Publications

Quantitative measurements of the total radiative heat transfer from high-pressure diesel spray flames under a range of conditions will enable engine modelers to more accurately understand and predict the effects of advanced combustion strategies on thermal loads and efficiencies. Moreover, the coupling of radiation heat transfer to soot formation processes and its impact on the temperature field and gaseous combustion pollutants is also of great interest. For example, it has been shown that reduced soot formation in diesel engines can result in higher flame temperatures (due to less radiative cooling) leading to greater NOx emissions. Whereas much of the previous work in research engines has evaluated radiation based on two- or three-color detection with limited spatial resolution, this work uses an imaging spectrometer in conjunction with a constant volume pre-burn vessel to quantify soot temperatures, optical thickness, and total radiation with spatial and spectral (360-700 nm) resolution along the flame axis. Sprays of n-dodecane were injected from a single hole, 90-m diameter orifice into a range of ambient temperature conditions while holding ambient density and oxygen concentration constant at 22.8 kg/m 3 and 15%, respectively. Soot surface temperatures derived by fitting a model to the spectral data were within 10 K of the stoichiometric computed adiabatic flame temperature for lower ambient temperature, lower sooting cases. As ambient temperature was increased, leading to greater soot formation, the spectrally derived peak soot temperature decreased relative to the calculated adiabatic flame temperature. For the highest ambient temperature case (1200 K), the spectrally derived soot surface temperature was more than 140 K lower than the calculated adiabatic flame temperature. Values of optical thickness, KL, were also derived by fitting the spectral data and these values were compared to extinction based KL measurements. The spectrally derived KL was within a factor of about 1.5 from the extinction based data for the higher sooting cases. Under lower sooting conditions the differences were larger. For the lowest sooting case, the radiant fraction-defined as the fraction of energy emitted by radiation relative to the chemical energy available from the fuel injection-was negligible at less than 0.01%. The highest temperature flame with the greatest optical thickness resulted in a radiant fraction of 0.46%. Copyright © 2014 SAE International.

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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.