Rolf Bombach - Academia.edu (original) (raw)
Papers by Rolf Bombach
Homogeneous ignition of CH4/air and H2O and schemes, which were ascribed to their ability to corr... more Homogeneous ignition of CH4/air and H2O and schemes, which were ascribed to their ability to correctly capture the p–T–u parameter range of the self-inhibited ignition behavior of methane. Comparisons between measured and predicted homogeneous The application of catalytically stabilized com-bustion (CST) to large-scale gas turbines has been actively pursued over the last years [1] as a means
Combustion and Flame
The gas-phase combustion of H2/O2/N2 mixtures over platinum was investigated experimentally and n... more The gas-phase combustion of H2/O2/N2 mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gasphase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase con...
Proceedings of the Combustion Institute, 2021
Colloquium topic: New Concepts Word count Text (using Microsoft Word Count) 3549 References 629 (... more Colloquium topic: New Concepts Word count Text (using Microsoft Word Count) 3549 References 629 (34 references + 2 x 2.3 x 7.6) Tables 129 (Table 1: 15 lines + 2 x 7.6) Equations 23 (Eq. (1), 1 line + 2 x 7.6
Proceedings of the Combustion Institute, 2021
Broadband combustion noise is a major part of the total noise radiated by modern jet engines. It ... more Broadband combustion noise is a major part of the total noise radiated by modern jet engines. It comprises two components: direct noise originating from the unsteady heat release of the flame, and indirect noise resulting from the acceleration of entropy fluctuations in the turbine stages. Not only do these entropy waves contribute to the noise pollution of aeroengines, they can also have a crucial role in the feedback loop leading to thermoacoustic instabilities, which induce vibrations and thermal loads that are highly detrimental for combustors and turbine components. Thus, there is a critical need to understand and model the complex mechanisms associated with the generation and the advection of these waves. This study presents quantitative measurements of the production of entropy waves in a technically premixed turbulent combustor, subject to acoustic forcing. Entropy transfer function (ETF) relating acoustic input, obtained with microphones, to entropy wave output, obtained with OH-LIF thermometry at a distance of four flame heights from the burner outlet, were measured between 40 Hz and 90 Hz. These ETF were obtained using two burners of same length with technical premixing of air and natural gas, operated at the same thermal power: a matrix burner producing an array of turbulent jet flames, and a burner producing a single swirled turbulent flame. It is found that the ETF of the matrix burner exhibits a low-pass behavior, with a gain ranging from approximately 0.7 at 40 Hz down to 0.25 at 90 Hz, while the gain of the swirled flame ETF was not exceeding 0.2. It is also demonstrated that entropy wave production with the matrix burner is highly nonlinear, with a dramatic drop of the ETF gain occurring beyond a certain level of acoustic forcing amplitude. These measurements can be used to derive predictive nonlinear models of thermoacoustic instabilities involving entropy wave feedback.
Proceedings of the Combustion Institute, 2019
The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was i... more The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was investigated in an optically accessible Rapid Compression-Expansion Machine at dual-fuel engine-like pressure/temperature conditions. The pilot fuel was admitted using a coaxial single-hole 100µm injector mounted on the cylinder periphery. Optical diagnostics include combined high-speed CH 2 O-PLIF (10kHz) and Schlieren (80kHz) imaging for detection of the firststage ignition, and simultaneous high-speed OH* chemiluminescence (40kHz) imaging for high-temperature ignition. The aim of this study is to enhance the fundamental understanding of the interaction of methane with the auto-ignition process of short pilot-fuel injections. Addition of methane into the air charge considerably prolongs ignition delay of the pilot spray with an increasing effect at lower temperatures and with higher methane/air equivalence ratios. The temporal separation of the first CH 2 O detection and high-temperature ignition was found almost constant regardless of methane content. This was interpreted as methane mostly deferring the cool-flame reactivity. In order to understand the underlying mechanisms of this interaction, experimental investigations were complemented with 1D-flamelet simulations using detailed chemistry, confirming the chemical influence of methane deferring the reactivity in the pilot-fuel lean mixtures. This shifts the onset of first-stage reactivity towards the fuel-richer conditions. Consequently, the onset of the turbulent cool-flame is delayed, leading to an overall increased high-temperature ignition delay. Overall, the study reveals a complex interplay between entrainment, low-T and high-T chemistry and micro-mixing for dual-fuel autoignition processes for which the governing processes were identified.
