Effect of Nozzle Orifice Geometry on Spray, Combustion, and Emission Characteristics under Diesel Engine Conditions

Diesel engine performance and emissions are strongly coupled with fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. Modern engines employ micro-orifices with different orifice designs. It is critical to characterize the effects of various designs on engine performance and emissions. In this study, a recently developed primary breakup model (KH-ACT), which accounts for the effects of cavitation and turbulence generated inside the injector nozzle is incorporated into a CFD software CONVERGE for comprehensive engine simulations. The effects of orifice geometry on inner nozzle flow, spray, and combustion processes are examined by coupling the injector flow and spray simulations. Results indicate that conicity and hydrogrinding reduce cavitation and turbulence inside the nozzle orifice, which slows down primary breakup, increasing spray penetration, and reducing dispersion. Consequently, with conical and hydroground nozzles, the vaporization rate and fuel air mixing are reduced, and ignition occurs further downstream. The flame lift-off lengths are the highest and lowest for the hydroground and conical nozzles, respectively. This can be related to the rate of fuel injection, which is higher for the hydroground nozzle, leading to richer mixtures and lower flame base speeds. A modified flame index is employed to resolve the flame structure, which indicates a dual combustion mode. For the conical nozzle, the relative role of rich premixed combustion is enhanced and that of diffusion combustion reduced compared to the other two nozzles. In contrast, for the hydroground nozzle, the role of rich premixed combustion is reduced and that of non-premixed combustion is enhanced. Consequently, the amount of soot produced is the highest for the conical nozzle, while the amount of NOx produced is the highest for the hydroground nozzle, indicating the classical tradeoff between them

A Reduced Mechanism for High-Temperature Oxidation of Biodiesel Surrogates

A skeletal mechanism with 118 species and 837 reactions was developed from a detailed LLNL mechanism that consisted of 3329 species and 10806 reactions for a tricomponent surrogate mixture, consisting of methyl decanoate, methy-9-decenoate, and <i>n</i>-heptane, which is suitable for combustion modeling of biodiesel derived from various feedstocks. The method of directed relation graph (DRG) for skeletal mechanism reduction was improved for mechanisms with large numbers of isomers. The improved DRG together with isomer lumping and DRG-aided sensitivity analysis (DRGASA) were subsequently applied to obtain a minimal skeletal mechanism from the detailed mechanism for the given error tolerance. The reduction was performed within a parameter range of pressure from 1 to 100 atm, equivalence ratio from 0.5 to 2, and temperature higher than 1000 K in autoignition and perfect stirred reactors (PSR). Although reduced in size almost by a factor of 30, the skeletal mechanism features high accuracy for high-temperature applications both in predicting the global system parameters, such as ignition delay and extinction time, and detailed profiles of species concentrations. Furthermore, numerical simulations of jet stirred reactors were compared with experimental measurements for rapeseed oil methyl esters. The temperature and species profiles in one-dimensional atmospheric counterflow diffusion flames were well predicted as well compared with experimental data in the literature

Development and validation of an n-dodecane skeletal mechanism for spray combustion applications

<div><p>n-Dodecane is a promising surrogate fuel for diesel engine study because its physicochemical properties are similar to those of the practical diesel fuels. In the present study, a skeletal mechanism for n-dodecane with 105 species and 420 reactions was developed for spray combustion simulations. The reduction starts from the most recent detailed mechanism for n-alkanes consisting of 2755 species and 11,173 reactions developed by the Lawrence Livermore National Laboratory. An algorithm combining direct relation graph with expert knowledge (DRGX) and sensitivity analysis was employed for the present skeletal reduction. The skeletal mechanism was first extensively validated in 0-D and 1-D combustion systems, including auto-ignition, jet stirred reactor (JSR), laminar premixed flame and counter flow diffusion flame. Then it was coupled with well-established spray models and further validated in 3-D turbulent spray combustion simulations under engine-like conditions. These simulations were compared with the recent experiments with n-dodecane as a surrogate for diesel fuels. It can be seen that combustion characteristics such as ignition delay and flame lift-off length were well captured by the skeletal mechanism, particularly under conditions with high ambient temperatures. Simulations also captured the transient flame development phenomenon fairly well. The results further show that ignition delay may not be the only factor controlling the stabilisation of the present flames since a good match in ignition delay does not necessarily result in improved flame lift-off length prediction.</p></div

Quantum Tunneling Affects Engine Performance

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