A physics-based approach to modeling real-fuel combustion chemistry - I. Evidence from experiments, and thermodynamic, chemical kinetic and statistical considerations
Introduction
Chemical reaction modeling of combustion processes requires a set of pre-specified thermodynamic conditions as the initial or boundary conditions. These conditions include the temperature and pressure, and the chemical identity of the reactant molecules and their initial concentrations. Conventional, petroleum-derived gasoline, aviation jet fuels, rocket fuels and diesel fuels have compositions that are not precisely defined, at least not to the level that can be treated by detailed chemistry modeling using the fuel composition as a part of thermodynamic input. These distillate fuels are usually comprised of hydrocarbons ranging in carbon numbers from 4 to 12, 7 to 18, and 8 to 20 for gasoline, jet and diesel fuels, respectively (e.g., [1], [2], [3]). Major classes of hydrocarbon compounds found in these fuels include normal paraffins, iso-paraffins, cycloparaffins, alkenes and aromatics. As an example, Fig. 1 presents typical compositions of three jet fuels.
Compositional complexities in real fuels usually preclude the possibility of identifying explicitly the molecular structure and concentration of every fuel constituent. For modeling their combustion behaviors, the principal approach adopted over the last decade is the surrogate-fuel approach (e.g., [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]). This approach attempts to mimic real-fuel combustion behaviors using a surrogate fuel comprised of several neat compounds of well-defined structure and composition to represent the chemical functionalities of a real fuel. A key advantage of the surrogate-fuel approach is that it removes the difficulty associated with the inability to define the composition of a fuel, thus transforming it into a problem that can be tackled, at least in principle, from fundamental reaction mechanisms and rates. There are, however, some drawbacks to the surrogate approach.
First, while the development of detailed reaction models of individual surrogate components can be carried out, building a surrogate mixture to mimic a real fuel is empirical. Matching the physicochemical properties (e.g., H/C ratio, average molecular weight, smoke point, and cetane number) does not necessarily yield a surrogate that accurately duplicates the combustion behavior of the real fuel. Only a careful selection of surrogate components and tuning of the surrogate mixture composition based on actual measured real-fuel combustion properties would recover the kinetic behavior over the range of conditions tested with real fuels. Since the condition space is usually large for practical combustors, experimental measurements must be extensive and are time consuming. Then, having tested the combustion behaviors of the real fuel over the range of relevant conditions, the need for the surrogate would itself diminish, since the combustion properties of the real fuel would have been known or acquired from the experiments. Second, typical surrogates are composed of four or five neat compounds (e.g., [10], [12]). Usually, detailed reaction models are developed and tested against experiments for single-component fuels. Kinetic coupling of the fragments of fuel components may occur in some combustion reaction processes. Hence, surrogate reaction models assembled by combining submodels of single-component hydrocarbons may have to be tested for this coupling. To fully verify the model accuracy, a wide range of experiments and validation tests are again needed in order to explore kinetic coupling of surrogate constituents on an exhaustive, combinatorial basis. Third, developing detailed reaction models for large hydrocarbons is by no means as fundamental as one would hope. The number of reactions could reach several thousands for a single hydrocarbon. It is daunting, if not impossible, to treat the great many reaction pathways and rate parameters by first-principles or experimentation.
The three considerations discussed above suggest that the surrogate approach is overall an empirical approach. It is also inefficient, if not impossible, to capture the combustion chemistry of real fuels over a wide range of thermodynamic condition space. Even more importantly, jet and diesel fuels are usually injected into an engine as a spray. The breakup and evaporation of the spray is critical to the subsequent combustion process. To this end, it is impossible to develop a four- or five-component surrogate that can match the distillation curve closely and produce a fuel vapor mixture that matches the chemical properties of a real fuel. If, for example, the lowest boiling-point hydrocarbon in the surrogate mixture belongs to a particular class of hydrocarbon compounds (e.g., an n-alkane), the ignition behavior of the surrogate would be sensitive only to that class of compounds as the fuel starts to evaporate. Yet, the distribution of the evaporated compounds toward the low-temperature part of the distillation curve are in fact similar to the distribution of the hydrocarbon compound classes of the entire distillate fuel, as demonstrated by Bruno and coworkers [15].
