In-nozzle Flow Research Articles

Effects of cavitation flow and stagnant bubbles on the initial temporal evolution of diesel spray

This study focuses on the in-nozzle flow characteristics and the primary spray breakup of diesel at the near-nozzle region under atmospheric conditions. A long working distance microscope and a Canon EOS 700D CCD Camera were employed to study the cavitation flow in the nozzle and the spray structure at near-nozzle region via photographic technique with backlighting. Images were obtained with short exposure time and sufficiently high spatial and temporal resolutions which allowed a detailed observation of the early phase of fuel injection from the nozzle orifice to near nozzle region. The images revealed that the formation of a variety of complex two-phase flow regimes. Stagnant bubbles were present inside the nozzle at the start of injection. The stagnant bubbles may have originated from the last injection because they have not been evacuated from the nozzle or sucked into the nozzle orifice as the needle valve opened. During the opening and closing stages of the needle valve, the cavitation occurred in the valve seat area. Subsequently, free cavitation was observed at the inlet orifice followed by film cavitation, cloud cavitation and string cavitation in the orifice. A dimensionless number S derived from the images of transparent nozzles was adopted to evaluate the cavitation intensity, which increased monotonically with the increase of injection pressure. In addition, mushroom shaped spray head was also found at the nozzle exit. A close inspection revealed that the stagnant bubbles or the fuel trapped in the injector orifice at the end of injection had an appreciable impact on the near nozzle spray structures. The spray angle, penetration length and the spray area also increased for every 10MPa rise by about 20%, 10% and 15% respectively.

Numerical analysis of deposit effect on nozzle flow and spray characteristics of GDI injectors

Injector deposit is a common phenomenon for gasoline direct ignition (GDI) engines that greatly affects the spray behavior and consequently the combustion performance and emissions. In this study, the effect of deposit on both the in-nozzle flow dynamics and downstream spray behaviors was numerically investigated. High-resolution X-ray microtomographic scans were performed first to obtain nozzle and deposit morphologies and topology. In-nozzle simulation was then carried out in the Euler-Euler framework with cavitation taken into account by a homogeneous relaxation model (HRM). Finally, the effect of deposit on spray behaviors was evaluated in the Euler-Lagrangian framework, coupled with the in-nozzle simulation results. Results of the nozzle flow simulations highlight that the rough surface of the deposits leads to additional cavitation inception and restricts the flow area, causing mass flow rate losses. Deposits inside the counterbore act as an extension to the inner orifice and constrain the air recirculation. Turbulent levels at the exit of the counterbore are lower for the coked injector due to the reduced air/fuel interaction. Spray simulations have shown that deposits would lead to longer spray penetration, a smaller spray cone angle and larger droplets diameters. Simulation results agree reasonably well with the available experimental data.

Effects of nozzle configuration on internal flow and primary jet breakup of flash boiling fuel sprays

Fuel spray plays more vital role in governing fuel economy and emission quality of IC engines due to the wide application of direct injection system in automotive engines. High temperature fuel spray, namely flash boiling spray, has been documented to improve fuel atomization and evaporation with less energy consumption compared with high pressure fuel spray. It is because the hot exhaust gas and coolant are two potential energy sources can be used to heat the fuel. Most of existing research works of flash boiling spray focus on investigating the spray behavior outside the nozzle, and limited studies can be found about the in-nozzle superheated flow. In this study, a two-dimensional transparent slit nozzle was designed to reveal the in-nozzle flow under various superheated conditions. Both internal flow and near-nozzle fuel jet were investigated using high-speed backlit imaging technique to acquire a better understanding of the primary breakup process of flash boiling sprays. The slit thickness was 40μm with well-adjusted nozzle length and inlet corner radius to investigate the effects of nozzle configuration. The ambient pressure ranged from 40kPa to 190kPa, and fuel temperature varied from 41°C to 71°C, which produced a wide range of superheated conditions. N-pentane was chosen as test fluid with an injection pressure of 0.6MPa. Experimental results showed that the bubbles occurring inside the nozzle near the nozzle exit affected the near-nozzle fuel jet characteristics. Stronger superheated conditions resulted in larger in-nozzle bubbles, which facilitated the jet break-up process significantly. Longer nozzle and nozzle with sharp inlet corner led to larger inner bubble volume fraction and narrower liquid core of near-nozzle fuel jet. In summary, the effects of nozzle configuration on in-nozzle flow and near-nozzle fuel jet were revealed with more fundamental understanding of the primary breakup process of flash boiling sprays.

