Search

Accurate and high-speed transient surface-temperature measurements of combustion devices including internal combustion (IC) engines, gas turbines, etc., provide validation targets and boundary conditions for computational fluid dynamics models, and are broadly relevant to technology advancements such as performance improvement and emissions reduction. Development and demonstration of a multi-infrared-channel pyrometry-based optical instrument for high-speed surface-temperature measurement is described. The measurement principle is based on multi-spectral radiation thermometry (MRT) and uses surface thermal radiation at four discrete spectral regions and a corresponding emissivity model to obtain surface temperature via non-linear least squares (NLLS) optimization. Rules of thumb for specifying the spectral regions and considerations to avoid interference with common combustion products are developed; the impact of these along with linear and non-linear MRT analysis are assessed as a function of temperature and signal-to-noise ratio. A multi-start method to determine the MRT-solution global optimum is described and demonstrated. The resulting multi-channel transient pyrometry instrument is described along with practical considerations including optical-alignment drift, matching intra-channel transient response, and solution-confidence indicators. The instrument demonstrated excellent >97% accuracy and >99% 2-sigma precision over the 400–800 °C range, with ~20 µs (50 kHz, equivalent to 0.2 cad at 2000 RPM IC-engine operation) transient response in the bench validation. [ABSTRACT FROM AUTHOR]

An optical diagnostic, based on laser-induced fluorescence (LIF), has been developed for on-engine measurements of real-time fuel dilution of engine oil or fuel in oil (FIO). Fuel dilution of oil is broadly relevant to advancing engine technology including durability, calibration, and catalyst-system management, and believed to promote destructive stochastic pre-ignition (SPI) during high-load engine operations. While standard (e.g., ASTM D3524-90) methods are not capable of real-time transient measurements, the LIF technique resolves transient dilution on the minutes time scale. We have expanded on our original FIO instrument development by introducing an improved analysis based on multivariate least square chemometrics analysis. The measurement uses a fuel dye (180–1300 parts per million, by mass) and monitors for its presence in the oil using 532 nm excitation and LIF. While the original FIO instrument utilized a two-color ratio method for analysis, the improved chemometric analysis uses the fully resolved LIF dye spectra to provide better predictive FIO accuracy (>92%) over a wide FIO range (1.5–14%) typical of engine application. We also investigate the effect of oil temperature on the LIF signal. Limited engine applications for demonstrating and validating the improved FIO instrument are shown, and the related data used to quantify practical detection limit and sensitivity. The improved analysis is insensitive to laser power fluctuation and change in detector integration time, providing an excellent FIO sensitivity (1–2%) and detection limit (0.01 %FIO) over a wide range of loads and injection timings, illustrating this updated approach to be a promising tool for advancing engine technology. Graphical Abstract [ABSTRACT FROM AUTHOR]

Biofuels are a promising ecologically viable and renewable alternative to petroleum fuels, with the potential to reduce net greenhouse gas emissions. However, biomass sourced fuels are often produced as blends of hydrocarbons and their oxygenates. Such blending complicates the implementation of these fuels in combustion applications. Variations in a biofuel's composition will dictate combustion properties such as auto ignition temperature, reaction delay time, and reaction pathways. A handful of novel drop-in replacement biofuels for conventional transportation fuels have recently been down selected from a list of over 10,000 potential candidates as part of the U.S. Department of Energy's (DOE) Co-Optimization of Fuels and Engines (Co-Optima) initiative. Diisobutylene (DIB) is one such high-performing hydrocarbon which can readily be produced from the dehydration and dimerization of isobutanol, produced from the fermentation of biomass-derived sugars. The two most common isomers realized, from this process, are 2,4,4-trimethyl-1-pentene (α-DIB) and 2,4,4-trimethyl-2-pentene (β-DIB). Due to a difference in olefinic bond location, the α- and β- isomer exhibit dramatically different ignition temperatures at constant pressure and equivalence ratio. This may be attributed to different fragmentation pathways enabled by allylic versus vinylic carbons. For optimal implementation of these biofuel candidates, explicit identification of the intermediates formed during the combustion of each of the isomers is needed. To investigate the combustion pathways of these molecules, tunable vacuum ultraviolet (VUV) light (in the range 8.1–11.0 eV) available at the Lawrence Berkeley National Laboratory's Advanced Light Source (ALS) has been used in conjunction with a jet stirred reactor (JSR) and time-of-flight mass spectrometry to probe intermediates formed. Relative intensity curves for intermediate mass fragments produced during this process were obtained. Several important unique intermediates were identified at the lowest observable combustion temperature with static pressure of 93,325 Pa and for 1.5 s residence time. As this relatively short residence time is just after ignition, this study is targeted at the fuels' ignition events. Ignition characteristics for both isomers were found to be strongly dependent on the kinetics of C4 and C7 fragment production and decomposition, with the tert-butyl radical as a key intermediate species. However, the ignition of α-DIB exhibited larger concentrations of C4 compounds over C7, while the reverse was true for β-DIB. These identified species will allow for enhanced engineering modeling of fuel blending and engine design. [ABSTRACT FROM AUTHOR]

