Nasir, Ehson Fawad; Farooq, Aamir(Proceedings of the Combustion Institute, Elsevier BV, 2018-12-14)[Article]
A sensor based on cavity-enhanced absorption spectroscopy (CEAS) was implemented for the first time in a rapid compression machine (RCM) for carbon monoxide concentration measurements. The sensor consisted of a pulsed quantum cascade laser (QCL) coupled to a low-finesse cavity in the RCM using an off-axis alignment. The QCL was tuned near 4.89μm to probe the P(23) ro-vibrational line of CO. The pulsed mode operation resulted in rapid frequency down-chirp (6.52 cm-1/μs) within the pulse as well as a high time resolution (10 μs). The combination of rapid frequency down-chirp and off-axis cavity alignment enabled a near complete suppression of the cavity coupling noise. A CEAS gain factor of 133 was demonstrated in experiments, resulting in a much lower noise-equivalent detection limit than a single-pass arrangement. The sensor thus presents many opportunities for measuring CO formation at low temperatures and for studying kinetics using dilute reactive environments; one such application is demonstrated in this work using dilute n-heptane/air mixtures in the RCM. The formation of CO during first-stage ignition of n-heptane was measured over 802-899K at a nominal pressure of 10bar. These conditions correspond to the NTC region of n-heptane and such results provide useful metrics to test and compare the predictions of low-temperature heat release by different kinetic models.
2,2,3-Trimethylbutane (i.e., triptane) is a potential gasoline octane booster with a research octane number (RON) of 112. Recent studies showed that it can be catalytically produced with high selectivity from methanol (CH3OH) and dimethyl ether (DME), which presents a promising route for utilizing biomass derivatives as transportation fuels. Understanding the ignition properties of triptane at engine relevant conditions is crucial for its further evaluation. In this work, a detailed kinetic model for triptane combustion is developed and validated. The rate rules for the low-temperature oxidation reactions are evaluated based on quantum chemistry calculations from literature, and thermochemical properties of all the species are assessed based on new thermodynamic group values with careful treatment of gauche interactions. In addition, alternative isomerization pathways for peroxy-alkylhydroperoxide species (ȮOQOOH) are incorporated in the model. The model is validated against new ignition delay data from facilities at King Abdullah University of Science and Technology (KAUST): rapid compression machine (RCM) experiments at pressures of 20 and 40 bar, equivalence ratios of 0.5 and 1 and across a temperature range of 620 to 1015 K, and shock tube experiments at 2 and 5 bar, 0.5 and 1 equivalence ratio and over 1000–1400 K. Moreover, the model prediction of various species is compared against species profiles from jet stirred reactor experiments at three equivalence ratios (0.5, 1 and 2) at atmospheric pressure. Finally, triptane is compared with its less branched isomers, n-heptane and 2-methylhexane, to evaluate the effect of branching on fuel reactivity and importance of alternative isomerization pathway.
KHALED, Fethi; Farooq, Aamir(Combustion and Flame, Elsevier BV, 2019-08-31)[Article]
Ignition delay times (IDTs) of fuels provide very important macro-information about the fuel reactivity and autoignition behavior. IDTs constitute a key metric for fuel/engine co-optimization studies. Chemical kinetic modeling pursuits rely on experimental IDTs as their primary validation target. There have been extensive works in literature on measuring, calculating, modeling and correlating IDTs of a wide range of hydrocarbons, oxygenates, mixtures of pure components and real fuels. Recently, some studies employed a simplified ignition model at high temperatures, comprising of a fast fuel decomposition step and a rate-determining small molecule oxidation step. This description suggests that high-temperature IDT is mainly controlled by the ignition of fuel fragments and is rather weakly dependent on the initial fuel composition. In this work, we study the validity of the hypothesis that IDT of multi-component fuels is weakly dependent on fuel composition under specific thermodynamic conditions. If so, high-temperature IDTs of practical fuels may be described by a universal Arrhenius type correlation. By combining experimental observations and chemical kinetic simulations, we determine the ranges of key parameters (temperature, pressure, equivalence ratio, composition) under which a universal IDT assumption is valid. We conclude that, for fairly random composition and within a P-T-ϕ constraint, IDTs of gasolines and jet fuels may be predicted with a high degree of certainty by the following modified Arrhenius expressions (P = 10–80 bar, P0 = 1 bar, ϕ = 0.5–2, fuel/air mixtures, units are ms, bar, K, mol, kcal): τgasoline=6.76*10−7( [Formula presented] )−1.01φ1.13− [Formula presented] exp( [Formula presented] ), forT> [Formula presented] τjetfuel=4.46*10−7( [Formula presented] )−1.21φ2.04− [Formula presented] *exp( [Formula presented] ), forT> [Formula presented]
An improved polycyclic aromatic hydrocarbon (PAH) model is developed to predict the decomposition of indene and the formation of large PAHs under pyrolytic conditions. This model is developed based on experimental study of pyrolytic kinetics of indene in a flow reactor at low and atmospheric pressures (30 and 760 Torr) by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). A general map of PAH growth is presented according to the observations in this study and those in literature. Indene dissociates via indanyl forming mono-cyclic aromatics and small intermediates, while its dominant decomposition product is indenyl. As a resonantly stabilized radical, indenyl serves as a platform molecule in PAH growth process which links small unsaturated hydrocarbons and mono-aromatic species to multi-cyclic ones. Reactions of indenyl radical are proposed to form commonly studied and recently observed PAHs. Rate constants of these reactions are evaluated by analyzing literature data of rate constant measurements, quantum chemical calculations and analogy to cyclopentadienyl radical. The main PAH formation pathways are the bi-molecular addition reactions of indenyl radical with indene and a series of intermediates, forming C10C18 and larger PAHs. Meanwhile, radical chain reactions provide huge passage for PAH growth form one resonantly stabilized radical (RSR) to larger ones. Particular contribution has been found from the reactions of RSRs that have five-member ring in their molecular structures, such as fluorenyl, benz-indenyl, cyclopenta-phenanthrenyl and benzo-fluorenyl.
