We study the role of individual reaction rates on engine performance, with an emphasis on the contribution of quantum tunneling. It is demonstrated that the effect of quantum tunneling corrections for the reaction HO2 + HO2 = H2O2 + O2 can have a noticeable impact on the performance of a high-fidelity model of a compression-ignition (e.g., diesel) engine, and that an accurate prediction of ignition delay time for the engine model requires an accurate estimation of the tunneling correction for this reaction. The three-dimensional model includes detailed descriptions of the chemistry of a surrogate for a biodiesel fuel, as well as all the features of the engine, such as the liquid fuel spray and turbulence. This study is part of a larger investigation of how the features of the dynamics and potential energy surfaces of key reactions, as well as their reaction rate uncertainties, affect engine performance, and results in these directions are also presented here.
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 Spray D single-hole injector. These results were presented at the 5th Workshop of the Engine Combustion Network 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 bi-modal 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.
A new diagnostic for the quantification of Sauter mean diameter in high-pressure fuel sprays has been recently developed using combined optical and X-ray measurements at the Georgia Institute of Technology and Argonne National Laboratory, respectively. This diagnostic utilizes liquid scattering extinction measurements from diffuse back-illumination imaging, conducted at Georgia Tech, and liquid absorption measurements from X-ray radiography, conducted at Argonne's Advanced Photon Source. The new diagnostic, entitled the scattering-absorption measurement ratio, quantifies two-dimensional distributions of path-integrated Sauter mean diameter, enabling the construction of the spatial history of drop size development within practical fuel sprays. This technique offers unique benefits over conventional drop-sizing methods in that it can be more robust in optically dense regions of the spray, while also providing high spatial resolution of the corresponding droplet field. The methodology for quantification of Sauter mean diameter distributions using the scattering-absorption measurement ratio technique has been previously introduced and demonstrated in diesel sprays using the Engine Combustion Network Spray D injector; however, a more detailed treatment of measurement uncertainties has been needed. In this work, we present a summary of the various sources of measurement uncertainty in the scattering-absorption measurement ratio diagnostic, like those due to the experimental setup, data processing methods, and theoretical assumptions, and assess how these sources of uncertainty affect the quantified Sauter mean diameter. The spatially resolved Sauter mean diameter measurements that result from the scattering-absorption measurement ratio diagnostic will be especially valuable to the engine modeling community for the quantitative validation of spray submodels in engine computational fluid dynamics codes. Careful evaluation and quantification of measurement uncertainties are important to support accurate model validation and to ensure the development of more predictive spray models.
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