Flash-boiling Spray Research Articles

Numerical analysis of flash-boiling spray characteristics with liquid ammonia

Numerical analysis of flash-boiling spray characteristics with liquid ammonia

Experimental study on generation and growth of bubbles in external flash-boiling spray regime

High-performance liquid propulsion systems, such as hypersonic scramjets and liquid rockets, utilize regenerative cooling by employing fuel as a coolant to shield components from thermal damage. The flash-boiling spray, resulting from superheated liquid injection and prevalent in regenerative cooling, has received considerable research interest for enhancing atomization and combustion efficiency. In this study, superheated water jets at various temperatures between 30 and 90 ℃ were injected into a vacuum chamber at different pressures above 5 kPa, and their spray patterns were acquired through diffusive backlight imaging. Images were analyzed to determine key parameters, such as bubble nucleation flux, bubble growth rate, and spray angle, in an external flash-boiling spray regime. The estimated nucleation flux agreed well with the predictions the model based on nucleation theory in the region of sufficiently high superheat. The dynamics of bubble growth was investigated and explained using a bubble growth model. The jet-expansion energy was computed by measuring the spray angle and compared with the available energy. By mapping the evolution of flash-boiling spray in relation to relevant nondimensional numbers, the study underscores the necessity of accounting for the pressure inside the injector to adequately model bubble generation inside the injector. Furthermore, this study substantiates the feasibility of visually recognizing and analyzing individual bubbles in the external flash-boiling regime, thereby enabling the generation of experimental data crucial for the validation of bubble nucleation and growth models.

EXPERIMENTAL STUDY ON FLASH BOILING SPRAY OF HIGH-PRESSURE LIQUID AMMONIA JET WITH ROUND AND ELLIPTICAL HOLE NOZZLES

Ammonia is an ideal alternative fuel for mitigating carbon emissions. High-pressure direct injection of liquid ammonia (LNH<sub>3</sub>) offers significant advantages in enhancing energy efficiency and minimizing emissions. Due to the high saturation vapor pressure, the injection of LNH<sub>3</sub> is susceptible to flash boiling. In this study, we used high-speed micro-imaging technology with backlight lighting to establish a high-pressure common-rail LNH<sub>3</sub> jet experimental platform and investigate the flash boiling spray characteristics of nozzles with round and elliptical holes. The results demonstrated that under non-flash boiling conditions, the residual LNH<sub>3</sub> in the sac chamber and nozzle can rapidly corrode the acrylic material of the nozzle, leading to deformation and failure of the nozzle structure. Under flash boiling conditions, LNH<sub>3</sub> ejected from the hole will produce spherical macroscopic spray morphology. Then, the spray gradually transitions from an elliptical profile to a conical profile as the back pressure increases. Compared to nozzles with round holes, nozzles with elliptical holes exhibit higher flow velocity, which enhances oil-gas mixing and promotes more pronounced flash boiling phenomena. Flash boiling occurs at an earlier stage with an increase in the spray cone angle, thereby improving the atomization characteristics under both flash and non-flash boiling conditions. The tail jet of nozzles with elliptical holes terminates earlier while exhibiting a higher decrease rate in the average gray value, which improves the atomization quality in the tail spray stage and meets the requirements of timing, quantification, and precise control of the fuel injection system.

