High-pressure Diesel Injection Research Articles

Experimental Investigation of High-Pressure Liquid Ammonia Injection under Non-Flash Boiling and Flash Boiling Conditions

Liquid ammonia is an ideal zero-carbon fuel for internal combustion engines. High-pressure injection is a key technology in organizing ammonia combustion. Characteristics of high-pressure liquid ammonia injection is lack of research. Spray behaviors are likely to change when a high-pressure diesel injector uses liquid ammonia as its fuel. This study uses high-speed imaging with a DBI method to investigate the liquid penetration, width, and spray tip velocity of high-pressure liquid ammonia injection up to 100 MPa. Non-flash and flash boiling conditions were included in the experimental conditions. Simulation was also used to evaluate the results. In non-flash boiling conditions, the Hiroyasu model provided better accuracy than the Siebers model. In flash boiling conditions, a phenomenon was found that liquid penetration and spray tip velocity were strongly suppressed in the initial stage of the injection process, this being the “spray resistance phenomenon”. The “spray resistance phenomenon” was observed when ambient pressure was below 0.7 MPa during 0–0.05 ms ASOI and was highly related to the superheated degree. The shape of near-nozzle sprays abruptly changed at 0.05 ms ASOI, indicating that strong cavitation inside the nozzle caused by needle lift effects is the key reason for the “spray resistance phenomenon”.

An analytical model of diesel injector’s needle valve eccentric motion

Past experimental studies have shown that the needle valve of high-pressure diesel injectors undergoes lateral movement and deformation, while the continuous increase in injection pressure enlarges the gap of the needle valve assembly. Two different analytical models, considering or omitting this change are presented here, linking the geometries of the needle valve assembly with the magnitude of needle valve tip lateral movement. It is found that the physical dimensions of the needle valve assembly and the injection pressure have a significant impact on the radial displacement of the needle. For example, for nominal clearances between the needle guidance and the needle valve of about 1–3 μm, the magnitude of the radial movement of the needle tip could reach tens of microns. The model that takes into account the variation of the gap between the needle guide and needle valve is found to give predictions closer to the experimental results.

Experimental Study of the Coking Phenomenon Effects on Spray Characteristics of the High Pressure Diesel Injector

Experimental Study of the Coking Phenomenon Effects on Spray Characteristics of the High Pressure Diesel Injector

Approaches for Detailed Investigations on Transient Flow and Spray Characteristics during High Pressure Fuel Injection

High pressure injection systems have essential roles in realizing highly controllable fuel injections in internal combustion engines. The primary atomization processes in the near field of the spray, and even inside the injector, determine the subsequent spray development with a considerable impact on the combustion and pollutant formation. Therefore, the processes should be understood as much as possible; for instance, to develop mathematical and numerical models. However, the experimental difficulties are extremely high, especially near the injector nozzle or inside the nozzle, due to the very small geometrical scales, the highly concentrated optical dense spray processes and the high speed and drastic transient nature of the spray. In this study, several unique and partly recently developed techniques are applied for detailed measurements on the flow inside the nozzle and the spray development very near the nozzle. As far as possible, the same three-hole injector for high pressure diesel injection is used to utilize and compare different measurement approaches. In a comprehensive section, the approach is taken to discuss the measurement results in comparison. It is possible to combine the observations within and outside the injector and to discuss the entire spray development processes for high pressure diesel sprays. This allows one to confirm theories and to provide detailed and, in parts, even quantitative data for the validation of numerical models.

An experimental study of a very high-pressure diesel injector (up to 5000 bar) by means of optical diagnostics

The aim of this work is to investigate a novel very high-pressure diesel injector under different working conditions. Furthermore, the aim of this work is also to analyse the behaviour of the evaporation, mixing and combustion of a diesel spray in a quiescent environment at very high injection pressures. In order to achieve this, the injector was first characterized from 2500 bar to 4000 bar of injection pressure. Then, an experimental matrix was designed to study the high-pressure spray under (i) cold non-evaporating, (ii) evaporating non-reactive and (iii) reactive conditions under different injection pressures up to 5000 bar. The experiments were carried out in a constant volume cell equipped with optical accesses. This allowed the application of the schlieren, mie-scattering and OH* chemiluminescence techniques at high speed to quantify the propagation of the spray through the ambient gases. The injector used was a prototype single-hole axial injector working with reference diesel as the injection fluid. The results showed a strong dependency of the spray tip penetration on the injection pressure. In some conditions creating shock waves inside the chamber that would disrupt the spray flow, causing it to mix better. Although the higher exit velocities made the fuel travel faster, resulting in longer penetrations of both phases, it also caused the fuel to evaporate faster due to slightly better mixing, resulting in similar liquid length values. This increased momentum enhanced mixing, resulting also in shorter ignition delay times. The higher velocities caused the flame base to stabilise further from the nozzle, which led to longer lift-off lengths.

