Ballistic Imaging of Transient Phenomena in Turbid Media

Ballistic imaging (BI) was developed as an optical diagnostic capable of ascertaining velocity and spatial information within dense sprays with relevance to liquid-fuel injection and combustion. This development includes a full model of light scattering within the complete imaging system, enabling the performance of the instrument to be examined, optimized and quantified. BI is a laser-based measurement for enhanced visualization of strong gradient disturbances within inhomogeneous highly scattering media. The technique is a specialized shadow-imaging method, closely related to schlieren and shadowgraph techniques, which focuses on eliminating stray and multiply-scattered source light from a line-of-sight integrated 2-D signal, detected in a forward-collection geometry. Experimental investigations of two turbulent sprays were conducted, and new spray behavior was observed over the course of both measurement campaigns: Bifurcation in a two-phase flow was observed for the first time, in a jet-in-crossflow, and overall flowrate effects were shown to influence the breakup characteristics in an effervescent spray. Analysis methods were developed to apply spatial correlation to BI results, enabling the determination of velocity vectors throughout a dense spray

Effect of asymmetrical orifice inlet geometry on spray kinematics and development

In diesel engines, fuel injection has a commanding effect on combustion. Thus studying diesel spray characteristics is beneficial for controlling and improving diesel combustion. However, information on diesel spray characteristics, especially those governed by injector needle lift, is lacking. This study investigates the near-nozzle spray kinematics for particular nozzle geometries over a range of injection pressures. The nozzles used in this research include a single-hole off-axis nozzle and a two-hole nozzle with deviated orifices. This study aims to observe the effect of asymmetrical orifice inlet on the spray kinematics and describe how sensitive they are to the injection pressure. First, we applied double-pulses time-gated ballistic imaging to obtain well-defined spray/gas interfaces. Then, by tracking these interface structures, we obtained spray kinematics. The results show that the two-hole nozzle generates slower sprays than the single-hole nozzle at the beginning of injection. However, the velocity differences between these sprays become less significant as the sprays develop to a quasi-steady state. In addition, the velocity diagrams show that the instabilities cause the flow to experience significant velocity alterations at the beginning of the injection. Moreover, we observed that the nominal spray axis shifts towards the sharper orifice inlet edge, which will affect the spray targeting. Finally, the injection pressure seems to have minimal effect on the spray profile, but it certainly changes spray evolution timing and shortens the transient phase

Collinear, two-color optical Kerr effect shutter for ultrafast time-resolved imaging

Imaging with ultrashort exposure times is generally achieved with a crossed-beam geometry. In the usual arrangement, an off-axis gating pulse induces birefringence in a medium exhibiting a strong Kerr response (commonly carbon disulfide) which is followed by a polarizer aligned to fully attenuate the on-axis imaging beam. By properly timing the gate pulse, imaging light experiences a polarization change allowing time-dependent transmission through the polarizer to form an ultrashort image. The crossed-beam system is effective in generating short gate times, however, signal transmission through the system is complicated by the crossing angle of the gate and imaging beams. This work presents a robust ultrafast time-gated imaging scheme based on a combination of type-I frequency doubling and a collinear optical arrangement in carbon disulfide. We discuss spatial effects arising from crossed-beam Kerr gating, and examine the imaging spatial resolution and transmission timing affected by collinear activation of the Kerr medium, which eliminates crossing angle spatial effects and produces gate times on the order of 1 ps. In addition, the collinear, two-color system is applied to image structure in an optical fiber and a gasoline fuel spray, in order to demonstrate image formation utilizing ballistic or refracted light, selected on the basis of its transmission time.Comment: 13 pages, 10 figure

In-nozzle flow and spray characteristics of large two-stroke marine diesel fuel injectors