Numerical study of the internal flow and initial mixing of diesel injector nozzles with V-type intersecting holes

A V-type intersecting hole nozzle, in which each hole is formed by the coalescence of a pair of sub-holes, has been developed to improve fuel-air mixing for diesel engines. In this paper, a numerical study has been conducted to examine the effects of the V-type intersecting hole structure on the internal flow of a nozzle and the initial stage of the fuel-air mixing processes. With this aim, a multi-phase flow calculation has been implemented on seven V-type intersecting hole nozzles with impact angles ranging from 20° to 50° and a referenced cylindrical hole nozzle under injection pressures varying from 60MPa to 240MPa. The comparison was made in terms of mass flow, momentum flux, effective velocity, discharge coefficient, area coefficient and velocity coefficient. The three-fluid model, which was validated using X-ray experimental data of an Engine Combustion Network (ECN) Spray A injector, was employed to calculate the associated multi-phase flow. The main results show that use of a V-type intersecting hole in a nozzle eliminates cavitation, leading to higher mass flow and momentum flux. Correspondingly, for these V-type intersecting hole nozzles, the discharge coefficients are insensitive to injection pressure, but decrease with an increasing impact angle, and are 20–30% higher than those of the cylindrical hole nozzle. These higher discharge coefficients mainly result from very high area coefficients that are approximately 0.98 at all injection pressure conditions because of the non-cavitating nature of the in-nozzle flow. Moreover, the impact effect of a V-type intersecting hole results in a fan-shaped jet with a very large spreading angle (25–40°) at the dispersion plane and a relatively narrower angle (approximately 12°) at the impact plane. The jet spreading angles at both planes are wider than that of the cylindrical hole nozzle (approximately 7°), indicating a noticeable improvement in the initial fuel-air mixing. Additionally, an increase in the impact angle of a V-type intersecting hole nozzle promotes initial mixing in terms of yielding a wider jet spreading angle, in spite of a slightly lower effective velocity.

Development of a compressible multiphase cavitation approach for diesel spray modelling

The influence of in-nozzle phenomena including cavitation on the morphology of the spray from a diesel injector with a sharp nozzle inlet is investigated numerically. A compressible, multi-phase Volume of Fluid Large Eddy Simulation is implemented in the OpenFOAM environment. The volume fraction transport equations for liquid and gas phases are reformulated to include mass transfer source terms. These source terms are modelled with two cavitation models by Schnerr and Kunz, which are extended to eliminate non-physical mass transfer rates. Validation is carried out only for the Schnerr cavitation model due to its independence of empirical parameters. The numerical method is validated by comparing the simulated mass flow rates, pressure and velocity profiles at different cavitation conditions against published experimental data obtained using a slightly converging square channel. Favourable comparison between simulations and experiments is achieved with minor discrepancies attributable to uncertainties in fuel properties, experimental artefacts and assumptions made in numerical models. Application of the method to calculation of in-nozzle phenomena and primary breakup of a diesel spray reveals that in-nozzle flow separation, wall shear and cavitation contribute greatly to the fragmentation of the jet. Comparison of the two cavitation models shows that after the onset of complete flow detachment, the Kunz implementation predicts higher air inflow at the nozzle outlet than the Schnerr model.

Cavitation Flow Visualization in Marine Diesel Injectors

Injector geometries of large marine two-stroke diesel engines differ extensively from configurations typically used in diesel engines for heavy duty and automotive applications. Fuel is usually injected into the cylinder using multiple injectors that are arranged around the cylinder wall. As a result, the injector orifices are asymmetrically arranged as all the bores face a similar direction. Due to this geometric setup, the orifices are also distributed eccentric with respect to the central bore of the injector. Experiments have shown that sprays from such orifices propagate nonsymmetrical to the nominal axis of the orifice. Those spray deviations can lead to wall wetting which increases fuel consumption, emissions, component temperatures and loss of lubrication film on the cylinder wall. In addition, preliminary work using CFD simulations with an advanced cavitation model indicates that the two-phase, highly compressible flow inside the nozzle may strongly influence the spray propagation. To further investigate the in-nozzle flow and how it affects the spray morphology for large marine two-stroke diesel engine injectors, experiments were performed using transparent nozzles made of PMMA. The transparent nozzles were mounted on a marine diesel injector which was operated under ambient conditions and run with diesel at a pressure of 50 MPa. The nozzle used was an orthogonally arranged 0.75 mm diameter, one-hole setup that matches large marine two-stroke diesel engines injector nozzle diameters. High-speed shadowgraphy using a far-field microscope was applied to visualize the cavitation in the nozzle during the quasi-steady state injection process. These imaging results are used to compute statistical evaluations of cavitation in the nozzle over a range of conditions.