Combustion and Flame, 2017
Hydrogen combustion over platinum at rich stoichiometries Impact of pressure on homogeneous ignit... more Hydrogen combustion over platinum at rich stoichiometries Impact of pressure on homogeneous ignition In situ Raman measurements Hot-O 2 and OH planar laser induced fluorescence (LIF) Catalytic-rich/gaseous-lean combustion concept a b s t r a c t The hetero-/homogeneous combustion of fuel-rich H 2 /O 2 /N 2 mixtures (equivalence ratios ϕ = 2.5-6.5) was investigated experimentally and numerically in a platinum-coated channel at pressures p = 1-14 bar. One-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer assessed the heterogeneous combustion processes, while planar laser induced fluorescence (LIF) of OH at pressures below ∼5 bar and of hot-O 2 at pressures above ∼5 bar (wherein OH-LIF was not applicable) determined the onset of homogeneous ignition. Simulations were carried out using a 2-D code with detailed hetero-/homogeneous chemical reaction schemes and transport. Both Raman measurements and numerical simulations attested a transport-limited catalytic conversion of the deficient O 2 reactant over the gas-phase induction zones. The agreement between measured and predicted homogeneous ignition distances was better than 12%, thus establishing the aptness of the employed hetero-/homogeneous chemical reaction mechanisms. Analytical homogeneous ignition criteria revealed that the catalytic reaction pathway introduced a scaling factor 1/ p to the homogeneous ignition distances. This outcome, in conjunction with the intricate pressure dependence of the gaseous ignition chemistry of hydrogen, yielded shorter homogeneous ignition distances at 14 bar compared to 1 bar. The practical implication for gas turbine burners utilizing the catalytic-rich/gaseous-lean combustion concept was that the high operating pressures of such systems promoted the onset of homogeneous ignition within the catalytic module. Sensitivity analysis has finally identified the key catalytic and gaseous reactions affecting homogeneous ignition.
Cars Temperature Measurements in a Lean, Turbulent, 120KW Natural Gas Flame
International Journal of Energetic Materials and Chemical Propulsion, 1994
Proceedings of the Combustion Institute, 2013
The catalytic oxidation of CO and CO/H 2 fuel mixtures over platinum was investigated experimenta... more The catalytic oxidation of CO and CO/H 2 fuel mixtures over platinum was investigated experimentally and numerically at fuel-lean stoichiometries, pressure of 5 bar and a surface temperature range relevant for large-turbine and micro-turbine based power generation systems (600-800 K). Experiments were performed in an optically accessible catalytic channel flow reactor with the application of in situ, spatially resolved 1-D Raman measurements of gas-phase species concentrations across the reactor boundary layer in order to determine the catalytic reactivity, and planar OH-LIF for monitoring the potential onset of homogeneous ignition. Numerical simulations were carried out with a 2-D elliptic CFD code in conjunction with a detailed heterogeneous kinetic scheme and transport. The reaction scheme, constructed by implementing recent updates to an existing kinetic model, reproduced the Raman-measured species profiles of either pure CO or combined CO/H 2 fuels under all examined conditions. The addition of hydrogen promoted kinetically the oxidation of CO at wall temperatures as low as 610 K, whereby the catalytic reactions of H 2 were fully lit and the CO conversion was mixed transport/kinetically controlled. This low temperature limit was of prime interest to idling operation for large gas turbines and to normal operation for recuperative microturbine systems. Moreover, kinetic analysis suggested that the promoting impact of H 2 addition on CO oxidation was due to indirect influence of hydrogen reactions on the surface species coverage, whereas direct coupling steps between CO and H 2 were of minor importance. Finally, simulations indicated that for wall temperatures less than 520 K, which were well-below the minimum reactant inlet temperatures in power generation systems, the added H 2 inhibited the oxidation of CO.
Sound Generating Flames of a Gas Turbine Burner Observed by Laser-Induced Fluorescence
In this contribution, an initial investigation of the ignition behavior of large two-stroke marin... more In this contribution, an initial investigation of the ignition behavior of large two-stroke marine diesel sprays has been performed. At engine-like conditions, the OH radical was traced with an intensified high speed camera and a sophisti-cated optical setup. A series of spectroscopic measurements showed, however, that the soot incandescence strongly con-tributes to the UV signal, superimposing with or even masking the chemiluminescence of the OH radical. As the com-bustion of typical fuels used in large two-stroke engines involves the formation of non-negligible amounts of soot, the signal is almost omnipresent during the oxidation process. A differentiation between the UV-light emitted by the OH radical and the UV-light emitted by soot incandescence is only possible when both signals are measured separately. Therefore, a second high speed camera recorded the light coming from soot incandescence. In addition, it recorded the background illuminated spray plume to make an exact posit...