Dimethyl methyl phosphonate (DMMP) is an organo-phosphorous compound (OPC) used as a fire suppressant and a simulant for sarin, a chemical warfare agent. There exists a critical need to gather combustion data at high heating rate and high temperatures conditions, similar to what exists during destruction process of chemical weapons. In the present work, DMMP pyrolysis and oxidation were carried out behind reflected shock waves at temperatures of 1300–1700 K and pressures of 1.5–1.8 atm. Lean, stoichiometric, and rich DMMP mixtures (Φ = 0.23, 0.5, 1, 2) were investigated for oxidation experiments. Laser absorption spectroscopy utilizing a quantum cascade laser near 4.9 µm was used to measure intermediate CO concentration formed during the pyrolysis and oxidation processes. To the best of our knowledge, we present the first intermediate concentration data at the reported conditions for DMMP. A tentative kinetic model, based on the AramcoMech2.0 mechanism with Lawrence Livermore National Lab (LLNL)'s OPC incineration chemistry, was utilized in Chemkin-Pro to predict CO yield during the processes. The model provided fair prediction of CO yield during DMMP pyrolysis, however, overpredicted the CO yield for oxidation. Sensitivity and rate of production analyses were carried out to understand important reactions leading to CO formation. [ABSTRACT FROM AUTHOR]

Graphical abstract Absorption cross section of OPC simulants in low frequency region (650–1500 cm−1) at 295 K and 1 atm (broadening gas: N 2). Highlights • High resolution measurements of the absorption cross section of organo phosphorus compounds. • Measurements taken using FTIR at 1 atm and room temperature. • The fundamental vibrational frequencies confirmed at high resolution. • Data is valuable for developing laser schemes for atmospheric radiation modeling. Abstract Due to extreme toxicity of chemical warfare agents, experiments to study their combustion behavior are commonly carried out using Organo Phosphorous Compounds (OPCs) as simulants. In this study, gas phase absorption cross sections of five common OPC simulants - TMP, DMMP, TEP, DEMP and DIMP have been measured within 650–3500 cm−1 wavenumber range (2.85–13.33 µm) at 295 K and 1 atm pressure (broadening gas: N 2). The quantitative absorption spectra reported in this study will be useful in setting up laser based diagnostics schemes to study their chemical kinetics in a shock tube. [ABSTRACT FROM AUTHOR]

Pyrolysis and oxidation of triethyl phosphate (TEP) were performed in the reflected shock region at temperatures of 1462-1673 K and 1213-1508 K, respectively, and at pressures near 1.3 atm. CO concentration time histories during the experiments were measured using laser absorption spectroscopy at 4580.4 nm. Experimental CO yields were compared with model predictions using the detailed organophosphorus compounds (OPC) incineration mechanism from the Lawrence Livermore National Lab (LLNL). The mechanism significantly underpredicts CO yield in TEP pyrolysis. During TEP oxidation, predicted rate of CO formation was significantly slower than the experimental results. Therefore, a new improved kinetic model for TEP combustion was developed, which was built upon the AramcoMech2.0 mechanism for C0-C2 chemistry and the existing LLNL submechanism for phosphorus chemistry. Thermochemical data of 40 phosphorus (P)-containing species were reevaluated, either using recently published group values for P-containing species or by quantum chemical calculations (CBS-QB3). The new improved model is in better agreement with the experimental CO time histories within the temperature and pressure conditions tested in this study. Sensitivity analysis was used to identify important reactions affecting CO formation, and future experimental/theoretical studies on kinetic parameters of these reactions were suggested to further improve the model. To the best of our knowledge, this is the first study of TEP kinetics in a shock tube under these conditions and the first time-resolved laser-based species time history data during its pyrolysis and oxidation. [ABSTRACT FROM AUTHOR]

