Simulation and Measurement of Transient Fluid Phenomena within Diesel Injection

Rail pressures of modern diesel fuel injection systems have increased significantly over recent years, greatly improving atomisation of the main fuel injection event and air utilisation of the combustion process. Continued improvement in controlling the process of introducing fuel into the cylinder has led to focussing on fluid phenomena related to transient response. High-speed microscopy has been employed to visualise the detailed fluid dynamics around the near nozzle region of an automotive diesel fuel injector, during the opening, closing and post injection events. Complementary computational fluid dynamic (CFD) simulations have been undertaken to elucidate the interaction of the liquid and gas phases during these highly transient events, including an assessment of close-coupled injections. Microscopic imaging shows the development of a plug flow in the initial stages of injection, with rapid transition into a primary breakup regime, transitioning to a finely atomised spray and subsequent vaporisation of the fuel. During closuring of the injector the spray collapses, with evidence of swirling breakup structures together with unstable ligaments of fuel breaking into large slow-moving droplets. This leads to sub-optimal combustion in the developing flame fronts established by the earlier, more fully-developed spray. The simulation results predict these observed phenomena, including injector surface wetting as a result of large slow-moving droplets and post-injection discharge of liquid fuel. This work suggests that post-injection discharges of fuel play a part in the mechanism of the initial formation, and subsequent accumulation of deposits on the exterior surface of the injector. For multiple injections, opening events are influenced by the dynamics of the previous injection closure; these phenomena have been investigated within the simulations

COMPUTATIONAL STUDY OF INTERNAL FLOW, NEAR NOZZLE AND EXTERNAL SPRAY OF A GDI INJECTOR UNDER FLASH-BOILING CONDITIONS

The early and late portions of transient fuel injection have proven to be a rich areaof research, especially since the end of injection can cause a disproportionate amountof emissions in direct injection internal combustion engines. While simulating theinternal flow of fuel injectors, valve opening and closing events are the perennialchallenges. A typical adaptive-mesh CFD simulation is extremely computationallyexpensive, as the small gap between the needle valve and the seat requires verysmall cells to be resolved properly. Capturing complete closure usually involves atopological change in the computational domain. Furthermore, Internal CombustionEngines(ICE) operating with Gasoline Direct Injection(GDI) principle are susceptibleto flash boiling due to the volatile nature of the fuel.The presented work simulates a gasoline direct injector operating under cavitatingconditions by employing a more gradual and easily implemented model of closure that avoids spurious water-hammer effects. The results show cavitation at low valve lift forboth flash boiling and non-flash boiling conditions. Further, this study reveals post-closure dynamics that result in dribble, which is expected to contribute to unburnthydrocarbon emissions. Flashing versus non-flashing conditions are shown to causedifferent sac and nozzle behavior after needle closure. In particular, a slowly boilingsac causes spurious injection behavior.Furthermore, a qualitative analysis of the injector tip-wetting phenomena underboth flash-boiling and non-flashing conditions are conducted and different wettingmechanisms are identified. The jet expansion mechanism is observed to dominate thewetting process during the main injection period, whereas the sac conditions drive thepost-closure wetting phenomena. Additionally, the effect of flash-boiling conditionson the near-nozzle spray during the quasi-steady period of the injection cycle is explored. The exploration captured hole-to-hole variations in the rate of injection (ROI), rate of momentum (ROM), and hydraulic coefficients of injection. Moreover, it also indicates influences of the in-nozzle variations on the near-nozzle spray behaviors.Finally, a novel plume-based coupling approach is developed to couple the Eulerian near nozzle simulations with the Lagrangian spray simulations under both non-flashing and flash-boiling conditions. Predictions from the novel coupling approachare validated with the experimental observations. This coupling approach requiresrunning an Eulerian primary atomization model, i.e., the Σ −Y model, to initializethe Lagrangian parcels for the secondary atomization process. Hence, this couplingapproach does not depend upon the linearized instability models to simulate the dense spray region