Realistic fuel spray modeling for gasoline direct injection engine applications

Fuel spray modeling plays a critical role during modern gasoline direct injection (GDI) engine development due to fuel injection’s dominant impact on engine performance and emissions as well as the complex physical processes involved. In engineering three-dimensional (3D) computational fluid dynamics (CFD) simulations, the liquid-phase fuel atomization, evaporation, and mixing are usually modeled with the discrete droplet model (DDM) adopting a Lagrangian approach for multiphase CFD simulations. To this end, general practices heavily depend on the reduced order characterization of the injector nozzle flow. However, such simplified injector modeling may lead to insufficient representations of the complex spray dynamics. To tackle this problem, this study proposes a novel workflow to numerically evaluate GDI sub-cooled and flash-boiling sprays under engine-relevant conditions using a side-mounted GDI injector together with real gasoline fuel properties. The workflow introduces a one-way coupling (OWC) method leveraging high-fidelity nozzle flow simulations to provide realistic boundary conditions to the Lagrangian injector model. The proposed workflow was first verified in a constant volume chamber (CVC) environment and then implemented in a practical GDI engine setup to study spray morphology, fuel-air mixing, and wall-wetting propensity. In addition, detailed comparison was performed between the OWC method and the conventional rate of injection (ROI) routine. Quantitative analysis of spray characteristics was conducted to highlight possible source of discrepancies of the conventional ROI method.

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.

Physics-based reduced-order modeling of flash-boiling sprays in the context of internal combustion engines

Flash-boiling injection is one of the most effective ways to accomplish improved atomization compared to the high-pressure injection strategy. The tiny droplets formed via flash-boiling lead to fast fuel–air mixing and can subsequently improve combustion performance in engines. While most of the previous studies related to the topic focused on modeling flash-boiling sprays using three-dimensional (3D) computational fluid dynamics (CFD) techniques such as direct numerical simulations (DNS), large-eddy simulations (LES), and Reynolds-averaged Navier–Stokes (RANS) simulations, the present work introduces a reduced-order cross-sectionally averaged spray (CAS) model with significantly reduced computational cost. The proposed CAS model incorporates several physical submodels in flash-boiling sprays such as those for air entrainment, drag, superheated droplet evaporation, flash-boiling induced breakup, and aerodynamic breakup models. The CAS model is then applied to different fuels to investigate macroscopic spray characteristics such as liquid and vapor penetration lengths under flash-boiling conditions. It is found that the newly developed CAS model captures the trends in global flash-boiling spray characteristics reasonably well for different operating conditions and fuels. Moreover, the CAS model is shown to be faster by up to four orders of magnitude compared with simulations of 3D flash-boiling sprays. The novelty of the present work lies in providing a reduced-order flash-boiling spray model that offers cost-effective computational representations of complex phenomena. The proposed model can be very valuable for applications, such as experimental design, fuel screening, and creating digital twins, thus playing a crucial role in the advancement of research and development in the field of spray combustion.

Experimental Investigation on Gasoline—Water Mixture Fuel Impingement Preparation Method and Spray Characteristics with High Injection Temperatures and Pressures

Gasoline–water mixed injections are of great interest because of their advantages for reduced manufacturing costs and improved atomization, with the potential to alleviate engine detonation and reduce emissions. In this work, based on the principle of impinging flow, a real-time gasoline–water mixture preparation system for internal combustion engines was designed and the preparation system performance was compared with the standard swirl mixing technique. An image processing method was established to quantify the uniformity of the prepared mixture. Based on the flash-boiling spray flash-boiling spray experiment, the spray characteristics of different gasoline–water mixtures were analyzed under different injection temperatures (30–160 °C) and pressures (5–15 MPa). The experiments showed that the impinging pressure was the main factor affecting the emulsification performance of the real-time gasoline–water mixture, and that the proposed real-time mixing system could produce a stable gasoline–water emulsion. For temperatures in the 30–160 °C range, the flash-boiling spray flash-boiling spray experiments showed that the spray penetration distance first decreases and then increases with the injection temperature, while the spray angle shows an opposite trend. The turning point corresponded to the flash-boiling point of each gasoline–water mixture.

