Spray Collapse Research Articles

CFD unified approach under Eulerian-Lagrangian framework for methanol and gasoline direct injection sprays in evaporative and flash boiling conditions

Innovative synthetic fuels for advanced propulsion systems, such as methanol and ammonia, and synthetic blended fuels (E00, E10, and E30), known for their high volatility, are often injected directly into combustion chambers. It follows that Eulerian-Lagrangian spray models need to accurately capture the spray collapse as a consequence of flash boiling onset and be capable of proficiently handling the preferential evaporation of multi-component fuels in evaporative scenarios.So, we performed the assessment of an Eulerian-Lagrangian CFD code for simulating methanol and E00 gasoline blend sprays in both early and late injection conditions involving flash boiling conditions and preferential evaporation. The adoption of an effervescent breakup model and of a non-equilibrium phase transition model for the discrete phase allows the adoption of a setup that is almost completely free from specific constant tuning, especially for what concerns the breakup model. We validated the simulations using experimental PLV maps of methanol and E00 sprays issued from the ECN Spray M injector. The results highlight a significantly different morphology of the methanol spray compared to the E00 one under late injection conditions. Under stratified combustion, low-volatile fuels are likely to be ignited first, and the flame propagates toward the high-volatile fuels. The spray collapse was also correctly reproduced, inducing the presence of a low-pressure zone and modifying the spray morphology.

Investigation of fuel temperature and injection timing effects on ammonia direct injection in an optical engine

This work investigates the effects of fuel temperature and injection timing on ammonia direct injection in an optical engine using a multi-hole injector. Flash-boiling may occur over various engine-relevant conditions due to ammonia’s high vapor pressure. This phenomenon impacts spray characteristics and, in severe cases, facilitates cavitation inside the nozzle. Both fuel temperature and injection timing (i.e., ambient conditions during injection) impact the intensity of flash-boiling during fuel injection. Therefore, this work varies fuel temperature (from 320K to 388K) and injection timing (from −60CADaTDC (crank angle degrees after top dead center) to −6CADaTDC) to assess their impact on injection mass flux, discharge coefficients, and macroscopic spray characteristics using an ammonia injection pressure of 200bar. For this purpose, the injection mass is measured by analyzing exhaust gas compositions during engine operation with ammonia injections but without combustion. In addition, diffuse background illumination (DBI) images capture the liquid phase of the fuel spray to characterize spray behavior. The findings reveal that increasing fuel temperature decreases ammonia’s injection mass by up to 12.8% but has little impact on discharge coefficients for late injection timings and high in-cylinder pressures. However, discharge coefficients decrease by up to 17.4% (from 0.58 to 0.48) for early injection timings if fuel temperatures are high. The individual sprays of the 6-hole GDI injector may collapse into a uniform spray at high-density conditions without flash-boiling or under strongly flash-boiling conditions. The findings clarify the impact of ammonia’s high vapor pressure on injection mass and prove the relevance of different spray collapse mechanisms in ammonia direct injection engines.

Numerical investigation on direct water injection characteristics under different injection and ambient conditions within oxygen/argon atmosphere

Improving efficiency and reducing emissions are essential to achieve carbon neutrality in transportation sector. The hydrogen-fueled argon power cycle (H2-APC) is a novel power system with high efficiency and zero emission by utilizing argon-oxygen mixture as working fluids. However, knock suppression is a problem within H2-APC engines. Direct water injection (DWI) is an effective method to inhibit detonation. Spray morphology greatly influences the effectiveness of DWI, the investigation of spray characteristics in oxygen/argon atmosphere is critical. This paper established a computational model based on experimental data to investigate DWI characteristics under different injection and ambient conditions within oxygen/argon atmosphere. The results reveal increased jet temperature leads to stronger superheated jet spray collapse, while Sauter mean diameter (SMD) decreases and atomization improves. Increasing jet pressure effectively reduces spray SMD and improves atomization effect and evaporation rate. Higher ambient temperature reduces ambient gas density, spray SMD and penetration are decreased and increased, respectively. Moreover, spray evaporation rate and evaporated mass are dropped with higher ambient pressure for both superheated and subcooled sprays. As ambient density rises, the capacity for jet perturbation and fragmentation is enhanced, and spray SMD decreases. The research results can provide an effective guide for the DWI strategy of H2-APC engines.

