I love email alerts. While they often
drop stuff into my inbox that does not particularly interest me, browsing the
results is relatively painless, and sometimes yields interesting reading. Here is a dissertation that landed in my
inbox recently. A summary appears below.
TIP 1: Set up Google® Scholar (http://scholar.google.com) alerts using key words that reflect your particular interests.
TIP 2: Browse the results on the day they come into your inbox. Don’t let them pile up. It only takes a minute of your time, and you can be pleasantly surprised.
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Reactivity of Hydrocarbon Fuels: Reaction Kinetics and Ignition Delay Times
Dissertation by Fethi Khaled In Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia
Driven by these needs, this work is a research through the kinetics of components of real fuels and their combustion intermediates. Particularly, it focuses on two main subjects. The first is about the study of elementary reactions and their rate coefficients. The second is about the study of the ignition delay time of fuels and fuel components and their dependence on thermodynamic conditions.
1.2 Focus of this work
A robust and satisfactory scientific knowledge of fuels require that we are armed with enough knowledge and tools to explain and predict how fuels perform under different reactive scenarios. Most of these scenarios involve either ignition timing, flame speed, extinction rate, energy release, burning rates or detonation and deflagration rates. All these scenarios involve a combination of many elementary chemical reactions that eventually lead to the macro behavior of the combustion system. A combustion kinetic model is the chemical model that describes how chemistry evolves during combustion and together with fluid dynamic equations, can allow reactive computational fluid dynamic (CFD) simulations of real combustion reactors. Nowadays, most detailed combustion kinetic models contain thousands of elementary reactions and hence it is impossible to perform detailed quantification of each reaction. A detailed experimental and quantum chemical study of the main sensitive reactions is the obvious way forward. A sensitivity study of the main combustion properties such as ignition delay time or flame speed, would reveal the importance of the reaction of fuel molecules or intermediates with the very reactive combustion radical, hydroxyl radical (OH). These reactions are critical in dictating the fate of the fuel molecule and the secondary chemistry that will eventually lead to ignition, energy release and emissions. The combustion phenomena depends on thermodynamic conditions of pressure, temperature and composition. Intermediate chemistry is important in defining the preferred kinetic routes through which energy is released and eventually final products are formed. Intermediate chemistry is controlled by intermediate combustion radicals that include, among others, O, H, CH3, HO2 and OH. Due to the high reactivity of OH radicals, reaction of OH with fuels is generally one or more order of magnitude higher than the reaction of other combustion radicals with fuels (see Figure 1-1). Figure 1-1: OH + fuel reaction is much faster than other radicals + fuel reactions: example of isobutene. Thereby, fuel + OH reactions represent a main consumption route for fuel consumption during the combustion process (see Figure 1-2). This makes these reactions key shapers of many combustion properties like ignition timing (ignition delay time) and burning rates (flame speed).
In my experimental study of the reaction of OH radicals with fuels, I measured the rate coefficient of the reaction of OH with different hydrocarbons and oxygenates (2-butanone and 3-buten-2-one [5], propyne and allene [6], furans [7, 8], dimethyl carbonate [9] , tetrahydrofuran [8] and ethyl formate [10] at high temperature conditions (800 K < T < 1400 K). This extensive study showed that reaction of OH with most fuel components goes through H-abstraction at high temperatures (T > 800 K). The availability of large experimental data on OH + fuels motivated us to improve the rate rules and estimation techniques for these type of rate coefficients, including the Next-Nearest-Neighbor (NNN) [11] and Structure Activity Relationship (SAR) [12]. This lead us to study the reaction of ethyl esters [9], alkanes [13], isobutene [14], ethane [15] and finally C2-hydrocarbons [16].