Effect of injection dynamic behavior on fuel spray penetration of common-rail injector

The spray tip penetration is an important parameter to evaluate the fuel injection quality. This study presents an experimental investigation on the high-pressure diesel injection process of a common-rail injector under condition of varying pressures, particularly on the effect of the dynamic injection behavior on the fuel spray penetration. The injector delivery process is tested on a common-rail test rig, with an optical test rig to examine the diesel spray based on a constant-volume bomb (CVB) system. This process includes two stages: i.e., the opening stage and the keeping stage, while the two-stage injection momentum causes crucial influence on the fuel spray. Correspondingly, the fuel spray penetration also has two stages. In Stage I (the dynamic stage), the spray penetration sharply increases. In Stage II (stable stage), the spray penetration stably increases. When the injection pressure rises, the spray tip penetration increases, and the two-stage turning-point time is shorter. Considering this two-stage characteristic, an equation is proposed to predicate the spray tip penetration: it is based on the dynamic injection pressure for the dynamic period but the constant pressure differential for the stable period. The predicated spray tip penetration has good agreement with the test data.

First observation and characterization of vortex flow in steel micronozzles for high-pressure diesel injection

Vortex flows can be observed in many industrial systems. One of the most intricate vortex flows is the one formed in high-pressure fuel injector nozzles, which dynamics and influence on initial jet formation have been veiled due to great difficulties in observation of micro-scale fluids enclosed with steel nozzles. In this study, an X-ray tracer imaging method was used to visualize the transient fluid motion in steel micronozzles for the high-pressure diesel injection. The 1 μm-size tungsten particles were added to the diesel so that well-contrasted X-ray images of in-nozzle flows can be recorded with high-energy polychromatic X-ray beams. The X-ray images revealed, for the first time, the transient behavior of vortex flow, vortex-type cavitation and flow separation inside the steel nozzle. The needle-lift dependence of in-nozzle vortex flow dynamics was then modeled and correlated to those observed in large-scale optical nozzles in previous studies. The results showed that the in-nozzle vortex and emerging jet flow characteristics of the practical nozzle were similar with those of large-scale nozzles, which suggested that the results of previous fundamental studies can be closely linked to practical cases.

Numerical investigation into primary breakup of diesel spray with residual bubbles in the nozzle

The paper presents a three-dimensional numerical investigation of the high-pressure diesel injection with residual bubbles in the nozzle using the Multi-fluid-Quasi-VOF (volume of fluid) model based on the OpenFOAM framework. The validity of the model to simulate the near-field spray was verified. It was found that the predicted primary breakup process was in good agreement with experimental imaging. Furthermore, the evolution of the instability waves at the gas-liquid interface, the formation of ligaments and the effects of bubbles initially residual in the nozzle on the primary breakup were analyzed. The results demonstrate that drag force is one of the main reasons to trigger the instability waves and consequently enhance the primary breakup. The residual bubble initially inside the nozzle can also improve the primary breakup and the turbulent perturbation especially in the mushroom-structure region. The large residual bubble near the nozzle inlet can lead to the formation of liquid filaments at the neck of the spray plume. On the other hand, the large residual bubble near the nozzle outlet considerably promotes the fragmentation of the mushroom structure. Meanwhile, residual bubbles will impact the development of cavitation and consequently enhance instability waves.

Effect of residual air bubbles on diesel spray structure at the start of injection

Experimental and numerical analyses of the effect of residual air bubbles in a single-hole high-pressure diesel injector nozzle are presented. Detailed information on spray structures and dynamics near nozzle exit at the Start of Injection(SOI) is described. Experimental measurements are performed using a laser-based backlit imaging technique through a long distance microscope by injecting diesel fuel into a constant volume high-pressure spray chamber. Numerical investigation of, in and near-nozzle fluid dynamics is conducted in an Eulerian framework using a Volume of Fluid(VOF) interface capturing technique integrated with Large Eddy Simulation(LES) turbulence modelling. The present flow setup includes residual air bubbles remaining from a previous injection event, in-nozzle turbulence with no-slip wall conditions. Experimental images show a toroidal starting vortex near the nozzle exit suggesting a partially filled nozzle; transparency in the emerging jet demonstrates the presence of air trapped inside the nozzle liquid from the previous injection event. The numerical model provides insight into the influence of residual air bubbles on the spray morphology and dynamics of the emerging jet at the SOI. A mathematical code is developed to replicate the backlit imaging approach with the numerical results. The virtual images demonstrate a transparent liquid jet emerging into the pressurized chamber gas showing improved agreement with experimental images. The inclusion of air bubbles in the nozzle liquid prior to injection in the numerical model also yields improved agreement in the penetration velocity profile of the jet. These results explain how inclusion of residual air bubbles inside the nozzle liquid affects the physics of the penetrating jet at the SOI. The air bubbles inclusion also provides an explanation for the transparency of the emerging jet, rough interfacial surfaces, and enhanced necking behind the jet tip captured at the SOI.