In-nozzle flow investigation of flash boiling fuel sprays

Abstract It is widely recognized that a well atomized spray promotes the fuel evaporation in engine cylinder and contributes to higher fuel efficiency and lower exhaust emission. Specifically, higher fuel temperature enhances the fuel spray evaporation due to the superheat phenomenon even at low injection pressure. In this study, both internal flow and near nozzle fuel jet of a two-dimensional transparent nozzle were investigated using high-speed backlit imaging technique to acquire a better understanding of the primary breakup process of flash boiling sprays. The two-dimensional slit transparent nozzle was designed to facilitate the enhanced visualization of bubbles inside the nozzle by squeezing small bubbles and eliminating overlapping bubbles. The ambient pressure ranged from 40 kPa to 190 kPa, and fuel temperature varied from 41 °C to 71 °C, to produce a wide range of superheated conditions. N-pentane was chosen as test fluid with an injection pressure was 0.5 MPa. Experimental results show that the bubbles occurring inside the nozzle near the nozzle exit under flash boiling conditions affect the shape and dynamic behavior of near nozzle fuel jet. It shows that higher fuel temperature and lower ambient pressure increase the inner bubble size and volume fraction, which led to narrower liquid core of near nozzle fuel jet. Inner bubbles facilitate the breakup process of fuel jet significantly. The breakup process of flash boiling spray can be elucidated with more fundamental understanding of bubble behavior inside the nozzle.

Large eddy simulation of a turbulent spray jet generated by high-pressure injection: impact of the in-nozzle flow

ABSTRACTThe turbulence properties of a gas spray jet generated through the injection of a high-pressure atomising spray were studied in the context of the Euler–Lagrange formulation using a large eddy simulation (LES) approach. The study's main aim was to investigate nozzle flow effects on flow and turbulence statistics for a two-phase jet flow in the near and far fields of the spray. The study investigated the injection of an existing in-nozzle flow at the spray inlet. Simulation results were compared with measurements obtained for non-evaporating sprays under quasi-steady conditions, which, in practice, correspond to a long-duration injection. Both simulated high-injection pressure cases showed a good agreement with experimental data. A two-stage evolution of the spray was observed. The flow of the gas jet first developed along the spray cone, followed by the appearance of highly vortical structures around and within the spray. Cavitation in the nozzle flow resulted in an observed asymmetry of the generated gas spray jet: on the gas spray jet side of the nozzle (relative to the cavitation side), stronger jet spreading occurred, and droplets were smaller and slower. The spray jet typically exhibited an ellipsoidal cross-sectional shape. The paper provides a characterisation of the asymmetry that persisted for the duration of the spray evolution (at least for the distances considered here).

Numerical Modelling of the In-Nozzle Flow of a Diesel Injector with Moving Needle during and after the End of a Full Injection Event

Numerical Modelling of the In-Nozzle Flow of a Diesel Injector with Moving Needle during and after the End of a Full Injection Event

In-nozzle flow and spray characteristics for mineral diesel, Karanja, and Jatropha biodiesels

Abstract Superior spray behavior of fuels in internal combustion engines lead to improved combustion and emission characteristics therefore it is necessary to investigate fuel spray behavior of new alternative fuels. This study discusses the evolution of the in-nozzle orifice parameters of a numerical simulation and the evolution of spray parameters of fuel spray in a constant-volume spray chamber during an experiment. This study compares mineral diesel, biodiesels (Karanja-and Jatropha-based), and their blends with mineral diesel. The results show that mineral diesel provides superior atomization and evaporation behavior compared to the biodiesel test fuels. Karanja biodiesel provides superior atomization and evaporation characteristics compared to Jatropha biodiesel. The qualitative comparison of simulation and experimental results in tandem shows that nozzle-hole design is a critical parameter for obtaining optimum spray behavior in the engine combustion chamber.