Experimental research of direct water injection characteristics over wide range of injection temperature and ambient pressure

Direct water injection (DWI) within internal combustion engine has become a promising technology due to its potential for suppressing knock, reducing emissions and lowering fuel consumption. The spray morphology shows a significant impact on the effectiveness of DWI, which makes it of great research significance. In this research, the spray characteristics of water jet under different conditions were investigated based on a constant volume vessel. The water injection temperature ranged from 30 °C to 240 °C, and the ambient pressure varied from 0.2 bar to 10 bar. The results showed that the spray width at near-field increases with water temperature. However, at ambient pressures of 0.8 bar–2.5 bar, higher superheat level resulted in severe spray collapse, manifested as significantly reduced spray width. It was identified that the collapse under flash boiling occurs as a result of the presence of the low-pressure zone at the center of the spray, and higher superheat level migrate the low-pressure zone closer to the nozzle exit, resulting in enhanced spray collapse. This could lead to excessive water mist concentration in partial region, negatively affecting combustion process and potentially causing oil emulsion in actual engine operation. This work also suggests a strong correlation between spray collapse and water injection pressure, in addition to superheat level and ambient pressure. The findings of this work have the potential to provide effective reference for high-temperature water injection strategies in internal combustion engines.

In-cylinder spray evolution in a motored central-injection gasoline engine: Imaging and simulating the effects of flash-boiling and intake crossflow

Accurate predictions of fuel spray behavior and mixture formation in simulations of direct-injection spark-ignition (DISI) engines are fundamental to ensure proper description of all subsequent processes including ignition, combustion, and emissions. In this work, the spray evolution in a single-cylinder optical DISI engine was studied experimentally and numerically with the goal of enabling predictive computational fluid dynamics (CFD) modeling of in-cylinder sprays. The authors explored a wide range of operating conditions characterized by several fuel injection temperatures and engine speeds, using a well-characterized nine-component gasoline surrogate known as PACE-20. The effect of flash boiling and intake crossflow on the spray is discussed, with a focus on evaluating the ability of the spray models to capture highly transient spray behavior. In the experiments, the fuel temperature was varied between 20°C and 80°C, allowing for non-flash- to flash-boiling transition to emerge with enhanced flashing intensity at the highest temperatures. Spray collapse resulted in vapor-rich regions, owing to the locally lower inertia of the fluid. Varying the engine speed from 650 to 1950 rpm promoted increasingly more turbulent in-cylinder crossflow which interacted with the spray during the injection event and resulted in enhanced spray dispersion. The CFD model was able to capture the spray morphology transition at different fuel temperatures and engine speeds adequately. It is shown that the spray breakup model could capture the transitional spray behavior induced by flash boiling atomization and intake flow via proper initialization of the spray cone angle and calibration of the spray models’ constants.

Influence of plume arrangement on spray collapsing process in lateral-cylinder-mounted gasoline injectors

Spray collapsing is a major factor that deteriorates combustion performance in gasoline direct-injection (GDI) engines. This study employed three different GDI injectors to investigate plume arrangement influence on spray dynamics. Novel optical diagnostics were applied to analyze complex spray collapse behaviors using LED-based diffusive back-illuminated extinction imaging (DBIEI) and 3D computed tomographic (CT) image reconstruction. The triangular pattern with 6 holes, commonly applied in modern GDI engines, was compared with two newly designed spray patterns: the inversed T pattern with 6 holes and the lambda pattern with 5 holes. Experimental results showed that the base triangular pattern, with plumes enclosed by other plumes, indicated significant spray collapse behavior under various conditions. The inversed T pattern, where plumes were arranged to not have a closed volume, had a robustness against collapse due to an open layout which prevents imbalance of the local pressure field that may causes collapse. The lambda pattern, which had a similar plume arrangement to the triangular pattern, except that one side of the diamond-shape closed volume is opened, also showed good robustness against collapse under most of the test conditions. This study demonstrated that spray collapse can be significantly mitigated by improved plume arrangement and provides spray patterns that can be applied to actual GDI engines with significantly improved spray robustness.