Experimental and analytical study on vapor phase and liquid penetration for a high pressure diesel injector

In this study, a macroscopic characterization has been performed on a solenoid diesel injector (2200 bar-8 hole nozzle) under various non-reacting but evaporative conditions. For vapor penetration a two pass Schlieren visualization set up was selected. A high speed camera was used to record high speed images of the injection event to analyze the transient evolution of the vapor phase of the spray. The transient liquid penetration of the spray has been measured via MIE-Scattering imaging technique using a high speed camera as well. Unsteady RANS based CFD Simulations have been performed to simulate the experimental conditions and correlation results are presented. Built-in models from commercial code StarCD have been used to model spray formation which includes submodels for turbulence, nozzle flow, break-up and fuel properties. A novel CAE process using an automation and optimization tool has been used to achieve robust model settings, and the final model prediction are compared with the experimental observation for the injector characterization with respect to the non-reacting spray penetration with change in ambient and injection conditions. The model correlates well with the sensitivities for temperature and injection pressures qualitatively however improvements required to capture the density effects mainly related to the mesh orientation, fixed time step size where further analysis required.

Modelling thermal effects in cavitating high-pressure diesel sprays using an improved compressible multiphase approach

In this study, the influence of in-nozzle phenomena including flow separation, cavitation, turbulence and hydraulic flip on the morphology of the spray emerging from a convergent-divergent-convergent diesel injector is investigated numerically. Non-linear equations of state for the liquid diesel, diesel vapour and chamber gas are employed for the simulation of high pressure diesel injection and atomisation processes. A modified multiphase mixture energy equation which takes into account enthalpy of phase change due to cavitation is integrated into a previously developed compressible, multiphase Volume of Fluid Large Eddy Simulation. The mass transfer source terms are modelled using a modified Schnerr and Sauer cavitation model. The numerical method is validated by comparing simulated mass flow rates, momentum fluxes, effective injection velocities and discharge coefficients at different injection conditions against published experimental data obtained using the same injector. Favourable comparison between simulations and experimental measurements is achieved with minor discrepancies attributable to unknown experimental uncertainties and assumptions made in numerical modelling. Calculation of in-nozzle flow and primary spray breakup reveals that interfacial instabilities generated due to in-nozzle flow separation, cavitation and liquid-wall shear contribute greatly to the jet fragmentation. The increase in sensible enthalpy due to wall shear induced viscous heating together with enthalpy of condensation increase the surface temperature of the exiting jet. Comparison of the flow physics before and after the onset of hydraulic flip indicates that wall shear is one of the main mechanisms inducing most of the energy for jet breakup. This modelling shows that vapour production at nozzle entrance remains after the onset of hydraulic flip, limiting the extent of ambient air influx. In addition, the onset of hydraulic flip causes production of near nozzle shockwaves as a result of significantly increased injection velocity attributable to minimised wall shear. This aspect needs more experimental evidence and simulations to confirm and validate.

END OF INJECTION PROCESS IN A SINGLE-HOLE DIESEL INJECTOR

The end of injection processes in a single-hole high-pressure diesel injector is investigated experimentally and numerically. Experimental measurements are performed using a laser-based backlit imaging technique. Numerical investigation of in- and near-nozzle fluid dynamics is conducted in a Eulerian framework using a volume-of-fluid interface capturing technique integrated with large eddy simulation turbulence modeling. An incompressible noncavitating and a compressible cavitating model are employed to gain a clearer understanding of the nozzle air ingestion mechanism at the end of injection. In the compressible model, a basic cavitation model is allowing liquid fuel to flash to gas at the fuel vapor pressure. The results show that upon needle valve closure, the high-energy core fluid maintains outward flow but the peripheral flow has reversed, as required by continuity, thus ingesting chamber air into the nozzle. The remaining unstable flow forms an asymmetry enhancing the air ingestion. Numerical results of the incompressible noncavitating model show a single bubble of chamber gas remains embedded within the liquid in the nozzle hole only after velocities have largely dissipated. The results of the compressible cavitating model demonstrate how chamber gas is entrained into the sac volume through the air passage previously generated by hydraulic flip. These results provide an explanation of a mechanism for air ingestion at the end of injection recently described using X-ray imaging. This mechanism also provides a possible explanation for the presence of air within the emerging jet of subsequent injections.