To further increase efficiency as well as to reduce emissions of large two-stroke marine Diesel engines, the understanding of the fuel injection processes and the resulting spray atomization characteristics is of high importance. The sheer dimensions, the uniflow scavenging design with the central exhaust valve position and the high swirl motion of the charge air is imposing the need for peripheral multiple fuel injector arrangement. The two or three fuel injectors are arranged by 180\ub0 resp. 120\ub0 and hence, a strongly asymmetrical and eccentrical atomizer design is given as all of the typically five orifices face a similar direction which is defined by the injector position and swirl flow. Experiments have shown that these characteristic nozzle tip bore arrangements lead to a strong spray deflection due to inhomogeneous velocity profiles induced by cavitation inside the orifice. Specifically designed transparent nozzles have been utilized to qualitatively investigate the in-nozzle cavitation flow phenomena. Furthermore, the influence on the subsequent atomization behaviour by simultaneously acquiring the spray morphology has been studied. A simplified transparent one-hole nozzle was used with a matching nozzle tip geometry representative for a large two-stroke marine Diesel engine injector. Fuel pressures of 50 MPa were applied to meet engine realistic injection conditions. Moreover, different degrees of hydro-erosive grinding were applied to the orifices to investigate the effects on the spray morphology with decreasing levels of in-nozzle flow cavitation derived by increasing inlet radii between main nozzle tip bore and orifice

Influence of nozzle geometry on spray and combustion characteristics related to large two-stroke engine fuel injection systems

As emission regulations for large two-stroke marine Diesel engines increase as well, the manufacturers face similar problems as small-sized engine builders. The fuel injection, spray formation and subsequent combustion process remain among the main drivers for emissions of direct-injection, compression-ignition internal combustion engines. Since the fuel injection nozzle design of large two-stroke marine Diesel engines differs significantly, not only in size but especially regarding their non-symmetricity and eccentrical orifice arrangement, compared with four-stroke engines, the bulk of research available related to this topic is very limited. To further deepen the understanding of how in-nozzle cavitation flow during the fuel injection process affects the combustion behaviour in large two-stroke marine Diesel engines, transparent nozzle geometries have been used to link the cavitation phenomena with spray and combustion characteristics. A total of six single-orifice nozzles based on the original five-orifice nozzle design of large two-stroke marine Diesel fuel injectors, with and without hydro-erosive grinding, have been experimentally investigated under realistic engine conditions using highspeed optical measurement techniques and a unique constant-volume spray chamber that geometrically represents a combustion chamber of a large two-stroke marine Diesel engine at top dead centre. The spray morphology and combustion results reveal significant differences between the non- and hydro-erosive ground nozzle geometries. While the standard and angled nozzle versions remain similar, the eccentrical nozzle with its distinctive in-nozzle swirl cavitation pattern behaves differently, leading to a very reliable start of ignition behaviour and extreme wide spray angle

Optical Arrangements for Time-Gated Ballistic Imaging

We report on a comparison of two optical setups used in time-gated ballistic imaging simulating monodisperse scattering environments with polystyrene spheres in different sizes and concentrations suspended in water

Experimental and numerical investigation of cavitation in marine Diesel injectors

To further increase the efficiency and decrease emissions of large two-stroke marine Diesel engines, the understanding of the fuel injection, spray breakup and the resulting combustion plays a vital role. Investigations have shown that the strongly asymmetrically and eccentrically arranged nozzle bores of the fuel injectors can lead to undesirable spray deflections that provoke increased component temperatures, emissions and fuel consumption. In order to investigate the origin of these spray deviations, transparent nozzles have been used to qualitatively visualize the in-nozzle flow under realistic geometrical and fuel pressure conditions. Three different, 0.75 mm diameter, single-hole nozzle geometries that represent typical geometrical characteristics have been used in cavitating nozzle flow experiments. The optical measurement technique Shadowgraphy has been applied to visualize the in-nozzle flow over the complete fuel injection process. The experiments have been performed with Diesel fuel at a rail pressure of 50 MPa with ambient back-pressure and temperature. Impingement measurements have been executed to compare the nozzle performance and validate CFD simulations using URANS with cavitation modeling in order to provide qualitative and quantitative support to the experimental results. The volume of fluid (VOF) method has been applied to simulate the multiphase flow with High Resolution Interface Capturing (HRIC). The cavitation model is based on a flash-boiling method with rapid heat transfer between the liquid and vapor phases. A Homogeneous Relaxation Model (HRM) has been utilized to describe the rate at which the instantaneous quality, the mass fraction of vapor in a two-phase mixture, will approach its equilibrium value. The numerical modeling of the cavitation inside the nozzle bore and the evaluated momentum flux have been compared to the experimental findings and show good agreement for the qualitative comparison of the cavitation patterns and differences of less than 6% for the quantitative momentum flux comparison