An Eulerian CFD model and X-ray radiography for coupled nozzle flow and spray in internal combustion engines

This paper implements a coupled approach to integrate the internal nozzle flow and the ensuing fuel spray using a Volume-of-Fluid (VOF) method in the finite-volume framework. A VOF method is used to model the internal nozzle two-phase flow with a cavitation description closed by the homogeneous relaxation model of Bilicki and Kestin (1990). An Eulerian single velocity field approach by Vallet et al. (2001) is implemented for near-nozzle spray modeling. This Eulerian approach considers the liquid and gas phases as a complex mixture with a highly variable density to describe near nozzle dense sprays. The liquid mass fraction is transported with a model for the turbulent liquid diffusion flux into the gas. Fully-coupled nozzle flow and spray simulations are performed in three dimensions and validated against the X-ray radiography measurements of Kastengren et al. (2014) for a diesel fuel surrogate. A standard k–∊ Reynolds Averaged Navier Stokes based turbulence model is used in this study and the influence of model constants is evaluated. First, the grid convergence study is performed. The effect of grid size is also evaluated by comparing the fuel distribution against experimental data. Finally, the fuel distribution predicted by the coupled Eulerian approach is compared against that by Lagrangian–Eulerian spray model along with experimental data. The coupled Eulerian approach provides a unique way of coupling the nozzle flow and sprays so that the effects of in-nozzle flow can be directly realized on the fuel spray. Both experiment and numerical simulations show non-cavitation occurring for this injector with convergent nozzle geometry. The study shows that the Eulerian approach has advantages over near-field dense spray distributions.

Development of a real-size optical injector nozzle for studies of cavitation, spray formation and flash-boiling at conditions relevant to direct-injection spark-ignition engines

High-pressure multi-hole injectors for direct-injection spark-ignition engines have shown enhanced fuel atomisation and flexibility in fuel targeting by selection of the number and angle of the nozzle holes. The nozzle internal flow is known to influence the characteristics of spray formation; hence, understanding its mechanisms is essential for improving mixture preparation. However, currently, no data exist for fuel temperatures representative of real engine operation, especially at low-load high-temperature conditions with early injection strategies that can lead to phase change due to fuel flash-boiling upon injection. This challenge is further complicated by the predicted fuel stocks, which may include new (e.g. bio-derived) components. The physical/chemical properties of such components can differ markedly from gasoline, and it is important to have the capability to study their effects on in-nozzle flow and spray formation, taking under consideration their different chemical compatibilities with optical materials as well. The current article presents the design and development of a real-size quartz optical nozzle, 200 µm in diameter, suitable for high-temperature applications and also compatible with new fuels such as alcohols. First, the internal geometry of a typical real multi-hole injector was analysed by electron microscopy. Mass flow was measured, and relevant fluid mechanics dimensionless parameters were derived. Laser and mechanical drilling of the quartz nozzle holes were compared. Abrasive flow machining of the optical nozzles was also performed and analysed by microscopy in comparison to the real injector. Initial validation results with a high-speed camera showed successful imaging of microscopic in-nozzle flow and cavitation phenomena, coupled to downstream spray formation, under a variety of conditions including high fuel temperature flash-boiling effects. The current work used gasoline and iso-octane to provide proof-of-concept images of the optical nozzle, and future work will include testing of a range of fuels, some of which will also be bio-derived.

Experimental and numerical momentum flux evaluation of high pressure Diesel spray

In the present paper, a detailed numerical and experimental investigation of high pressure Diesel spray evolution is reported. The analysis is mainly focused on the spray momentum flux measurement, which is increasingly used to characterise both the in-nozzle flow in terms of cavitation intensity and the flow uniformity among the different jets emerging from the same nozzle. In the present study, the potentially critical aspects of the spray momentum flux measurement methodology based on the impact force on a flat target have been investigated comparing CFD-3D results and experimental data pertaining to a wide range of operating conditions in terms of injection pressure, energizing time and back-pressure. The available experimental data, pertaining to a commercial common-rail injector, include the spray momentum flux time-histories and steady values, the injection rate and the spray imaging during momentum flux tests and free-jet evolution. Globally, a good agreement was found among experimental and numerical momentum flux histories; furthermore, CFD allowed to evaluate the significant contribution of the spray gaseous phase to the measured momentum flux, which rapidly tends to become predominant as the measurement station distance from the nozzle exit increases. The obtained results also suggest that some details of the measurement system can significantly affect the spray momentum detection accuracy. Hence, a detailed numerical analysis of the spray impact force was developed to evaluate the target intrusiveness, pointing out the irrelevance in terms of momentum flux time-history of the target presence into the simulated domain with respect to the free jet evolution. A practical strategy to define the optimal combination of target-diameter and nozzle–target distance, for each set of operating conditions, as a compromise between the extreme configurations of the involved parameters, is discussed.