General understanding on spray collapse process of an asymmetrical multi-hole direct injection gasoline injector under wide flash-boiling conditions

How the under-expansion impacts the flash-boiling spray collapse process over wide superheat levels ( Rp, defined as the ratio of saturation pressure to ambient pressure) is not well understood. In the present study, n-hexane flash-boiling sprays issued from a five-hole asymmetrical injector were experimentally and numerically studied to obtain a more general understanding of the spray collapse with wide Rp variation. The experimental results proved that the collapse in the transitional region occurs in the far field, unlike the fully collapse that occurred in the near-nozzle region. The numerical results demonstrated the complexity of individual jet evolutions and their interactions over wide Rp. For individual flash-boiling jets, there were different behaviors in the near-nozzle region. In the case with Rp slightly larger than 1, no shock waves can be observed, but a set of compression-expansion chains. The further increase in Rp caused the generation of shock waves, and resultantly the primary cells were established. For the multi-jet sprays, the further increase in Rp enlarged the primary cells, leading to their interactions and the generation of secondary cells. When Rp was sufficiently higher, the further interactions among primary and secondary cells could cause the generation of tertiary cell. Orderly interactions of shock cells were observed with increasing Rp, that is, the interactions initially occur between the adjacent jets with smaller distances, and then other jets were involved. Based on the results: It was found that the compression-expansion chains caused the low- Rp flash-boiling spray collapse in the far field; With the increase in Rp, the shock waves and shock-to-shock interaction become the main contributor to spray collapse, leading collapse appearing in the near-nozzle region.

Near-field plume-to-plume interaction of multi-hole GDI injector under elevated ambient density conditions

Spray collapse can occur in the multi-hole injector under elevated ambient density conditions due to the plume-to-plume interaction. Numerous studies have investigated spray collapse in high ambient density conditions, but limited literature is available regarding the near-field plume-to-plume interaction triggering the spray collapse due to the optical density of the spray in the near field. In this study, an X-ray phase-contrast imaging (XPCI) technique that can visualize the spray structure and dynamics in the near field is applied to resolve the optical difficulties. With this method, the plume-to-plume interaction of a multi-hole gasoline direct injection injector is analyzed by measuring the transverse spray droplet size and axial velocity distributions under various ambient densities and injection pressures. The results showed that in the high ambient density condition with the low injection pressure, the spray droplet size started to increase, and the axial velocity remained stagnant from around 3 mm axial location as a result of plume-to-plume interaction and following spray collapse. The region of large droplets appeared asymmetric based on the hole arrangement. As the injection pressure increased in the high ambient density condition, the plume-to-plume interaction was suppressed in the near field and the spray collapse appeared farther downstream. Through the results, the spray collapse phenomenon and associated mechanism in the near field were discussed considering the droplet collision/coalescence and pressure structure in the spray.

Complete Workflow of Internal Nozzle Flow and Engine Simulation Using Multi-Component Fuel at Flash Boiling Conditions

Article Complete Workflow of Internal Nozzle Flow and Engine Simulation Using Multi-Component Fuel at Flash Boiling Conditions Bejoy Mandumpala Devassy 1, Yidan Zhang 2, * , Enyuan Zhang 2, and Long Zhou 2 1 AVL LIST GmbH, Hans-List-Platz 1, Graz 8020, Austria 2 AVL List Technical Center (Shanghai) Co., LTD. Shanghai 201206, China * Correspondence: yidan.zhang@avl.com Received: 17 May 2023 Accepted: 1 September 2023 Published: 14 September 2023 Abstract: At present 3D CFD simulation tools are getting more and more importance for addressing complex physical problems. This article presents a complete virtual analysis of the in-cylinder workflow in gasoline engine at flash boiling condition. Using the standard gasoline surrogate fuel PACE-20, properties of different fuel components was considered both the nozzle and in-cylinder flow. The flow, evaporation, spray jet interaction and spray collapse under flash boiling condition was studied. Molar density fraction was used to describe fuel component distribution for both gas phase and liquid phase. Based on nozzle flow simulation result, in-cylinder simulation has been done, the distribution of different fuel components and the influence on wall film, flame propagation, emission distribution has been studied. It shows that the simulation results have good agreement with testing data, and it proves the model validation for gasoline engine virtual development method under flash boiling conditions.

