City Research Online

Abstract

The current experimental research work is concerned to address different types of cavitation inside the multi-hole nozzle and their impact on the emerging spray stability and atomisation as the effect of cavitation on atomisation is not yet fully understood. The previous studies have provided experimental data addressing unresolved questions about string cavitation origin, area of formation, lifetime and influence on the nozzle hole flow [1] [2] [3] [4] [5] [6]. More importantly, it is aimed to fully characterize the spray structure generated from the new generation stepped multi hole injectors [7] [8]. Thus, this experimental research work has been planned in four phases to address specific issues. The first phase of the experimental investigation was to visualise the in-nozzle flow and cavitation development inside a 15-times transparent enlarged model of a conventional multi-hole injector (6-holes symmetric) using high-speed visualisation (Mie Scattering) technique. A new enlarged model injector was designed that was geometrically similar to phase 1 model but 7-times larger than the real injector. In this regard, cavitation types and development at different flow conditions were visualised. It was found that the geometric and vortex cavitation can significantly affect the spray angle and can induce instability in the spray structure. The underlying mechanisms relating to the interaction of vortex and geometric cavitation and the resulting impact of the bubble pockets and their collapse in the erosion sites was obtained. Two counter rotating vortex cavitation found at the top and bottom of the nozzle inlet which can contribute to erosion at the erosion sites. The results also revealed that the hydraulic flip happens much earlier than expected and at lower cavitation numbers compared to the 15 times enlarged model. It also reveals that a stochastic ligament spray with much lower velocities is being formed at the vicinity close to the upper part of the nozzle exit where the air entrainment seems to be maximum. The wetting phenomena can happen inside the counter bore stepped-hole region of the nozzle or on the curved surface on the tip of the injector nose. In the third phase of the experiment, a real-size stepped injector test rig was designed and manufactured. It also enabled the measurement of the spray tip penetration and cone angle from different viewing angles. It further allowed the visualisation of the overall spray behaviour and very near nozzle exit spray. It was found that stepped-nozzle compared to straight nozzle injector may have flapping of the jets creating snake shape pattern. Unique A shaped with no visible jet boundary was also seen. Higher jet to jet interactions and higher air entrainment were observed compared to other injectors. Tip penetrations of 22mm at 0.25ms, 50mm at 0.75ms ASOI are almost similar to other injectors. Increasing the pressure from 50 bar to 100 bar increases the cone angle significantly from 64 degrees to 72 degrees. In the fourth phase of the experiment, a Fiberflow Dantec PDA measurement system was setup to measure spray characteristics including droplet diameters and velocities at the very near-nozzle area of the injector (1mm from it) up to distance of 35mm from injector tip, to investigate early breakup and spray characteristics. Velocities up to 120m/s at 1mm away from nozzle exit at 100bar are in good agreement with micro-PIV measurements of the in-nozzle flow of transparent model of a similar type of injector at the same injection pressure. Comparison of the average droplet diameter at 1mm away from nozzle exit of the current stepped multi-hole injector (7.5μm) with conventional straight multi-hole (15μm) and outward pintle-type (13μm) at same operation condition shows better atomization performance of this model. The improved atomization performance can be due to the stepped part of the current injector where the fuel undergoes through a sudden expansion process whereby the flow becomes 3-D and highly turbulent and becomes susceptible to earlier breakup and rapid atomization. It was also found that as Jet 1 is slightly contracted, the center of the jet moves slightly downward towards the axis of the spray during the main injection event. Also existence of 4 local peaks in the instantaneous velocity contour plot and the change in their location at different ASOI during the main injection event indicates that there are instabilities in the velocity. Maximum droplet mean diameter, SMD, and Weber number decreases during the main injection event. The injector has Weber number of 8 at 1mm from nozzle exit while outward pintle-type model had Weber number of 25 at 2.5mm away from nozzle exit.

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

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