Volume 225, Issue 11-12 - Special Issue Prof. Kohse-Höinghaus

Rate constants for the overall reactions of OH with n -pentane, n -heptane, and n -nonane were measured in shock tube experiments behind reflected shock waves. Narrow-linewidth laser absorption by OH at 306.7 nm was used in pseudo first-order experiments with temperatures between 869 to 1364 K. tert -Butyl hydroperoxide (TBHP) was used as the OH precursor. Experiments were also performed to study the kinetics of the TBHP decomposition and resulting product chemistry, and an accurate mechanism describing OH precursor chemistry effects was developed to model OH concentration time-history in the n -alkane + OH experiments. The experimental results for the n -alkane + OH rate constant measurements can be expressed as rate constants in Arrhenius form as k n -pentane + OH = 2.10 × 10 -10 exp(-2038/T[K]) (869–1364 K), k n -heptane + OH = 2.43 × 10 -10 exp(-1804/T[K]) (869–1364 K), k n -nonane + OH = 3.17 × 10 -10 exp(-1801/T[K]) (884–1352 K), each in units of cm 3 molecule -1 s -1 . The present rate constants measured for OH with n -pentane and n -heptane show agreement within 20% with recent work by Sivaramakrishnan and Michael [ J. Phys. Chem. A , 113 (2009) 5047]. The measurements of the rate constant for n -nonane + OH presented here represent the first in the literature to depict the temperature dependence of the rate constant above 800 K. The measurements of each n -alkane + OH rate constant studied were compared with two models in the literature used to estimate the rate constants of n -alkane + OH reactions. The Structure-Activity Relationship of Kwok and Atkinson [ Atmos. Environ. , 29 (1995) 1685] shows the best agreement with the current data for all three n -alkanes over the entire temperature range studied, demonstrating that this model is capable of predicting the overall rate constants for reactions of OH with n -pentane, n -heptane, and n -nonane for temperatures up to 1364 K.

Ferrocene-doped low-pressure, laminar, premixed propene/oxygen/argon flames of different stoichiometries were investigated experimentally using laser induced fluorescence (LIF) and the results were compared with numerical simulations. The influence of ferrocene on the flame temperature proved to be minimal both in the measurement and in the simulation. The measured flame temperatures were 100–250 K below the adiabatic flame temperature, as expected. However, the modeled flame temperatures were around 450 K below the measurements. It is assumed that the reason for this lies in the incorrect modeling of the flame speed since systematic errors in the LIF measurements should be small, as discussed in the paper. Iron atom concentrations were also measured using LIF. The results show the influence of the stoichiometry on the iron atom concentration, which is also reproduced by the model. The majority of the added iron was observed to exist as atomic iron in the flame. As more oxygen becomes available in leaner flames, iron will tend to form more FeO so that the iron atom concentration decreases with decreasing equivalence ratio.

Experimental investigation of nanosecond pulsed discharge in premixed CH 4 /air mixtures at atmospheric pressure has been carried out using laser diagnostics. Electron temperature and number density are measured using laser Thomson scattering. Temperature of neutral molecules is measured by CARS. Finally, OH, CH and CH 2 O are probed using PLIF to identify their role in the reduction of ignition delay and in the improvement of lean burn capability relative to conventional spark ignition. Measurements are compared with numerical simulations performed using CHEMKIN-based code.

The initial stage of coke formation in steam reforming of natural gas over rhodium/alumina catalysts was studied microscopically. A well-defined model catalyst prepared by an aerosol technique was placed in a flow reactor to very mildly coke the catalyst sample. Therefore, a natural gas–steam mixture at steam-to-carbon ratios of unity was fed to the reactor operated for thirty minutes at atmospheric pressure and moderate temperatures of 650 ºC. Fresh and used catalyst samples were characterized by SEM-EDX and a recently developed AFM technique, the Oscillating Friction Microscopy (OFM), to analyze the friction characteristics of the sample. OFM combined with SEM-EDX allowed to distinguish between coke depositions, alumina support (Al 2 O 3 ), and Rh particles and to locate the initial carbon deposition in the process. It was found that coke formation starts on the catalyst particle. The carbonaceous overlayer can be removed from the catalyst and the closely surrounding support by multiples scans with the AFM tip.

