Combustion Recession after End of Injection in Diesel Spray

This work contributes to the understanding of physical mechanisms that control flashback, or more appropriately combustion recession, in diesel-like sprays. Combustion recession is the process whereby a lifted flame retreats back towards the injector after end-of-injection under conditions that favor autoignition. The motivation for this study is that failure of combustion recession can result in unburned hydrocarbon emissions. A large dataset, comprising many fuels, injection pressures, ambient temperatures, ambient oxygen concentrations, ambient densities, and nozzle diameters is used to explore experimental trends for the behavior of combustion recession. Then, a reduced-order model, capable of modeling non-reacting and reacting conditions, is used to help interpret the experimental trends. Finally, the reduced-order model is used to predict how a controlled ramp-down rate-ofinjection can enhance the likelihood of combustion recession for conditions that would not normally exhibit combustion recession. In general, fuel, ambient conditions, and the spray rate-of-injection transient during the end-of-injection determine the success or failure of combustion recession. The likelihood of combustion recession increases for higher ambient temperatures and oxygen concentrations as well as for higher reactivity fuels. In the transition between high and low ambient temperature (or oxygen concentration), the behavior of combustion recession changes from spatially sequential ignition to separated, or isolated, ignition sites that eventually merge. In contradistinction to typical diesel ignition delay trends where the autoignition times are longer for increasing injection pressure, the time required for combustion recession increases with injection pressure.Knox, BW.; Genzale, C.; Pickett, L.; García-Oliver, JM.; Vera-Tudela, WM. (2015). Combustion Recession after End of Injection in Diesel Spray. SAE International Journal of Fuel and Lubricants. 8(2):1-17. doi:10.4271/2015-01-0797S1178

Experimental and Computational Investigation of Subcritical Near-Nozzle Spray Structure and Primary Atomization in the Engine Combustion Network Spray D

[EN] In order to improve understanding of the primary atomization process for diesel-like sprays, a collaborative experimental and computational study was focused on the near-nozzle spray structure for the Engine Combustion Network (ECN) Spray D single-hole injector. These results were presented at the 5th Workshop of the ECN in Detroit, Michigan. Application of x-ray diagnostics to the Spray D standard cold condition enabled quantification of distributions of mass, phase interfacial area, and droplet size in the near-nozzle region from 0.1 to 14 mm from the nozzle exit. Using these data, several modeling frameworks, from Lagrangian-Eulerian to Eulerian-Eulerian and from Reynolds-Averaged Navier-Stokes (RANS) to Direct Numerical Simulation (DNS), were assessed in their ability to capture and explain experimentally observed spray details. Due to its computational efficiency, the Lagrangian-Eulerian approach was able to provide spray predictions across a broad range of conditions. In general, this "engineering-level" simulation was able to reproduce the details of the droplet size distribution throughout the spray after calibration of the spray breakup model constants against the experimental data. Complementary to this approach, higher-fidelity modeling techniques were able to provide detailed insight into the experimental trends. For example, interface-capturing multiphase simulations were able to capture the experimentally observed bimodal behavior in the transverse interfacial area distributions in the near-nozzle region. Further analysis of the spray predictions suggests that peaks in the interfacial area distribution may coincide with regions of finely atomized droplets, whereas local minima may coincide with regions of continuous liquid structures. The results from this study highlight the potential of x-ray diagnostics to reveal salient details of the near-nozzle spray structure and to guide improvements to existing primary atomization modeling approaches.Battistoni, M.; Magnotti, GM.; Genzale, CL.; Arienti, M.; Matusik, KE.; Duke, DJ.; Giraldo-Valderrama, JS.... (2018). Experimental and Computational Investigation of Subcritical Near-Nozzle Spray Structure and Primary Atomization in the Engine Combustion Network Spray D. SAE International Journal of Fuel and Lubricants. 11(4):337-352. https://doi.org/10.4271/2018-01-0277S33735211

Transient Microscopy of Primary Atomization in Gasoline Direct Injection Sprays

Abstract Understanding the physics governing primary atomization of high pressure fuel sprays is of paramount importance to accurately model combustion in direct injection engines. The small length and time scales of features that characterize this process falls below the resolution power of typical grids in CFD simulations, which necessitates the inclusion of physical models (sub-models) to account for unresolved physics. Unfortunately current physical models for fuel spray atomization are based on significant empirical scaling because there is a lack of experimental data to understand the governing physics. The most widely employed atomization sub-model used in current CFD simulations assumes the spray atomization process to be dominated by aerodynamically-driven surface instabilities, but there has been no quantitative experimental validation of this theory to date. The lack of experimental validation is due to the high spatial and temporal resolutions required to simultaneously to image these instabilities, which is difficult to achieve. The present work entails the development of a diagnostic technique to obtain high spatial and temporal resolution images of jet breakup and atomization in the near nozzle region of Gasoline Direct Injection (GDI) sprays. It focuses on the optical setup required to achieve maximum illumination, image contrast, sharp feature detection, and temporal tracking of interface instabilities for long-range microscopic imaging with a high-speed camera. The resolution and performance of the imaging system is characterized by evaluating its modulation transfer function (MTF). The setup enabled imaging of GDI sprays for the entire duration of an injection event (several milliseconds) at significantly improved spatial and temporal resolutions compared to historical spray atomization imaging data. The images show that low to moderate injection pressure sprays can be visualized with a high level of detail and also enable the tracking of features across frames within the field of view (FOV)

Engine combustion network: comparison of spray development, vaporization, and combustion in different combustion vessels

Development and mixing of Diesel sprays are long known to be key factors for combustion and pollutant emissions but the related measurements in a real engine is not an easy task. This fact led researchers to simulate engine conditions in special facilities that allow the use of high-fidelity diagnostics. The Engine Combustion Network (ECN) has focused on overcoming the variability from one institution to the next by testing nominally identical Diesel injectors in four different facilities for the first time, including constant-pressure flow and constant-volume preburn chambers. Liquid- and vapor-phase penetration, ignition delay, and lift-off length measurements are compared with similar experimental setups and processing methodologies. The consistency of the data obtained indicates a good level of repeatability between the test rigs employed, and no deviation of the results can be associated with the facility type. Comparison of liquid length measurements via Mie scattering shows that this diagnostic is sensitive to the orientation of the light source. For more repeatable results between facilities, diffused back-illumination imaging is recommended. A novel image processing method has been employed to detect spray boundaries obtained in high-speed schlieren imaging: the method showed high accuracy and robustness to the different schlieren setups employed by the institutions. High-speed broadband chemiluminescence, as schlieren imaging, shows the onset of cool flame, and moreover when the combustion is stabilized, it provides an important reference to define ignition delay and lift-off length. The methodology put in place by the ECN participants in this work allows an important step forward in two directions. The first is to understand the repeatability related to experimental data in high-pressure, high-temperature environments. The second is to advance the understanding of the different diagnostics applied, thereby providing more quantitative measurements that yield to a more suitable datasets for computational fluid-dynamic model evaluation.Bardi, M.; Payri Marín, R.; Malbec, LM.; Bruneaux, G.; Pickett, LM.; Manin, J.; Bazyn, T.... (2012). Engine combustion network: comparison of spray development, vaporization, and combustion in different combustion vessels. Atomization and Sprays. 22(10):807-842. doi:10.1615/AtomizSpr.2013005837S807842221