COMODIA2012 Wartsila A.Schmid
Clean Air: International Journal on Energy for a Clean Environment, 2004
The applicability of various hetero/homogeneous chemical reaction schemes in the catalytically st... more The applicability of various hetero/homogeneous chemical reaction schemes in the catalytically stabilized combustion (CST) of hydrogen/air mixtures over platinum was investigated experimentally and numerically in channelflow configurations. The present work stems from the internationally intensified effort to commercialize power generation systems with ultra-low NOx (< 3 ppm), CSTbased gas turbines. Crucial in the development of such systems is the understanding of the heterogeneous (catalytic) kinetics, the homogeneous (gas-phase) kinetics, and their respective coupling. Experiments were performed at atmospheric pressure and laminar flow conditions in an optically-accessible catalytic channel combustor: planar laser induced fluorescence (PLIF) of the OH radical was used to monitor the onset of homogeneous ignition and line-Raman measurements provided the boundary-layer profiles of the temperature and major species. Measured homogeneous ignition distances were compared against numerical predictions obtained with a two-dimensional elliptic fluid mechanical model. Six homogeneous (elementary, reduced and single-step schemes) and three heterogeneous schemes were tested in the model. The comparisons have revealed substantial differences between measured and predicted homogeneous ignition distances (ranging from 8% to 85%) and, moreover, these differences were attributed primarily to the homogeneous reaction pathway.
Study of a Rich/Lean Staged Combustion Concept for Hydrogen at Gas Turbine Relevant Conditions
Volume 1A: Combustion, Fuels and Emissions, 2013
The combustion of hydrogen-rich fuels (> 80 % vol. H2), relevant for gas turbine cycles with “... more The combustion of hydrogen-rich fuels (> 80 % vol. H2), relevant for gas turbine cycles with “pre-combustion” carbon capture, creates great challenges in the application of standard lean premix combustion technology. The significant higher flame speed and drastically reduced auto-ignition delay time of hydrogen compared to those of natural gas, which is normally burned in gas turbines, increase the risk of higher NOX emissions and material damage due to flashback. Combustion concepts for gas turbines operating on hydrogen fuel need to be adapted to assure safe and low-emission combustion. A rich/lean (R/L) combustion concept with integrated heat transfer that addresses the challenges of hydrogen combustion has been investigated. A sub-scale, staged burner with full optical access has been designed and tested at gas turbine relevant conditions (flame temperature of 1750 K, preheat temperature of 400 °C and a pressure of 8 bar). Results of the burner tests have confirmed the capabi...
Proceedings of the Combustion Institute, 2013
The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigate... more The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigated experimentally and numerically at pressures of 1-14 bar, equivalence ratios of 0.1-0.5, and surface temperatures ranging from 700 to 1300 K. Experiments were carried out in an optically accessible channel-flow reactor and included in situ 1-D Raman measurements of major gas phase species concentrations across the channel boundary layer for determining the catalytic reactivity, and planar laser induced fluorescence (LIF) of the OH radical for assessing homogeneous ignition. Numerical simulations were performed with a 2-D CFD code with detailed hetero-/homogeneous C 2 kinetic mechanisms and transport. An appropriately amended heterogeneous reaction scheme has been proposed, which captured the increase of ethane catalytic reactivity with rising pressure. This scheme, when coupled to a gas-phase reaction mechanism, reproduced the combustion processes over the reactor extent whereby both heterogeneous and homogeneous reactions were significant and moreover, provided good agreement to the measured homogeneous ignition locations. The validated hetero-/homogeneous kinetic schemes were suitable for modeling the catalytic combustion of ethane at elevated pressures and temperatures relevant to either microreactors or large-scale gas turbine reactors in power generation systems. It was further shown that the pressure dependence of the ethane catalytic reactivity was substantially stronger compared to that of methane, at temperatures up to 1000 K. Implications for high-pressure catalytic combustion of natural gas were finally drawn.
An experimental and numerical investigation of the hetero-/homogeneous combustion of fuel-rich hydrogen/air mixtures over platinum
Proceedings of the Combustion Institute, 2013
Abstract The hetero-/homogeneous combustion of hydrogen/air mixtures over platinum was investigat... more Abstract The hetero-/homogeneous combustion of hydrogen/air mixtures over platinum was investigated experimentally and numerically in a channel-flow configuration at fuel-rich equivalence ratios ranging from 2 to 7, pressures up to 5 bar and wall temperatures 760–1200 K. Experiments involved in situ one-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical, while simulations included an elliptic 2-D model with detailed heterogeneous and homogeneous reaction mechanisms. The employed reaction schemes reproduced the measured catalytic reactant consumption, the onset of homogeneous ignition, and the post-ignition flame shapes at all examined conditions. Although below a critical pressure, which depended on temperature, the intrinsic gas-phase kinetics of hydrogen dictated lower reactivity for the fuel-rich stoichiometries when compared to fuel-lean ones, homogeneous ignition was still more favorable for the rich stoichiometries due to the lower molecular transport of the deficient oxygen reactant that resulted in modest catalytic reactant consumption over the gaseous induction zone. Above the critical pressure, the intrinsic gaseous hydrogen kinetics yielded higher reactivity for the rich stoichiometries, which resulted in vigorous gaseous combustion at pressures up to 5 bar, in contrast to lean stoichiometry studies whereby homogeneous combustion was altogether suppressed above 3 bar. Computations at fuel-rich stoichiometries in practical channel geometries indicated that homogeneous combustion was not of concern for reactor thermal management, since the larger than unity Lewis number of the deficient oxygen reactant confined the flames to the core of the channel, away from the solid walls.