Multi-line NO-LIF thermometry is used to determine the two-dimensional temperature distribution inside a low-pressure plasma reactor. The applicability of multi-line temperature measurements to non-equilibrium plasma environments was evaluated. Temperatures between 300 and 3000 K have been observed, while microwave power, and pressure show a strong effect on the temperature distribution. Metal-organic precursors added for particle synthesis additionally influence the temperature through the heat release during particle formation and the oxidation of organic ligands.

Premixed low-pressure tetrahydrofuran/oxygen/argon flames are investigated by photoionization molecular-beam mass spectrometry using vacuum-ultraviolet synchrotron radiation. For two equivalence ratios ( φ = 1.00 and 1.75), mole fractions are measured as a function of distance from the burner for almost 60 intermediates with molar masses ranging from 2 (H 2 ) to 88 (C 4 H 6 O 2 ), providing a broad database for flame modeling studies. The isomeric composition is resolved by comparisons between experimental photoionization efficiency data and theoretical simulations, based on calculated ionization energies and Franck - Condon factors. Special emphasis is put on the resolution of the first reaction steps in the fuel destruction. The photoionization experiments are complemented by electron-ionization molecular-beam mass-spectrometry measurements that provide data with high mass resolution. For three additional flames with intermediate equivalence ratios ( φ = 1.20 , 1.40 and 1.60), mole fractions of major species and photoionization efficiency spectra of intermediate species are reported, extending the database for the development of chemical kinetic models.

Although the propene + OH reaction has been in the center of interest of numerous experimental and theoretical studies, rate coefficients have never been determined experimentally between ∼ 600 and ∼750 K , where the reaction is governed by the complex interaction of addition, back-dissociation and abstraction. In this work OH time-profiles are measured in two independent laboratories over a wide temperature region (200–950 K) and are analyzed incorporating recent theoretical results. The datasets are consistent both with each other and with the calculated rate coefficients. We present a simplified set of reactions validated over a broad temperature and pressure range, that can be used in smaller combustion models for propene + OH. In addition, the experimentally observed kinetic isotope effect for the abstraction is rationalized using ab initio calculations and variational transition-state theory. We recommend the following approximate description of the OH + C 3 H 6 reaction C 3 H 6 + OH ⇆ C 3 H 6 OH (R1a,R-1a) C 3 H 6 + OH → C 3 H 5 + H 2 O (R1b) k 1 a (200 K ≤ T ≤ 950 K; 1 bar ≤ P ) = 1.45 × 10 -11 ( T /K) -0.18 e 460 K/ T cm 3 molecule -1 s -1 k -1 a (200 K ≤ T ≤ 950 K; 1 bar ≤ P ) = 5.74 × 10 12 e -12690 K/ T s -1 k 1 b (200 K ≤ T ≤ 950 K) = 1.63 × 10 -18 ( T /K) 2.36 e -725 K/ T cm 3 molecule -1 s -1

Environmentally friendly biodiesel is a mixture of saturated and unsaturated methyl (or ethyl) esters of long-chain fatty acids. To experimentally examine the effect of C=C double bonds on the combustion chemistry of fatty acid esters, low-pressure premixed laminar flames of four small esters have been studied using flame-sampling molecular-beam mass spectrometry. Mole fraction profiles of reactants, products, and well-identified stable and reactive combustion intermediates in flames of the saturated species methyl propanoate (CH 3 CH 2 COOCH 3 ) and its isomer ethyl acetate (CH 3 COOCH 2 CH 3 ) have been compared with results from flames of the unsaturated fuels methyl propenoate (CH 2 CHCOOCH 3 ) and vinyl acetate (CH 3 COOCHCH 2 ) flames. A total of eight flames have been studied, with two fuel-rich flame conditions investigated (fuel-equivalence ratios φ =1.2 and 1.56) for each fuel. In addition, the underlying oxidation chemistry at these premixed flame conditions has been investigated using a detailed chemical kinetic reaction mechanism, which is largely based on a previously proposed model for saturated esters [B. Yang et al. , Phys. Chem. Chem. Phys. 13 (2011) 7205]. The combined results provide a detailed understanding of the similarities and differences between the combustion of saturated vs. unsaturated esters. Meanwhile, the isomeric and stoichiometric effects on their combustion chemistry are also addressed. In this paper, experimental and modeling details are discussed with a special focus on the different reaction pathways.

