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.
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.
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.
Sarathy, Mani; Tingas, Alexandros; Nasir, Ehson Fawad; Detogni, Alberta; Wang, Zhandong; Farooq, Aamir; Im, Hong G.(Proceedings of the Combustion Institute, Elsevier BV, 2018-09-17)[Article]
Multi-stage heat release is an important feature of hydrocarbon auto-ignition that influences engine operation. This work presents findings of previously unreported three-stage heat release in the auto-ignition of n-heptane/air mixtures at lean equivalence ratios and high pressures. Detailed homogenous gas-phase chemical kinetic simulations were utilized to identify conditions where two-stage and three-stage heat release exist. Temperature and heat release profiles of lean n-heptane/air auto-ignition display three distinct stages of heat release, which is notably different than two-stage heat release typically reported for stoichiometric fuel/air mixtures. Concentration profiles of key radicals (HO2 and OH) and intermediate/product species (CO and CO2) also display unique behavior in the lean auto-ignition case. Rapid compression machine measurements were performed at a lean equivalence ratio to confirm the existence of three-stage heat release in experiments. Laser diagnostic measurements of CO concentrations in the RCM indicate similar concentration-time profiles as those predicted by kinetic modeling. Computational singular perturbation was then used to identify key reactions and species contributing to explosive time scales at various points of the three-stage ignition process. Comparisons with two-stage ignition at stoichiometric conditions indicate that thermal runaway at the second stage of heat release is inhibited under lean conditions. H + O2 chain branching and CO oxidation reactions drive high-temperature heat release under stoichiometric conditions, but these reactions are suppressed by H, OH, and HO2 radical termination reactions at lean conditions, leading to a distinct third stage of heat release.
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.
Giri, Binod; AlAbbad, Mohammed A.; Barker, John R.; Farooq, Aamir(Proceedings of the Combustion Institute, Elsevier BV, 2018-06-28)[Article]
This work reports thermal decomposition of cyclopentanone behind reflected shock waves over 1150 - 1590 K and 750 - 1800 Torr. Carbon monoxide is one of the main reaction products and its formation was monitored using a quantum cascade laser operating near 4.56 μm. Our results show that cyclopentanone undergoes decomposition, under the present experimental conditions, via reaction channels that produce CO almost exclusively. A recent ab initio study by Zaras et al. revealed that cyclopentanone decomposes to produce CO and ethylene by two channels: ring-opening to form a di-radical which subsequently decomposes, and concerted elimination to produce CO and C 2 H 4 directly; their predicted rate constants are much slower than literature experimental data. To resolve the rate constant discrepancy and to determine whether keto- enol tautomerism plays a significant role, we performed master equation simulations which produced results in good agreement both with the previous ab initio study and with the experimental data obtained in the present work.
Liu, Dapeng; Giri, Binod; Farooq, Aamir(Proceedings of the Combustion Institute, Elsevier BV, 2018-06-28)[Article]
Methanol (CHOH) is the simplest alcohol and is considered to be a future fuel, produced by solar-driven reduction of carbon dioxide. The reaction of methanol and hydroxyl radicals is important in both combustion and atmospheric systems because this reaction is the dominant consumption pathway for methanol oxida- tion. Hydrogen abstraction at the CH or OH site of CH OH leads to different radical intermediates. The relative importance of these two channels is critical for combustion modeling as the subsequent chemistries of the product radicals (CHO and CHOH) are markedly different. In this work, we measured overall rate coefficients for the reaction of methanol (CHOH), methanol-d (CD OH) and methanol-d (CH DOH) with OH radicals over the temperature range of 900 -1300 K and pressures near 1.3 atm by employing shock tube/UV laser absorption technique. Combining our results with literature data, we recommend following three-parameter Arrhenius expressions (cm molecule s ): k1 (CH3OH + OH ) = 3.25 × 10 (T/300 K) exp(297.8K/T) 210 - 1344 K k2 (CDOH + OH ) = 4.69 × 10 (T/300 K) exp (-59.8K/T)293 - 1287 K Using our measured total rate coefficients, we determined site-specific H-abstraction rate coefficients and hence, branching ratios of the two abstraction channels. Our results show that abstraction at the CH site is the dominant channel, contributing more than 80% throughout our temperature range. Our calculated site-specific rate coefficients (per H atom) over 900-1300K are given by (cm molecule s ): k (CH2OH channel) = 2.55 × 10 exp (-2287.1 K/T ) k (CH3O channel) = 4.30 × 10 exp (-3463.2 K/T )
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.
Zhang, Guangle; Khabibullin, Kuanysh; Farooq, Aamir(Proceedings of the Combustion Institute, Elsevier BV, 2018-06-27)[Article]
Extended wavelength tuning of an IH-QCL (integrated heater quantum cascade laser) is exploited for simultaneous detection of methane and acetylene using direct absorption spectroscopy. The integrated heater, placed within few microns of the laser active region, enables wider wavelength tuning than would be possible with a conventional DFB (distributed feedback) QCL. In this work, the laser current and heater resistor current are modulated simultaneously at 25 kHz to tune the laser over 1279.6-1280.1 cm, covering absorption transitions of methane and acetylene. The laser is characterized extensively to understand the dependence of wavelength tuning on modulation frequency, modulation amplitude and phase difference between laser/heater modulation. Thereafter, the designed sensor is validated in both room-temperature static cell experiments and non-reactive high-temperature-measurements in methane-acetylene-argon gas mixtures in the shock tube. Finally, the sensor is applied for simultaneous detection of methane and acetylene during the high-temperature pyrolysis of iso-octane behind reflected shock waves.
The export option will allow you to export the current search results of the entered query to a file. Different
formats are available for download. To export the items, click on the button corresponding with the preferred download format.
By default, clicking on the export buttons will result in a download of the allowed maximum amount of items.
For anonymous users the allowed maximum amount is 50 search results.
To select a subset of the search results, click "Selective Export" button and make a selection of the items you want to export.
The amount of items that can be exported at once is similarly restricted as the full export.
After making a selection, click one of the export format buttons. The amount of items that will be exported is indicated in the bubble next to export format.