Parametric Investigations of the Induced Shear Stress by a Laser-Generated Bubble

The present paper focuses on the simulation of the growth and collapse of a bubble in the vicinity of a wall. Both liquid and gas phases are assumed compressible, and their interaction is handled with the volume-of-fluid method. The main interest is to quantify the influence of the induced shear stress and pressure pulse in the vicinity of the wall for a variety of bubble sizes and bubble–wall distances. The results are validated against prior experimental results, such as the measurements of the bubble size, induced pressure field, and shear stress on the wall. The simulation predictions indicate that the wall in the vicinity of the bubble is subjected both to high shear stresses and large pressure pulses because of the growth and collapse of the bubble. In fact, pressure levels of 100 bar or more and shear stresses up to 25 kPa have been found at localized spots on the wall surface, at the region around the bubble. Moreover, the simulations are capable of providing additional insight to the experimental investigation, as the inherent limitations of the latter are avoided. The present work may be considered as a preliminary investigation in optimizing bubble energy and wall generation distance for ultrasound cleaning applications

Numerical investigation of multiple injection strategy on the development of high-pressure Diesel sprays

Computational fluid dynamics results are presented providing information on the influence of multiple injection strategy on fuel vaporization characteristics under conditions typical of direct injection, turbocharged, high-speed automotive diesel engines. The fuel is assumed to be injected from a high-pressure common rail injector nozzle. Focus is given on the number of multiple injections and dwell-time on the evaporating spray plume development. Comparison between the different cases is performed in terms of liquid and vapour penetration curves, the spatial distribution of the air–fuel equivalence ratio and the fuel vapour spatial distribution difference between the cases considered. The results confirm that, under the operating conditions investigated, the liquid penetration length, known to freeze at a distance from the nozzle exit, is not significantly affected by the injection strategy, while vapour penetration follows the time-shift of the dwell-time. Longer dwell-times retard the diffusion of the vapour in the carrier gas. Although injection of small fuel quantities prior to the main pulse does not affect the liquid penetration, it contributes up to 5 per cent more stoichiometric fuel vapour present in the area of observed auto-ignition sites. Post injection and splitting of the main injection in two pulses modify the vapour distribution by creating two spatially separated fuel-rich zones

Experimental and numerical analysis of the single droplet impact onto stationary ones

The present paper investigates experimentally and numerically the impact of a spherical water droplet onto a stationary sessile one lying onto a substrate. The experiments were performed with two different film thicknesses, three different We numbers and two surface contact angles (two substrates, aluminium and glass). For this purpose a CCD camera was used and the corresponding qualitative and quantitative characteristics regarding the time evolution of the phenomenon, such as the diameter and height of the evolving crown, were obtained by image analysis. The aforementioned investigation was extended applying also the V.O.F (Volume Of Fluid) numerical methodology for the prediction of the temporal evolution of the phenomenon, so as to identify important characteristics of the induced flow field, not easy to be measured. This permits the in depth understanding of the governing flow laws, which resemble to those in the case of a droplet impact onto shallow films. The governing Navier-Stokes equations are solved both for the gas and liquid phase coupled with an additional equation for the transport of the liquid interface. An unstructured numerical grid is used along with an adaptive local grid refinement technique, increasing the numerical accuracy along the liquid-gas interface with the minimum computational cost. The numerical model is validated against the corresponding experimental data showing a good agreement. The regimes of deposition and splashing are identified as a function of We number and of the maximum thickness of the steady film, which is affected by the surface wettability properties. Moreover, following an analysis of the controlling parameters describing the temporal evolution of the lamella spreading, the role of We and Oh numbers as also of the wetting contact angle were identified, providing analytical expressions for the main dimensions characterizing the phenomenon

Effect of dwell-time on multi-component fuel vaporisation of high-pressure Diesel sprays injected from cylindrical and reverse tapered multi-hole nozzles

The effect of dwell-time, fuel composition and nozzle hole shape on the development of dense Diesel sprays injected from high-pressure multi-hole common rail injector nozzles is evaluated using a validated computational fluid dynamics spray model. The initial conditions required as input to the model have been estimated by a multiphase nozzle hole cavitation model. The subsequent liquid plume development is predicted using an Eulerian- Lagrangian spray model, which accounts for liquid-core atomisation, droplet aerodynamic break-up, turbulent dispersion, droplet-to-droplet interaction and multi-component fuel vaporisation. The physical properties of the liquid fuel follow those of specified composition of pure hydrocarbons; the effect of different composition on the spray development during pilot and main injection periods is assessed. In the absence of experimental data to characterise the detailed spray structure under such operating conditions, the computational results presented in this work aim to provide some useful information about the effect of multi-injection strategy on fuel vaporisation characteristics under conditions typical of direct injection, turbocharged, high-speed Diesel engines