A method for measurements of planar temperature distributions based on planar laser-induced fluorescence (PLIF) of the OH radiacal is described. The technique was developed specifically for the application in lean combustion systems, where OH equilibrium concentrations are largely independent on equivalence ratio and a function of temperature only. It is thus possible to derive a temperature information from measurements of absolute OH concentration, which can be obtained from a combined PLIF/absorption measurement. This paper discusses the basics of the method, and describes validation experiments in high pressure laminar premixed flames which were performed to asses its applicability and accuracy. Therefore, we compared our LIF based results with CARS measurements performed in the same flames. Finally, an example for the application in a lean gas turbine model combustor is discussed.

We highlight the capabilities and potential of diode laser induced fluorescence for measurements in gas-phase reacting flows. Many applications of diode lasers in practical sensing are based on absorption spectroscopy. Fluorescence-based diagnostics possess similar advantages in terms of practicality and implementation-cost but additionally are capable of achieving excellent spatial resolution. Diode laser fluorescence instruments have been employed for high-sensitivity trace gas monitoring in applications ranging from plasma physics to atmospheric chemistry. This article begins by describing the UV-visible diode laser technology used to perform fluorescence. The principles of diode laser induced fluorescence are then reviewed and a comparison is made with absorption spectroscopy. Examples are given of concentration measurements of both atomic and molecular trace gases. Recent work on using diode laser induced atomic fluorescence for precision measurements of flame temperature is also reviewed. We conclude by a discussion of future opportunities for diode laser fluorescence spectroscopy drawing attention to interesting potential target species as well as novel application areas, such as monitoring of synthesis processes for nanomaterials.

Experiments were conducted in a gas-driven shock tube to investigate shock wave-induced damage to Bacillus thuringiensis Al Hakam endospores over a wide range of post-shock temperatures in non-oxidative gas environments. The results were compared with previous studies on B. atrophaeus and B. subtilis and demonstrate that B. thuringiensis Al Hakam exhibited a qualitatively similar response to rapid shock heating, even though this strain has a significantly different endospore structure. B. thuringiensis Al Hakam endospores were nebulized into an aqueous aerosol, which was loaded into the Stanford aerosol shock tube, and subjected to shock waves of controlled strength. Endospores experienced uniform test temperatures between 500 and 1000 K and pressures ranging from 2 atm to 7 atm for approximately 2.5 ms. During this process the bio-aerosol was monitored using in situ time-resolved laser absorption and scattering diagnostics. Additionally, shock-treated bio-aerosol samples were extracted for ex situ analysis including viability plating, flow cytometry and scanning electron microscopy (SEM) imaging. B. thuringiensis Al Hakam endospores lost the ability to form colonies at post-shock temperatures above 500 K while significant breakdown in morphology was observed only for post-shock temperatures above 700 K. While viability loss and endospore morphological deterioration adhere to a similar framework across all endospore species studied, phenomena unique to B. thuringiensis Al Hakam were noted in the SEM images and optical extinction data. This initial characterization of the response of B. thuringiensis Al Hakam spores treated with shock/blast waves shows that these methods have potential for spore inactivation and detection.

Ultrananocrystalline Diamond (UNCD) Films were deposited by MW-CVD from an Ar/H 2 /CH 4 plasma. The plasma properties were measured in situ by optical emission spectroscopy and mass spectroscopy. The intensity of the C 2 emission line was systematically varied as a key plasma parameter and a correlation with resulting film properties was found.

Many technical combustion devices are susceptible to thermoacoustic instabilities. A deeper understanding of this transient phenomenon as well as the underlying physical processes is desirable. In this context, a turbulent enclosed flame is investigated numerically within this work by means of a hybrid LES/CAA (Large Eddy Simulation/Computational Aero Acoustics) approach as a step towards a numerical prediction of combustion instability. A thermoacoustically stable as well as a thermoacoustically unstable operating point are investigated. The LES computations reveal good overall agreement between simulation and experimental data. However, the recirculation zone, which is created by a backward facing step, is underestimated for the unstable case. The amplitudes as well as the frequencies of the simulated acoustic pressure are in good agreement with experimental data for the stable case. Considering the unstable case, the frequency of the oscillation is captured accurately, while the amplitudes are underpredicted.