Design and Manufacture of Experimental Equipment for Flash Boiling Spray

Design and Manufacture of Experimental Equipment for Flash Boiling Spray

Effect of flash-boiling spray on ignition and flame-holding characteristics under sub-atmospheric pressure

To prevent the performance degradation and flameout of an afterburner/ramjet combustor under sub-atmospheric pressure, the flash-boiling spray by twin-orifice nozzle is attempted to incorporate with the pilot flameholder for reliable ignition and flame stability. Herein, the effects of flash boiling spray by twin-orifice nozzle on the ignition and flame-holding characteristics of evaporating V-gutter flameholder under sub-atmospheric pressure are investigated. According to the analysis of flow patterns and fuel concentrate distribution, the flame stabilization mechanisms of evaporating V-gutter flameholder under lean-state and rich-state are concluded. Compared with subcooled spray, the significant improvement of flash boiling spray on fuel atomization and evaporation under sub-atmospheric pressure can effectively broaden the maximum ignitable velocity of the evaporating V-gutter flameholder, and the lean ignition and blowout fuel-to-air ratios can be reduced by 68.2% and 71.3%, respectively. Furthermore, the flash-boiling spray can promote the establishment of initial flame kernel and accelerate the flame-front propagation, and make the flame stabilization mechanism transform from the rich-flame stabilization mechanism under subcooled spray to the lean-flame stabilization mechanism, resulting in a better performance of flame stability. The results provide a theoretical and technical basis for the practical application of flash-boiling spray in engineering afterburner/ramjet combustors under extreme conditions.

Characterizing under-expansion behaviors induced by rapid phase change of flash-boiling jets

Flash-boiling sprays issued from multi-hole gasoline direct injection (GDI) injectors could collapse, which is recently proved to be caused by under-expansion, but very few experimental data are available on this sort of under-expansion. In this study, high-speed microscopic Schlieren photography was used to capture the near-field shock structures in flash-boiling n-hexane jets. The tests were carried out in a constant volume vessel with ambient pressures (Pamb) from 10 to 101 kPa, fuel temperatures (Tfuel) from 30 to 170 ℃, and injection pressures (Pinj) from 2 to 20 MPa. The shock structure was carefully analyzed in terms of shock width and length. Generally, both the shock width and length decreased with the increase in Pamb. With the increase in Tfuel, the shock length was relatively stable and the shock width overall increased, but a decreasing trend in shock width could be observed at 170 ℃, possibly caused by the near-critical behaviors. Further analysis showed that the shock width shows a strong linear correlation with Δμ·Pamb−0.5 (Δμ denoting chemical potential of phase change) for a given Pinj, and the shock length shows a strong quadratic polynomial relationship with ηinj (injection pressure ratio, defined as the ratio of Pinj to Pamb). Additionally, some new shock structures without Mach disk were observed, indicating that the phase change induced under expansion is different from the typical gaseous under-expansion.

The influence of intake flow and coolant temperature on gasoline spray morphology during early-injection DISI engine operation

Multi-hole gasoline injectors operating at conditions spanning throttled early-intake stroke operation produce spray plumes that either remained separated or merge and collapse due to flash boiling. Flash boiling occurs due to the sudden expansion of gas bubbles in the liquid fuel at high fuel temperature and low ambient pressure. This study records high-speed images of spray-morphology changes due to in-cylinder flow, thereby revealing operating conditions that do and do not affect the self-induced morphology observed in quiescent vessels. Specifically, in a central-injection, four-valve, high-tumble engine, where the thermodynamic state and in-cylinder cross flow are dynamic. Motivated by cold start and hot restart operation, the fuel pressure, coolant temperature, in-cylinder air pressure, and engine rpm were systematically varied over relevant operating conditions, which bracketed the range from non- to flash-boiling sprays. The results reveal the operating conditions at which the in-cylinder cross flow disrupts the spray morphology as well as the extent of the disruption. At 650 rpm, the spray morphology was similar to that observed in quiescent vessels at nominally equivalent fuel temperature and in-cylinder pressure, indicating that the spray’s self-induced entrainment flow dominated the in-cylinder flow. However, for fuel temperature and ambient pressure near the transition between non- and flash-boiling, the intake cross flow at higher engine speed (1950 rpm) significantly disrupted the spray morphology. The high cross-flow velocity appears to induce plume merging and collapse, whereas none was evident at low rpm (650 rpm). This study led to the postulate that the spray merging and collapse are governed by the rate of atomization near the nozzle exit, presumed to be controlled by either or both aerodynamic atomization and flash-boiling intensity. It would then follow that spray modeling in CFD requires atomization models that blend the effects of both physical processes.

