Andreas Brockhinke

Sampling Probe Influences on Temperature and Species Concentrations in Molecular Beam Mass Spectroscopic Investigations of Flat Premixed Low-pressure Flames

Abstract New operating regimes for engines and combustors and the advocated use of non-conventional transportation fuels demand investigation of the combustion chemistry of different classes of chemicals, especially under premixed conditions. Detailed species compositions during combustion are needed to estimate hazardous emissions, and models for their prediction must be validated for the intended combustion conditions.Molecular-beam mass spectrometry (MBMS) is a common technique to measure quantitative species concentrations in flames. It is widely employed to characterize the flame chemistry of laminar premixed combustion, and it has been complemented with optical measurements for the detection of a number of molecular species and radicals. Significant progress has been made in recent studies through the introduction of synchrotron-based MBMS instruments. They have improved the identification process by using tunable vacuum-ultraviolet radiation for photoionization of the species to be detected, and isomer-specific measurements are now almost routinely possible. Along with quantitative species measurements, the temperature profile is needed as input parameter for chemical kinetic modeling. It is usually determined either using thermocouples or laser spectroscopic techniques.It is an ongoing discussion how sampling probes affect these measurements, and how MBMS results can be compared to combustion modeling. The present article is intended to contribute to this discussion by providing optical and MBMS results obtained with several sampling configurations.

Luminescence properties of C-diazaborolyl-ortho-carboranes as donor-acceptor systems.

Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C(2)B(10)H(12)) with a 1,3-diethyl- or 1,3-diphenyl-1,3,2-benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low-energy fluorescence emissions with large Stokes shifts of 15100-20260 cm(-1) and quantum yields (Φ(F)) of up to 65% in the solid state. The low-energy fluorescence emission, which was assigned to a charge-transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C-C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660-18090 cm(-1) for the CT bands and 1960-5540 cm(-1) for the high-energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ(F) values between 8-32%. Two distinct excited singlet-state (S(1)) geometries, denoted S(1)(LE) and S(1)(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (ψ). TD-DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C-diazaborolyl-ortho-carboranes were viewed as donor-acceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.

Quantitative one-dimensional single-pulse multi-species concentration and temperature measurement in the lift-off region of a turbulent H2/air diffusion flame

In this article, we report on one-dimensional single-pulse measurements of temperature and major-species concentration (O2, N2, H2O and H2) in a turbulent H2/air jet diffusion flame using Raman and Rayleigh scattering of KrF* excimer-laser radiation. Spatial resolution of ≈ 0.5 mm along a 6mm long line has been obtained, with reasonable error limits for mole fraction (Δχ = 5 % for N2 detection) and temperature (ΔT = 8 %) determination at flame temperatures. We present various profiles showing the composition and temperature along a line at different heights in the flame with particular emphasis on the lift-off region (i.e. lowx/D). In this zone, temperature and mixture fraction can be determined simultaneously — from a single laser pulse — in a spatial region extending from unburnt gas in the center of the jet across the flame front into the cool air of the surrounding atmosphere. This allows for the first time the systematic study of the shape and width of the high-temperature region and the corresponding concentration and temperature gradients. The comparison of averaged data and scatter plots with previous pointwise measurements shows good agreement.

Structural changes in the Ras protein revealed by fluorescence spectroscopy

The Y32W mutant of the Ras protein which has a tryptophan residue close to the guanine nucleotide binding site is studied using two fluorescence spectroscopic techniques. Two-dimensional mapping of all emission and all fluorescence spectra using excitation–emission spectroscopy (EES) in conjunction with time-resolved laser-induced fluorescence (LIF) is used to analyze and assign the contribution of the different fluorophores to the total fluorescence. Time-resolved LIF is shown to be a method that allows to follow the slight conformational changes of Ras binding to the nucleotides GDP, GTP, or the non-hydrolyzable analogues GppNHp, GppCH2p and GTP-γS and allows to distinguish between the active and inactive form. Additionally, a variant of the EES technique is used for the investigation of the intrinsic GTPase function of Ras and the determination of kinetic constants for this reaction.

Structural properties of lifted hydrogen jet flames measured by laser spectroscopic techniques

The region near the lift-off height of several turbulent H-2/air and H-2/N-2/air diffusion flames with Reynolds numbers between 3600 and 17,300 was investigated to study the effects of chemical composition, large-scale structures, and gradients on the flame stabilization process. Using Raman and Rayleigh scattering, quantitative single-pulse one-dimensional profiles of 911 major species concentrations and temperature have been measured with high accuracy and good spatial resolution. The local mixture fraction has been determined from these images; postprocessing of the data allowed the identification of large-scale structures, the accurate determination of the position of reaction zones, and of regions with high scalar dissipation or large temperature gradients. Double-pulse experiments allowed the direct determination of the local heat release. This is illustrated by individual examples. The interpretation of the data, in view of current flame stabilization theories, suggests an extended analysis with respect to statistical criteria. Evaluation of all images shows that fuel and air at the lift-off height are generally mixed over a region that is several mm wide and that these mixtures have stoichiometries well within the H-2 flammability limits. The majority of images exhibiting a distinct high-temperature zone also show the presence of large-scale structures, which appear to be related to the flame stabilization process. The scalar dissipation at the lift-off height is one order of magnitude lower than the critical value for flame extinction A statistical analysis of several thousands of images shows that maxima in the scalar dissipation rate are not correlated to temperature gradients or to the position of the instantaneous flame front. The observed structural features and their statistical relevance are discussed in the context of recent advances in flame stabilization theories