The absorption cross-sections of CH 4 at two wavelengths in the mid-IR region: λ peak  = 3403.4 nm and λ valley  = 3403.7 nm were measured. Data were taken using three different compositions of non-reactive gas mixtures comprising CH 4 /Ar/CO 2 between 700 < T < 2000 K and 0.1 < P < 1.5 atm in a shock tube utilizing a continuous-wave distributed-feedback quantum cascade laser. Also, broadband room temperature methane cross section measurements were performed using a Fourier transform infrared spectrometer and the cascade laser to gain a better insight into the changes of the line shapes in various bath gasses (Ar, CO 2 , and N 2 ). An application of the high-temperature cross-section data was demonstrated to determine the concentration of methane during oxy-methane combustion in a mixture of CO 2 , O 2 , and Ar. Current measurements will be valuable addition to the spectroscopy database for methane- an important fuel used for power generation and heating around the world. [ABSTRACT FROM AUTHOR]

Soot emissions in combustion are unwanted consequences of burning hydrocarbon fuels. The presence of soot during and following combustion processes is an indication of incomplete combustion and has several negative consequences including the emission of harmful particulates and increased operational costs. Efforts have been made to reduce soot production in combustion engines through utilizing oxygenated biofuels in lieu of traditional nonoxygenated feedstocks. The ongoing Co-Optimization of Fuels and Engines (Co-Optima) initiative from the US Department of Energy (DOE) is focused on accelerating the introduction of affordable, scalable, and sustainable biofuels and high-efficiency, low-emission vehicle engines. The Co-Optima program has identified a handful of biofuel compounds from a list of thousands of potential candidates. In this study, a shock tube was used to evaluate the performance of soot reduction of five high-performance biofuels downselected by the Co-Optima program. Current experiments were performed at test conditions between 1,700 and 2,100 K and 4 and 4.7 atm using shock tube and ultrafast, time-resolve laser absorption diagnostic techniques. The combination of shock heating and nonintrusive laser detection provides a state-of-the-art test platform for high-temperature soot formation under engine conditions. Soot reduction was found in ethanol, cyclopentanone, and methyl acetate; conversely, an α-diisobutylene and methyl furan produced more soot compared to the baseline over longer test times. For each biofuel, several reaction pathways that lead towards soot production were identified. The data collected in these experiments are valuable information for the future of renewable biofuel development and their applicability in engines. [ABSTRACT FROM AUTHOR]

Ethanol oxidation was studied by measuring CO time-histories and ignition delay times behind reflected shockwaves at elevated pressures. In this study, experimental conditions included a temperature range of 960–1580 K, pressure range of 17.8–23.9 atm, and initial fuel concentrations of 6.54% and 0.25% with nitrogen and argon used as bath gasses, respectively. The equivalence ratio for high fuel loading was kept constant at 1.0, and for low fuel loading equivalence ratios were 1.0 and 0.5. For high fuel loading, early heat/energy release was observed in nearly all ignition delay time measurements, indicative of preignition; however, data collected are in good agreement with model predictions. These events are interpreted as a transition from mild to strong ignition. For the low fuel loading cases (ϕ = 1.0 and 0.5), no early heat/energy release was observed. Comparisons of measured CO concentration profiles with the predictions from kinetic mechanisms of Metcalfe et al. (2013), Mittal et al. (2014), and Zhang et al. (2018) were made assuming constant internal energy and volume for the test gas. Such comparisons in addition with performed sensitivity and pathway analyses for CO formation revealed lower temperature sensitivity to the bimolecular reaction of methyl and hydroperoxyl radicals given by CH 3 + HO 2 CH 3 O + OH, and H-atom abstraction by the hydroperoxyl radical at the alpha site on ethanol given by elementary reaction C 2 H 5 OH + HO 2 sC 2 H 4 OH + H 2 O 2. The current study provides important validation targets for ethanol chemical kinetic mechanisms and highlights the benefits of time-history measurements at various temperatures and pressures in a shock tube, which are scare in the literature. [ABSTRACT FROM AUTHOR]