Detached Eddy Simulation Simulation of Asymmetrical Flow in a High Pressure Diesel Injector

Recent experiments have shown that the lateral motion of a high pressure injector needle can lead to significant asymmetrical flow in the sac and asymmetric spray pattern in the combustor, which in turn degrades the combustion efficiency and results in spray hole damage. However, the underlying cause of the lateral needle motion is not understood. In this paper, we numerically studied the complex transient flow in a high pressure diesel injector using the detached eddy simulation to understand the cause of the lateral needle motion. The flow field was described by solving the compressible Navier–Stokes equations. The mass transfer between the liquid and vapor phases of the fuel was modeled using the Zwart–Gerber–Belamri equations. Our study revealed that the vortical flow structures in the sac are responsible for the lateral needle motion and the hole-to-hole flow variation. The transient motion of the vortical structure also affected vapor formation variations in spray holes. Further analysis showed that the rotational speed of the vortical flow structure is proportional to the lateral force magnitude on the lower needle surfaces.

Quantitative predictions of cavitation presence and erosion-prone locations in a high-pressure cavitation test rig

Experiments and numerical simulations of cavitating flow inside a single-orifice nozzle are presented. The orifice is part of a closed flow circuit, with diesel fuel as the working fluid, designed to replicate the main flow pattern observed in high-pressure diesel injector nozzles. The focus of the present investigation is on cavitation structures appearing inside the orifice, their interaction with turbulence and the induced material erosion. Experimental investigations include high-speed shadowgraphy visualization, X-ray micro-computed tomography (micro-CT) of time-averaged volumetric cavitation distribution inside the orifice as well as pressure and flow rate measurements. The highly transient flow features that are taking place, such as cavity shedding, collapse and vortex cavitation (also known as ‘string cavitation’), have become evident from high-speed images. Additionally, micro-CT enabled the reconstruction of the orifice surface, which provided locations of cavitation erosion sites developed after sufficient operation time. The measurements are used to validate the presented numerical model, which is based on the numerical solution of the Navier–Stokes equation, taking into account compressibility of both the liquid and liquid–vapour mixture. Phase change is accounted for with a newly developed mass transfer rate model, capable of accurately predicting the collapse of vaporous structures. Turbulence is modelled using detached eddy simulation and unsteady features such as cavitating vortices and cavity shedding are observed and discussed. The numerical results show agreement within validation uncertainty with the obtained measurements.

A numerical study of the thermal transient in high-pressure diesel injection

A study of n-dodecane spray atomization, following the prescribed unseating of the injector’s needle tip, is presented for a high-pressure, non-cavitating Bosch Diesel injector (“Spray A”, in the Engine Combustion Network denomination). In the simulations discussed here, the internal and external multiphase flows are seamlessly calculated across the injection orifice using an interface-capturing approach for the liquid fuel surface together with an embedded boundary formulation for the injector’s walls. Another novelty is the capability to model the compressibility of the liquid and the gas phase while maintaining a sharp interface between the two. This setting makes it possible to directly relate time-dependent spray characteristics to the moving internal geometry of the injector and to the exit thermodynamic state of the fuel. Using the Tait’s equation of state calibrated for n-dodecane, we examine the differences in jet atomization between adiabatic and isothermal wall conditions.

Novel insights into the dynamic structure of biodiesel and conventional fuel sprays from high-pressure diesel injectors

The spray dynamics of biodiesel has not been thoroughly investigated in previous studies. Understanding the dynamic structure is important for successful modeling of biodiesel sprays and the proper use of biodiesel in modern engines. This study compares the dynamic structure of biodiesel and conventional fuel sprays from single- and multi-hole diesel injectors using a synchrotron X-ray velocimetry technique. Three fuels, biodiesel, diesel and Viscor16br, were used in this study. The results showed that the high viscosity and density of biodiesel decreased the injection velocity compared to conventional fuels. The biodiesel effect on injection velocity was less significant for the multi-hole injector. For the single-hole injector, the biodiesel slowed down the flow breakup and increased the intact core length that caused the lower velocity decay rate and turbulence intensity along the spray center. Meanwhile, in the case of the multi-hole injector, the flow breakup, and the velocity decay rate and turbulence intensity along the spray center appeared equivalent regardless of the fuel. The fuel viscosity did not play a dominant role in the spray dynamics of the multi-hole injector and the dynamic structure of the biodiesel and conventional fuel sprays can be scaled based on the momentum conserving gas jet theory.