Heat and mass transfer effects in the nozzle of a fuel injector from the start of needle lift to after the end of injection in the presence of fuel dribble and air entrainment

The design of fuel injectors is key to achieving high-efficiency engine combustion with low tailpipe emissions. The small dimensions of injector nozzle holes make the manufacturing of real-size optical injectors aimed at fundamental understanding of in-nozzle processes at design stage very challenging, especially for operation under realistic in-cylinder thermodynamic conditions. Therefore, faithful numerical predictions based on complete multiphase flow simulations upstream and downstream of the nozzle exit of a real injector geometry are highly sought after. In this paper, numerical studies of a Diesel injector nozzle with moving needle were performed using transient Reynolds Averaged Navier-Stokes (RANS) modelling with compressibility of all phases accounted for. A Volume of Fluid (VOF) method was employed, coupled to cavitation and evaporation submodels, along with a complete set of pressure and temperature dependent thermophysical fuel properties. The aim was to understand the flow inside the nozzle both during injection and after the end of injection, including fuel dribble and air backfilling effects. A range of fuel injection and air chamber pressures and temperatures were simulated, namely 400 and 900 bar upstream and 1, 35 and 60 bar downstream. Fuel, air and wall temperatures were varied in the range 300 K to 550 K. The results showed that the flow during injection carried hysteresis effects. After the end of injection, the state of the nozzle varied from being filled with a large amount of liquid to being filled mostly with air. Some form of immediate fuel dribble existed in all test cases, whilst late liquid fuel mass expulsion was also predicted under certain conditions. The latter prediction highlighted sensitivity to the models enabled. The use of a transient pressure outlet based on an engine's expansion stroke pressure trace affected the process of late fuel expulsion by pulling fuel out of the nozzle in multiphase form faster. These processes are of particular importance as they can contribute directly to unburned hydrocarbon emissions and/or the formation of deposits inside the holes. Starting a second injection from the resulting state of the nozzle at the end of the original injection resulted in a deformed liquid jet tip without the classic mushroom shape and a temporarily lower liquid jet penetration

Time-resolved fuel injector flow characterisation based on 3D laser Doppler vibrometry

In order to enable investigations of the fuel flow inside unmodified injectors, we have developed a new experimental approach to measure time-resolved vibration spectra of diesel nozzles using a three dimensional laser vibrometer. The technique we propose is based on the triangulation of the vibrometer and fuel pressure transducer signals, and enables the quantitative characterisation of quasi-cyclic internal flows without requiring modifications to the injector, the working fluid, or limiting the fuel injection pressure. The vibrometer, which uses the Doppler effect to measure the velocity of a vibrating object, was used to scan injector nozzle tips during the injection event. The data were processed using a discrete Fourier transform to provide time-resolved spectra for valve-closed-orifice, minisac and microsac nozzle geometries, and injection pressures ranging from 60 to 160MPa, hence offering unprecedented insight into cyclic cavitation and internal mechanical dynamic processes. A peak was consistently found in the spectrograms between 6 and 7.5kHz for all nozzles and injection pressures. Further evidence of a similar spectral peak was obtained from the fuel pressure transducer and a needle lift sensor mounted into the injector body. Evidence of propagation of the nozzle oscillations to the liquid sprays was obtained by recording high-speed videos of the near-nozzle diesel jet, and computing the fast Fourier transform for a number of pixel locations at the interface of the jets. This 6-7.5kHz frequency peak is proposed to be the natural frequency for the injector's main internal fuel line. Other spectral peaks were found between 35 and 45kHz for certain nozzle geometries, suggesting that these particular frequencies may be linked to nozzle dependent cavitation phenomena.Comment: 12 pages, 10 figure