Quantitative determination of combustion intermediates with cavity ring-down spectroscopy: systematic study in propene flames near the soot-formation limit

Cavity ring-down spectroscopy (CRDS) was applied in several fuel-rich, one-dimensional, premixed C3H6/O2/Ar flames at 50 mbars (37.5 torr) to measure absolute OH, HCO, and 1CH2 concentration as well as temperature as a function of stoichiometry. Although these flames near the sooting limit present a complex chemical environment, significant spectral interferences were found to be absent. Specific aspects of the CRDS technique for measurement of temperature and radical concentration profiles are discussed; and the results are analyzed in comparison with flame model simulations.

Rotationally resolved single-photon ionization of HCl and DCl

The rotationally resolved photoionization yields of jet‐cooled HCl and DCl have been measured in the energy range between their spin–orbit split 2Π ionic thresholds. For single‐photon excitation, narrow‐band vuv radiation is generated by resonant frequency mixing. The spectra are complex due to an interaction of autoionizing resonances belonging to series converging to different rotational states of the ion core. This is taken into account using the multichannel quantum defect theory in a treatment capable to handle a transition in angular momentum coupling from Hund’s case (c) to case (e). Even for Rydberg orders n≳35 the experimental data cannot be explained using a pure case (e) picture.

Combination of Laser- and Mass-Spectroscopic Techniques for the Investigation of Fuel-Rich Flames

Abstract Soot is one of the most important pollutants originating from combustion. Despite recent advances in the measurement of size and composition of soot particles, their actual formation mechanism is still under debate. It depends on fuel, stoichiometry, temperature, flow conditions and the concentration of a large number of intermediate species. An adequate characterization of this complex reaction system generally requires the use of several complementary techniques. In this article, we present measurements aiming to study reactions in fuel-rich flames using several complementary techniques. Only with a combination of optical and mass-spectrometric measurements, important features of the early polyaromatic hydrocarbon (PAH) and soot formation chemistry are accessible in detail. Three different techniques are combined to investigate one-dimensional laboratory flames on the same low-pressure burner and their respective merits are discussed: (i) cavity ring-down spectroscopy (CRDS) for the detection of small radicals and measurement of the temperature, (ii) mass spectrometry with electron-impact (EI) ionization in order to measure species with molecular weights up to m/e = 90, and (iii) mass spectrometry with resonantly-enhanced multi-photon ionization (REMPI) to distinguish isomers with masses up to m/e = 178. Measurements of this type may prove a valuable input to improve kinetic and combustion models.

Double-pulse one-dimensional Raman and Rayleigh measurements for the detection of temporal and spatial structures in a turbulent H 2 –air diffusion flame

A novel spectroscopic technique for the observation of the temporal development of small spatial structures of temperature and major-species mole fractions in turbulent f lames has been developed. It uses two independent lasers and linewise, single-pulse detection systems for Raman-Rayleigh-scattered light. With this technique, all major-species concentrations, temperature, and positions of vortices and f lame fronts, combined with scalar dissipation in two directions or heat release, can be measured, for the first time to our knowledge, as a result of one pair of laser pulses separated either in space or time. This yields, to our knowledge, the highest number of correlated quantities that have been obtained in probing turbulent f lames of simple chemistry.

Quantitative laser diagnostic and modeling study of C2 and CH chemistry in combustion.

Quantitative concentration measurements of CH and C(2) have been performed in laminar, premixed, flat flames of propene and cyclopentene with varying stoichiometry. A combination of cavity ring-down (CRD) spectroscopy and laser-induced fluorescence (LIF) was used to enable sensitive detection of these species with high spatial resolution. Previously, CH and C(2) chemistry had been studied, predominantly in methane flames, to understand potential correlations of their formation and consumption. For flames of larger hydrocarbon fuels, however, quantitative information on these small intermediates is scarce, especially under fuel-rich conditions. Also, the combustion chemistry of C(2) in particular has not been studied in detail, and although it has often been observed, its role in potential build-up reactions of higher hydrocarbon species is not well understood. The quantitative measurements performed here are the first to detect both species with good spatial resolution and high sensitivity in the same experiment in flames of C(3) and C(5) fuels. The experimental profiles were compared with results of combustion modeling to reveal details of the formation and consumption of these important combustion molecules, and the investigation was devoted to assist the further understanding of the role of C(2) and of its potential chemical interdependences with CH and other small radicals.