Droplet characteristics of multi-plume flash boiling spray evaluation using SLIPI-LIEF/Mie planar imaging technique

Flash boiling atomization is a promising approach whose primary breakup heavily relies on the phase change inside the nozzle. Droplet size measurement is crucial for fundamental studies of flash boiling sprays. However, the interference of the vapor phase and multiple scatterings in droplet-dense regions notably impact the accuracy of spray droplet size measurement. In this work, we utilized a three-phase structured laser illumination planar imaging, laser-induced exciplex fluorescence/Mie scattering (3p-SLIPI-LIEF/Mie) method in evaluating the droplet size distribution for a two-hole fuel injector and an ECN Spray G injector. The SMD characteristics of the fuel spray under various temperature and pressure boundary conditions were examined, and flash boiling spray collapse conditions were emphasized. The results show that the proposed SLIPI-LIEF/Mie method is superior to PDI techniques in providing more comprehensive results while maintaining satisfactory accuracy. The SLIPI-LIEF/Mie results show that the near-field plume expansion and SMD distribution strongly correlate with the flash boiling phase transition process. The droplet in the collapsed region was larger than that in the primary plume regions. However, whether or not the spray collapses, the SMD of the spray reduces as the fuel temperature increases, ambient pressure decreases, and injection pressure increases.

Evaluation of GDI spray structures under flash boiling and strong collapse conditions with diverse fuels

GDI (Gasoline Direct Injection) sprays are a matter of continuous research due to their potential for higher thermal efficiency and power output. Injections in GDI engines tend to happen at relatively high temperatures, which, combined with volatile fuels, are a perfect environment for flashing sprays, which are present in recently developed GDI engine passenger cars. Phenomena like flash boiling and especially spray collapse are still being investigated due to their complexity and impact on the engine performance. In the present research, the impact of different levels of flash boiling in the sprays is analyzed. Four surrogate mono-component fuels were tested with different volatile properties. Two optical techniques were carried out simultaneously: light extinction images (DBI) and frontal MIE scattering. The spray width was used to represent the radial expansion and study its relation with the spray collapse due to flash boiling. The degree of collapse was quantified by means of the instantaneous spray width variation for which three levels of collapse were identified. A novel MIE frontal approach was employed to study the spray surface evolution and the collapsing time by monitoring the nozzle tip clouding (opaquing) by the fuel. Results showed an increment of the width with the ratio of saturation to ambient pressure (Rp), as well as a strong correlation between the spray surface and the Rp at each fuel temperature when the spray is stabilized. Based on the MIE technique, three collapse instants were identified throughout the injection time depending on the Rp.

Spray collapse resistance of GDI injectors with different hole structures under flash boiling conditions

Spray collapse resistance of GDI injectors with different hole structures under flash boiling conditions

MACROSCOPIC SPRAY CHARACTERISTICS OF GASOLINE, METHANOL, AND ETHANOL IN DIRECT INJECTION SPARK IGNITION ENGINE-LIKE CONDITIONS

In recent decades, stringent emission norms have been enforced upon the engine research community and OEMs to encourage them to develop new spark ignition engine technologies, such as variable valve lifts, turbocharging, and direct injection spark ignition (DISI) engines. For further development, greater control of parameters such as in-cylinder air motion, spray characteristics, injection, and ignition events is required. Spray characterizations are crucial for understanding the mixing phenomena in heated and pressurized engine combustion chamber conditions. Spray pattern, fuel injection pressure (FIP), rate shape, and thermodynamic conditions of the combustion chamber play a vital role in the mixture preparation. The present study uses Mie-Scattering techniques to examine spray structures of fuels like methanol and ethanol and compare them to gasoline, which is of great interest to DISI engines. Three different temperatures of 50, 100, and 200°C and two chamber pressures, 4 and 8 bar, are considered to simulate typical engine-cylinder conditions. It is observed that the initial chamber conditions greatly influence the spray structure. Spray collapse is lesser for alcohol than gasoline. Three semi-empirical models for predicting spray penetration are analyzed: Dent, Hiroyasu and Arai, and Arrègle. These models could not differentiate between the test fuels, particularly methanol and ethanol, for predicting spray penetration length. The degree of deviation in predictions is the lowest in the Hiroyasu and Arai model and the highest in the Dent model. Spray penetration length increased with an increasing FIP regardless of ambient conditions; however, the spray penetration length decreased with increasing chamber pressure.

