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Title:Autoignition and combustion chemistry of kerosene type fuels and components
Author(s):Valco, Daniel Joseph
Director of Research:Lee, Tonghun
Doctoral Committee Chair(s):Lee, Tonghun; Yang, Hong
Doctoral Committee Member(s):Diao, Ying; Flaherty, David W.
Department / Program:Chemical & Biomolecular Engr
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Rapid compression machine
Shock tube
Ignition delay
Hydroprocessed renewable jet (HRJ)
Fischer-Tropsch (FT)
Alcohol to jet (ATJ)
Direct sugar to hydrocarbon (DSHC)
Jet fuel
Abstract:The research in this thesis is focused on the combustion of conventional (petroleum-based) jet fuels, alternative (synthetic or bio-derived) fuels, and pure fuel components at low combustion temperatures (500 – 725K) and under stoichiometric to fuel lean conditions. These conditions are of interest because they occur at the edges of stable engine operation, which are sensitive to ignition and combustion processes. Alternative fuels from various feedstocks are proposed as substitutes for conventional fuels. These alternative fuels can vary significantly in physical and/or chemical properties. In order to incorporate alternative fuels into current energy conversion systems, certification of these fuels must be obtained, which requires detailed understanding of the fuel properties and combustion characteristics. This research examines aviation fuels, where combustion experiments were conducted utilizing a rapid compression machine and a shock tube that enable acquisition of zero-dimensional ignition delay data for subsequent analysis of gas-phase reactivity. The data collected captures the effect of the chemical ignition process, which supports the creation and/or refinement of kinetic models as a validation comparison to enable optimization of current and future engine technologies. Over the six-year course of this project, ignition delay measurements of conventional jet fuels, alternative fuels, specially formulated surrogate fuels, and 50/50 fuel blends were conducted over a wide pressure (5 – 20 bar) and equivalence ratio (0.25 ≤ ϕ ≤ 1.0) range in the low temperature region. All measurements were accompanied with gas chromatography and mass spectrometer (GC/MS) data to link performance characteristics with chemometric signatures of the fuel. The low temperature combustion environment fosters a complex mix of chemical reactions which can significantly influence the combustion characteristics under engine operating conditions. At the lowest temperatures (T<725K), classical kinetic modeling for alkanes suggests that reactivity is controlled by low-temperature chain branching: R + O2 ↔ RO2 ↔ QOOH (+O2) ↔ OOQOOH → 2OH + products. The variation and/or similarities in ignition behavior observed amongst fuels is attributed to their different concentrations of fuel components, whose fractions control the global rate of low-temperature chain branching. From the array of fuels tested at the various conditions, the findings included the presence of negative-temperature coefficient behavior especially at low pressures and lean fuel mixtures. In addition, the similarity in reactivity of all conventional aviation fuels studied (Jet-A, JP-5, and JP-8) at low temperatures was noted. The petroleum based aviation fuels clearly exhibit longer ignition delays than most alternative fuels. The shorter ignition delays and corresponding enhanced reactivity of the alternative fuels is generally due to the high paraffinic content relative to conventional fuels. Other notable behaviors of alternative fuels and blends include the effect of branching in isoparaffins, where low levels of branching showed similar effects to highly n-paraffinic fuels, while significant branching showed a strong reduction in reactivity. For specially formulated surrogate fuels, the influence of aromatic content on ignition delay was apparent with increased delays due to radical scavenging. Despite the dominating effects of some chemical structures, in general, the results indicate the possibility for future use of blends in actual flight conditions. In addition, exploring fuel concentration with the equivalence ratio at stoichiometric and lean (ϕ = 1.0 and ϕ = 0.5) conditions yields achievement of higher temperatures after the first-stage heat release which then rapidly accelerates the main ignition event. At further lean conditions (ϕ = 0.25), experiments revealed interesting three-stage ignition results which were examined further in kinetic simulations. Several mechanisms were examined to compare current jet fuel surrogate in the literature to experimental results. Jet-A and kerosene-type surrogate fuels and kinetic models were used to model JP-5 ignition delay times. The Dooley Jet-A and Aachen surrogates examined predicted the general trends found experimentally, but did not replicate the ignition delay values found. These results were consistent with others in the research community. In addition, kinetic models were used to elaborate on three-stage ignition behavior noted at low-temperature, lean combustion of conventional and alternative jet fuels. Of the three mechanisms evaluated the results from the Ranzi mechanism [1] were in close agreement to experimental results with respect to capturing the ignition delay time of the main ignition event and first order correlations of the ignition profiles. CO and H2 reactions appear to be the main cause of the third-stage of ignition. An analysis of the reactions suggests that CO−H2−O2 kinetics are the driving force for the third-stage ignition seen in the ϕ = 0.25 cases. The final aspect of my graduate research focuses on pure components in the kerosene fuel range, with a goal of improving understanding of the effects of specific chemical structures on the ignition of fuels. The pure components selected either replicate prominent species in petroleum-based jet fuel or are species of interest to the research community. The pure components autoignition properties were compared to current kinetic models available for the species themselves or ones of similar bonding; to provide insight into the accuracy of the models at low temperature conditions. Overall the models did a good job of replicating trends, but differences were apparent in the prediction of the ignition delays. This study prompts future work to create a jet fuel surrogate that better matches the molecular weight of components found in conventional aviation fuels, where to date, other surrogates contain components at the limits of the jet fuel carbon number range or neglect significant components completely.
Issue Date:2017-11-28
Rights Information:Copyright 2017 Daniel Joseph Valco
Date Available in IDEALS:2018-03-13
Date Deposited:2017-12

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