Study of the H+O+M reaction forming OH∗: Kinetics of OH∗ chemiluminescence in hydrogen combustion systems (original) (raw)
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The temporal variation of OH * chemiluminescence in hydrogen oxidation chemistry has been studied behind re- flected shock waves at temperatures of 1100-3000 K, at a pressure of 1 bar. The aim of the present wor k was to obtain a validated reaction scheme to describe OH * formation. The main pathway of OH * formation in hydrogen oxidation at the measured temperature is attributed to H + O + M → OH * + M with a derived rate coefficient of 1.5 ×10 13 exp(-25 kJmol -1/RT ) cm 6mol -2s-1. The numerical simulation allows the qualitative a nalysis for the absolute OH * concentration based on laminar flame data from the literature.
Experimental study of chemiluminescence in UV and VIS range at hydrogen-oxygen mixtures ignition
MATEC Web of Conferences, 2018
The nonequilibrium radiation in the spectral range of 210-415 nm at ignition of a 10% stoichiometric hydrogen-oxygen mixture with additives of combustion inhibitors diluted with argon behind shock waves was registered. The detected chemiluminescence is presumably attributed to electronically excited H2O * and H2O2 *. Instead of the expected quenching of excited radicals and molecules in the ignition zone, with the addition of halogenated hydrocarbons inhibitors, the increase of radiation, particularly in the range of 330-415 nm, was observed. The possible reasons of this phenomenon are discussed.
Applied Physics B, 2012
The temporal variation of chemiluminescence emission from OH * (A 2 Σ +) and CH * (A 2 Δ) in reacting Ar-diluted H 2 /O 2 /CH 4 , C 2 H 2 /O 2 and C 2 H 2 /N 2 O mixtures was studied in a shock tube for a wide temperature range at atmospheric pressures and various equivalence ratios. Time-resolved emission measurements were used to evaluate the relative importance of different reaction pathways. The main formation channel for OH * in hydrocarbon combustion was studied with CH 4 as benchmark fuel. Three reaction pathways leading to CH * were studied with C 2 H 2 as fuel. Based on well-validated groundstate chemistry models from literature, sub-mechanisms for OH * and CH * were developed. For the main OH *forming reaction CH + O 2 = OH * + CO, a rate coefficient of k 2 = (8.0 ± 2.6) × 10 10 cm 3 mol −1 s −1 was determined. For CH * formation, best agreement was achieved when incorporating reactions C 2 + OH = CH * + CO (k 5 = 2.0 × 10 14 cm 3 mol −1 s −1) and C 2 H + O = CH * + CO (k 6 = 3.6 × 10 12 exp(−10.9 kJ mol −1 /RT) cm 3 mol −1 s −1) and neglecting the C 2 H + O 2 = CH * + CO 2 reaction. 1 Introduction Spontaneous light emission from chemically excited species in combustion processes is frequently used for detecting
Experimental Studies on Ignition Delay Time for Hydrogen – Oxygen Mixtures at High Concentrations
This study addresses the ignition delay time for hydrogen-oxygen mixtures at high concentrations at the sudden rise in pressure. Using a shock tube facility, measurements on ignition delay times of hydrogen-oxygen mixtures diluted with argon (20-30%) were conducted. The experiments have been carried out under normal shock wave conditions at temperatures of 1000-1300 K, pressure of 0.450 bar for rich mixtures as well as lean mixtures.
Ignition Processes in Hydrogen-Oxygen Mixtures
Ignition processes in the hydrogen-oxygen system were simulated by solving the corresponding conservation equations (i.e., conservation of mass, energy, momentum, and species mass) for one-dimensional geometries using a detailed reaction mechanism and a mUltispecies transport model. An additional source term in the energy conservation allowed the treatment of induced ignition, and a realistic model for the destruction of reactive species at the vessel surface was used to treat auto-ignitions in static reactors. Spatial discretization using finite differences and an adaptive grid point system led to a differential-algebraic equation system, which was solved numerically by extrapolation or by backward differencing codes. Comparisons with experimental works show that one common reaction mechanism is able to simulate shock-tube-induced ignitions (modeled by treating the reaction system as a homogeneous mixture heated up by the shock wave) as well as the three explosion limits of the hydrogen-oxygen system. Minimum ignition energies are calculated for various mixture compositions, pressures, radii of the external energy source, and ignition times, and it is shown that for long ignition times the "uniform pressure assumption" is a quite good approximatiori for computin"g minimum ignition energies. .