Eulerian CFD Modeling of Multiphase Internal Injector Flow and External Sprays

The improvement of combustion systems which use sprays to atomize liquid fuel requires an understanding of that atomization process. Although the secondary break up mechanisms for the far-field of an atomizing spray have been thoroughly studied and well understood for some time, understanding the internal nozzle flow and primary atomization on which the far-field spray depends has proven to be more of a challenge. Flow through fuel injector nozzles can be highly complex and heavily influenced by factors such as turbulence, needle motion, nozzle imperfections, nozzle asymmetry, and phase change. All of this occurs within metallic injectors, making experimental characterization challenging. A review of computational studies in literature shows a trend towards engineering models based on the Eulerian description of the fluid, rather than the Lagrangian. With this approach, the internal and external flow can be simulated together. This allows for the influence of nozzle geometry on the spray to be captured. Developments and advancements applicable to these Eulerian solvers are discussed. This includes a new constraint on the turbulent mixing model, as well as the inclusion of a vaporization model and a non-equilibrium phase change model. Additionally, issues regarding thermodynamic and hydrodynamic consistency of compressible flows using a segregated solution approach are addressed, as are issues regarding dynamic mesh motion in a compressible solver. Finally, an efficient way of accounting for high pressure thermodynamic properties is presented. Applied case studies of diesel direct injection are then described. The single-field Eulerian approach is shown to perform very well at the high Reynolds and Weber numbers present in diesel DI conditions, capturing spray characteristics such as density distribution, penetration, and velocity profiles with a moderate level of accuracy. While transient needle motion is shown to cause interesting internal flow features, in converging, axisymmetric diesel DI nozzles, modeling of the internal flow is shown to only marginally benefit the solution. Finally, applied case studies of gasoline direct injection are presented, first with a parameter study varying counterbore depth, pressure drop, and the ambient to saturated pressure ratio. The significant influence of flash-boiling under a low ambient to saturated pressure ratio is shown. Next, a detailed analysis of internal nozzle and near-field flow in flashing and non-flashing multi-hole injection is presented. Excellent agreement to experimental rate of injection is achieved with transient needle motion and qualitative agreement to experimental imaging in the near-field is shown. Complex internal nozzle flow is analyzed and shown to result in string flash-boiling, perturbations and expansions of the spray angle, and oscillation in the ROI. Single-field multiphase Eulerian modeling is a useful tool for understanding and designing DI atomizers. The next generation of spray models will rely heavily on this approach to better understand and predict the influence of internal and near-field flow on the combustion system

Multiphase phenomena in Diesel fuel injection systems

Fuel Injection Equipment (FIE) are an integral component of modern Internal Combustion Engines (ICE), since they play a crucial role in the fuel atomization process and in the formation of a fuel/air combustible mixture, consequently affecting efficiency and pollutant formation. Advancements and improvements of FIE systems are determined by the complexity of the physical mechanisms taking place; the spatial scales are in the order of millimetres, flow may become locally highly supersonic, leading to very small temporal scales of microseconds or less. The operation of these devices is highly unsteady, involving moving geometries such as needle valves. Additionally, extreme pressure changes imply that many assumptions of traditional fluid mechanics, such as incompressibility, are no longer valid. Furthermore, the description of the fuel properties becomes an issue, since fuel databases are scarce or limited to pure components, whereas actual fuels are commonly hydrocarbon mixtures. Last but not least, complicated phenomena such as phase change or transition from subcritical to transcritical/supercritical state of matter further pose complications in the understanding of the operation of these devices

A numerical study on the effect of cavitation erosion in a Diesel injector

The consequences of geometry alterations in a Diesel injector caused by cavitation erosion are inves-tigated with numerical simulations. The differences in the results between the nominal design geometryand the eroded one are analyzed for the internal injector flow and spray formation. The flow in the in-jector is modeled with a 3–phase Eulerian approach using a compressible pressure–based multiphase flowsolver. Cavitation is simulated with a non–equilibrium mass transfer rate model based on the simplifiedform of the Rayleigh–Plesset equation. Slip velocity between the liquid–vapor mixture and the air isincluded in the model by solving two separate momentum conservation equations. The eroded injector isfound to result to a loss in the rate of injection but also lower cavitation volume fraction inside the nozzle.The injected sprays are then simulated with a Lagrangian method considering as initial conditions thepredicted flow characteristics at the exit of the nozzle. The obtained results show wider spray dispersionfor the eroded injector and shorter spray tip penetration

