as the origin of the spray collapse under flashing conditions for multi-hole GDI injectors. At high superheat levels, shock wave structures similar to those in highly under-expanded gaseous jets were identified. However, the transonic position located at some distances from the nozzle rather than at the throttle. Besides, vapor fraction played the dominant role in the onset of expansion, while the expansion was ended by the pressure difference between the two sides of the Mach disk. [ABSTRACT FROM AUTHOR]

In this research, n -hexane jets discharged from a single-hole gasoline direct injector (GDI) were studied in a constant volume vessel. The injection temperature ranged from 20 °C to 100 °C, and the ambient pressure varied from 4 kPa to 101 kPa. The jets were in the external flashing regime over all the superheated conditions with the maximum R p of 60.23. At the ambient pressures lower than 10 kPa, a liquid core can be observed inside the sprays, surrounded by the mist composed of fine droplets. With the increase in the injection temperature, the length of the liquid core decreased and the mist area became wider. By correlating the nucleation rate with the spray width, it was found that the intensity of bubble blast was positively related to the nucleation rate when the nucleation rates were relatively low. However, as the nucleation rate reached a critical point, the thermal energy for vaporization became the rate-controlling factor. Furthermore, realizing external flashing was proposed as a potential way to mitigate the collapse of multi-jet flashing GDI sprays because it could suppress the severe jet overlap near the nozzle exit, which was believed as one of the necessary conditions for the spray collapse. [ABSTRACT FROM AUTHOR]

The main objective of this study is to investigate the effect of ambient pressure ( P amb ) on the morphology of multi-jet flash boiling sprays and to promote the understanding on the collapse mechanism. With the high-speed imaging technique, the n-hexane spray process in a constant volume vessel was captured under the ambient pressures from 2 kPa to 101.3 kPa and the fuel temperatures ( T fuel ) from 20 °C to 120 °C. The spray width within the axial distances of 20 nozzle diameters ( do ) could be increased by increasing T fuel and/or decreasing P amb . Beyond 20 do , the change in spray width against superheated degree ( ΔT SD ) was fairly sensitive to P amb . It decreased with the increase in ΔT SD at P amb larger than 50 kPa, and increased with ΔT SD at P amb less than 50 kPa. Based on the new understanding that the collapse in multi-jet flash-boiling sprays was induced by vapor condensation at the nozzle exit and the resultant local pressure drop (i.e. generation of low-pressure zone), the sensitivity of spray width on P amb could be well explained as follows: relatively larger P amb (>50 kPa for the used injector) would be conducive to the vapor coagulation and cause stronger vapor condensation (i.e. larger pressure drop) while the vapor dispersion may become dominant at lower P amb (<50 kPa for the used injector), negatively influencing the intensity of vapor condensation. [ABSTRACT FROM AUTHOR]

• A non-equilibrium phase change model for multicomponent mixtures is presented. • Nozzle flow simulations of the ECN Spray G injector with PACE-20 were performed. • The new model captures mixing-driven vaporization and preferential vaporization. • The new model is capable of representing different phase change mechanisms. The homogenous relaxation model (HRM) is one of the most widely used models to describe the liquid‑gas phase transition. However, in its original formulation, it is unable to handle multispecies vapor-liquid equilibrium (VLE), which limits its applicability to single-component fluids. In this work, a unified non-equilibrium phase change model that considers the VLE of multicomponent mixtures is proposed building upon the HRM's structure. A time factor is introduced to mimic the effect of different phase change timescales due to different mechanisms, e.g., cavitation, flash-boiling, and evaporation. To assess the model's performance, computational fluid dynamics simulations of the internal and near-nozzle injection flow of the Engine Combustion Network's Spray G injector were performed using the nine-component PACE-20 fuel with both the unified model and the original HRM. The predicted fuel density in the near-nozzle region matched well with X-ray tomography measurements. The simulation results indicated that, whereas the HRM failed to capture the vaporization due to convective mixing between the fuel and ambient gas, the unified model performed well in predicting the mixing-driven vaporization and the corresponding evaporative cooling. Further comparisons using the nine-component fuel formula and a single-component fuel surrogate demonstrated the unified model's ability to predict preferential vaporization, which affects the predictions of local mixture composition and rate of vaporization. Finally, it is shown that the unified model is capable of representing multiple phase change mechanisms, and the relaxation time factor plays an important role in determining the degree of phase change due to the different mechanisms. [ABSTRACT FROM AUTHOR]

