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Vortex flow realized inside the sac volume and the injection holes of automotive and heavy duty injectors plays an important role in the formation and development of cavitation and the near-nozzle structure of the emerging fuel sprays. Large-scale vortical flow structures are mainly induced by the geometric details of the injector. Vortex flow may be also induced by eccentric needle opening as well as the manufacturing tolerances of locations critical to the nozzle geometry such as the hole entry shape. The present paper assesses the predictive capability of a Large Eddy Simulation model against LDV measurements of the flow velocity obtained inside a transparent nozzle replica. Model predictions are compared also with RANS model predictions obtained using the standard k-ε model.

High pressurization of Diesel fuel in modern common-rail injectors, in addition to its effect on spray atomization, can result to increase of fuel density and viscosity in comparison to atmospheric conditions; moreover, due to the sharp de-pressurization experienced by the fuel at the inlet of the injection holes significant gradients of the above properties are established. Consequently, the characteristics of cavitation taking place at the entrance to the injection holes are affected. The present study quantifies the role of these effects in automotive Diesel injectors operating at pressures in excess of 1500 bar through use of a cavitation CFD model. The flow solver is accordingly modified to account for such effects during the solution of the conservation equations. Two different injector designs have been considered, both based on the same sac-type nozzle body; one with sharp-inlet cylindrical holes and one with tapered holes with inlet rounding.

The internal flow in Diesel injector nozzles significantly affects the spray formation, atomisation and air/fuel mixing rates. A multi-dimensional model has been developed to numerically predict the spray evolution patterns with particular focus on capturing the influence of the injector nozzle flow on the near-nozzle spray dispersion. The link to the internal flow is established by using as initial conditions for the injected fuel, the transiently and spatially resolved distribution of the flow field at the nozzle hole exit plane as calculated from a multi-dimensional and multi-phase nozzle flow simulation model. The local spray dispersion angle is estimated by assuming that the disintegration of the liquid jet is function of the distribution of liquid velocity, cavitation vapour volume fraction and liquid turbulence level at the exit of the injection hole.

The predictive capability of Lagrangian and Eulerian multi-dimensional computational fluid dynamics models accounting for the onset and development of cavitation inside Diesel nozzle holes is assessed against experimental data. These include cavitation images available from a real-size six-hole mini-sac nozzle incorporating a transparent window as well as high-speed/CCD images and LDV measurements of the liquid velocity inside an identical large-scale fully transparent nozzle replica. Results are available for different cavitation numbers, which correspond to different cavitation regimes forming inside the injection hole. Discharge coefficient measurements for various real-size nozzles operating under realistic injection pressures are also compared and match well with models' predictions.

Cavitation formation and development inside Diesel injector nozzles suffering from erosion damage has been investigated using enlarged transparent nozzle replicas and computational fluid dynamics (CFD) simulations. Cavitation erosion has been observed at different locations within the nozzle. These have included the top surface inside the nozzle hole next to its entry, the 3o'clock and 9c'clock hole side-inlets as well as at the needle seat area. Instantaneous and time-averaged high-speed CCD images of cavitation have verified that cavitation erosion sites are found in areas of cavitation bubble collapse. This has been further supported by CFD predictions obtained using the measured injection pressure and needle lift traces, both for the pilot and main injection events. The cavitating flow regimes associated with these erosion sites correspond to geometrically-induced hole cavitation, the string cavitation and the needle seat cavitation, respectively.

The performance of multi-hole injectors designed for use in second-generation direct-injection gasoline engines has been characterised in a constant-volume chamber. Two types of multi-hole injector have been used: the first has 11 holes, with one hole on the axis of the injector and the rest around the axis at 30 degrees apart, and the second has 6 asymmetric holes located around the nozzle axis. Measurements of droplet axial and radial velocity components and their diameter were obtained using a 2-D phase Doppler anemometer (PDA) at injection pressures up to 120 bar, chamber pressures from atmospheric to 8 bar, and ambient temperatures. Complementary spray visualisation made use of a pulsed light and a CCD camera synchronised with the injection process.

Cavitation formation and development inside various types of nozzles for close-spacing spray-guided fuel injection systems is predicted using a computational fluid dynamics cavitation model. The fuel injection systems investigated include generic geometries of multi-hole nozzles and outwards opening pintle injectors. Model validation is performed against experimental data reported elsewhere in large-scale transparent nozzle replicas. The results confirm that cavitation strongly depends on the geometry of the nozzle and the operating conditions. For multi-hole nozzles, cavitation structures similar to those realised in Diesel injectors are formed. These include the needle seat cavitation realised at low needle lifts, the geometrically-induced hole entry cavitation and string cavitation developing inside the sac volume. A more chaotic and less understood cavitation pattern develops at the sealing area of inward seal band outwards opening nozzles.