Experimental Setup of a Laser Diagnostics System for a High-Temperature Solar Receiver/Reactor
Journal of Solar Energy Engineering, 1994
A solar receiver/reactor has been designed specifically to study high-temperature gas phase chemi... more A solar receiver/reactor has been designed specifically to study high-temperature gas phase chemical reactions using a laser based metrology. It is a cavity-type receiver, lined with stabilized ZrO2, and operated at temperatures up to 2000 K. The gas temperature is measured in situ using the coherent anti-Stokes Raman spectroscopy (CARS) of N2. Optical access for the CARS measurement is accomplished via two side windows, each subtending a 118-mrad cone angle at the center of the cavity, providing enough clearance for the input laser beams and the output signal carrying the temperature information. Two endothermic processes were used for the initial evaluation of this method: the NH3 dissociation into N2 and H2, and the CO2-reforming of CH4 into synthesis gas. The process flow was directly exposed to high solar fluxes in addition to infrared radiation emitted by the hot reactor walls. The laser-based metrology performed satisfactorily in spite of the presence of the intense radiation...
Branching ratios and partition of the excess energy for the predissociation of CO+2 C̃ 2Σ+g molecular cations
The Journal of Chemical Physics, 1983
He–Iα photoelectron–photoion coincidence spectroscopy is used to study the decay of CO+2 molecula... more He–Iα photoelectron–photoion coincidence spectroscopy is used to study the decay of CO+2 molecular cations, initially prepared in various vibrational levels of the C̃ 2Σ+g state. The branching ratios for the energetically accessible fragmentation pathways are determined. Owing to the precisely known energetics and to an accurate measurement of the kinetic energies released, the partition of the respective excess energy among internal and external degrees of freedom of the separating fragments could be established. The formation of highly rotationally excited diatomic fragments must be proposed in order to explain the experimental results. The disappearance of O+ ions in favor of CO+ fragments as soon as the energy exceeds the dissociation limit for CO+ formation is consistent with a reinterpretation of previous ab initio calculations of potential-energy surfaces. The observed branching ratio for the population of the v=0 and v=1 levels of the CO fragment can be accounted for by an a...
Photoelectron–photoion coincidence spectroscopy of gas-phase clusters
The Journal of Chemical Physics, 1982
A photoelectron–photoion coincidence technique for obtaining the photoelectron spectrum of a sing... more A photoelectron–photoion coincidence technique for obtaining the photoelectron spectrum of a single component of a gas-phase mixture has been developed. It utilizes a newly designed instrument which measures the ion mass in coincidence with the photoelectron kinetic energy. Initial experiments were carried out on Xe2 and Xe3 produced in mixture of clusters (plus monomer) in a free-jet supersonic expansion. These measurements determined the photoelectron appearance potential (i.e., the lowest binding energy for which photoelectrons are detected) to be 11.30(5) eV for Xe3. It was also found that fragmentation of cluster ions strongly affects the coincidence spectra. This was investigated by varying the stagnation pressure, and thus the beam composition, in order to assess fragmentation contributions to coincidence spectra of the cluster under study. One case studied in detail indicated the energy levels of Xe3 near the ionization potential of 11.6 eV, corresponding to 0.7 eV of intern...
Fundamental Aspects of Ionic Dissociations: The Fragmentation Pathways of Excited Bicyclobutane Cations
Helvetica Chimica Acta, 1983
A most recently developed method to quantify the fragmentation pathways of excited radical cation... more A most recently developed method to quantify the fragmentation pathways of excited radical cations is presented. Using bicyclobutane cation as an illustrative example, the RRKM analysis of the breakdown diagram determined by He‐Iα photoelectron‐photoion coincidence spectroscopy is outlined. The results imply complete isomerization to 1,3‐butadiene cation preceding the dissociative processes. The rate‐energy functions of four competitive primary fragmentation reactions, leading to C3H, C4H, C4H and C2H are established. There is compelling evidence that the production of C2H fragment ions does not compete effectively with these four reactions. The extent of kinetic and competitive shift effects is determined. The derived enthalpies of formation are in excellent accord with the available high quality reference data. The relative importance of different fragmentation pathways which ultimately lead to fragment ions of identical mass to charge ratio is assessed.