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.

A morphological study of supercritical fuel flash-boiling spray

The optimization of fuel injection is one of the key technologies to achieve cleaner and more efficient engines. Extensive studies have demonstrated that the flash-boiling of superheated fuel would dramatically improve the atomization, mixing and combustion process. Further increasing the fuel temperature can make the fuel at a supercritical state, the flash-boiling spray of which has rarely been noticed in existing studies. The relevant data and analysis of the behavior and characteristics of supercritical fuel flash-boiling spray are still lacking. In this study, the supercritical flash-boiling injection was achieved by heating the fuel inside the injector, and its spray behavior was examined by high-speed backlit imaging. This study found that the supercritical fuel flash-boiling spray has a transient formation process different from common sprays due to the supersonic effect during the initial injection. The spray structure was carefully analyzed based on its thermodynamic state and hydrodynamic state. The spray penetration, radial expansion, location of Mach disk was also systematically investigated. The characteristics of the supercritical fuel flash-boiling spray are very similar to that of the highly under-expanded jet.

Difficulties in Defining the Degree of Superheat in Flash Boiling Liquid Nitrogen Sprays

Difficulties in Defining the Degree of Superheat in Flash Boiling Liquid Nitrogen Sprays

Formation and Structure of the Supercritical Fuel Flash-boiling Spray

Formation and Structure of the Supercritical Fuel Flash-boiling Spray

Comparison of spray characteristics of gasoline and water-in-gasoline mixture at elevated fuel temperature conditions

With the advent of the era of ultra-high thermal efficiency and ultra-low emission internal combustion engines, water injection technology is the key to solving the knocking problem and breaking the technical bottleneck. Water-in-oil mixture shows great potential as a special water injection technology for avoiding the huge cost of engine modification and the application of the flash-boiling spray, which brings more possibilities for the improvement of atomization quality, the formation of fuel–air mixture and the combustion. In this work, the spray development process of gasoline(G100) and water-in-gasoline(G70) was explored. Spray characteristics, including spray penetration, spray cone angle, spray area, minimum energy for atomization and spray particle size distribution, were investigated at elevated fuel temperature conditions (25 ~ 140 °C). The results show that as the temperature of the mixture rises, the spray pattern gradually assumes a collapsed shape, which will be prominent at the initial injection by the intervention of water after 100 °C. There are critical temperatures of 75 °C(G100) and 90 °C(G70), above which the spray penetration is remarkably reduced and the spray cone angle and spray area of G70 are enormously increased but that of G100 are diminished. The minimum energy for atomization and SMD (Sauter Mean Diameter) decrease as the temperature of the mixture rises. There exists a critical temperature of 90 °C, above which the particle size span decreases sharply and the small particle size droplets disappear quickly.

Machine-learning enabled prediction of 3D spray under engine combustion network spray G conditions

