Cyril Crua

The initial stage of fuel spray penetration

Effects of droplet evaporation, break-up and air entrainment on diesel fuel spray penetration have been studied theoretically at the initial stage of spray penetration when the influence of air entrainment is small (up to 0.1–0.2 ms after the start of injection). Theoretical plots of spray penetration versus time are compared with experimental results obtained using an optical single cylinder rapid compression test rig based on a Ricardo Proteus engine. Three models of spray penetration have been compared. In the first, neither break-up nor air entrainment are taken into account. The break-up processes (bag and stripping) are taken into account in the second model, while in the third model both bag break-up and air entrainment processes are considered. It has been found that the agreement between the predictions of the third model with experimental measurements is better than that for the first two models.

The Influence of Injector Parameters on the Formation and Break-Up of a Diesel Spray

The influences of injector nozzle geometry, injection pressure and ambient air conditions on a diesel fuel spray were examined using back-lighting techniques. Both stills and high-speed imaging techniques were used. Operating conditions representative of a modern turbocharged aftercooled HSDI diesel engine were achieved in an optical rapid compression machine fitted with a common rail fuel injector. Qualitative differences in spray structure were observed between tests performed with short and long injection periods. Changes in the flow structure within the nozzle could be the source of this effect. The temporal liquid penetration lengths were derived from the high-speed images. Comparisons were made between different nozzle geometries and different injection pressures. Differences were observed between VCO (Valve Covers Orifice) and mini-sac nozzles, with the mini-sac nozzles showing a higher rate of penetration under the same conditions.

Laser-induced incandescence study of diesel soot formation in a rapid compression machine at elevated pressures

Simultaneous laser-induced incandescence (LII) and laser-induced scattering (LIS) were applied to investigate soot formation and distribution in a single cylinder rapid compression machine. The fuel used was a low sulfur reference diesel fuel with 0.04% volume 2-ethylhexyl nitrate. LII images were acquired at time intervals of 1 CA throughout the soot formation period, for a range of injection pressures up to 160 MPa, and in-cylinder pressures (ICP) up to 9 MPa. The data collected shows that although cycle-to-cycle variations in soot production were observed, the LII signal intensities converged to a constant value when sufficient cycles were averaged. The amount of soot produced was not significantly affected by changes in in-cylinder pressure. Soot was observed distributed in definite clusters, which were linked to slugs of fuel caused by oscillations in the injector needle. The highest injection pressure exhibited lower soot productions and more homogeneous soot distributions within the flame. Despite diffusion flames lasting longer with lower injection pressure, it appeared that the extended oxidation time was insufficient to oxidize the excess production of soot. In addition, soot particles were detected closer to the nozzle tip with higher injection pressures. The recording of LII sequences at high temporal resolutions has shown that three distinct phases in soot formation can be observed. First, high soot formation rates are observed before the establishment of the diffusion flame. Second, a reduced soot formation rate is apparent from the start of diffusion flame until the end of injection. Finally, high soot oxidation rates occur after the end of injection and for the duration of the flame.

High-Speed Microscopic Imaging of the Initial Stage of Diesel Spray Formation and Primary Breakup

The formation and breakup of diesel sprays was investigated experimentally on a common rail diesel injector using a long range microscope. The objectives were to further the fundamental understanding of the processes involved in the initial stage of diesel spray formation. Tests were conducted at atmospheric conditions and on a rapid compression machine with motored in-cylinder peak pressures up to 8 MPa, and injection pressures up to 160 MPa. The light source and long range imaging optics were optimised to produce blur-free shadowgraphic images of sprays with a resolution of 0.6 µm per pixel, and a viewing region of 768×614 µm. Such fine spatial and temporal resolutions allowed the observation of previously unreported shearing instabilities and stagnation point on the tip of diesel jets. The tip of the fuel jet was seen to take the shape of an oblate spheroidal cap immediately after leaving the nozzle, due to the combination of transverse expansion of the jet and the physical properties of the fuel. The spheroidal cap was found to consist of residual fuel trapped in the injector hole after the end of the injection process. The formation of fuel ligaments close to the orifice was also observed, ligaments which were subsequently seen to breakup into droplets through hydrodynamic and capillary instabilities. An ultra-high speed camera was then used to capture the dynamics of the early spray formation and primary breakup with fine temporal and spatial resolutions. The frame rate was up to 5 million images per second and exposure time down to 20 ns, with a fixed resolution of 1280×960 pixels covering a viewing region of 995×746 µm. A vortex ring motion within the vapourised spheroidal cap was identified, and resulted in a slipstream effect which led to a central ligament being propelled ahead of the liquid jet.