Experiments in Fluids, 2008
Homogeneous ignition of CH4/air and H2O and schemes, which were ascribed to their ability to corr... more Homogeneous ignition of CH4/air and H2O and schemes, which were ascribed to their ability to correctly capture the p–T–u parameter range of the self-inhibited ignition behavior of methane. Comparisons between measured and predicted homogeneous The application of catalytically stabilized com-bustion (CST) to large-scale gas turbines has been actively pursued over the last years [1] as a means
Combustion and Flame
The gas-phase combustion of H2/O2/N2 mixtures over platinum was investigated experimentally and n... more The gas-phase combustion of H2/O2/N2 mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gasphase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase con...
Proceedings of the Combustion Institute, 2021
Colloquium topic: New Concepts Word count Text (using Microsoft Word Count) 3549 References 629 (... more Colloquium topic: New Concepts Word count Text (using Microsoft Word Count) 3549 References 629 (34 references + 2 x 2.3 x 7.6) Tables 129 (Table 1: 15 lines + 2 x 7.6) Equations 23 (Eq. (1), 1 line + 2 x 7.6
Proceedings of the Combustion Institute, 2021
Broadband combustion noise is a major part of the total noise radiated by modern jet engines. It ... more Broadband combustion noise is a major part of the total noise radiated by modern jet engines. It comprises two components: direct noise originating from the unsteady heat release of the flame, and indirect noise resulting from the acceleration of entropy fluctuations in the turbine stages. Not only do these entropy waves contribute to the noise pollution of aeroengines, they can also have a crucial role in the feedback loop leading to thermoacoustic instabilities, which induce vibrations and thermal loads that are highly detrimental for combustors and turbine components. Thus, there is a critical need to understand and model the complex mechanisms associated with the generation and the advection of these waves. This study presents quantitative measurements of the production of entropy waves in a technically premixed turbulent combustor, subject to acoustic forcing. Entropy transfer function (ETF) relating acoustic input, obtained with microphones, to entropy wave output, obtained with OH-LIF thermometry at a distance of four flame heights from the burner outlet, were measured between 40 Hz and 90 Hz. These ETF were obtained using two burners of same length with technical premixing of air and natural gas, operated at the same thermal power: a matrix burner producing an array of turbulent jet flames, and a burner producing a single swirled turbulent flame. It is found that the ETF of the matrix burner exhibits a low-pass behavior, with a gain ranging from approximately 0.7 at 40 Hz down to 0.25 at 90 Hz, while the gain of the swirled flame ETF was not exceeding 0.2. It is also demonstrated that entropy wave production with the matrix burner is highly nonlinear, with a dramatic drop of the ETF gain occurring beyond a certain level of acoustic forcing amplitude. These measurements can be used to derive predictive nonlinear models of thermoacoustic instabilities involving entropy wave feedback.
Proceedings of the Combustion Institute, 2019
The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was i... more The ignition behavior of n-dodecane micro-pilot spray in a lean-premixed methane/air charge was investigated in an optically accessible Rapid Compression-Expansion Machine at dual-fuel engine-like pressure/temperature conditions. The pilot fuel was admitted using a coaxial single-hole 100µm injector mounted on the cylinder periphery. Optical diagnostics include combined high-speed CH 2 O-PLIF (10kHz) and Schlieren (80kHz) imaging for detection of the firststage ignition, and simultaneous high-speed OH* chemiluminescence (40kHz) imaging for high-temperature ignition. The aim of this study is to enhance the fundamental understanding of the interaction of methane with the auto-ignition process of short pilot-fuel injections. Addition of methane into the air charge considerably prolongs ignition delay of the pilot spray with an increasing effect at lower temperatures and with higher methane/air equivalence ratios. The temporal separation of the first CH 2 O detection and high-temperature ignition was found almost constant regardless of methane content. This was interpreted as methane mostly deferring the cool-flame reactivity. In order to understand the underlying mechanisms of this interaction, experimental investigations were complemented with 1D-flamelet simulations using detailed chemistry, confirming the chemical influence of methane deferring the reactivity in the pilot-fuel lean mixtures. This shifts the onset of first-stage reactivity towards the fuel-richer conditions. Consequently, the onset of the turbulent cool-flame is delayed, leading to an overall increased high-temperature ignition delay. Overall, the study reveals a complex interplay between entrainment, low-T and high-T chemistry and micro-mixing for dual-fuel autoignition processes for which the governing processes were identified.