A dense-particle Eulerian-Lagrangian stochastic methodology, able to resolve the dense spray formed at the nozzle exit has been applied to the simulation of evaporating diesel sprays. Local grid refinement at the area where the spray evolves allows use of cells having sizes from 0.6 down to 0.075mm. Mass, momentum and energy source terms between the two phases are spatially distributed to cells found within a distance from the droplet centre; this has allowed for grid-independent interaction between the Eulerian and the Lagrangian phases to be reached. Additionally, various models simulating the physical processes taking place during the development of sprays are considered. The cavitating nozzle flow is used to estimate the injection velocity of the liquid while its effect on the spray formation is considered through an atomisation model predicting the initial droplet size.

The cavitating flow characteristics inside multi-hole injectors for large Diesel engines have been investigated. Flow imaging obtained inside transparent nozzle replicas using simultaneously two high speed cameras and various illumination light sources has shown that cavitation is formed not only at the hole entrance due to the local pressure drop induced by nozzle inlet geometry, but also inside the volume of nozzle tip just upstream of the injection holes. CFD calculations of the internal nozzle flow have shown that cavitation strings are formed in the areas where large vortical structures are present. Processing of the acquired images has allowed estimation of the mean location and probability of appearance of the cavitating strings in the 3-D space. Parametric studies have also revealed the effect of the needle lift, cavitation number and Reynolds number.

A mathematical methodology is proposed for the computational inverse design of nozzle shapes producing controlled geometric cavitation. The proposed methodology uses an unstructured RANS flow solver, with the ability to compute sensitivity derivatives via an Adjoint algorithm and independently of the shape parameterisation. The method is used to develop and evaluate conceptual shapes for nozzle hole cavitation reduction. The localised region at the hole inlet where geometric-cavitation is produced, is parameterised using its radius of curvature. The parameterisation method is an empirical curvature fit method suitable for the design and manufacturing of such nozzles. In order to validate the efficiency of the proposed method, the optimisation problem is handled as an inverse design one. The objective function is formed using a target pressure distribution where the negative pressure area is narrowed or even eliminated.

The effect of multiple-injection strategy on nozzle hole cavitation has been investigated both experimentally and numerically. A common-rail Diesel injection system, used by Toyota in passenger car engines, has been employed together with a double-shutter CCD camera in order to visualise cavitation inside a submerged and optically accessible (in one out of the six holes) real-size VCO nozzle. Initially the cavitation development was investigated in single injection events followed by flow images obtained during multiple injections consisting of a pilot and a main injection pulse. In order to identify the effect of pilot injection on cavitation development during the main injection, the dwell time between the injection events was varied between 1.5-5ms for different pilot injection quantities. The extensive test matrix included injection pressures of 400 and 800bar and back pressures ranging from 2.4 up to 41bar.

The initiation and development of cavitation in enlarged transparent acrylic models of six-hole nozzles for direct injection Diesel engines has been visualised by a high-speed digital video camera in a purpose-built refractive index matching test rig. The obtained high temporal resolution images have allowed improved understanding of the origin of the cavitation structures in Diesel injector nozzles and clarification of the effect of sac geometry (conical mini-sac vs. VCO) on cavitation initiation and development in the nozzle holes. The link between cavitation and flow turbulence in the sac volume and, more importantly, in the injection holes has been quantified through measurements of the flow by laser Doppler velocimetry (LDV) at a number of planes as a function of the Reynolds and cavitation numbers.

The numerical methodology used to predict the flow inside pressure-swirl atomizers used with gasoline direct injection engines and the subsequent spray development is presented. Validation of the two-phase CFD models used takes place against film thickness measurements obtained from high resolution CCD-based images taken inside the discharge hole of a pressure swirl atomizer modified to incorporate a transparent hole extension. The transient evolution of the film thickness and its mean axial and swirl velocity components as it emerges from the nozzle hole is then used as input to a spray CFD model predicting the development of both non-evaporating and evaporating sprays under a variety of back pressure and temperature conditions. Model predictions are compared with phase Doppler anemometry measurements of the temporal and spatial variation of the droplet size and velocity as well as CCD spray images.

