High-pressure Shock Tube Facility Research Articles

Ignition Delay Times and Chemical Kinetic Model Validation for Hydrogen and Ammonia Blending With Natural Gas at Gas Turbine Relevant Conditions

Abstract Ignition delay times from undiluted mixtures of natural gas (NG)/H2/Air and NG/NH3/Air were measured using a high-pressure shock tube at the University of Central Florida. The combustion temperatures were experimentally tested between 1000 and 1500 K near a constant pressure of 25 bar. As mentioned, mixtures were kept undiluted to replicate the same chemistry pathways seen in gas turbine combustion chambers. Recorded combustion pressures exceeded 200 bar due to the large energy release, hence why these were performed at the high-pressure shock tube facility. The data are compared to the predictions of the NUIGMech 1.1 mechanism for chemical kinetic model validation and refinement. An exceptional agreement was shown for stoichiometric conditions in all cases but strayed at lean and rich equivalence ratios, especially in the lower temperature regime of H2 addition and all temperature ranges of the baseline NG mixture. Hydrogen addition also decreased ignition delay times by nearly 90%, while NH3 fuel addition made no noticeable difference in ignition time. NG/NH3 exhibited similar chemistry to pure NG under the same conditions, which is shown in a sensitivity analysis. The reaction CH3 + O2 = CH3O + O is identified and suggested as a possible modification target to improve model performance. Increasing the robustness of chemical kinetic models via experimental validation will directly aid in designing next-generation combustion chambers for use in gas turbines, which in turn will greatly lower global emissions and reduce greenhouse effects.

Evaluation of high-pressure syngas ignition under high-CO2 dilution in shock tubes

Due to its importance as a candidate fuel for power cycles for future energy production, there have been active efforts to improve our understanding of the chemical kinetics of the Allam-Fetvedt cycle that utilizes syngas highly diluted with CO2. Shock-tube ignition delay times (IDT) of syngas-CO2 mixtures over a range of pressures, syngas blends, and equivalence ratios are now available in the literature, but their unique behavior deserves further study before the resulting data can be used to modify existing syngas kinetics models. In this study, an analysis of CO2-diluted syngas ignition behind reflected shock waves was conducted at the High-Pressure Shock Tube facility at Texas A&M University using an endwall imaging system. A syngas blend of 1:1 H2:CO at an equivalence ratio of 1 was studied at pressures around 12 atm for CO2 dilutions of 85, 88, and 94 % by volume. The combined results of the high-speed OH* imaging through the endwall and OH* time histories from sidewall windows indicate that the initial ignition event appears to be homogeneous and occurs first near the endwall, with no apparent premature ignition or localized flame kernels. An important finding of this investigation is that the existing syngas-CO2 IDT data in the literature are likely free from shock-tube ignition inhomogeneities. Therefore, they can be used to improve existing chemical kinetics mechanisms, which currently have trouble modeling the existing literature data over the range of temperatures of the shock-tube studies (∼1000 – 1400 K). Additionally, the lack of a measured post-combustion pressure rise makes it necessary to use a constant-pressure-enthalpy constraint when modeling the shock-tube IDT event.

Ignition kinetics of nitrocyclohexane behind reflected shock waves in inert and air environments

Nitrocyclohexane is an important propellant within the class of nitroalkane based energetic fuels characterized by the nitro functional group. Although several studies were reported on the ignition of small nitroalkanes, very few kinetic studies are performed on their ignition characteristics under oxygen-free conditions. Herein, the ignition kinetics of nitrocyclohexane in inert and air environments are studied using a high-pressure shock tube facility. The experiments are performed at pressure of 2, 5, 10, and 20 bar with equivalence ratios of 0.5, 1.0 and 2.0 under the temperature range from 950 K to 1420 K. The effect from fuel concentrations at 0.61%, 1.14% and 2.05% with nitrogen as the inert gas are also studied. Comparisons of the ignition kinetics with other fuels including nitromethane, 1-nitropropane, methylcyclohexane, and cyclohexylamine are performed. The present manuscript provides the first experimental data for nitroalkanes in inert conditions, and should be valuable to understand the combustion chemistry of nitroalkanes.

