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Experimental Study of Fuel Concentration and Droplet Size in an Isothermal Diesel Spray

With currently growing demands on spray performance, the need for improved understanding of atomization and spray/flow interaction becomes more important.
An important characteristic of diesel spray is the spatial and temporal fluctuation of droplet size and concentration. A common way to indicate the average particle size in fluid dynamics is the use of the Sauter Mean Diameter (SMD), that is defined as the diameter of a sphere that has the same volume to surface area ratio as a particle of interest. It was originally developed by german scientist J. Sauter in the late 1920s.
Several method have been devised to obtain a good estimate of the SMD. A recent method is a planar droplet sizing called Planar Laser Induced Fluorescence (PLIF) technique, which makes use of the fluorescence intensity emitted from the dye added or being present in liquid droplets and the scattered light intensity from droplets to obtain droplet sizing information. The scattered light and fluorescence intensities are measured simultaneously during the illumination of a spray with a laser sheet. The principle of the PLIF technique relies on the assumption that the fluorescence intensity, IPLIF, emitted by the fluorescent dye, is proportional to the volume of the droplet and that the scattered light intensity, IMie, is proportional to its surface area. As a consequence, the ratio of the two intensities on an illuminated plane of a spray is proportional to the Sauter Mean Diameter (SMD).
This type of measurements is ideally non-intrusive and typically relies on laser interactions with the droplet field. To monitor spray properties close to the diesel injector tip, the experimental technique must be able to monitor droplet sizes in spite of high number densities and high levels of attenuation. There is a main problem associated with this technique that is related to multiple scattering in a dense spray because implicit is the approximation that each photon that reaches the detector is scattered only once. Multiple scattering occurs when the spacing is sufficiently small that a single photon readily interacts with more than one droplet before reaching the detector.
Hence, the main objective of the present work is the improvement of the PLIF technique by the calibration of the measuring method, as well as the theoretical and experimental analysis of physical phenomena that takes place. In particular we attempted to create a predictive model of multiple light scattering by small particles in order to better understand such phenomenon, as well as to predict limit conditions of its appearance and principal noisy effects in the image recording.
Moreover the current work extends the experimental and theoretical analysis to evaluate the sizing accuracy of the PLIF technique and to investigate possible solutions to reduce the great uncertainty that this technique demonstrates to have although the use of state-of-art technology.
En error analysis has been done to establish the sizing uncertainty and to suggest a novel approach to improve the accuracy of Sauter Mean Diameter measurement in sprays

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6 1 Introduction In the last thirty five years compression-ignition engines have become the prime movers for the road transportation. Actually modern compression-ignition engines produce dramatically lower particulate and NOx emissions than two decades ago, while maintaining their main advantage over other types of engines: unequalled fuel economy and the consequent minimised CO2 emissions. The oil crisis in the mid 70 s has triggered three logical reactions: investigation work directed at improving the current engines fuel consumption, the research for alternative fuels to use in the already-available engines, like methanol, ethanol, hydrogen etc. and, as a more radical solution, the search for alternative sources of energy like the wind and solar energy. In the quest of improving in particular the diesel engine performance and efficiency a great effort should be concentrated on the injection process, which is the main responsible for achieving a good air and fuel mixture. Injection pressures have risen from around 50MPa in the 70 s to around 200MPa and more in the present days. Multi-orifice nozzles have been employed in order to achieve a homogenous mixture and to decrease the droplet size and thus increase the liquid-gas contact surface. The change from indirect injection to direct injection and the employ of electronics allowed for great flexibility of the injection strategies, whose main purpose in the recent years has been to comply with the ever more stringent emissions regulations. Studying the diesel sprays outside the engine in steady and controlled conditions has always been of special interest because of its easiness. Direct studies on the diesel spray in its usual environment, i.e. the engine, are more complicated because optical access is restricted physically and the eventual windows are easily dirtied when running the engine. One of the compromise solutions is to reproduce completely or partially the engine environment in a purpose-built test rig which can have larger dimensions and proper optical access. Finally a more scientific approach, typical of pure research, is to isolate only few phenomena and reproduce them in strictly controlled conditions, maybe in a fully different experiment, that allow to investigate the effect of each single parameter affecting the physical phenomenon.

Laurea liv.II (specialistica)

Facoltà: Ingegneria

Autore: Daniele Carboni Contatta »

Composta da 196 pagine.


Questa tesi ha raggiunto 408 click dal 14/06/2007.

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