A comprehensive set of computational and experimental results for high- pressure diesel sprays are presented and discussed. The test cases investigated include injection of diesel into air under both atmospheric and high pressure/temperature chamber conditions, injection against pressurized and cross-flowing CF6 simulating respectively the density and flow conditions of a diesel engine at the time of injection, as well as injection into the piston bowl of both research and production turbocharged high-speed DI diesel engines. A variety of high-pressure injection systems and injector nozzles have been used including mechanical and electronic high-pressure pumps as well as common-rail systems connected to nozzles incorporating a varying number of holes with diameters ranging from conventional to micro-size.

A combined two-phase CFD nozzle model and 1-D fuel injection system model is used to predict the flow development inside the discharge hole of a pressure-swirl atomizer connected to a common-rail based fuel injection system for DISI engines. The fuel injection model accounts for the transient pressure pulses developing inside the common-rail and the injector upstream of the nozzle tip and predicts the fuel injection rate through the nozzle. This is then used as input to a 3-D single-phase CFD model estimating the transient development of the swirl velocity inside the pressure-swirl atomizer, as a function of the geometric characteristics of nozzle.

A production six-hole conical sac-type nozzle incorporating a quartz window in one of the injection holes has been used in order to visualize the flow under cavitating flow conditions. Simultaneous variation of both the injection and the back chamber pressures allowed images to be obtained at various cavitation and Reynolds numbers for two different fixed needle lifts corresponding to the first- and the second-stage lift of two-stage injectors. The flow visualization system was based on a fast and high resolution CCD camera equipped with high magnification lenses which allowed details of the various flow regimes formed inside the injection hole to be identified. From the obtained images both hole cavitation initiated at the top inlet corner of the hole as well as string cavitation formed inside the sac volume and entering into the hole from the bottom corner, were identified to occur at different cavitation and Reynolds numbers.

A one-dimensional, transient and compressible flow model was used in order to simulate the flow and pressure distribution in advanced high-pressure fuel injection systems; these include electronic distributor-type pumps with either axial or radial plungers and a common-rail system. Experimental data for the line pressure, needle lift, injection rate and total fuel injection quantity obtained over a wide range of operating conditions (from idle to high speed/full load) were used to validate the model. The FIE system used for validation comprised an electronic high-pressure pump connected to two-stage injectors of different type including 6-hole vertical and 5-hole inclined conical-sac and VCO nozzles.

A new simulation approach to the modeling of the whole fuel injection process within a common-rail fuel injection system for direct-injection gasoline engines, including the pressure-swirl atomizer and the conical hollow-cone spray formed at the nozzle exit, is presented. The flow development in the common-rail fuel injection system is simulated using an 1-D model which accounts for the wave dynamics within the system and predicts the actual injection pressure and injection rate throughout the nozzle. The details of the flow inside its various flow passages and the discharge hole of the pressure-swirl atomizer are investigated using a two-phase CFD model which calculates the location of the liquid-gas interface using the VOF method and estimates the transient formation of the liquid film developing on the walls of the discharge hole due to the centrifugal forces acting on the swirling fluid.

An enlarged transparent model of a six-hole vertical diesel injector has been used to allow visualization of the flow at Reynolds and cavitation numbers matching those of real size injectors operating under normal Diesel engine conditions. The visualization system comprised a CCD camera, high-magnification lenses and a spark light source which allowed high-resolution images to be obtained. The flow conditions examined in terms of flow rates and pressures covered the range from low to full load of the real size injector while the needle lift position corresponded to that of full lift of the first- and second- stage in two-stage injectors. In addition, different values of needle eccentricity were tested in order to examine its effect on the cavitation structures within the injection holes.

An enlarged transparent model of a six-hole vertical diesel injector has been manufactured in order to allow flow measurements inside the sac volume and the injection holes to be obtained using a combination of laser Doppler velocimetry (LDV) and the refractive index matching technique under steady state conditions. The measurement points were concentrated in the sac volume close to the entrance of the injection holes as well as inside them on a vertical plane passing through the axis of two injection holes for two different needle lifts. The velocity flow field was characterized in terms of the mean velocity and the turbulent intensity. The results revealed that, under certain conditions, cavitation may occur in the recirculation zone formed at the entrance to the hole since the pressure in this region can reach the value of the vapor pressure of the flowing liquid; this was found to strongly depend on the needle lift and eccentricity.