Assessment of transient effects in diesel injectors affected by fouling and cavitation erosion

An explicit density-based solver of the compressible Navier-Stokes (NS) and energy conservation equations has been developed and implemented in the open-source CFD code OpenFOAM®®; the flow solver is combined with two thermodynamic closure models for the liquid, vapor and vapor liquid equilibrium (VLE) property variation as function of pressure and temperature. The first is based on tabulated data for a 4-component Diesel fuel surrogate, derived from the Perturbed-Chain, Statistical Associating Fluid Theory (PC-SAFT) Equation of State (EoS), allowing for thermal effects to be quantified. The second thermodynamic closure is based on the widely used barotropic Equation of State (EoS) approximation between density and pressure and neglects viscous heating. The Wall Adapting Local Eddy viscosity (WALE) LES model was used to resolve sub-grid scale turbulence while a cell-based mesh deformation Arbitrary Lagrangian–Eulerian (ALE) formulation is used for modelling the injector’s needle valve movement. Numerical predictions of the fuel heating and cavitation erosion location indicators occurring during the opening and closing periods of the needle valve inside a five-hole common rail Diesel fuel injector are presented. Model predictions are found in close agreement against 0-D estimates of the temporal variation of the fuel temperature difference between the feed and hole exit during the injection period. Two mechanisms affecting the temperature distribution within the fuel injector have been revealed and quantified. The first is ought to wall friction-induced heating, which may result to local liquid temperature increase up to fuel’s boiling point while superheated vapor is formed. At the same time, liquid expansion due to the depressurisation of the injected fuel results to liquid cooling relative to the fuel’s feed temperature; this is occurring at the central part of the injection orifice. The formed spatial and temporal temperature and pressure gradients induce significant variations in the fuel density and viscosity, which in turn, affect the formed coherent vortical flow structures. It is found, in particular, that these affect the locations of cavitation formation and collapse, that may lead to erosion of the surfaces of the needle valve, sac volume and injection holes. Model predictions are compared against corresponding X-ray surface erosion images obtained from injector durability tests, showing good agreement. Further, investigation of the fuel heating, vapor amount formation and cavitation erosion location patterns occurring during the early opening period of the needle valve (from 2μm to 80μm) inside a five-hole common rail Diesel fuel injector discharging at 180MPa, 350MPa and 450MPa, are presented. These have been obtained using an explicit density-based solver of the compressible Navier-Stokes (NS) and energy conservation equations; the flow solver is combined with tabulated property data for a 4-component Diesel fuel surrogate, derived from the Perturbed-Chain, Statistical Associating Fluid Theory (PC-SAFT) Equation of State (EoS), allowing for the significant variation of the fuel’s physical and transport properties to be quantified. The Wall Adapting Local Eddy viscosity (WALE) LES model was used to resolve sub-grid scale turbulence while a cell-based mesh deformation Arbitrary Lagrangian–Eulerian (ALE) formulation is used for modelling the injector’s needle valve movement. Emphasis is placed on the temperature and vapor volume fraction evolution in needle seat passage. Friction-induced heating has been found to increase significantly with increasing pressure drop, especially at needle valve lifts from 2μm to 40μm. At the same time, liquid cooling is occurring due to fuel expansion at the areas of bulk flow away from walls; up to 25 degrees local fuel temperature drop relative to the fuel’s feed temperature are calculated. As the needle valve reaches 80 μm the fuel vapor volume, the average temperature into this flow passage and at the exit of the orifice converge to the same values for all injection pressures. The extreme injection pressures induce fuel’s jet velocity magnitude of the order of 1100 m/s, which in turn, affect the formation of coherent vortical flow structures into the nozzle’s sac volume. It is found, in particular, that the fuel jet velocity variations with increasing discharge pressure, affect the locations of cavitation formation and collapse, which in turn, lead to different potential locations of erosion of the surface of the needle valve