Study of Hydrogen Combustion in an Oxygen Environment
High Temperature, 2018
In this paper, we analyze the hydrogen combustion and heat transfer in the combustion chamber in an oxygen environment with forced cooling of the combustion chamber. We simulated the combustion of a previously unmixed mixture of fuel and oxidizer with allowance for dissociation processes. The main parameters and composition of the combustion products are determined for different geometric dimensions of the combustion chamber and the excess oxidant coefficients. An increase in the internal diameter and length of the combustion chamber with an unchanged hydrogen consumption is shown to lead to a reduction in the actual underburning. In this case, the actual underburning is significant due to the dissociation of combustion products at high temperatures. The effect of oxidizer excess on underburning and the temperature of combustion products is studied.
Shock-tube investigation of key reactions for chemiluminescence in various combustion systems
2013
Existing combustion systems, especially gas turbines in power generation applications must be optimized with regard to the reduction of pollutant emission and increase of efficiency. Combustion under fuel-lean conditions is beneficial for a significant reduction of NO x and soot formation. However, these operating conditions can lead to undesired combustion phenomena such as combustion-induced oscillations and flame flash back which must be avoided. For this purpose, fundamental knowledge of the underlying chemical processes is required. Non-intrusive optical methods such as the use of chemiluminescence are potential practical approaches to provide combustion relevant information for the development of combustion apparatus and process control. This requires knowledge of the formation reactions of chemiluminescence as well as adequate kinetics models that link the light intensity to relevant combustion parameters such as local heat release. An accurate description of chemiluminescence fundamentally depends on the corresponding ground-state chemistry. For small hydrocarbons such as CH 4 and C 2 H 2 detailed reaction mechanisms already exist which were used as a base for the development of OH* and CH* sub-mechanisms in the present work. The present work was devoted to study the formation reactions of OH* and CH* chemiluminescence in shock tubes time-resolved detection of the emission with a photomultiplier with narrowband interference filters. The signals were compared to the corresponding excited-state species concentrations from simulations where based on established ground-state mechanisms, OH* and CH* kinetics models were compiled and validated with the experimental data from the present work. Based on the present work, the reactions H + O + M = OH* + M and CH + O 2 = OH* + CO are identified as the main OH* formation channels in hydrogen and hydrocarbon oxidation and their corresponding rate coefficients are determined as (1.5±0.45)×10 13 exp(−25.0 kJ mol −1 /RT) cm 6 mol −2 s −1 and (8.0±2.56)×10 10 cm 3 mol −1 s −1 , respectively. For CH* chemiluminescence the reactions C 2 + OH = CH* + CO and C 2 H + O = CH* + CO are the most important formation reactions and their underlying rate coefficients are (5.7±3.02)×10 13 cm 3 mol −1 s −1 and (1.0±0.53)×10 12 exp(−10.9 kJ mol −1 /RT) cm 3 mol −1 s −1 , respectively. While for small hydrocarbons well-known ground-state mechanisms are available, reliable kinetics models for ethanol oxidation, especially for high temperatures, are sparse. Therefore, the formation of important intermediates and products (e.g., OH, C 2 H 2 , and CO 2) was studied for ethanol oxidation by time-of-flight mass spectrometry and ring-dye laser absorption spectroscopy under shock-tube conditions. The experimental data were compared to simulations using different reaction mechanisms from the literature and recommendations for the improvement of the corresponding mechanisms were suggested. V Zusammenfassung Bestehende Verbrennungssysteme, insbesondere Gasturbinen für die Erzeugung von Strom, müssen in Hinblick auf die Reduzierung des Rohstoffeinsatzes und des Ausstoßes von Emissionen optimiert werden. Hierbei kann die Verbrennung unter mageren Mischungsbedingungen zu einer signifikanten Reduzierung der