An extended hybrid chemistry approach for complex hydrocarbons is developed to capture high-temperature fuel chemistry beyond the pyrolysis stage. The model may be constructed based on time-resolved measurements of oxidation species beyond the pyrolysis stage. The species’ temporal profiles are reconstructed through an artificial neural network (ANN) regression to directly extract their chemical reaction rate information. The ANN regression is combined with a foundational C0-C2 chemical mechanism to model high-temperature fuel oxidation. This new approach is demonstrated for published experimental data sets of 3 fuels: n-heptane, n-dodecane and n-hexadecane. Further, a perturbed numerical data set for n-dodecane, generated using a detailed mechanism, is used to validate this approach with homogeneous chemistry calculations. The results demonstrate the performance and feasibility of the proposed approach.
The oxidation chemistry of complex hydrocarbons involves large mechanisms with hundreds or thousands of chemical species and reactions. For practical applications and computational ease, it is desirable to reduce their chemistry. To this end, high-temperature fuel oxidation for large carbon number fuels may be described as comprising two steps, fuel pyrolysis and small species oxidation. Such an approach has recently been adopted as ‘hybrid chemistry’ or HyChem to handle high-temperature chemistry of jet fuels by utilizing time-series measurements of pyrolysis products. In the approach proposed here, a shallow Artificial Neural Network (ANN) is used to fit temporal profiles of fuel fragments to directly extract chemical reaction rate information. This information is then correlated with the species concentrations to build an ANN-based model for the fragments’ chemistry during the pyrolysis stage. Finally, this model is combined with a C0-C4 chemical mechanism to model high-temperature fuel oxidation. This new hybrid chemistry approach is demonstrated using homogeneous chemistry calculations of n-dodecane (n-C12H26) oxidation. The experimental uncertainty is simulated by introducing realistic noise in the data. The comparison shows a good agreement between the proposed ANN hybrid chemistry approach and detailed chemistry results.
Ignition delay times for 1,3-butadiene oxidation were measured in five different shock tubes and in a rapid compression machine (RCM) at thermodynamic conditions relevant to practical combustors. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10, 20 and 40 atm in both the shock tubes and in the RCM. Additional measurements were made at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in argon, at pressures of 1, 2 and 4 atm in a number of different shock tubes. Laminar flame speeds were measured at unburnt temperatures of 295 K, 359 K and 399 K at atmospheric pressure in the equivalence ratio range of 0.6–1.7, and at a pressure of 5 atm at equivalence ratios in the range 0.6–1.4. These experimental data were then used as validation targets for a newly developed detailed chemical kinetic mechanism for 1,3-butadiene oxidation.
\nA detailed chemical kinetic mechanism (AramcoMech 3.0) has been developed to describe the combustion of 1,3-butadiene and is validated by a comparison of simulation results to the new experimental measurements. Important reaction classes highlighted via sensitivity analyses at different temperatures include: (a) ȮH radical addition to the double bonds on 1,3-butadiene and their subsequent reactions. The branching ratio for addition to the terminal and central double bonds is important in determining the reactivity at low-temperatures. The alcohol-alkene radical adducts that are subsequently formed can either react with HȮ2 radicals in the case of the resonantly stabilized radicals or O2 for other radicals. (b) HȮ2 radical addition to the double bonds in 1,3-butadiene and their subsequent reactions. This reaction class is very important in determining the fuel reactivity at low and intermediate temperatures (600–900 K). Four possible addition reactions have been considered. (c) 3Ö atom addition to the double bonds in 1,3-butadiene is very important in determining fuel reactivity at intermediate to high temperatures (> 800 K). In this reaction class, the formation of two stable molecules, namely CH2O + allene, inhibits reactivity whereas the formation of two radicals, namely Ċ2H3 and ĊH2CHO, promotes reactivity. (d) Ḣ atom addition to the double bonds in 1,3-butadiene is very important in the prediction of laminar flame speeds. The formation of ethylene and a vinyl radical promotes reactivity and it is competitive with H-atom abstraction by Ḣ atoms from 1,3-butadiene to form the resonantly stabilized Ċ4H5-i radical and H2 which inhibits reactivity. Ab initio chemical kinetics calculations were carried out to determine the thermochemistry properties and rate constants for some of the important species and reactions involved in the model development. The present model is a decent first model that captures most of the high-temperature IDTs and flame speeds quite well, but there is room for considerable improvement especially for the lower temperature chemistry before a robust model is developed.
