Introduction

Foundational Fuel Chemistry Model (FFCM-1) is mainly the result of a long-term, ongoing research collaboration between Hai Wang’s research group at Stanford University and Gregory Smith of the SRI International.  Its primary objective is to advance a reaction model for the combustion of small hydrocarbon fuels using up-to-date kinetic knowledge and with well-defined predictive uncertainties. This web page documents FFCM-1, an initial model for a subset of small hydrocarbon fuels, the process by which the model is derived, and what we learned from this exercise. 

We prefer to use the wording “reaction model” rather than “reaction mechanism”.  The latter has been used traditionally in the reaction chemistry community to describe a model that mimics a certain phenomenon governed by the mechanism of reactions.

 

What are the foundational fuels and foundational fuel chemistry model?

Reliable chemical kinetic models for the combustion of hydrocarbon fuels are key components towards rational design and optimization of practical combustors. Owing to the hierarchical nature of combustion chemistry, the high-temperature reactions of a basis set of compounds that may include H₂, CO, and C_nH_m (1 ≤ n ≤ ~4) form the foundation of combustion reactions of higher hydrocarbons. In high-temperature flames, real liquid fuels tend to undergo rapid decomposition in the preheat zone of the flame.  The decomposition produces smaller, intermediate species [1]. The oxidation of the intermediate species is almost universally the rate limiting step of the overall reaction process [2].  Because of the disparity in the lifetimes of the intermediates, n is typically assumed to be around four, although depending on the initial fuel structure and composition, small aromatic species such as benzene and toluene must be also considered in the foundational fuel set.

Comprised of the species listed in Table I, the foundational fuels are those found in the decomposition of the higher-hydrocarbon fuels, whereas the secondary species are common fuels themselves. The set of hydrocarbons (along with hydrogen and carbon monoxide) are referred to as the foundational fuels.  The corresponding reaction model is called Foundational Fuel Chemistry Model.

 

Table I. List of foundational chemistry fuels.*

 

A brief history

There is a rather long history of combustion reaction model development by the combustion chemistry community.  The work started by Dixon-Lewis in the 1960s’ [3, 4], followed by the pioneering work of, among others, Warnatz (e.g., [5, 6]), Miller (e.g., [7, 8]), Westbrook [9, 10], and Dryer [9, 11]. Their work led to a capability that a wide range of low-dimensional flame and ignition phenomena now can be modeled from fundamental reaction mechanisms and rates.  Aside from predictions of basic phenomena associated with global combustion responses, including laminar flame speed or heat release rate and ignition delay, significant progress has been made in the area of predicting the concentrations of certain species or combustion byproducts ranging from NO_x [12] and polycyclic aromatic hydrocarbons [13] to soot [14].

A unique advance in reaction model development came about some twenty years ago.  As the result of a decade-long collaboration among several institutions, a natural-gas reaction model was advanced.  Known as the GRI Mech effort [15, 16], the resulting model has been the industry standard for the last two decades.  Unique to this effort was a collaborative approach involving advanced experimentation involving laser diagnostics, reaction pathway and rate evaluations using reaction rate theories, and global constrained optimization within the reaction rate parameter uncertainties [17].  The combined approach ensures the physical soundness of the rate assignments based on the best kinetic knowledge at the time, and at the same time, the predictive capability of the reaction model against a wide set of combustion target data.

Much progress has been made since the GRI Mech effort.  Today, shock tube/laser diagnostics techniques have been advanced to the extent that the rate coefficient of some reactions can be measured with an accuracy of ±20% [16].  The application of ab initio electronic structure calculations and reaction rate theories allow rate coefficients of many reactions to be calculated with chemical accuracy [18-20].  Given these advances, many of the reaction rate coefficients must be updated to reflect the current kinetic knowledge.  More recent work in the area include a wide range of examination of the H2/CO submodels (see, e.g., [21-26], and the development of the AramcoMech [27] – an effort led by Professor Henry Curran of National University of Ireland Galway.

 

What’s new and unique about the FFCM effort?

The current FFCM effort is unique.  It differs in its approaches, objectives, and scopes from other studies in several aspects.

·    In addition to the customary process of rate coefficient evaluation and model compilation followed by test against a set of target combustion data, we have collaborated with experimentalists on an ongoing basis to determine relevant kinetic data when necessary and while the development work took place.  For this reason, the current release has had contributions of many of our colleagues in experimental and theoretical chemical kinetics and researchers who are involved in fundamental combustion theories and the measurement of flame properties.

·    Although FFCM-1 is subject to optimization in a manner similar to the GRI Mech effort, we additionally carried out a comprehensive uncertainty quantification (UQ) analysis, including forward uncertainty propagation and uncertainty minimization against a target set of fundamental combustion dataset over a wide range of conditions and phenomena, and lastly, validation against the target combustion dataset and test against an even wider array of available combustion data.  Most of the model validation results are expressed as a simulated uncertainty band, rather than a simple, nominal prediction and comparison against the underlying experiment.

·    A key objective of the FFCM effort is to pinpoint the weakness of our kinetic knowledge at the current time and help to direct future research.  To this end, we used the reaction model, both trial and optimized versions, to explore the remaining uncertainty and unresolved issues in the reaction rate coefficients and the fundamental combustion data.  In addition, we also discuss the UQ method that must be improved for future model development.

 

What is included in the current release?

The current release, FFCM-1, considered the reactions of C_0-2 species and combustion targets of H₂, H₂O₂, CO, CH₂O, CH₄, and a limited set of C₂H₆ data.  The release should be used for predicting H₂, H₂/CO, CH₂O and CH₄ combustion only.

 

Acknowledgements

The FFCM effort started in 2011 as a part of the work of the DOE Combustion Energy Frontier Research Center (CEFRC). The work continued with support from the Air Force Office of Scientific Research (AFOSR) since 2012. During the process, many researchers have contributed to the outcome of the model, notably

1) Dr. Enoch Dames contributed to the original version of the trial model through a critical review of some of the rate coefficient assignments;

2) Dr. Elke Goos of DLR-Institute of Combustion Technology and Dr. Branko Ruscic of Argonne National Laboratory provided the thermochemical data;

3) Professor Ronald K. Hanson and Dr. David F. Davidson of Stanford University took measurements on some of the key reaction rate coefficients;

4) Professor Fokion N. Egolfopoulos of University of Southern California provided useful discussion of the laminar flame speed data.

 

References

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[27]  AramcoMech 2.0. http://www.nuigalway.ie/c3/aramco2/frontmatter.html. 2016.