Evaluation of friction heating in cavitating high pressure Diesel injector nozzles

Variation of fuel properties occurring during extreme fuel pressurisation in Diesel fuel injectors relative to those under atmospheric pressure and room temperature conditions may affect significantly fuel delivery, fuel injection temperature, injector durability and thus engine performance. Indicative results of flow simulations during the full injection event of a Diesel injector are presented. In addition to the Navier-Stokes equations, the enthalpy conservation equation is considered for predicting the fuel temperature. Cavitation is simulated using an Eulerian-Lagrangian cavitation model fully coupled with the flow equations. Compressible bubble dynamics based on the R-P equation also consider thermal effects. Variable fuel properties function of the local pressure and temperature are taken from literature and correspond to a reference so-called summer Diesel fuel. Fuel pressurisation up to 3000bar pressure is considered while various wall temperature boundary conditions are tested in order to compare their effect relative to those of the fuel heating caused during the depressurisation of the fuel as it passes through the injection orifices. The results indicate formation of strong temperature gradients inside the fuel injector while heating resulting from the extreme friction may result to local temperatures above the fuel's boiling point. Predictions indicate bulk fuel temperature increase of more than 100°C during the opening phase of the needle valve. Overall, it is concluded that such effects are significant for the injector performance and should be considered in relevant simulation tools.

Planar near-nozzle velocity measurements during a single high-pressure fuel injection

In order to reduce the fuel consumption and exhaust emissions of modern Diesel engines, the high-pressure fuel injections have to be optimized. This requires continuous, time-resolved measurements of the fuel velocity distribution during multiple complete injection cycles, which can provide a deeper understanding of the injection process. However, fuel velocity measurements at high-pressure injection nozzles are a challenging task due to the high velocities of up to 300 m/s, the short injection durations in the $$\upmu\text{s}$$ range and the high fuel droplet density especially near the nozzle exit. In order to solve these challenges, a fast imaging Doppler global velocimeter with laser frequency modulation (2D-FM-DGV) incorporating a high-speed camera is presented. As a result, continuous planar velocity field measurements are performed with a measurement rate of 200 kHz in the near-nozzle region of a high-pressure Diesel injection. The injection system is operated under atmospheric surrounding conditions with injection pressures up to 1400 bar thereby reaching fuel velocities up to 380 m/s. The measurements over multiple entire injection cycles resolved the spatio-temporal fluctuations of the fuel velocity, which occur especially for low injection pressures. Furthermore, a sudden setback of the velocity at the beginning of the injection is identified for various injection pressures. In conclusion, the fast measurement system enables the investigation of the complete temporal behavior of single injection cycles or a series of it. Since this eliminates the necessity of phase-locked measurements, the proposed measurement approach provides new insights for the analysis of high-pressure injections regarding unsteady phenomena.

Twin-jet sprays to improve high-pressure diesel injection systems

Twin-jet sprays to improve high-pressure diesel injection systems

Transient heating effects in high pressure Diesel injector nozzles

The tendency of today’s fuel injection systems to reach injection pressures up to 3000bar in order to meet forthcoming emission regulations may significantly increase liquid temperatures due to friction heating; this paper identifies numerically the importance of fuel pressurization, phase-change due to cavitation, wall heat transfer and needle valve motion on the fluid heating induced in high pressure Diesel fuel injectors. These parameters affect the nozzle discharge coefficient (Cd), fuel exit temperature, cavitation volume fraction and temperature distribution within the nozzle. Variable fuel properties, being a function of the local pressure and temperature are found necessary in order to simulate accurately the effects of depressurization and heating induced by friction forces. Comparison of CFD predictions against a 0-D thermodynamic model, indicates that although the mean exit temperature increase relative to the initial fuel temperature is proportional to (1−Cd2) at fixed needle positions, it can significantly deviate from this value when the motion of the needle valve, controlling the opening and closing of the injection process, is taken into consideration. Increasing the inlet pressure from 2000bar, which is the pressure utilized in today’s fuel systems to 3000bar, results to significantly increased fluid temperatures above the boiling point of the Diesel fuel components and therefore regions of potential heterogeneous fuel boiling are identified.