PDA characterisation of dense Diesel sprays using a common-rail injection system

To meet the future low emission targets for diesel engines, engineers are optimizing both the fuel injection and aftertreatment systems fitted to diesel engines. In order to optimize the fuel injection system there is a need to characterize the fuel spray for a given injection nozzle geometry and injection pressure/duration. Modern diesel common-rail systems produce very dense sprays, making in-cylinder investigation particularly difficult. In this study the measurement of droplet sizes and velocities in dense diesel sprays has been investigated using Phase Doppler Anemometry (PDA). PDA has been proven to be a valuable technique in providing an understanding of the structure and characteristics of liquid sprays in many studies. It is often applied to finely atomized and dispersed particle flows. However, the application of PDA to dense sprays is complex and therefore the measurements reported in the literature are performed under conditions that are not representative of modern diesel engines. This paper reports both on the processes undertaken to optimize a classic PDA system so that it may be used to gather data in such difficult conditions and on the interpretation of the results obtained. The PDA technique was applied to the instantaneous measurement of diesel droplet sizes and velocities in a rapid compression machine operated at realistic engine conditions. Results are presented for in-cylinder pressures ranging from 1.6 MPa to 6 MPa and injection pressures from 60 to 160 MPa.

In-Cylinder Penetration and Break-Up of Diesel Sprays Using a Common-Rail Injection System

As part of an ongoing investigation, the influence of in-cylinder charge density, and injector nozzle geometry on the behavior of diesel sprays were examined using high-speed imaging. Both liquid and vapor penetration profiles were investigated in operating conditions representative of a modern turbocharged after-cooled HSDI diesel engine. These conditions were achieved in an optical rapid compression machine fitted with a common-rail fuel injection system. Differences in spray liquid and vapor penetrations were observed for different nozzle geometries and in-cylinder conditions over a range of injection fuelling representative of those in a typical engine map. Investigation into the differences in spray structure formed by multi-hole and single-hole injections were also undertaken. The results of the spray penetration profiles from the experiments were compared to empirical correlations in the literature and differences observed were attributed to flow structures within the nozzle, which are not taken into account by these correlations.

Spray penetration in a turbulent flow

Analytical expressions for mass concentration of liquid fuel in a spray are derived taking into account the effects of gas turbulence, and assuming that the influence of droplets on gas is small (intitial stage of spray development). Beyond a certain distance the spray is expected to be fully dispersed. This distance is identified with the maximum spray penetration. Then the influence of turbulence on the spray stopping distance is discussed and the rms spray penetration is computed from a trajectory (Lagrangian) approach. Finally, the problem of spray penetration is investigated in a homogeneous two-phase flow regime taking into account the dispersion of spray away from its axis. It is predicted that for realistic values of spray parameters the spray penetration at large distances from the nozzle is expected to be proportional to t2/3 (in the case when this dispersion is not taken into account this distance is proportional to t1/2). The t2/3 law is supported by experimental observations for a high pressure injector.

Diesel fuel spray penetration, heating, evaporation and ignition: modelling vs. experimentation

The modified WAVE droplet breakup model, taking into account the transient processes during spray injection, the Effective Thermal Conductivity (ETC) liquid phase model, the gas phase model suggested by Abramzon and Sirignano, and the customised version of the Shell autoignition model have been implemented into the KIVA 2 CFD code. The observed Diesel spray tip penetration and Sauter Mean Radii show much better agreement with the prediction of the modified WAVE model compared with other droplet breakup models. The difference in the autoignition delay times predicted using the Infinite Thermal Conductivity (ITC) and ETC models is important for practical computations.

Diesel autogignition at elevated in-cylinder pressueres

Abstract The autoignition of diesel sprays at in-cylinder pressures from 5 to 9 MPa and injection pressures from 100 to 160 MPa was investigated. A pseudo three-dimensional view was obtained by using two high-speed video cameras recording the autoignition process simultaneously from two different viewpoints, so that the positional ambiguity of the ignition sites may be removed. The autoignition recordings were related to spray liquid core and vapour phase video recordings reported previously, in order to obtain a detailed understanding of the structure of the sprays. The autoignition delay was found to decrease with an increase of in-cylinder pressure up to 7 MPa. However, contrary to common belief, a reverse in this trend was observed at higher pressures, with a rapid increase in the delay at in-cylinder pressures above 7 MPa. This effect was related to a decrease in spray penetration, a decrease of diffusion coefficient and an increase in chemical delay.

Puffing and Microexplosion Behavior of Water in Pure Diesel Emulsion Droplets During Leidenfrost Effect

ABSTRACT The microexplosion evolution phenomenon of single droplets of water in pure diesel emulsion under Leidenfrost effect has been studied. The tested emulsions were stabilized with a blend of commercial surfactants with three different water contents of 9%, 12%, and 15%. A high speed camera synchronized with backlight technique was used to capture the evolution of microexplosion and puffing. Three different droplet diameters of approximately 2.6 mm, 2 mm, and 0.2 mm were analyzed. It was found that the tendency of microexplosion and puffing frequency was influenced by the droplet diameter. Coalescence was the dominating factor in inducing microexplosion in bigger droplets. It was observed that the child droplets ejected from the parent droplet undergoes further puffing processes. The size of the secondary droplets after microexplosion were also found to be slightly influenced by the parent droplet size.The waiting time for microexplosion and puffing were compared for different droplets size.