In this study, syngas combustion was investigated behind reflected shock waves in CO2 bath gas to measure ignition delay times (IDT) and to probe the effects of CO2 dilution. New syngas data were taken between pressures of 34.58-45.50 atm and temperatures of 1113-1275 K. This study provides experimental data for syngas combustion in CO2 diluted environments: ignition studies in a shock tube (59 data points in 10 datasets). In total, these mixtures covered a range of temperatures T, pressures P, equivalence ratios φ, H2/CO ratio θ, and CO2 diluent concentrations. Multiple syngas combustion mechanisms exist in the literature for modeling IDTs and their performance can be assessed against data collected here. In total, twelve mechanisms were tested and presented in this work. All mechanisms need improvements at higher pressures for accurately predicting the measured IDTs. At lower pressures, some of the models agreed relatively well with the data. Some mechanisms predicted IDTs which were two orders of magnitudes different from the measurements. This suggests that there is behavior that has not been fully understood on the kinetic models and is inaccurate in predicting CO2 diluted environments for syngas combustion. To the best of our knowledge, current data are the first syngas IDTs measurements close to 50 atm under highly CO2 diluted (85% per vol.) conditions. [ABSTRACT FROM AUTHOR]

The chemical kinetics of n-heptane (n-C 7 H 16 ) – an important reference compound for real fuels – oxidation are well studied at stoichiometric and lean conditions. However, there is only limited information on the n-heptane chemical kinetics in fuel-rich combustion. In order to verify the accuracy of chemical kinetic models at these conditions, the oxidation of rich n-heptane mixtures has been investigated. Combustion of n-C 7 H 16 /O 2 /Ar mixtures at equivalence ratios, φ, of 2.0 and 3.0 behind reflected shock waves has been studied at temperatures ranging from 1066 to 1502 K and at pressures ranging from 1.4 to 6.2 atm. Reaction progress was monitored by recording pressure and absorption time-histories of ethylene (C 2 H 4 ) and n-heptane at a location 2 cm from the endwall of a 14-cm inner diameter shock tube. Ethylene and n-heptane absorption time-histories were measured, respectively, using absorption spectroscopy at 10.532 μm from a tunable CO 2 laser and at around 3.4 μm from a continuous wave distributed feedback interband cascade laser (ICL). The measured absorption time-histories were compared with modeled predictions from the Lawrence Livermore National Lab (LLNL) detailed n-heptane reaction mechanism. To the best of our knowledge, current data are the first time-resolved n-heptane and ethylene concentration measurements conducted in a shock tube at these conditions. [ABSTRACT FROM AUTHOR]

Due to the high toxicity of chemical warfare (CW) agents, laboratory experiments are carried out using CW surrogates. Diisopropyl Methylphosphonate (DIMP), an organophosphate compound (OPC), is a crucial CW surrogate that has a chemical structure similar to the deadly nerve agent Sarin (GB). In this work, high-temperature pyrolysis of DIMP is conducted in a shock tube within a temperature range of 1400 - 1800 K at near 1 atm pressure. Laser absorption spectroscopy was used to obtain carbon monoxide mole fraction time-histories during high-temperature pyrolysis for DIMP. In order to predict the CO mole fraction time-histories during DIMP pyrolysis, a reaction mechanism was developed using the Lawrence Livermore National Lab (LLNL)'s OPC mechanism. Several important reactions of DIMP and its important intermediates during pyrolysis were studied at the CBS-QB3 level of theory, and corresponding reaction rates were determined using transition state theory. The reactions related to methyl phosphonic acid (MPA) and methyl(oxo)phosphoniumolate (MOPO) were added from literature and by analogies with similar compounds. Thermochemical properties of crucial species involved in DIMP pyrolysis were calculated using the CBS-QB3 composite method and were updated in the model. A reaction mechanism was compiled for DIMP pyrolysis using these rates. Two versions of this mechanism ('Model A' and 'Model B') were generated to understand the significance of new reactions in predicting CO mole fraction time histories. Significant improvement in the prediction of CO was observed compared to the LLNL model with either of these mechanisms. A reaction path analysis was conducted to understand the formation pathways of CO during DIMP pyrolysis. Finally, sensitivity analysis was performed to get insights into important reactions leading to CO formation. It was found that the H-abstraction from MOPO by methyl radical plays an important role in CO formation. [ABSTRACT FROM AUTHOR]