Combustion and Flame, 2017
Hydrogen combustion over platinum at rich stoichiometries Impact of pressure on homogeneous ignit... more Hydrogen combustion over platinum at rich stoichiometries Impact of pressure on homogeneous ignition In situ Raman measurements Hot-O 2 and OH planar laser induced fluorescence (LIF) Catalytic-rich/gaseous-lean combustion concept a b s t r a c t The hetero-/homogeneous combustion of fuel-rich H 2 /O 2 /N 2 mixtures (equivalence ratios ϕ = 2.5-6.5) was investigated experimentally and numerically in a platinum-coated channel at pressures p = 1-14 bar. One-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer assessed the heterogeneous combustion processes, while planar laser induced fluorescence (LIF) of OH at pressures below ∼5 bar and of hot-O 2 at pressures above ∼5 bar (wherein OH-LIF was not applicable) determined the onset of homogeneous ignition. Simulations were carried out using a 2-D code with detailed hetero-/homogeneous chemical reaction schemes and transport. Both Raman measurements and numerical simulations attested a transport-limited catalytic conversion of the deficient O 2 reactant over the gas-phase induction zones. The agreement between measured and predicted homogeneous ignition distances was better than 12%, thus establishing the aptness of the employed hetero-/homogeneous chemical reaction mechanisms. Analytical homogeneous ignition criteria revealed that the catalytic reaction pathway introduced a scaling factor 1/ p to the homogeneous ignition distances. This outcome, in conjunction with the intricate pressure dependence of the gaseous ignition chemistry of hydrogen, yielded shorter homogeneous ignition distances at 14 bar compared to 1 bar. The practical implication for gas turbine burners utilizing the catalytic-rich/gaseous-lean combustion concept was that the high operating pressures of such systems promoted the onset of homogeneous ignition within the catalytic module. Sensitivity analysis has finally identified the key catalytic and gaseous reactions affecting homogeneous ignition.
Cars Temperature Measurements in a Lean, Turbulent, 120KW Natural Gas Flame
International Journal of Energetic Materials and Chemical Propulsion, 1994
Proceedings of the Combustion Institute, 2013
The catalytic oxidation of CO and CO/H 2 fuel mixtures over platinum was investigated experimenta... more The catalytic oxidation of CO and CO/H 2 fuel mixtures over platinum was investigated experimentally and numerically at fuel-lean stoichiometries, pressure of 5 bar and a surface temperature range relevant for large-turbine and micro-turbine based power generation systems (600-800 K). Experiments were performed in an optically accessible catalytic channel flow reactor with the application of in situ, spatially resolved 1-D Raman measurements of gas-phase species concentrations across the reactor boundary layer in order to determine the catalytic reactivity, and planar OH-LIF for monitoring the potential onset of homogeneous ignition. Numerical simulations were carried out with a 2-D elliptic CFD code in conjunction with a detailed heterogeneous kinetic scheme and transport. The reaction scheme, constructed by implementing recent updates to an existing kinetic model, reproduced the Raman-measured species profiles of either pure CO or combined CO/H 2 fuels under all examined conditions. The addition of hydrogen promoted kinetically the oxidation of CO at wall temperatures as low as 610 K, whereby the catalytic reactions of H 2 were fully lit and the CO conversion was mixed transport/kinetically controlled. This low temperature limit was of prime interest to idling operation for large gas turbines and to normal operation for recuperative microturbine systems. Moreover, kinetic analysis suggested that the promoting impact of H 2 addition on CO oxidation was due to indirect influence of hydrogen reactions on the surface species coverage, whereas direct coupling steps between CO and H 2 were of minor importance. Finally, simulations indicated that for wall temperatures less than 520 K, which were well-below the minimum reactant inlet temperatures in power generation systems, the added H 2 inhibited the oxidation of CO.
Sound Generating Flames of a Gas Turbine Burner Observed by Laser-Induced Fluorescence
In this contribution, an initial investigation of the ignition behavior of large two-stroke marin... more In this contribution, an initial investigation of the ignition behavior of large two-stroke marine diesel sprays has been performed. At engine-like conditions, the OH radical was traced with an intensified high speed camera and a sophisti-cated optical setup. A series of spectroscopic measurements showed, however, that the soot incandescence strongly con-tributes to the UV signal, superimposing with or even masking the chemiluminescence of the OH radical. As the com-bustion of typical fuels used in large two-stroke engines involves the formation of non-negligible amounts of soot, the signal is almost omnipresent during the oxidation process. A differentiation between the UV-light emitted by the OH radical and the UV-light emitted by soot incandescence is only possible when both signals are measured separately. Therefore, a second high speed camera recorded the light coming from soot incandescence. In addition, it recorded the background illuminated spray plume to make an exact posit...