A comprehensive experimental and kinetic modeling study of di-isobutylene isomers: Part 2

A wide variety of high temperature experimental data obtained in this study complement the data on the oxidation of the two di-isobutylene isomers presented in Part I and offers a basis for an extensive validation of the kinetic model developed in this study. Due to the increasing importance of unimolecular decomposition reactions in high-temperature combustion, we have investigated the di-isobutylene isomers in high dilution utilizing a pyrolysis microflow reactor and detected radical intermediates and stable products using vacuum ultraviolet (VUV) synchrotron radiation and photoelectron photoion coincidence (PEPICO) spectroscopy. Additional speciation data at oxidative conditions were also recorded utilizing a plug flow reactor at atmospheric pressure in the temperature range 725–1150 K at equivalence ratios of 1.0 and 3.0 and at residence times of 0.35 s and 0.22 s, respectively. Combustion products were analyzed using gas chromatography (GC) and mass spectrometry (MS). Ignition delay time measurements for di-isobutylene were performed at pressures of 15 and 30 bar at equivalence ratios of 0.5, 1.0, and 2.0 diluted in ‘air’ in the temperature range 900–1400 K using a high-pressure shock-tube facility. New measurements of the laminar burning velocities of di-isobutylene/air flames are also presented. The experiments were performed using the heat flux method at atmospheric pressure and initial temperatures of 298–358 K. Moreover, data consistency was assessed with the help of analysis of the temperature and pressure dependencies of laminar burning velocity measurements, which was interpreted using an empirical power-law expression. Electronic structure calculations were performed to compute the energy barriers to the formation of many of the product species formed. The predictions of the present mechanism were found to be in adequate agreement with the wide variety of experimental measurements performed.

Ignition delay time measurements of primary reference fuel blends

Ignition delay times of four different primary reference fuels (PRF), mixtures of n-heptane and iso-octane, were measured behind reflected shock waves in a high-pressure shock tube facility. The PRFs were formulated to match the RON of two high-octane gasolines (RON 95 and 91) and two prospective low-octane naphtha fuels (RON 80 and 70). Experiments were carried out over a wide range of temperatures (700–1200K), pressures (10, 20, and 40bar) and equivalence ratios (0.5 and 1). Kinetic modeling predictions from four chemical kinetic mechanisms are compared with the experimental data. Ignition delay correlations are developed to reproduce the measured ignition delay times. Brute force sensitivity analyses are carried out to identify reactions that affect ignition delay times at specific temperature, pressure and equivalence ratio. The large experimental data set provided in the current work will serve as a benchmark for the validation of chemical kinetic mechanisms of primary reference fuel blends.

Mixed butanols addition to gasoline surrogates: Shock tube ignition delay time measurements and chemical kinetic modeling

The demand for fuels with high anti-knock quality has historically been rising, and will continue to increase with the development of downsized and turbocharged spark-ignition engines. Butanol isomers, such as 2-butanol and tert-butanol, have high octane ratings (RON of 105 and 107, respectively), and thus mixed butanols (68.8% by volume of 2-butanol and 31.2% by volume of tert-butanol) can be added to the conventional petroleum-derived gasoline fuels to improve octane performance. In the present work, the effect of mixed butanols addition to gasoline surrogates has been investigated in a high-pressure shock tube facility. The ignition delay times of mixed butanols stoichiometric mixtures were measured at 20 and 40 bar over a temperature range of 800–1200 K. Next, 10 vol% and 20 vol% of mixed butanols (MB) were blended with two different toluene/n-heptane/iso-octane (TPRF) fuel blends having octane ratings of RON 90/MON 81.7 and RON 84.6/MON 79.3. These MB/TPRF mixtures were investigated in the shock tube conditions similar to those mentioned above. A chemical kinetic model was developed to simulate the low- and high-temperature oxidation of mixed butanols and MB/TPRF blends. The proposed model is in good agreement with the experimental data with some deviations at low temperatures. The effect of mixed butanols addition to TPRFs is marginal when examining the ignition delay times at high temperatures. However, when extended to lower temperatures (T < 850 K), the model shows that the mixed butanols addition to TPRFs causes the ignition delay times to increase and hence behaves like an octane booster at engine-like conditions.

