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Influence of the Spatially Resolved Nozzle Hole Exit Flow Distribution on Diesel Spray Development

The City University, London, UK-M. Spathopoulou, D. Papoulias, E. Giannadakis, M. Gavaises
Fluid Research Co., Athens, Greece-A. Theodorakakos
Published 2007-09-16 by Consiglio Nazionale delle Ricerche in Italy
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. Results confirm that the internal nozzle flow and in particular formation of cavitation, significantly affects the initial spray dispersion angle in a transient manner, in agreement with flow visualisation experiments inside transparent nozzle replicas.
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Cavitation in Fuel Injection Systems for Spray-Guided Direct Injection Gasoline Engines

The City University, London, UK-D. Papoulias, E. Giannadakis, N. Mitroglou, M. Gavaises
Fluid Research Co., Athens, Greece-A. Theodorakakos
Published 2007-04-16 by SAE International in United States
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. Vapour pockets have been found to develop around the circumferential area of the needle sealing area in a transient mode. Parametric studies obtained under realistic injection and back pressure conditions reveal the effect of nozzle design on the different nozzle flow patterns that may form during…
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Spray Structure Generated by Multi-Hole Injectors for Gasoline Direct-Injection Engines

The City University, London, UK-N. Mitroglou, J. M. Nouri, Y. Yan, M. Gavaises, C. Arcoumanis
Published 2007-04-16 by SAE International in United States
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.Comparison of the results between the two injectors has allowed quantification of the influence of the number of holes and nozzle length-to-diameter ratio on the spray structure and spray tip penetration, droplet velocity and size distribution as a function of injection and chamber pressures of relevance to gasoline passenger cars.
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Internal Flow and Cavitation in a Multi-Hole Injector for Gasoline Direct-Injection Engines

The City University, London, UK-J. M. Nouri, N. Mitroglou, Y. Yan, C. Arcoumanis
Published 2007-04-16 by SAE International in United States
A transparent enlarged model of a six-hole injector used in the development of emerging gasoline direct-injection engines was manufactured with full optical access. The working fluid was water circulating through the injector nozzle under steady-state flow conditions at different flow rates, pressures and needle positions. Simultaneous matching of the Reynolds and cavitation numbers has allowed direct comparison between the cavitation regimes present in real-size and enlarged nozzles. The experimental results from the model injector, as part of a research programme into second-generation direct-injection spark-ignition engines, are presented and discussed. The main objective of this investigation was to characterise the cavitation process in the sac volume and nozzle holes under different operating conditions. This has been achieved by visualizing the nozzle cavitation structures in two planes simultaneously using two synchronised high-speed cameras.Imaging of the flow inside the injector nozzle identified the formation of three different types of cavitation as a function of the cavitation number, CN. The first is needle cavitation, formed randomly at low CN (0.5-0.7) in the vicinity of the needle, which penetrates into…
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Internal Flow and Spray Characteristics of Pintle-Type Outwards Opening Piezo Injectors for Gasoline Direct-Injection Engines

The City University, London, UK-A. Marchi, J. M. Nouri, Y. Yan, C. Arcoumanis
Published 2007-04-16 by SAE International in United States
The near nozzle exit flow and spray structure generated by an enlarged model of a second generation pintle type outwards opening injector have been investigated under steady flow conditions as a function of flow-rate and needle lift. A high resolution CCD camera and high-speed video camera have been employed in this study to obtain high-magnification images of the internal nozzle exit flow in order to identify the origin of string ligaments/droplets formation at the nozzle exit. The images of the flow around the nozzle seat area showed clearly that air was entrained from outside into the nozzle seat area under certain flow operating conditions (low cavitation number, CN); the formed air pockets inside the annular nozzle proved to be the main cause of the breaking of the fuel liquid film into strings as it emerged from the nozzle with a structure consisting of alternating thin and thick liquid filaments. As the flow rate increased, the air pockets were suppressed, reduced in size and pushed towards the exit of the nozzle resulting in a smoother spray.The…
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