Analysis of Diesel Spray Momentum Flux Spatial Distribution

In the present paper the results of an experimental and numerical analysis of a common -rail, high pressure Diesel spray evolving in high counter pressure conditions is reported. The experimental study was carried out mainly in terms of spray momentum flux indirect measurement by the spray impact method; the measurement of the impact force time-histories, along with the CFD analysis of the same phenomenon, gave interesting insight in the internal spray structure. As well known, the overall spray structure momentum flux along with the injection rate measurements can be used to derive significant details about the in-nozzle flow and cavitation phenomena intensity. The same global spray momentum and momentum flux measurement can be useful in determining the jet -tojet ununiformities also in transient, engine-typical injection conditions which can assist in the matching process between the injection system and the combustion chamber design. In the present paper, the potential in the internal spray structure analysis by means of local spray momentum flux distribution investigation is evaluated. To obtain such momentum flux distribution maps, a variant in the global spray momentum measurement technique is evaluated. The use of an adaptor to the momentum flux measurement rig is discussed, numerically analyzed and tested on a dedicated experimental bench. Some results pertaining to solenoid-actuated common rail injectors are presented and discussed in terms of spray distribution irregularities, momentum profiles and differences in the spray structure obtained for the different jets emerging from the nozzle.

Highly-resolved LES and PIV Analysis of Isothermal Turbulent Opposed Jets for Combustion Applications

Turbulent opposed jet (TOJ) burners are an interesting test case for fundamental combustion research and a good benchmark for the available modelling approaches. However, these opposed jet flames strongly depend on the turbulence generation inside the nozzle, which is usually achieved through a perforated plate upstream of the nozzle exit. The present work investigates the flow from these perforated plates and the subsequent turbulence generation in great detail. We present results from highly-resolved large eddy simulations (LES) of the in-nozzle flow in turbulent opposed jets alongside state-of-the-art particle image velocimetry (PIV) at standard and high repetition rates taken inside a glass nozzle. The in-nozzle PIV data provides the LES inflow conditions with unprecedented detail, which are used to follow the initial jet development behaviour known from PIV, before jet coalescence, turbulence production and decay further downstream in the nozzles are successfully predicted. In regions where the PIV experiment suffers from inherent limitations like reflections and the velocity bias, the LES data is available to still obtain a detailed picture of the flow. The sensitivity of the simulations to various physical and numerical parameters is discussed in detail. Results from LES and PIV are compared qualitatively and quantitatively in terms of first and second moments of velocity, temporal autocorrelations, and energy density spectra. Significant deviations are found in the frequency (20%) and strength of vortex shedding from the inlet plane only, whereas the qualitative and quantitative agreement between simulation and experiment is otherwise excellent throughout, implying that a successful large eddy simulation of a turbulent opposed jet can be performed in a domain that includes the perforated plates.

Effect of fuel temperature on in-nozzle cavitation and spray formation of liquid hydrocarbons and alcohols from a real-size optical injector for direct-injection spark-ignition engines

High-pressure multi-hole injectors for direct-injection spark-ignition engines offer some great benefits in terms of fuel atomisation, as well as flexibility in fuel targeting by selection of the number and angle of the nozzle holes. The flow through the internal passages of injectors is known to influence the characteristics of spray formation. In particular, understanding how in-nozzle cavitation phenomena can be used to improve atomisation is essential for improving mixture preparation quality under homogeneous or stratified engine operating conditions. However, no data exist for injector body temperatures representative of real engine operation, especially at low-load conditions with early injection strategies that can also lead to phase change due to fuel flash-boiling upon injection. This challenge is further complicated by the predicted fuel stocks which will include a significant bio-derived component presenting the requirement to manage fuel flexibility. The physical/chemical properties of bio-components, like various types of alcohols, can differ markedly from gasoline and it is important to study their effects. This work outlines results from an experimental imaging investigation into the effects of fuel properties, temperature and pressure conditions on the extent of cavitation, flash-boiling and, subsequently, spray formation. This was achieved by the use of real-size transparent nozzles, replica of an injector from a modern direct-injection spark-ignition combustion system. Gasoline, iso-octane, n-pentane, ethanol and butanol were used at 20, 50 and 90 °C injector body temperatures for ambient pressures of 0.5 bar and 1.0 bar in order to simulate early homogeneous injection strategies for part-load and wide open throttle engine operation. The fuel matrix also included a blend of 10% ethanol with 90% gasoline (E10) because the vapour pressure of E10 is higher than the vapour pressure of either ethanol or gasoline and the distillation curve of E10 reflects strongly this effect. Therefore, the distillation curves of the fuels, the vapour pressures, as well as density, viscosity and surface tension were obtained and the Reynolds, Weber, Ohnesorge and Cavitation numbers were considered in the analysis. The in-nozzle flow regime and spray formation was found to be sensitive to the fuel temperature and gas pressure as a result of the vapour pressure and temperature relationships.