Numerical simulations of internal nozzle flow that include transient needle valve motion offer the potential to better predict spray penetration, mixing and liquid breakup. For example, the level of gas initially inside the sac and holes, as well as the rate of needle movement, influence the initial fuel delivery rate and spray development, thereby affecting ignition position and combustion. In this study, needle movement and gas exchange inside operating transparent fuel injectors are imaged at high speed, and CFD simulations with fine resolution (2-micrometers) in the needle-seat area are performed to understand the impact of needle movement and initial gas in the sac on ramp-up in rate of injection. The injector bodies and sac geometries are replicas of the Engine Combustion Network Spray A and Spray D injectors. Imaging shows that gas is ingested into the injector at the beginning of needle movement, an unexpected results given the high injection pressure above the needle valve. Finite element analysis simulations accounting for the elastic properties of the metal seat and needle are performed to explain this result. As forces on the needle and seat are relieved at the beginning of injection, the sac volume enlarges while contact between sealing surfaces remains. Needle and nozzle wall measurements confirm that the needle tip may move roughly 5-10 micrometers before the passage opens at the needle seat to allow flow and pressurization of the sac. Measured needle movement from an experiment (optical or X-ray) must be corrected to achieve a different "needle gap" profile for simulations with no elasticity. This elasticity-corrected profile should be used for CFD simulations, otherwise, early and incorrect spray development will be predicted. Simulations with the corrected needle-lift profile and gas initially within the sac show that the mass flow rate at the start of injection includes cycling in flow rate caused primarily by sac pressure fluctuations, which are recommended for future Lagrangian CFD simulations.
Sforzo, Brandon A.; Matusik, Katarzyna E.; Powell, Christopher F.; Kastengren, Alan L.; Daly, Shane D.; Skeen, Scott A.; Cenker, Emre C.; Pickett, Lyle M.; Crua, Cyril C.; Manin, Julien M.
The collapse or merging of individual plumes of direct-injection gasoline injectors is of fundamental importance to engine performance because of its impact on fuel-air mixing. However, the mechanisms of spray collapse are not fully understood and are difficult to predict. The purpose of this work is to study the aerodynamics in the inter-spray region, which can potentially lead to plume collapse. High-speed (100 kHz) particle image velocimetry is applied along a plane between plumes to observe the full temporal evolution of plume interaction and potential collapse, resolved for individual injection events. Supporting information along a line of sight is obtained using simultaneous diffused back illumination and Mie-scatter techniques. Experiments are performed under simulated engine conditions using a symmetric eight-hole injector in a high-temperature, high-pressure vessel at the “Spray G” operating conditions of the engine combustion network. Indicators of plume interaction and collapse include changes in counter-flow recirculation of ambient gas toward the injector along the axis of the injector or in the inter-plume region between plumes. The effect of ambient temperature and gas density on the inter-plume aerodynamics and the subsequent plume collapse are assessed. Increasing ambient temperature or density, with enhanced vaporization and momentum exchange, accelerates the plume interaction. Plume direction progressively shifts toward the injector axis with time, demonstrating that the plume interaction and collapse are inherently transient.
Designers of direct-injection compression-ignition engines use a variety of strategies to improve the fuel/charge-gas mixture within the combustion chamber for increased efficiency and reduced pollutant emissions. Strategies include the use of high fuel-injection pressures, multiple injections, small injector orifices, flow swirl, long-ignition-delay conditions, and oxygenated fuels. This is the first journal publication on a new mixing-enhancement strategy for emissions reduction: ducted fuel injection. The concept involves injecting fuel along the axis of a small cylindrical duct within the combustion chamber, to enhance the mixture in the autoignition zone relative to a conventional free-spray configuration (i.e., a fuel spray that is not surrounded by a duct). The results described herein, from initial proof-of-concept experiments conducted in a constant-volume combustion vessel, show dramatically lower soot incandescence from ducted fuel injection than from free sprays over a range of charge-gas conditions that are representative of those in modern direct-injection compression-ignition engines.
Whilst the physics of both classical evaporation and supercritical fluid mixing are reasonably well characterized and understood in isolation, little is known about the transition from one to the other in the context of liquid fuel systems. The lack of experimental data for microscopic droplets at realistic operating conditions impedes the development of phenomenological and numerical models. To address this issue we performed systematic measurements using high-speed long-distance microscopy, for three single-component fuels (n-heptane, n-dodecane, n-hexadecane), into gas at elevated temperatures (700–1200 K) and pressures (2–11 MPa). We describe these high-speed visualizations and the time evolution of the transition from liquid droplet to fuel vapour at the microscopic level. The measurements show that the classical atomization and vaporisation processes do shift to one where surface tension forces diminish with increasing pressure and temperature, but the transition to diffusive mixing does not occur instantaneously when the fuel enters the chamber. Rather, subcritical liquid structures exhibit surface tension in the near-nozzle region and then, after time surrounded by the hot ambient gas and fuel vapour, undergo a transition to a dense miscible fluid. Although there was clear evidence of surface tension and primary atomization for n-dodecane and n-hexadecane for a period of time at all the above conditions, n-heptane appeared to produce a supercritical fluid from the nozzle outlet when injected at the most elevated conditions (1200 K, 10 MPa). This demonstrates that the time taken by a droplet to transition to diffusive mixing depends on the pressure and temperature of the gas surrounding the droplet as well as the fuel properties. We summarise our observations into a phenomenological model which describes the morphological evolution and transition of microscopic droplets from classical evaporation through a transitional mixing regime and towards diffusive mixing, as a function of operating conditions. We provide criteria for these regime transitions as reduced pressure–temperature correlations, revealing the conditions where transcritical mixing is important to diesel fuel spray mixing.