In this study, the spray characteristics of a five-hole gasoline direct injector were investigated in a constant volume vessel. The absolute ambient pressure ranged from 0.5 to 10.0 bar, and the fuel temperatures were 20 °C and 80 °C. High-speed imaging and phase Doppler measurement technique were utilized to investigate the spray morphology and the droplet dynamics, respectively. Spray collapses were observed in the near field under the elevated ambient pressure (higher than 1.0 bar) conditions and in the far field under the flash boiling conditions in both macroscopic and microscopic levels. In addition, under the elevated ambient pressure conditions, the droplets at the inner side of the target jet were larger than those at the outer side, due to the increased probability of droplet coalescence as a result of spray collapse. Whilst, slight increase in droplet diameter in the inner side of the target jet was also found under the flash boiling conditions. Furthermore, the near-field collapse at the elevated ambient pressure conditions was attributed to the jet-air interaction, termed as jet-induced spray collapse; and the far-field collapse under the flash boiling conditions was attributed to the dramatic temperature drop and the resultant vapor condensation, termed as condensation-induced spray collapse. [ABSTRACT FROM AUTHOR]

Spray impingement adversely affects fuel mixture preparation, combustion performance and emissions and more studies are required to understand this process. The isooctane spray impingement process and characteristics were investigated by ultrahigh speed imaging technique with the employment of highly spatially resolved long distance microscope. The effects of impact surface temperature were also studied. It was found that during the initial stage and steady stage of spray impingement, a large proportion of droplets splashed due to high velocity. The droplet size after impingement generally reduced because of the strong collision. For the end stage of impingement, droplets tended to stick on the impact surface and float on the fuel film due to the low droplet velocity and the existence of built liquid fuel film. It was also found that hot impact surface could only improve the impingement and reduced the film building-up rate in the initial stage. The steady stage and end stage of spray impingement were less affected by the variation in impact surface temperature. [ABSTRACT FROM AUTHOR]

The flash boiling sprays in a single-cylinder optical gasoline direct injection (GDI) engine were studied using phase Doppler particle analyzer (PDPA) and high-speed photography. Initially, the factors which may influence the in-cylinder measurement reliability of PDPA were carefully examined under atmospheric conditions. Afterwards, engine experiments were conducted under idle and wide open throttle (WOT) conditions with the fuel temperature ranging from 20 °C to 90 °C. The results show that under idle operation, the spray collapsed towards the injector axis as the fuel temperature increased. At high fuel temperatures, the spray penetration increased so that the probability of fuel-wall impingement rises. As the fuel temperature increased from 20 °C to 60 °C, the AMD and SMD slumped by 43.6% and 33.2%, indicating the dramatic improvement in atomization quality. Concurrently, the droplet uniformity decreased sharply. As the temperature further increased to 90 °C, the AMD and SMD only dropped slightly, but the droplet uniformity became better. Under WOT operation, no distinct evidence of spray deformation was found except for the case at 90 °C. The AMD and SMD dropped steadily with the increase in fuel temperature, which is mainly attributed to the reduction in viscosity and surface tension. Besides, the droplet uniformity increased gradually along with the fuel temperature. [ABSTRACT FROM AUTHOR]

In this study, one of the five jets issued from a 5-hole Direct Injection (DI) injector was studied in detail using Phase Doppler Particle Analyzer (PDPA) and high speed imaging techniques. The tests were carried out in a constant volume vessel with absolute ambient pressures ranging from 0.05 to 0.5 MPa and injection pressures ranging from 6.0 to 15.0 MPa. Throughout the test conditions, the jet trajectory deviation of the target jet is observed due to the gas pressure imbalance between the inner and outer sides of the target jet. Both the results from spray morphology and the time-resolved droplet velocity have demonstrated the bifurcation of the target jet during the spray process at sub-atmospheric and atmospheric ambient pressures. It is attributed to the joint effects of the pressure imbalance between the two sides of the target jet and the velocity distribution change at the nozzle exit which relates to the vapor pressure of gasoline. The increase in injection pressure advances and enhances the deviation of the jet trajectory due to the larger jet speed. The deviation and/or bifurcation of the target jet also changed the size distribution. [ABSTRACT FROM AUTHOR]