COMPARATIVE STUDY ON THE COLLAPSE MODES UNDER FLASH-BOILING AND NON-FLASH-BOILING CONDITIONS USING A TWO-HOLE GDI INJECTOR

Several mechanisms are available for the multi-jet GDI spray collapse under low ambient pressure (≤ 1.0 bar) flash-boiling (LP-FB) and non-flash-boiling (NFB) conditions. The present investigation is to clarify the applicability of the available mechanisms under different thermodynamic states. The tests were carried out in an optically accessible constant volume vessel using a two-hole GDI injector over fuel temperature (T<sub>fuel</sub>) ranging from 50° to 130°C and ambient pressure (P<sub>amb</sub>) ranging from 0.2 to 9.0 bar. The collapse could be found both under LP-FB and NFB conditions, but their characteristics changing with P<sub>amb</sub> and T<sub>fuel</sub> showed the opposite trends. The LP-FB collapse enhanced as P<sub>amb</sub> decreased or T<sub>fuel</sub> increased while the NFB collapse weakened as P<sub>amb</sub> decreased and could disappear once Tfuel was beyond 70°C. Further analysis into the temporal spray development showed that the LP-FB collapse occurred at the very beginning of injection accompanied with the occurrence of a new jet between the original jets (termed as "inter-jet jet") while the NFB collapse gradually occurred with the time elapsed. The NFB collapse was attributed to the low-pressure zone caused by the jet entrainment. The disappearance of NFB collapse as T<sub>fuel</sub> increased demonstrated that the enhanced evaporation would counteract this low-pressure zone. This also provided the strong support that the LP-FB collapse could not be induced by the jet entrainment because the flashing evaporation was much stronger. After careful examination of the available mechanisms, it was more likely that the occurrence of under-expansion in LP-FB jets and their interactions at the nozzle exit caused the collapse.

Investigation into shock-to-shock interactions induced by flash boiling and the impact on spray behaviors

Through recent numerical studies, the shock-to-shock interactions between flash-boiling jets are believed to be the reason for multi-jet spray collapse in gasoline direct injection (GDI) engines. But this sort of interaction was not well understood through the experimental investigation. In the present study, a two-hole GDI injector was used to examine this process via optical diagnostics and CFD simulations. With the increase in superheat level (Rp), the shock waves (primary cells) in individual jets became observable. Further increase in Rp could cause shock-to-shock interactions, and a secondary cell could be formulated between two primary cells. The different intensity of shock-to-shock interactions would result in different jet-to-jet interaction behaviors. In the moderate interaction region, the secondary jet appeared inside and downstream of the secondary cell, but the sprays did not collapse. In the highly interaction region, both the secondary jet and spray collapse could be observed, where the secondary jet would laterally develop perpendicular with the plane of the two jets. The detailed numerical results showed that the secondary cell was small in the moderate interaction region, but became much larger in the highly interaction region. The occurrence of secondary cell directed the fuel from primary cells into the secondary cell, contributing to the generation of secondary jet. After flowing out of the secondary cell, the incipient secondary jet could subsequently entrain the nearby low-velocity fuel. Further, the strong shock-to-shock interactions would alter the shape of primary cells, reducing the transverse velocity component of jets and promoting collapse. Based on the experimental and numerical results, the existence of shock-to-shock interactions and its important role in jet-to-jet interactions and spray collapse were fully illustrated, and this understanding would be of help in nozzle optimization.