In this thesis, I am focusing on unsaturated hydrocarbons and intermediates. For unsaturated hydrocarbons (alkenes, alkynes), we have seen a clear contribution of addition pathways (addition of OH radicals to the unsaturated carbon-carbon (CC) bond) in the overall OH + fuel reaction rate. For that, we performed a comprehensive comparative study on unsaturated hydrocarbons from room temperatures up to combustion temperatures in order to evaluate the contribution of addition pathways to the overall OH + fuel reactivity. This study started with propyne and allene [6], and we later extended it with studies on the rate coefficients of OH with different alkenes, including butenes, pentenes, hexenes [17] and allyl radicals [18]. A study on the importance of the OH addition pathways and the correct estimation of the thermodynamics and stability of formed adducts for combustion kinetics models was deeply studied and detailed in a detailed experimental work on two carbons (C2) unsaturated hydrocarbons (ethylene and acetylene) [16]. To proceed in a logical manner going from small and simple to large and complex fuel + OH systems, the first results chapter (chapter 3) of this thesis is about the reactivity of OH with C2 hydrocarbons (ethane, ethylene and acetylene). The last experimental investigation on this subject (chapter 6) is about the dienes reactions with OH where the presence of two unsaturated CC bonds in the chemical structure further intensifies the contribution of the addition pathways in the overall reactivity of these hydrocarbons with OH radicals. Although I have been fortunate enough to be involved in many fuel + OH investigations in our laboratory, this thesis will focus on the reactions of OH with unsaturated hydrocarbons (alkenes, dienes and C2 hydrocarbons) and resonantly stable radicals (allyl radical) to illustrate the effect of the presence of the CC unsaturated bond on their reaction kinetics.
Elementary reactions studied in the first section of this thesis are important in improving the predictive capability of combustion kinetic models to capture macro behavior, such as ignition delay time, of fuels. Ignition delay time (IDT) of fuels is a main property in an engine/fuel co-optimization task. Diesels have very low ignition delay times compared to gasolines to the point that they auto-ignite at the post-compression conditions without the need for spark assistance. That is the main reason why diesels are used in a compression ignition engine whereas gasolines are used in spark ignition engines.
Ignition delay times of commercial fuel components can range several orders of magnitudes for highly reactive components such as n-dodecane to highly unreactive components like ethanol. Our investigation on this topic covered experimental measurements of the ignition delay time of several fuel components in the two shock tube facilities available at the chemical kinetics and laser sensors laboratory at KAUST. We started by measuring the ignition delay time of propene [19] and isobutene [3] at different conditions of pressure, temperature and equivalence ratio. This work was part of a vast collaboration between various institutions, and was led by Curran and coworkers at the National University of Ireland at Galway (NUIG) to develop the C0-C4 AramcoMech model. Our ignition delay time data were used as a validation for the proposed C0-C4 kinetic model.
Furthermore, we have also performed ignition delay time measurements of two longer chain molecules; 2-methylhexane [20] and 2-methylbutanol [21] in collaboration with Sarathy and coworkers at KAUST. These ignition measurements were aimed at the validation of a kinetic model for FACE (Fuels for Advanced Combustion Engines) gasoline surrogates [4].
In many engine CFD tasks, such as knock and pre-ignition studies, the sole information needed from the reactive system is the ignition timing. This fact, combined with the high dependence of ignition delay time of fuels on the thermodynamic conditions of pressure, temperature, equivalence ratio and composition, motivated us to develop a universal correlation for ignition delay times of practical fuels [22]. The methodology is intended to be used in CFD engine simulations to predict ignition timing and initiation locations. As a first step, the developed correlation was tested against homogeneous charge compression ignition (HCCI) simulation cases and the results showed great promise with the ability to detect combustion phasing within one crank angle degree (CAD) at different engine operating conditions. An online tool for ignition delay time prediction, based on this work, is embedded in the Cloudflame website [23]. We extended this work further by investigating the experimental observation that high-temperature ignition delay times of many fuels collapses into a narrow range of values. We have approached such observations from purely mathematical thinking and verified it using various available ignition delay time (IDT) measurements of gasolines, jet fuels and diesels. Results are promising in that the high-temperature ignition delay times of real fuels are weakly dependent on composition and allow us to recommend Arrhenius-like expressions for the IDTs of these three families of commercial fuels.
Free full text source: https://repository.kaust.edu.sa/bitstream/handle/10754/630072/FethiKhaledThesisNov2018.pdf?sequence=3&isAllowed=y
source: https://repository.kaust.edu.sa/bitstream/handle/10754/630072/FethiKhaledThesisNov2018.pdf?sequence=3&isAllowed=y
TIP 1: Set up Google® Scholar (http://scholar.google.com) alerts using key words that reflect your particular interests.