Shock tube and kinetic study of C2H6/H2/O2/Ar mixtures at elevated pressures

Using a high-pressure shock tube facility, the ignition delay times of stoichiometric C2H6/H2/O2 diluted in argon were obtained behind reflected shock wave at elevated pressures (p = 1.2, 4.0 and 16.0 atm) with ethane blending ratios from 0 to 100%. The measured ignition delay times were compared to the previous correlations, and the results show that the ignition delay times of ethane from different studies exhibit an obvious difference. Meanwhile, numerical studies were conducted with three generally accepted kinetic mechanisms and the results show that only NUIG Aramco Mech 1.3 agrees well with the measurements under all test conditions. Sensitivity analysis was made to interpret the poor prediction of the other two mechanisms. Furthermore, the effect of ethane blending ratio on the ignition delay times of the mixtures was analyzed and the results show that ethane blending ratio gives a non-linear effect on the auto-ignition of hydrogen. Finally, chemical interpretations on this non-linear effect were made from the reaction pathway analysis and normalized H radical consumption analysis.

Ignition and kinetic modeling of methane and ethane fuel blends with oxygen: A design of experiments approach

A series of shock-tube experiments and chemical kinetics modeling calculations were performed to investigate the ignition behavior of methane and ethane with oxygen in regions which are not presently well understood and in a much more comprehensive manner than what has been done previously. Test conditions were determined using a statistical Design of Experiments approach which allows the experimenter to probe a wide range of variable factors with a comparatively low number of experimental trials. A matrix of 22 mixtures was developed using this statistical approach for binary fuel blends of 100% methane to 100% ethane; pressure ranges of 1, 11–16, and 25–31atm; equivalence ratios of 0.5, 1.0, and 2.0; over a temperature range of 1154–2248K, and argon dilutions of 98%, 95%, 85%, and 75%. Details on the relatively new high-pressure shock-tube facility are also provided. The experimental results were used to validate a detailed chemical kinetics model. The model considers hydrocarbons C1–C4 and has been developed in a hierarchical manner grounded with fundamental kinetics and experimentally validated by data from shock tubes and rapid compression machines, flow and jet-stirred reactors, and flame speed measurements. The important reactions are highlighted and the pertinent rate constants are described.

High-pressure shock tube studies on graphite oxidation reactions with carbon dioxide and water

Heterogeneous reactions involving graphite particles reacting with gaseous carbon dioxide and steam were studied under high-pressure and high-temperature conditions. The high-pressure shock tube facility at the University of Illinois at Chicago was utilized to perform the heterogeneous experiments with post-shock pressures ranging from 209 to 363 atm, temperatures ranging between 1546 and 2692 K, and nominal reaction times of 1 ms. Graphite particles were injected into the shock tube, by means of a particle injector, containing mixtures of varying concentrations of H 2O or CO 2 in Ar as balance. The overall reaction rate coefficients from these graphite experiments have been calculated and compared to previous work performed with carbon black particles as well as theoretical and experimental values from prior investigators.

High-Pressure Shock Tube Studies on Carbon Oxidation Reactions with Carbon Dioxide and Water

The heterogeneous reactions of solid carbon reacting with gaseous carbon dioxide and steam have been studied under high-pressure and high-temperature conditions. The high-pressure shock tube facility at the University of Illinois at Chicago was used to perform experiments with post-shock pressures ranging from 40 to 412 atm, temperatures ranging between 1404 and 2534 K, and nominal reaction times of 1 ms. Amorphous carbon particles were injected into the shock tube by means of a particle injector specifically designed for these experiments. The carbon particles were injected into the shock tube containing mixtures of varying concentrations of H2O or CO2 in Ar as balance. The overall reaction rates have been calculated and compared to rates of previous theoretical and experimental values. The wide temperature range of the experimental conditions allowed for the observation of both reaction- and transport-limiting regimes. Using theoretical techniques for computing the species transport properties, the transition temperatures where transport dominates the reaction were determined and match the experimental results.