Spray Collapse in a Side-mount Gasoline Direct Injection Injector with Various Injection Conditions and Injector Nozzle Configurations

Spray Collapse in a Side-mount Gasoline Direct Injection Injector with Various Injection Conditions and Injector Nozzle Configurations

ECN Spray G: Coupled Eulerian internal nozzle flow and Lagrangian spray simulation in flash boiling conditions

Flash-boiling of fuel sprays occurs when a super-heated fuel is discharged into an environment whose pressure is lower than the saturation pressure of the fuel and can dramatically alter spray formation due to complex two-phase flow effects and rapid droplet vaporization. In gasoline direct injection (GDI) engines, typically, it occurs during the injection process when high fuel temperatures make its saturation pressures higher than the in-cylinder one. Flash boiling significantly affects the spray structure and fuel-air mixture formation, with, potentially, if spray collapse is avoided, positive consequences for the engine performance and pollutant emissions. Hence, this paper developed a modelling approach based on the coupling of Eulerian multi-phase and Eulerian-Lagrangian simulations to reproduce flash boiling sprays physics. It allows a comprehensive description of the flash-boiling phenomena that begins within the injector’s ducts and continues outside, where a particular mechanism for primary breakup occurs. Experimental data of the Engine Combustion Network (ECN) Spray G injector, covering various operating conditions, were exploited to assess the reliability of the numerical technique adopted. The validation was performed taking into account various parameters such as axial liquid and vapour penetrations, droplet size and images regarding the near-nozzle zone. The numerical model achieves a pretty good level of agreement with the experimental data and, in particular, it is sensitive to the spray behaviour in the different operating conditions.

Spray collapse characteristics of practical GDI spray for lateral-mounted GDI engines

Spray collapsing and plume merging processes were investigated using a lateral-mounted gasoline direct injection (GDI) injector with a practical 'triangular' spray pattern. High-speed diffusive back illumination extinction imaging followed by computed tomography reconstruction was applied to understand the spatiotemporal plume dynamics under engine-like conditions. The spray chamber and injector conditions include (1) cold, subcooled standard temperature and pressure (STP) used by the injector manufacturer, (2) practical gasoline fuels with full-range distillation, (3) flash-boiling with fuel temperature and vacuum gas pressure, and (4) high gas pressure and temperature typical of injection during compression. The novel experiments permit tracking of plume merging at different times and axial distances downstream of the nozzle. A triangular 6-hole pattern, which is widely used in lateral-mounted GDI engines, was found to be prone to having the centrally located plumes move close to each other thus leading to spray collapse with these plumes at all practical test conditions (2)–(4). Variations of air entrainment and local pressure with different conditions were identified as dominant factors for the timing and position of spray collapse.

Understanding the collapse of flash-boiling sprays formed by multi-hole injectors operating at low injection pressures

Sprays consisting of initially separated plumes, which are generated by multi-hole injectors in strong flash-boiling conditions, may collapse and form a single spray cloud. It has been shown that for high-pressure injection systems (≥5 MPa), such behaviour is dependent on both the superheat and the geometrical configuration of the injector; while the spray collapse in low-pressure systems (≤1 MPa) is not yet understood. To the best of the author's knowledge, this is the first study aimed at filling this information gap. The principal novelty of this work is related to understanding the collapse in low-pressure systems, in terms of the different numbers of nozzles and the difference in the effects from flash boiling between low- and high-pressure sprays.The results showed that the flash-boiling sprays formed by the two- and six-hole injectors were substantially different, which is similar behaviour as for high-pressure sprays. The major difference from the sprays formed at high pressure is that the flash boiling does not result in an increased penetration (in relation to the subcooled conditions). This distinct behaviour was linked with a different jet break-up regime, and different droplet size reduction due to flash boiling, both depending on the injection pressure. This work discusses that the importance of the aerodynamic drag force due to a stronger droplet size reduction in the case of low-pressure sprays can be increased more than twofold compared to the high-pressure sprays.The lack of increased penetration leads to an important finding in terms of practical applications. In contrast to high-pressure systems, flash boiling will not lead to enhanced wall wetting regardless of the number of nozzles.This study has revealed that the interaction of the spray plumes is not sufficient to cause a strong spray collapse and the formation of the single jet-like spray structure. The necessary additional factor is the flow acceleration in the axis of the injector. The results highlighted the crucial role of the circular pattern, which already in subcooled cases led to the creation of flow conditions promoting axial propagation of a gaseous medium; while in the event of flash boiling, it prevented the surrounding air from entering the centre region of the spray cloud.

Spray Collapse Characteristics of Practical Gdi Spray for Lateral-Mounted Gdi Engines

Spray Collapse Characteristics of Practical Gdi Spray for Lateral-Mounted Gdi Engines