TIP 2: Browse the results on the day they come into your inbox. Don’t let them pile up. It only takes a minute of your time, and you can be pleasantly surprised.
///////
Reactivity of Hydrocarbon Fuels: Reaction Kinetics and Ignition Delay Times
Dissertation by Fethi Khaled In Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy, King Abdullah University of Science and Technology, Thuwal, Kingdom of Saudi Arabia
Driven by these needs, this work is a research through the kinetics of components of real fuels and their combustion intermediates. Particularly, it focuses on two main subjects. The first is about the study of elementary reactions and their rate coefficients. The second is about the study of the ignition delay time of fuels and fuel components and their dependence on thermodynamic conditions.
1.2 Focus of this work
A robust and satisfactory scientific knowledge of fuels require that we are armed with enough knowledge and tools to explain and predict how fuels perform under different reactive scenarios. Most of these scenarios involve either ignition timing, flame speed, extinction rate, energy release, burning rates or detonation and deflagration rates. All these scenarios involve a combination of many elementary chemical reactions that eventually lead to the macro behavior of the combustion system. A combustion kinetic model is the chemical model that describes how chemistry evolves during combustion and together with fluid dynamic equations, can allow reactive computational fluid dynamic (CFD) simulations of real combustion reactors. Nowadays, most detailed combustion kinetic models contain thousands of elementary reactions and hence it is impossible to perform detailed quantification of each reaction. A detailed experimental and quantum chemical study of the main sensitive reactions is the obvious way forward. A sensitivity study of the main combustion properties such as ignition delay time or flame speed, would reveal the importance of the reaction of fuel molecules or intermediates with the very reactive combustion radical, hydroxyl radical (OH). These reactions are critical in dictating the fate of the fuel molecule and the secondary chemistry that will eventually lead to ignition, energy release and emissions. The combustion phenomena depends on thermodynamic conditions of pressure, temperature and composition. Intermediate chemistry is important in defining the preferred kinetic routes through which energy is released and eventually final products are formed. Intermediate chemistry is controlled by intermediate combustion radicals that include, among others, O, H, CH3, HO2 and OH. Due to the high reactivity of OH radicals, reaction of OH with fuels is generally one or more order of magnitude higher than the reaction of other combustion radicals with fuels (see Figure 1-1). Figure 1-1: OH + fuel reaction is much faster than other radicals + fuel reactions: example of isobutene. Thereby, fuel + OH reactions represent a main consumption route for fuel consumption during the combustion process (see Figure 1-2). This makes these reactions key shapers of many combustion properties like ignition timing (ignition delay time) and burning rates (flame speed).
In my experimental study of the reaction of OH radicals with fuels, I measured the rate coefficient of the reaction of OH with different hydrocarbons and oxygenates (2-butanone and 3-buten-2-one [5], propyne and allene [6], furans [7, 8], dimethyl carbonate [9] , tetrahydrofuran [8] and ethyl formate [10] at high temperature conditions (800 K < T < 1400 K). This extensive study showed that reaction of OH with most fuel components goes through H-abstraction at high temperatures (T > 800 K). The availability of large experimental data on OH + fuels motivated us to improve the rate rules and estimation techniques for these type of rate coefficients, including the Next-Nearest-Neighbor (NNN) [11] and Structure Activity Relationship (SAR) [12]. This lead us to study the reaction of ethyl esters [9], alkanes [13], isobutene [14], ethane [15] and finally C2-hydrocarbons [16].