The current study seeks to advance an alternative concept. The approach, called HyChem (Hybrid Chemistry), employs a physics-based understanding of the primary reaction pathways in fuel combustion. It combines an experimentally constrained fuel-pyrolysis model with a detailed, foundational chemistry model for the oxidation of pyrolysis products to describe and predict the combustion behaviors of real, multi-component fuels. Historically, ideas and elements of the HyChem approach have existed for some time. For example, lumped reaction models have been used in fuel combustion and chemical process research for a long time (see, for example, the pioneering work of Ranzi [16]). Williams and coworkers have advocated a “simplified” reaction modeling approach for some time now and demonstrated such an approach to modeling JP-10 combustion [17], [18]. In the current paper, we provide experimental evidence as well as thermodynamic, chemical kinetic and statistical justifications to support the HyChem approach. We also present a sample HyChem reaction model for a typical Jet A fuel (POSF10325) to illustrate its ability to predict the real-fuel combustion behavior. The discussion of the current paper focuses on high-temperature chemistry only. In the follow-up paper [19], we will present HyChem models for three jet fuels and two rocket fuels, including a discussion about the feasibility of treating the low-temperature chemistry in the negative temperature coefficient (NTC) region.
Section snippets
Simulation methods
Two modeling approaches were taken in the present work. The first one is a Monte Carlo simulation of the multi-component effect on the combustion properties of fuel mixtures using JetSurF 1.0 [20] and 2.0 [21] that were expanded to include reactions of aromatics, including ethyl-, n-propyl-, and n‑butyl‑benzene compounds and highly branched iso-paraffinic hydrocarbons, including neohexane and 2,2,4-trimethylpentane. The JetSurF model considers the high-temperature combustion chemistry of n
Flow reactor facility
A flow reactor facility was used to investigate the oxidative pyrolysis kinetics of Jet A. The flow reactor is comprised of a vertical quartz reactor tube enclosed in a pressure vessel; detailed descriptions are provided in a recent study [31]. A liquid fuel was injected into a vaporizer by a syringe pump before being introduced into the reactor in a nitrogen carrier gas. The reaction products sampled by a cooled probe were sent to a 4-column micro gas chromatograph (Inficon microGC 3000) that
Flame structure and species time histories during high-temperature oxidation of single-component fuels
In a high-temperature combustion process, large fuel molecules first undergo decomposition into several small pyrolysis fragments, followed by the oxidation of these fragments to produce final combustion products. This generally is true regardless of whether or not molecular oxygen is present in the system. To illustrate this point, Fig. 2 depicts the calculated structure of an adiabatic laminar premixed flame of n-butylcyclohexane in air. The computation was carried out using JetSurF 2.0 at
Principle of large component number – multi-component real fuels are not more complex than neat fuels
The combustion chemistry of multi-component real fuels is historically considered more complex than that of a single-component fuel. Here, we examine the validity of this notion. For this purpose, Monte Carlo simulations were carried out for hydrocarbon fuel mixtures. The simulations reveal a central rule for real, multi-component fuel combustion and dispels the preconceived notion concerning the impact of the chemical complexity associated with multi-component real fuels on our understanding.
The HyChem approach
The approach builds on the observations and rules discussed above. It combines an experimentally constrained, “one-species” fuel pyrolysis model with a detailed foundational fuel chemistry model for the oxidation of the pyrolysis fragments. The USC Mech II [51] is comprised of 111 species and 784 reaction and is used here for this purpose. Detailed application of the approach to several jet fuels will be discussed in a companion paper [19]. Here we discuss the underlying assumptions and
Conclusions
The following conclusions can be made from the experiments and analyses presented herein:
- 1.
For combustion processes occurring above the temperature where the NTC chemistry is relevant, large hydrocarbon fuels undergo pyrolysis first, followed by the oxidation of pyrolysis products. The decoupled description is applicable to phenomena governed by radical pool buildup and in flames.
- 2.
The second step, i.e., the oxidation of pyrolysis products, is rate limiting. Hence, the composition of the
Acknowledgment
The authors acknowledge Dr. Sayak Banerjee for obtaining some of the flow reactor data. The significant technical involvement by the AFOSR program manager, Dr. Chiping Li, is acknowledged. This research was funded by the Air Force Office of Scientific Research under grant numbers FA9550-14-1-0235 (CTB, RKH and HW), FA9550-16-1-0195 (CTB, RKH and HW), FA9550-12-1-0472 (HW), FA9550-15-1-0409 (FNE), and FA9550-16-1-0079 (KB). The work was also supported by the National Aeronautics and Space
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