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Abstract

The focus of the present study was on the spray characterization of an outwards opening pintle-type injector to be used for spray-guided direct injection spark-ignition (DISI) engines. This spray-guided approach requires excellent spray performance and stability over a wide range of operating conditions. To achieve these goals it is necessary to have a good understanding of the internal nozzle flow and its link with the spray characteristics. In particular, the knowledge of the in-nozzle two-phase flow structure (air entrainment and cavitation), the spray development (spray angle, penetration and recirculation zones) and droplets velocity and size distribution is essential. Therefore, this study was planned and carried out in three main phases:

The first phase of the investigation was focused on a comprehensive study of the internal flow of the injector in order to understand not well-clarified flow/spray phenomena such as the longitudinal string formation and the spray-to-spray variation (flapping) which play a very important role in the behaviour of the overall spray-guided DISI system. To overcome the problems related to the small dimension of the injector and its optical accessibility, an enlarged transparent model about 23 times the real size injector was manufactured. Quantitative flow analysis performed with the Laser Doppler Velocimetry (LDV) technique was matched with 3D Mie scattering visualization to provide full information about the internal flow behaviour and allow correlation of the flow upstream and downstream of the nozzle exit.

The flow between the needle guide and the nozzle seat in the enlarged injector consisted of four liquid jets and four pairs of unstable counter rotating vortices which were found to be responsible for the tangential oscillation of the flow downstream of the nozzle exit. The emerging hollow cone spray exhibited a string type structure similar to that of the real size injector with the string spacing depending on the flow velocity. The origin of these strings has been attributed to the formation of a two-phase flow inside the nozzle due to flow separation just upstream of the nozzle exit giving rise to air entrainment in the form of small bubbles. The presence of these bubbles in the nozzle outlet was correlated with the exiting flow and the formation of large longitudinal strings. The initiation and development of cavitation at the nozzle seat was also identified and was found to enhance the spray breakup and droplet atomisation.

The second part of the investigation was focused on the spray characterization of the real size injector by 3D Mie scattering visualization and a parametric study of the velocity field and droplet size by Phase Doppler Anemometry (PDA), Particle Image Velocimetry (PIV) and LDV measurements. The post processed data have characterised the string-to-string velocity a droplet size distribution as a function of injection and backpressure, injection duration, chamber temperature and pintle needle lift. An alternative arrangement set up made it possible to overcome problems associated with the signal attenuation due to the high spray density and also allow characterisation of the internal and external spray recirculations where ignition takes place. By introducing very low velocity atomized water mist against the spray, the velocity field of the air entrainment around the nozzle exit of the injector was measured and showed the flow development during the spray injection process which may influence the string instability and contribute to the primary droplet break up phenomenon. Validation of the results concerning recirculation and air entrainment was obtained by a PIV investigation.

The performance of three prototype pintle-type injectors having different nozzle exit designs was investigated and, in particular, the interaction of the spray with the in-cylinder flow was observed by mounting the injectors in an optical engine and visualizing the effects of the injection and engine parameters on the spray stability, spray angle variation and spray flow recirculation. Overall, the classification of the three prototypes has shown that the Inward Seal Band positive step design produced the most robust spray angle which is ideally suited for stratified fuel mixture formation in spray-guided configurations for DISI engines which offer promise for outstanding efficiency and reduced C02 emissions, approaching the levels of passenger car diesels.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology > Engineering School of Science & Technology > School of Science & Technology Doctoral Theses

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