COMODIA2012 Wartsila A.Schmid
Clean Air: International Journal on Energy for a Clean Environment, 2004
The applicability of various hetero/homogeneous chemical reaction schemes in the catalytically st... more The applicability of various hetero/homogeneous chemical reaction schemes in the catalytically stabilized combustion (CST) of hydrogen/air mixtures over platinum was investigated experimentally and numerically in channelflow configurations. The present work stems from the internationally intensified effort to commercialize power generation systems with ultra-low NOx (< 3 ppm), CSTbased gas turbines. Crucial in the development of such systems is the understanding of the heterogeneous (catalytic) kinetics, the homogeneous (gas-phase) kinetics, and their respective coupling. Experiments were performed at atmospheric pressure and laminar flow conditions in an optically-accessible catalytic channel combustor: planar laser induced fluorescence (PLIF) of the OH radical was used to monitor the onset of homogeneous ignition and line-Raman measurements provided the boundary-layer profiles of the temperature and major species. Measured homogeneous ignition distances were compared against numerical predictions obtained with a two-dimensional elliptic fluid mechanical model. Six homogeneous (elementary, reduced and single-step schemes) and three heterogeneous schemes were tested in the model. The comparisons have revealed substantial differences between measured and predicted homogeneous ignition distances (ranging from 8% to 85%) and, moreover, these differences were attributed primarily to the homogeneous reaction pathway.
Study of a Rich/Lean Staged Combustion Concept for Hydrogen at Gas Turbine Relevant Conditions
Volume 1A: Combustion, Fuels and Emissions, 2013
The combustion of hydrogen-rich fuels (> 80 % vol. H2), relevant for gas turbine cycles with “... more The combustion of hydrogen-rich fuels (> 80 % vol. H2), relevant for gas turbine cycles with “pre-combustion” carbon capture, creates great challenges in the application of standard lean premix combustion technology. The significant higher flame speed and drastically reduced auto-ignition delay time of hydrogen compared to those of natural gas, which is normally burned in gas turbines, increase the risk of higher NOX emissions and material damage due to flashback. Combustion concepts for gas turbines operating on hydrogen fuel need to be adapted to assure safe and low-emission combustion. A rich/lean (R/L) combustion concept with integrated heat transfer that addresses the challenges of hydrogen combustion has been investigated. A sub-scale, staged burner with full optical access has been designed and tested at gas turbine relevant conditions (flame temperature of 1750 K, preheat temperature of 400 °C and a pressure of 8 bar). Results of the burner tests have confirmed the capabi...
Proceedings of the Combustion Institute, 2013
The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigate... more The hetero-/homogeneous combustion of fuel-lean ethane/air mixtures over platinum was investigated experimentally and numerically at pressures of 1-14 bar, equivalence ratios of 0.1-0.5, and surface temperatures ranging from 700 to 1300 K. Experiments were carried out in an optically accessible channel-flow reactor and included in situ 1-D Raman measurements of major gas phase species concentrations across the channel boundary layer for determining the catalytic reactivity, and planar laser induced fluorescence (LIF) of the OH radical for assessing homogeneous ignition. Numerical simulations were performed with a 2-D CFD code with detailed hetero-/homogeneous C 2 kinetic mechanisms and transport. An appropriately amended heterogeneous reaction scheme has been proposed, which captured the increase of ethane catalytic reactivity with rising pressure. This scheme, when coupled to a gas-phase reaction mechanism, reproduced the combustion processes over the reactor extent whereby both heterogeneous and homogeneous reactions were significant and moreover, provided good agreement to the measured homogeneous ignition locations. The validated hetero-/homogeneous kinetic schemes were suitable for modeling the catalytic combustion of ethane at elevated pressures and temperatures relevant to either microreactors or large-scale gas turbine reactors in power generation systems. It was further shown that the pressure dependence of the ethane catalytic reactivity was substantially stronger compared to that of methane, at temperatures up to 1000 K. Implications for high-pressure catalytic combustion of natural gas were finally drawn.
An experimental and numerical investigation of the hetero-/homogeneous combustion of fuel-rich hydrogen/air mixtures over platinum
Proceedings of the Combustion Institute, 2013
Abstract The hetero-/homogeneous combustion of hydrogen/air mixtures over platinum was investigat... more Abstract The hetero-/homogeneous combustion of hydrogen/air mixtures over platinum was investigated experimentally and numerically in a channel-flow configuration at fuel-rich equivalence ratios ranging from 2 to 7, pressures up to 5 bar and wall temperatures 760–1200 K. Experiments involved in situ one-dimensional Raman measurements of major gas-phase species concentrations over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical, while simulations included an elliptic 2-D model with detailed heterogeneous and homogeneous reaction mechanisms. The employed reaction schemes reproduced the measured catalytic reactant consumption, the onset of homogeneous ignition, and the post-ignition flame shapes at all examined conditions. Although below a critical pressure, which depended on temperature, the intrinsic gas-phase kinetics of hydrogen dictated lower reactivity for the fuel-rich stoichiometries when compared to fuel-lean ones, homogeneous ignition was still more favorable for the rich stoichiometries due to the lower molecular transport of the deficient oxygen reactant that resulted in modest catalytic reactant consumption over the gaseous induction zone. Above the critical pressure, the intrinsic gaseous hydrogen kinetics yielded higher reactivity for the rich stoichiometries, which resulted in vigorous gaseous combustion at pressures up to 5 bar, in contrast to lean stoichiometry studies whereby homogeneous combustion was altogether suppressed above 3 bar. Computations at fuel-rich stoichiometries in practical channel geometries indicated that homogeneous combustion was not of concern for reactor thermal management, since the larger than unity Lewis number of the deficient oxygen reactant confined the flames to the core of the channel, away from the solid walls.
