Effects of Pressure on the Fundamental Physics of Fuel Injection in Diesel Engines

This paper provides an analysis of high-pressure phenomena and its potential effects on the fundamental physics of fuel injection in Diesel engines. We focus on conditions when cylinder pressures exceed the thermodynamic critical pressure of the injected fuel and describe the major differences that occur in the jet dynamics compared to that described by classical spray theory. To facilitate the analysis, we present a detailed model framework based on the Large Eddy Simulation (LES) technique that is designed to account for key high-pressure phenomena. Using this framework, we perform a detailed analysis using the experimental data posted as part of the Engine Combustion Network (see www.sandia.gov/ECN): namely the “Baseline n-heptane” and “Spray-A (n-dodecane)” cases, which are designed to emulate conditions typically observed in Diesel engines. Calculations are performed by rigorously treating the experimental geometry, operating conditions and relevant thermo-physical gas-liquid mixture properties. Results are further processed using linear gradient theory, which facilitates calculations of detailed vapor-liquid interfacial structures, and compared with the high-speed imaging data. Analysis of the data reveals that fuel enters the chamber as a compressed liquid and is heated at supercritical pressure. Further analysis suggests that, at certain conditions studied here, the classical view of spray atomization as an appropriate model is questionable. Instead, nonideal real-fluid behavior must be taken into account using a multicomponent formulation that applies to arbitrary hydrocarbon mixtures at high-pressure supercritical conditions. Intr oduction Research over the past decade has provided significant insights into the structure and dynamics of multiphase flows at high pressures [1‐12]. Most of this research has been done in the context of liquid-rocket propulsion, which involves direct injection of both liquid fuel and oxidizer into the combustion chamber. However, the observed trends are equally valid for other liquid fueled devices. Here we focus on Diesel engines at conditions where the fuel is injected at conditions that exceed the thermodynamic critical pressure. Injection of liquid fuel in systems where the working fluid exceeds the thermodynamic critical pressure of the liquid phase is not well understood. Depending on pressure, injected jets can exhibit two distinctly different sets of evolutionary processes. At low subcritical pressures, the classical situation exists where a well-defined molecular interface separates the injected liquid from ambient gases due to the presence of surface tension. Interactions between dynamic shear forces and surface tension promote primary atomization and secondary breakup processes that evolve from a dense state, where the liquid exists as sheets filaments or lattices intermixed with sparse pockets of gas; to a dilute state, where drop-drop interactions are negligible and dilute spray theory can be used. As ambient pressures approach or exceed the critical pressure of the liquid, however, the situation may become quite different. Under these conditions, interfacial diffusion layers can develop as a consequence of both vanishing surface tension forces and locally diminishing gas-liquid interfaces. The lack of inter-molecular forces and a distinct interfacial structure promotes diffusion dominated mixing processes prior to atomization. As a consequence, injected jets evolve in the presence of exceedingly large but continuous thermo-physical gradients in a manner that is markedly different from the classical assumptions. Modeling either of the two extremes described above poses a variety of challenges. To enhance our understanding of these processes in the context of Diesel engines, we have performed a series of calculations using the Large Eddy Simulation (LES) technique and combined this with 1) key experimental observations, and 2) the development of a detailed theoretical framework to explain the observed trends. We use the experimental data provided by Pickett et al. as part of the Engine Combustion Network (see www.ca.sandia.gov/ECN [13]) using the “Baseline n-heptane” and “Spray-A (n-dodecane)” cases as key targets. Significant attention is focused on corroborating measured and modeled results as a function of distinctly different phenomenological processes that occur as a function of pressure. This is accomplished by rigorously treating the experimental geometry (injector and vessel) and operating conditions with a fully integrated state-of-the-art model framework.