In this thesis, I am focusing on unsaturated hydrocarbons and intermediates. For unsaturated hydrocarbons (alkenes, alkynes), we have seen a clear contribution of addition pathways (addition of OH radicals to the unsaturated carbon-carbon (CC) bond) in the overall OH + fuel reaction rate. For that, we performed a comprehensive comparative study on unsaturated hydrocarbons from room temperatures up to combustion temperatures in order to evaluate the contribution of addition pathways to the overall OH + fuel reactivity. This study started with propyne and allene [6], and we later extended it with studies on the rate coefficients of OH with different alkenes, including butenes, pentenes, hexenes [17] and allyl radicals [18]. A study on the importance of the OH addition pathways and the correct estimation of the thermodynamics and stability of formed adducts for combustion kinetics models was deeply studied and detailed in a detailed experimental work on two carbons (C2) unsaturated hydrocarbons (ethylene and acetylene) [16]. To proceed in a logical manner going from small and simple to large and complex fuel + OH systems, the first results chapter (chapter 3) of this thesis is about the reactivity of OH with C2 hydrocarbons (ethane, ethylene and acetylene). The last experimental investigation on this subject (chapter 6) is about the dienes reactions with OH where the presence of two unsaturated CC bonds in the chemical structure further intensifies the contribution of the addition pathways in the overall reactivity of these hydrocarbons with OH radicals. Although I have been fortunate enough to be involved in many fuel + OH investigations in our laboratory, this thesis will focus on the reactions of OH with unsaturated hydrocarbons (alkenes, dienes and C2 hydrocarbons) and resonantly stable radicals (allyl radical) to illustrate the effect of the presence of the CC unsaturated bond on their reaction kinetics.
Elementary reactions studied in the first section of this thesis are important in improving the predictive capability of combustion kinetic models to capture macro behavior, such as ignition delay time, of fuels. Ignition delay time (IDT) of fuels is a main property in an engine/fuel co-optimization task. Diesels have very low ignition delay times compared to gasolines to the point that they auto-ignite at the post-compression conditions without the need for spark assistance. That is the main reason why diesels are used in a compression ignition engine whereas gasolines are used in spark ignition engines.
Ignition delay times of commercial fuel components can range several orders of magnitudes for highly reactive components such as n-dodecane to highly unreactive components like ethanol. Our investigation on this topic covered experimental measurements of the ignition delay time of several fuel components in the two shock tube facilities available at the chemical kinetics and laser sensors laboratory at KAUST. We started by measuring the ignition delay time of propene [19] and isobutene [3] at different conditions of pressure, temperature and equivalence ratio. This work was part of a vast collaboration between various institutions, and was led by Curran and coworkers at the National University of Ireland at Galway (NUIG) to develop the C0-C4 AramcoMech model. Our ignition delay time data were used as a validation for the proposed C0-C4 kinetic model.
Furthermore, we have also performed ignition delay time measurements of two longer chain molecules; 2-methylhexane [20] and 2-methylbutanol [21] in collaboration with Sarathy and coworkers at KAUST. These ignition measurements were aimed at the validation of a kinetic model for FACE (Fuels for Advanced Combustion Engines) gasoline surrogates [4].
In many engine CFD tasks, such as knock and pre-ignition studies, the sole information needed from the reactive system is the ignition timing. This fact, combined with the high dependence of ignition delay time of fuels on the thermodynamic conditions of pressure, temperature, equivalence ratio and composition, motivated us to develop a universal correlation for ignition delay times of practical fuels [22]. The methodology is intended to be used in CFD engine simulations to predict ignition timing and initiation locations. As a first step, the developed correlation was tested against homogeneous charge compression ignition (HCCI) simulation cases and the results showed great promise with the ability to detect combustion phasing within one crank angle degree (CAD) at different engine operating conditions. An online tool for ignition delay time prediction, based on this work, is embedded in the Cloudflame website [23]. We extended this work further by investigating the experimental observation that high-temperature ignition delay times of many fuels collapses into a narrow range of values. We have approached such observations from purely mathematical thinking and verified it using various available ignition delay time (IDT) measurements of gasolines, jet fuels and diesels. Results are promising in that the high-temperature ignition delay times of real fuels are weakly dependent on composition and allow us to recommend Arrhenius-like expressions for the IDTs of these three families of commercial fuels.
Free full text source: https://repository.kaust.edu.sa/bitstream/handle/10754/630072/FethiKhaledThesisNov2018.pdf?sequence=3&isAllowed=y
source: https://repository.kaust.edu.sa/bitstream/handle/10754/630072/FethiKhaledThesisNov2018.pdf?sequence=3&isAllowed=y
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