• Spray collapse of the ECN Spray G injector was studied under flashing conditions. • The flow development of flashing sprays was strongly tied with the fuel properties. • With propane, the flow was transonic and a unique shock structure was formed. • The underlying physics of the spray collapse process was discussed in detail. This work features a numerical study of the spray collapse process that occurs under flash-boiling, gasoline-direct-injection conditions. The Engine Combustion Network's Spray G injector was modeled, and iso-octane, n-hexane, and propane were used as fuels with the goal to investigate fuel property effects. The development of individual jets and jet-to-jet interaction were analyzed to reveal the physical mechanisms of spray collapse. The results agreed qualitatively well with the experimental data of propane available from the literature and the predictions revealed the model's ability to capture the near-nozzle shock structures witnessed in the experiments. Simulations showed that the flow development of flashing sprays was strongly tied with the fuel properties. Specifically, under-expanded jets were not observed with iso-octane due to the fuel's low vapor pressure. In the case of n-hexane, the jets were under-expanded, and a low-pressure core was observed inside the jets due to the flow expansion. With propane, the very strong expansion caused the flow to become transonic and generate a unique shock structure near the nozzle exit. The shock structure revealed the formation of primary shock cells within the jets and secondary shock cells induced by jet-to-jet interaction. Based on the results, the spray collapse under flash boiling conditions was mainly attributed to two effects, i.e., the decrease in pressure at the spray center induced by the low-pressure cores inside and between the jets, and the isolation effect due to the jet-to-jet interaction which contributed to maintain the pressure differential between the spray center and the ambient. [ABSTRACT FROM AUTHOR]

• The flow development of under-expanded flash boiling jets was investigated. • Differences between flashing jets and frozen two-phase jets were revealed. • Key factors determining the characteristics of flashing jets were discussed. • Two flow choking mechanisms have been identified for flashing jets. In this research, the n-hexane jets discharged from a straight channel were studied numerically to reveal the development and characteristics of under-expanded flashing jets. Firstly, the development of flash boiling jets was investigated and compared to that of two-phase jets without phase transition. It was shown that the two-phase jets without phase transition can be under-expanded only if the flow is choked. In contrast, flash boiling jets can be under-expanded whether or not the choked conditions are verified. Secondly, the factors affecting the flashing jet behaviors were discussed. The relaxation time in phase transition, the expansion ratio, and the injection pressure have been identified as three key factors determining the jet characteristics. With high enough expansion ratio and low enough relaxation time, the expansion was so strong that the flow became transonic and shock structures similar to those of under-expanded gaseous jets appeared near the orifice exit. Finally, two types of choking have been identified for flash boiling jets, i.e., vaporization-induced choking and expansion-induced choking. The former one is mainly ascribed to the vapor formation in the orifice which reduces the local speed of sound, while the latter one is due to the expansion of under-expanded two-phase flow and the resultant flow acceleration. [ABSTRACT FROM AUTHOR]

Microscopic and macroscopic impingement characteristics of a single jet were investigated under various flash boiling conditions. The influences of split injection strategy and the related interaction between split injections were studied. The fuel concentration distribution during the impingement process was also quantified. It was found that non-flash boiling condition lead to large secondary droplets and ligaments, resulting in a large amount of liquid fuel deposited on the impact surface. The boosted flash boiling could effectively reduce the sizes of primary droplets and secondary droplets and the resultant deposited fuel. The employment of split injection strategy could further decrease the deposited fuel. The second split injection built up the fuel film more quickly than the first injection due to the existence of the built fuel film by the first injection, however, increasing flash boiling strength decreased the building-up rate by improving dispersion and evaporation. In addition, the concentration of the deposited fuel on the surface could effectively be reduced by enhanced flash boiling and the lifetime of dense fuel was shortened by increasing dwell interval between split injections. [ABSTRACT FROM AUTHOR]