Shu, B.; Vallabhuni, S.K.; He, X.; Issayev, Gani; Moshammer, K.; Farooq, Aamir; Fernandes, R.X.(Proceedings of the Combustion Institute, Elsevier BV, 2018-08-08)[Article]
Ammonia (NH3) has been considered as a promising alternative energy carrier for automobile engines and gas turbines due to its production from renewable sources using concepts such as power-to-gas. Knowledge of the combustion characteristics of NH3/air and the formation of pollutants, especially NOx and unburned NH3, at intermediate temperatures is crucially important to investigate. Detailed understanding of ammonia reaction mechanism is still lacking. The present study reports ignition delay times of NH3/air mixtures over a temperature range of 1100–1600 K, pressures of 20 and 40 bar, and equivalence ratios of 0.5, 1.0, and 2.0. The experimental results are compared to the literature mechanism of Mathieu and Petersen (2015) and reasonable agreement is observed. Detailed modeling for ammonia emissions is performed, and the NH3/air combustion is found to be potentially free from NOx and unburned NH3 at fuel-rich conditions.
KHALED, Fethi; Giri, Binod; Farooq, Aamir(Proceedings of the Combustion Institute, Elsevier BV, 2018-06-28)[Article]
The reaction of hydroxyl radicals with fuel components and combustion intermediates is one of the most important steps for fuel oxidation. These reactions constitute the primary consumption pathways for hy- drocarbons at atmospheric and combustion conditions. Depending on the chemical structure and thermo- dynamic conditions, different chemical pathways are available for the reaction of OH with hydrocarbons. Primarily, OH may abstract an H atom directly or may undergo addition reaction forming a complex which may produce various bimolecular products. The knowledge of the branching fractions and competition of these channels is crucial to understand the combustion behavior of practical fuels. In this work, we report experimental study on the reaction of two C2 hydrocarbons, ethylene and acetylene, with OH radicals and combine it with our previous work on ethane to draw conclusions on the effect of C �C bond type on the competition between association and abstraction/bimolecular channels over a wide range of thermodynamic conditions . Experiments were carried out behind reflected shock waves over 800�1300 K and the reaction progress was monitored by probing OH radicals using UV laser absorption near 306 nm. To discern association channel from C �H bond breaking channels (direct H-abstraction and bimolecular channels), reaction of OH radicals was studied with ethylene, deuterated ethylene, acetylene and deuterated acetylene. We pre- viously showed that ethane + OH reaction expectedly follows solely direct H-abstraction pathway. Here, we found that ethylene + OH reaction presents a competition between association, bimolecular channels and direct H-abstraction of the vinylic H atoms, where association pathway becomes negligible for T > 700 K. On the other hand, acetylene is found to react with OH mainly through the association channel which dominates till temperatures as high as 1050 K.
Figueroa Labastida, Miguel; Badra, Jihad; Elbaz, Ayman M.; Farooq, Aamir(Combustion and Flame, Elsevier BV, 2018-09-26)[Article]
Understanding premature ignition or preignition is of great importance as this phenomenon influences the design and operation of internal combustion engines. Preignition leading to super-knock restricts the efficiency of downsized boosted engines. To gain a fundamental understanding of preignition and how it affects an otherwise homogeneous ignition process, a shock tube may be used to decipher the influence of fuel chemical structure, temperature, pressure, equivalence ratio and bath gas on preignition. In a previous work by Javed et al. (2017), ignition delay time measurements of n-heptane showed significantly expedited reactivity compared to well-validated chemical kinetic models in the intermediate-temperature regime. In the current work, ethanol is chosen as a representative fuel that, unlike n-heptane, does not exhibit negative temperature coefficient (NTC) behaviour. Reactive mixtures containing 2.9% and 5% of ethanol at equivalence ratios of 0.5 and 1 were used for the measurement of ignition delay times behind reflected shock waves at 2 and 4 bar. Effect of bath gas was studied with mixtures containing either Ar or N2. In addition to conventional side-wall pressure and OH* measurements, a high-speed imaging setup was utilized to visualize the shock tube cross-section through a transparent quartz end-wall. The results suggest that preignition events are more likely to happen in mixtures containing higher ethanol concentration and that preignition energy release is more pronounced at lower temperatures. High-speed imaging shows that low-temperature ignition process is usually initiated from an individual hot spot that grows gradually, while high-temperatures ignition starts from many spots simultaneously which consume the reactive mixture almost homogeneously.
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