Experimental Setup of a Laser Diagnostics System for a High-Temperature Solar Receiver/Reactor
Journal of Solar Energy Engineering, 1994
A solar receiver/reactor has been designed specifically to study high-temperature gas phase chemi... more A solar receiver/reactor has been designed specifically to study high-temperature gas phase chemical reactions using a laser based metrology. It is a cavity-type receiver, lined with stabilized ZrO2, and operated at temperatures up to 2000 K. The gas temperature is measured in situ using the coherent anti-Stokes Raman spectroscopy (CARS) of N2. Optical access for the CARS measurement is accomplished via two side windows, each subtending a 118-mrad cone angle at the center of the cavity, providing enough clearance for the input laser beams and the output signal carrying the temperature information. Two endothermic processes were used for the initial evaluation of this method: the NH3 dissociation into N2 and H2, and the CO2-reforming of CH4 into synthesis gas. The process flow was directly exposed to high solar fluxes in addition to infrared radiation emitted by the hot reactor walls. The laser-based metrology performed satisfactorily in spite of the presence of the intense radiation...
Branching ratios and partition of the excess energy for the predissociation of CO+2 C̃ 2Σ+g molecular cations
The Journal of Chemical Physics, 1983
He–Iα photoelectron–photoion coincidence spectroscopy is used to study the decay of CO+2 molecula... more He–Iα photoelectron–photoion coincidence spectroscopy is used to study the decay of CO+2 molecular cations, initially prepared in various vibrational levels of the C̃ 2Σ+g state. The branching ratios for the energetically accessible fragmentation pathways are determined. Owing to the precisely known energetics and to an accurate measurement of the kinetic energies released, the partition of the respective excess energy among internal and external degrees of freedom of the separating fragments could be established. The formation of highly rotationally excited diatomic fragments must be proposed in order to explain the experimental results. The disappearance of O+ ions in favor of CO+ fragments as soon as the energy exceeds the dissociation limit for CO+ formation is consistent with a reinterpretation of previous ab initio calculations of potential-energy surfaces. The observed branching ratio for the population of the v=0 and v=1 levels of the CO fragment can be accounted for by an a...
Photoelectron–photoion coincidence spectroscopy of gas-phase clusters
The Journal of Chemical Physics, 1982
A photoelectron–photoion coincidence technique for obtaining the photoelectron spectrum of a sing... more A photoelectron–photoion coincidence technique for obtaining the photoelectron spectrum of a single component of a gas-phase mixture has been developed. It utilizes a newly designed instrument which measures the ion mass in coincidence with the photoelectron kinetic energy. Initial experiments were carried out on Xe2 and Xe3 produced in mixture of clusters (plus monomer) in a free-jet supersonic expansion. These measurements determined the photoelectron appearance potential (i.e., the lowest binding energy for which photoelectrons are detected) to be 11.30(5) eV for Xe3. It was also found that fragmentation of cluster ions strongly affects the coincidence spectra. This was investigated by varying the stagnation pressure, and thus the beam composition, in order to assess fragmentation contributions to coincidence spectra of the cluster under study. One case studied in detail indicated the energy levels of Xe3 near the ionization potential of 11.6 eV, corresponding to 0.7 eV of intern...
Fundamental Aspects of Ionic Dissociations: The Fragmentation Pathways of Excited Bicyclobutane Cations
Helvetica Chimica Acta, 1983
A most recently developed method to quantify the fragmentation pathways of excited radical cation... more A most recently developed method to quantify the fragmentation pathways of excited radical cations is presented. Using bicyclobutane cation as an illustrative example, the RRKM analysis of the breakdown diagram determined by He‐Iα photoelectron‐photoion coincidence spectroscopy is outlined. The results imply complete isomerization to 1,3‐butadiene cation preceding the dissociative processes. The rate‐energy functions of four competitive primary fragmentation reactions, leading to C3H, C4H, C4H and C2H are established. There is compelling evidence that the production of C2H fragment ions does not compete effectively with these four reactions. The extent of kinetic and competitive shift effects is determined. The derived enthalpies of formation are in excellent accord with the available high quality reference data. The relative importance of different fragmentation pathways which ultimately lead to fragment ions of identical mass to charge ratio is assessed.
Experiments in Fluids, 2008