Stickoxid-und Rußbildung führen. Diese Betriebszustände führen jedoch teilweise zu unerwünschten Schwingungen und Flammenrückschlag innerhalb der Brennkammer, die vermieden werden müssen. Hierfür ist ein grundlegendes Wissen über den zugrundeliegenden Verbrennungsprozess erforderlich. Nichtinvasive optische Methoden wie das Flammenleuchten sind potentielle Ansätze zur Bereitstellung von verbrennungsrelevanten Informationen für die Entwicklung von Verbrennungskonzepten und deren Regelung. Dies erfordert jedoch zum einen die Kenntnis über die Bildungsreaktionen der Chemilumineszenz und zum anderen sind geeignete Kinetikmodelle zur Beschreibung erforderlich. Die Beschreibung der Chemilumineszenz erfordert genaue Kenntnis über die zugrundeliegende Grundzustandschemie. Für einfache Kohlenwasserstoffverbindungen wie z.B. CH 4 oder C 2 H 2 existieren bereits gut validierte Modelle, die in der vorliegenden Arbeit als Basis für die Entwicklung von OH*-und CH*-Mechanismen verwendet wurden. Im Rahmen dieser Arbeit wurden die Bildungsreaktionen der OH*-und CH*-Chemilumineszenz in Stoßwellenreaktoren mit Hilfe von Emissionsmessungen untersucht. Hierbei wurde das Flammleuchten mit einer Kombination aus Photomultiplier und schmalbandigem Interferenzfilter zeitaufgelöst gemessen. Basierend auf etablierten Mechanismen zur Beschreibung der Grundzustandschemie wurden Kinetikmodelle für OH*-und CH*-Chemilumineszenz aufgestellt und mithilfe der experimentellen Daten validiert. Die Reaktionen H + O + M = OH* + M und CH + O 2 = OH* + CO wurden als Hauptreaktionen für die Bildung von OH* bei der Oxidation von Wasserstoff oder Kohlenwasserstoffen identifiziert und ihre zugrundeliegenden Geschwindigkeitskoeffizienten wurden ermittelt mit (1.5±0.45)×10 13 exp(−25.0 kJ mol −1 /RT) cm 6 mol −2 s −1 bzw. (8.0±2.56)×10 10 cm 3 mol −1 s −1. Für CH*-Chemilumineszenz wurden die Reaktionen C 2 + OH = CH* + CO und C 2 H + O = CH* + CO als wichtigste Bildungsreaktionen identifiziert und mit den Geschwindigkeitskoeffizient (5.7±3.02)×10 13 cm 3 mol −1 s −1 bzw. (1.0±0.53)×10 12 exp(−10.9 kJ mol −1 /RT) cm 3 mol −1 s −1. Während für kleine Kohlenwasserstoffe etablierte Mechanismen vorliegen, ist der Reaktionsmechanismus der Verbrennung von Ethanol, insbesondere bei hohen Temperaturen, nur unzureichend bekannt. Daher wurde im Rahmen dieser Arbeit die Bildung von wichtigen Intermediaten und Produkten (u.a. OH, C 2 H 2 , CO 2) bei der Oxidation von Ethanol im Stoßwellenrohr mittels Flugzeit-Massenspektrometrie und Farbstoff-Ringlaser-Absorptionsspektroskopie untersucht und mit verschiedenen Reaktionsmechanismen verglichen, die zusätzliche Daten zur Verbesserung und weiteren Validierung der bestehenden Modelle liefern. was successfully applied to characterize and to quantify spatially-resolved CH 2 O and OH concentrations. These species are combustion-relevant intermediates and their combined concentrations correlate with the local heat release. The benefit of optical measurements is its non-intrusive nature which allows to study combustion processes without disturbing them. In harsh environments of practical applications, laser-diagnostic techniques, however, are not suitable for in-situ measurements. These techniques require an external light source and optical ports to couple the laser beam into the combustion chamber. Common industrial combus-Reaction kinetics 2 tors have limitations in the available geometry and are originally not designed to provide optical accessibility. Moreover, additional technical modifications would affect the combustion process. The required laser and imaging system make optical diagnostic very complex and expensive for practical applications. These disadvantages rule out conventional laser-based diagnostics for many field applications. Hence, less costly and straightforward optical techniques are desired. Luminescence of flames from chemical excitation of specific intermediate species, the so called chemiluminescence (CL), is a promising tool that can potentially provide information about local heat release [6-8] and equivalence ratios [9-11] once the underlying mechanisms are well enough understood. Emission of UV-and visible light from electronically-excited species is a characteristic of hydrocarbon combustion. The most common chemiluminescent