Spray and air–fuel mixing in gasoline direct-injection (GDI) engines play a crucial role in combustion and emission characteristics. While a variety of phenomenological spray models and computational fluid dynamics (CFD) simulations have been applied to identify air–fuel mixture distribution, most research efforts so far were concentrated on single axial-nozzle injectors and limited range of ambient conditions. Especially, the prediction of flash-boiling sprays in multi-hole injectors remains a great challenge due to the lack of understanding of the complicated two-phase flow dynamics. For the specific conditions, the question can arise concerning the capability of machine-learning algorithms to predict complex flash-boiling sprays. We developed a machine-learning algorithm, as a simple variant of linear regression, that is capable of predicting the spray 3D topology for various fuels and ambient conditions. A series of spray experiments were carried out in a constant-flow spray vessel coupled with high-speed diffused back-illumination extinction imaging to produce a data set for algorithm training. Nine different test fuels, including single component iso-octane (ic8) and multi-component EEE gasoline, that cover a wide range of fuel properties were injected using Engine Combustion Network (ECN) Spray G injector under ECN G2 (50 kPa absolute), G3 (100 kPa absolute), and G3HT (G3 with 393 K ambient temperature) conditions. Among the test fuels, ic8ib2 (ic8 80%, iso-butanol 20% v/v) and EEE gasoline were specified as target fuels for spray prediction by the machine-learning algorithm, thus they were not included in the training data. The macroscopic spray analysis based on projected liquid volume (PLV) and computed tomographic (CT) reconstruction showed that the spray prediction by the machine-learning algorithm showed excellent agreement with true values from the experimental data. The maximum differences in liquid penetration for ic8ib2 and EEE fuel were 3.6 mm (7.3% error) and 1.3 mm (2.32% error), respectively. The 3D spray predicted had a consistent trend to experimental data showing slight plume movement for ic8ib2 but complete spray collapsing for EEE gasoline fuel. The plume direction angle enabled by the CT data showed differences up to 2° compared to true values during the injection period. The quantitative validation results showed that the machine-learning algorithm is capable of predicting spray performance with nine input features (fuel properties and ambient conditions), and is actually superior to CFD performance for these same number of spray parameters.

Macroscopic characteristics of flash-boiling spray focused on plume interaction

In gasoline direct injection (GDI) engines, plume-plume interaction usually occurs when the superheated fluid discharges from multi-hole injectors. To mitigate plume interaction for further utilizing the merits of flash-boiling phenomena, it is essential to obtain the criteria of plume interaction and their effects on flashing spray characteristics. An 8-hole injector from Engine Combustion Network (ECN) is used to obtain the plume interaction criteria. Iso-octane, pentane, hexane, and their blends were tested, and experiments using the method of diffused back illumination (DBI) were conducted in a constant volume chamber (CVC). The new nominal parameter calculated by the pressure-enthalpy graph was introduced to analyze the spray morphologies and macroscopic characteristics. The new criteria considering fuel properties, injector geometries, and ambient pressure were obtained and validated by mono- /multi-component fuels. Based on the new criteria, spray characteristics in various stages of plume interaction are investigated. The tunning point of tip penetration and spray angles occur at the critical point defined by the new criteria in all tested fuels.

Nozzle Tip Wetting in GDI Injector at Flash-boiling Conditions

The flash-boiling spray has become a timely topic of seeking the high mixture quality and clean combustion of the gasoline-direction-injection (GDI) engines. The flash-boiling spray, which has the enlarged individual spray cone angles and enhanced plume-plume interactions, are likely to favor the fuel-film formation, otherwise known as the nozzle tip wetting. Recently, the nozzle tip wetting has caused widespread concern because it is considered to be an essential source of particle emissions in GDI engines. In this study, an experimental investigation was proposed to clarify the possible influence of flash-boiling spray on nozzle tip wetting by taking advantage of the X-ray phase-contrast imaging (XPCI) technique. Due to the high brilliance, high energy, and low emittance of X-ray, the fuel flow inside the metal nozzle and spray characteristics at the nozzle exit was observed simultaneously, and results were discussed quantitatively to understand the underlying mechanism. It is found that the post-injected fuel caused by the needle bouncing is one primary source that results in tip wetting at non-flash-boiling conditions. Rising fuel temperature at non-flash-boiling conditions would not significantly affect spray dynamics and tip wetting. At flash-boiling conditions, tip wetting rises quickly during fuel injection, and a maximum appears under transitional flash-boiling conditions. This phenomenon can be understood by the enhanced droplet collision against the nozzle wall and boosted droplet evaporation under flash-boiling.

Study on Flame Synthesis Method for Nanoparticles by Flash Boiling Spray

Study on Flame Synthesis Method for Nanoparticles by Flash Boiling Spray