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| Flowchart showing the surrogate formulation method. Credit: ACS, Anand et al. Click to enlarge. |
In January 2006, the Fuels for Advanced Combustion Engines (FACE) Group was chartered as a working group of the Coordinating Research Council, tasked with the mission of recommending sets of test fuels well-suited for research so that researchers evaluating advanced combustion systems may compare results from different laboratories using the same set (or sets) of fuels for consistency. Examples of advanced combustion systems are low temperature combustion (LTC), homogenous charge compression ignition
(HCCI), high efficiency, and clean combustion (HECC).
FACE developed nine fuels. Researchers at the Engine Research Center, University of Wisconsin—Madison, are now proposing surrogate models for these nine fuels developed for studying low-emission, high-efficiency advanced diesel
engine concepts. A paper describing their work was published in the ACS journal Energy & Fuels.
The FACE group identified three major fuel properties—cetane number, aromatic content and 90% distillation temperature—that are of primary importance in the performance of advanced combustion engines. It built a matrix of eight fuels with two different target levels of these three properties (cetane numbers of 30 and 55, aromatics of
20 and 45 vol %, and 90% distillation temperatures of 270 and 340 °C around a center fuel with a target cetane number of 42.5, aromatics of 32.5 vol %, and 90% distillation temperature of 305 °C.
Numerical simulations of the spray and combustion processes
of fuels in internal-combustion engines using multi-dimensional
computational fluid dynamics (CFD) codes, such as KIVA-3V,
require accurate values of the fuel properties, including density,
vapor pressure, surface tension, latent heat of vaporization,
liquid- and vapor-phase specific heat capacity, viscosity, and
thermal conductivity over a range of temperatures from the
ambient condition to above the critical temperature conditions.
Experimental determination of these fuel properties at varying
temperature conditions is difficult, especially at very high temperatures.
Hence, it is important to accurately estimate the fuel
properties for carrying out spray and combustion simulation
studies of the real fuels. Further, accurate chemical kinetic
modeling to describe the fuel-air gas-phase oxidation reactions
is crucial to the development and improvement of advanced
combustion engine operations, such as HCCI. Because some
transportation fuels may include thousands of hydrocarbon
compounds, theoretical investigations of spray and combustion
processes of these fuels can only be realized with representative
surrogate fuel models.—Anand et al.
A surrogate fuel is a fuel composed of a smaller
number of known concentrations of selected pure compounds
whose behavior matches certain characteristics of the target
fuel. Surrogates have a wide range of application targets, the team notes; as an example, a surrogate tailored for modeling the ignition process can be different from a surrogate applied for soot modeling.
The UW-Madison team developed its surrogate fuel compositions for
the nine FACE fuels by modeling their distillation profiles so that the chosen surrogate compositions cover the range and
measured concentrations of the various hydrocarbon classes in
the fuels, and also represent properties important for modeling spray
and combustion processes.
The requirement of a large number of surrogates
(~14) to accurately simulate the composition and properties of
the fuels may prohibit the inclusion of all of the surrogates in
chemical reaction mechanisms for studying the gas-phase oxidation
of the fuels, even if mechanisms were available for each
surrogate (which they are not, at least for most of the surrogates
considered in this work). Hence, a hybrid surrogate modeling
approach is proposed, wherein the physical properties of the fuels
are represented by a “physical property” surrogate model, as in
the present study, and a separate “chemistry” surrogate model
(with fewer representative hydrocarbon species) is used to
describe the gas-phase oxidation of the fuel. The concentrations
of the hydrocarbon species from the fuel property model could
be distributed to the fuel oxidation surrogates based on chemical
class grouping. This paper focuses on accurately matching
chemistry class, distillation, and physical properties. The subject
of matching chemistry will be the subject of a second paper.—Anand et al.
They found that the optimum number of surrogate components required to obtain a good agreement
of the distillation profiles varied between 9 (for FACE 6) and 14
(for FACE 2 and 5). The authors determined that their surrogate models were adequate for representing
the physical properties of the real FACE fuels. In particular, they found excellent agreement between the predicted and measured specific gravity, lower heating value, and distillation temperatures.
Resources
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K. Anand, Y. Ra, R. D. Reitz, B. Bunting (2011) Surrogate Model Development for Fuels for Advanced Combustion Engines. Energy & Fuels Article ASAP doi: /10.1021/ef101719a