A TRAnsparent cylindrical (TRAC) shock tube test section has been developed to explore the ignition structure behind reflected shockwaves via simultaneous lateral and endwall high-speed imaging using three fuel/oxidizer mixtures: 0.1% n-C 7 H 16 /1.10% O 2 /4.14% N 2 /94.66% Ar, 0.1% iC 8 H 18 /1.25% O 2 /4.69% N 2 /93.96% Ar, and 5% CH 4 /10% O 2 /85% CO 2. During mild ignition, the first exothermic centers were located outside of the observable test section. It is shown that with increasing temperature the far-wall ignition events move closer to the endwall, until a strong ignition is achieved. The temperature gradients that promote mild ignition were quantified and were found to have a slight dependence on fuel with the RMS temperature gradients being 0.80% for n-heptane and 1.07% for isooctane. The impact of temperature gradients on metrics used to improve kinetic models, such as species time histories, is noteworthy. From this temperature gradient, an experimental correlation is provided to estimate the transition temperature from mild to strong ignition for a mixture. The effect of diagnostic location in non-homogeneous reactions is quantified in reference to ignition delay. The unique and insightful perspective on the ignition process under heavy shock bifurcation is also discussed. [ABSTRACT FROM AUTHOR]

The direct-fired supercritical CO2 (sCO2) cycles promise high efficiency and reduced emissions while enabling complete carbon capture. However, there is a severe lack of fundamental combustion kinetics knowledge required for the development and operation of these cycles, which operate at high pressures and with high CO2 dilution. Experiments at these conditions are very challenging and costly. In this study, a shock tube was used to investigate the auto-ignition tendencies of several mixtures under high carbon dioxide dilution and high fuel loading. Individual mixtures of oxy-syngas and oxy-methane fuels were added to CO2 bath gas environments and ignition delay time data were recorded. Reflected shock pressures neared 100 atm, above the critical pressure of carbon dioxide into the supercritical regime. In total, five mixtures were investigated with a pressure range of 70-100 atm and a temperature range of 1050-1350 K. Measured ignition delay times of all mixtures were compared with two leading chemical kinetic mechanisms for their predictive accuracy. The mixtures included four oxy-syngas and one oxy-methane compositions. The literature mechanisms tended to show good agreement with the data for the methane mixture, while these models were not able to accurately capture all behavior for syngas mixtures tested in this study. For this reason, there is a need to further investigate the discrepancies. To the best of our knowledge, we report the first ignition data for the selected mixtures at these conditions. Current work also highlights the need for further work at high pressures to fully understand the chemical kinetic behavior of these mixtures to enable the sCO2 power cycle development. [ABSTRACT FROM AUTHOR]

Carbon monoxide time-histories, ignition delay times, and laminar burning velocity measurements are reported for the oxidation of 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol). These prenols are fuel candidates outlined by the U.S. Department of Energy's Co-Optimization of Fuels and Engines (Co-Optima) program. The laminar burning velocity measurements were conducted for two fuels with synthetic air within a constant-volume spherical combustion chamber at initial conditions of 428 K and 1 atm for a range of equivalence ratios from 0.75 to 1.50. The laminar burning velocities of the two fuels were found to be similar, and the maximum value occurred at an equivalence ratio near 1.0. Carbon monoxide time-histories and ignition delay times were recorded behind reflected shockwaves in a double-diaphragm, heated shock tube over the temperature range 1269–1472 K near 9.4 atm with a mixture of 0.05% fuel/0.35% O 2 /99.6% Ar. Comparisons with predictions of a detailed chemical kinetic mechanism from the literature were provided. Current model predictions overpredicted both the ignition delay time and the max CO yield; however, the model captured the profile of CO formation well. Detailed uncertainty and sensitivity analyses were carried out to identify important reactions that need attention for accurate prediction of these fuel's chemistry. Further investigation into the rate of C 3 H 3 + O 2 = CH 2 CO + HCO reaction was suggested based on current experiments. The experimental data and analysis presented here is critical in the development, validation and improvement of kinetic models of these promising Co-Optima fuels. [ABSTRACT FROM AUTHOR]