Development of a real-size optical injector nozzle for studies of cavitation,          spray formation and flash-boiling at conditions relevant to direct-injection          spark-ignition engines

Flash-boiling of sprays may occur when a superheated liquid is discharged into an ambient environment with lower pressure than its saturation pressure. Such conditions normally exist in direct-injection spark-ignition engines operating at low incylinder pressures and/or high fuel temperatures. The addition of novel high volatile additives/fuels may also promote flashboiling. Fuel flashing plays a significant role in mixture formation by promoting faster breakup and higher fuel evaporation rates compared to non-flashing conditions. Therefore, fundamental understanding of the characteristics of flashing sprays is necessary for the development of more efficient mixture formation. The present computational work focuses on modelling flash-boiling of n-Pentane and isoOctane sprays using a Lagrangian particle tracking technique. First an evaporation model for superheated droplets is implemented within the computational framework of STAR-CD, along with a full set of temperature dependent fuel properties. Then the computational tool is used to model the injection of flashing sprays through a six-hole asymmetric injector. The computational results are validated against optical experimental data obtained previously with the same injector by high-speed imaging techniques. The effects of ambient pressure (0.5 and 1.0 bar) and fuel temperature (20-180° C) on the non-flashing and flashing characteristics are examined. Effects of initial droplet size and break-up submodels are also investigated. The computational methodology is able to reproduce important physical characteristics of flashboiling sprays like the onset and extent of spray collapse. Based on the current observations, further improvements to the mathematical methodology used for the flash-boiling model are proposed.

Section: Flash-boiling Evaporation Modelmentioning

confidence: 99%

“…This was measured for the same injection system at an injection pressure, P inj of 150 bar, e.g. see [25], [26]. A variation in mass flow rate of less than 5% was seen in the experimental data at varying ambient conditions, hence it was decided to fix the flow rate at all conditions within the bounds of the current study.…”

Section: Simulation Setupmentioning

confidence: 99%

Aspects of Numerical Modelling of Flash-Boiling Fuel Sprays

Price¹,

Hamzehloo²,

Aleiferis³

et al. 2015

“…To facilitate the analysis, impressions were made of the internal surfaces of the nozzle holes using a specially formulated low viscosity silicone impression material called Vinyl-Poly-Siloxane (VPS), the same material used for taking high accuracy dental and archaeological surface impressions, using a similar technique according to [7] and [8].…”

Section: Scanning Electron Microscope Analysismentioning

confidence: 99%

“…Further electron microscope images of this particular sparkdrilled injector can be found in [8]. Fig.…”

Section: Scanning Electron Microscope Analysismentioning

confidence: 99%

Characterisation of Spray Development from Spark-Eroded and Laser-Drilled Multi-Hole Injectors in an Optical DISI Engine and in a Quiescent Injection Chamber

Butcher¹,

Aleiferis²,

Richardson³

2015

This paper addresses the need for fundamental understanding of the mechanisms of fuel spray formation and mixture preparation in direct injection spark ignition (DISI) engines. Fuel injection systems for DISI engines undergo rapid developments in their design and performance, therefore, their spray breakup mechanisms in the physical conditions encountered in DISI engines over a range of operating conditions and injection strategies require continuous attention. In this context, there are sparse data in the literature on spray formation differences between conventionally drilled injectors by spark erosion and latest Laser-drilled injector nozzles. A comparison was first carried out between the holes of sparkeroded and Laser-drilled injectors of same nominal type by analysing their in-nozzle geometry and surface roughness under an electron microscope. Then the differences in their spray characteristics under quiescent conditions, as well as in a motoring optical engine, are discussed on the basis of highspeed imaging experiments and image processing methods. Specifically, the spray development mechanism was quantified by spray tip penetration and cone angle data under a range of representative low-load and high-low engine operating conditions (0.5 bar and 1.0 bar absolute, respectively), as well as at low and high injector body temperatures (20 °C and 90 °C) to represent cold and warm engine-head conditions. Droplet sizing was also performed with the two injectors using Phase Doppler Anemometry in a quiescent chamber.

“…Such a type of injector has been used by at University College London for optical studies of hydrogen combustion in a single-cylinder DI research engine [14]. The geometry of the holes was characterized by creating castings of the injector holes, using techniques described in [46] as shown in Figure 1, and the diameter of the outer and inner hole was measured to be 0.2 mm and 0.4 mm, respectively. The depth of the injector hole was measured to be ~0.66 mm, divided equally between the narrow and wide nozzle-hole sections.…”

Section: Nozzle-hole Geometry and Gridmentioning

confidence: 99%

Computational Study of Hydrogen Direct Injection for Internal Combustion Engines

Hamzehloo¹,

Aleiferis²

2013

Hydrogen has been largely proposed as a possible fuel for internal combustion engines. The main advantage of burning hydrogen is the absence of carbon-based tailpipe emissions. Hydrogen's wide flammability also offers the advantage of very lean combustion and higher engine efficiency than conventional carbon-based fuels. In order to avoid abnormal combustion modes like pre-ignition and backfiring, as well as air displacement from hydrogen's large injected volume per cycle, direct injection of hydrogen after intake valve closure is the preferred mixture preparation method for hydrogen engines. The current work focused on computational studies of hydrogen injection and mixture formation for direct-injection spark-ignition engines. Hydrogen conditions at the injector's nozzle exit are typically sonic. Initially the characteristics of under-expanded sonic hydrogen jets were investigated in a quiescent environment using both Reynolds-Averaged NavierStokes (RANS) and Large-Eddy Simulation (LES) techniques. Various injection conditions were studied, including a reference case from the literature. Different nozzle geometries were investigated, including a straight nozzle with fixed cross section and a stepped nozzle design. LES captured details of the expansion shocks better than RANS and demonstrated several aspects of hydrogen's injection and mixing. Incylinder simulations were also performed with a side 6-hole injector using 70 and 100 bar injection pressure. Injection timing was set to just after inlet valve closure with duration of 6 μs and 8 μs, leading to global air-to-fuel equivalence ratios  typically in the region of 0.2-0.4. The engine intake air pressure was set to 1.5 bar absolute to mimic boosted operation. It was observed that hydrogen jet wall impingement was always prominent. Comparison with non-fuelled engine conditions demonstrated the degree of momentum exchange between in-cylinder hydrogen injection and air motion. LES highlighted details of hydrogen's spatial distribution throughout the injection duration and up to ignition timing. Higher peak velocities were predicted by LES, especially on the tumble plane. With the employed injection strategy, the areas closer to the cylinder wall were richer in fuel than the centre of the chamber close to the end of compression.