species are OH*, CH*, C 2 *, and CO 2 *, where the asterisk denotes electronic excitation as a consequence of chemical reactions. Chemiluminescence investigation is an important tool in the field of combustion research. The correlation of combustion relevant parameters such as heat release and fuel/air ratio with the chemiluminescence emission of excited state species was subject of many investigations [6, 12-15]. These studies showed that chemiluminescence can be used to spatially-resolve flame fronts [16] and to measure heat release [5, 17] and local equivalence ratios [6, 18-20]. Due to their simplicity, chemiluminescence sensors are desirable and can be easily designed for practical applications. However, the fundamental chemical kinetics leading to chemiluminescence, which is required for these applications, is still under debate. Overall, the capability to provide combustion-relevant information in combination with the simplicity of the detection system makes chemiluminescence very attractive for practical applications. This, however, requires the coupling of chemiluminescence signals with the underlying chemical processes in a quantitative manner. A quantitative and direct coupling between measured light intensity and the relevant combustion parameter (chemiluminescent species concentration, heat release rate or local equivalence ratio) is not straightforward. An interpretation of the measured signals can be done by linking the measured chemiluminescence intensities with the corresponding species concentrations taken from kinetics mechanisms. In conventional ground-state mechanisms, chemiluminescence and its formation pathways are not considered because electronically excited species are several orders of magnitude less abundant compared to ground-state species. Therefore, chemiluminescent species have no influence on the global combustion process and are mostly not included in the ground-state mechanisms. For a quantitative investigation of the chemiluminescence, the available groundstate mechanisms must be extended by sub-models to describe the chemiluminescence pathways. Based on the low concentrations of chemiluminescent species and the sophisticated interpretation, the characterization of the responsible formation reactions leading to chemiluminescence
A Shock Tube Study of OH + H2O2→ H2O + HO2and H2O2+ M → 2OH + M using Laser Absorption of H2O and OH
The Journal of Physical Chemistry A, 2010
The rate constants of the reactions were measured in shock-heated H 2 O 2 /Ar mixtures using laser absorption diagnostics for H 2 O and OH. Timehistories of H 2 O were monitored using tunable diode laser absorption at 2550.96 nm, and time-histories of OH were achieved using ring dye laser absorption at 306 nm. Initial H 2 O 2 concentrations were also determined utilizing the H 2 O diagnostic. On the basis of simultaneous time-history measurements of OH and H 2 O, k 2 was found to be 4.6 × 10 13 exp(-2630 K/T) [cm 3 mol-1 s-1 ] over the temperature range 1020-1460 K at 1.8 atm; additional measurements of k 2 near 1 atm showed no significant pressure dependence. Similarly, k 1 was found to be 9.5 × 10 15 exp(-21 250 K/T) [cm 3 mol-1 s-1 ] over the same temperature and pressure range.
Studies of premixed and non-premixed hydrogen flames
Combustion and Flame, 2014
The hydrogen oxidation chemistry constitutes the foundation of the kinetics of all carbon- and hydrogen-containing fuels. The validation of rate constants of hydrogen-related reactions can be complicated by uncertainties associated with experimental data caused by the high reactivity and diffusivity of hydrogen. In the present investigation accurate experimental data on flame propagation and extinction were determined for premixed and non-premixed hydrogen flames at pressures between p = 1 and 7 atm. The experiments were designed to sensitize the three-body H + O2 + M → HO2 + M reaction, whose rate is subject to notable uncertainty. This was achieved by increasing the pressure and by adding to the reactants H2O and CO2 whose collision efficiencies are high compared to other species. In the present study, directly measured flame properties were compared against computed ones, in order to eliminate uncertainties associated with extrapolations, as is the case for laminar flame speeds. ...