Volume 229, Issue 4 - Special Issue: Combustion, Part 1 Dedicated to Professor Henning Bockhorn on the occasion of his 70th birthday / Eva Gutheil, Hans-Jürgen Troe

The formulation of a comprehensive flamelet model to consider detailed chemical reaction mechanisms in the simulation of turbulent spray flames is a very challenging task due to the inherent multi-regime structure of spray flames. Non-premixed, premixed, and evaporation-controlled combustion regimes may be found in a single spray flame. Recently, attempts have been made to extend classical single regime flamelet models to more complex situations, where at least two combustion regimes coexist. The objective of this work is to develop a framework in which two-regime flamelet models can be described and combined in order to advance the development of a comprehensive flamelet model for turbulent spray flames. For this purpose, a set of spray flamelet equations in terms of the mixture fraction and a reaction progress variable is derived, which includes the evaporation, characterizing the spray flames, and which describes all combustion regimes appearing in spray flames. The two-regime and single regime flamelet equations available in the literature are retrieved from these multi-dimensional spray flamelet equations as special cases. The derived set of spray flamelet equations is then used to evaluate structures of laminar ethanol/air spray flames in the counterflow configuration in order to determine the significance of different combustion regimes. The present study concerns spray flames with no pre-vaporized liquid in the oxidizing gas phase, and it is found that only non-premixed and evaporation-controlled combustion regimes exist, so that premixed effects may be neglected. Moreover, an exact transport equation for the scalar dissipation rate is derived, which explicitly takes spray evaporation and detailed transport into account. This equation is then used to evaluate assumptions commonly adopted in the literature. The results show that the spatial variation of the mean molecular weight of the mixture may be neglected in the formulation of the mixture fraction, but it may be significant for its scalar dissipation rate. The assumption of unity Lewis number may lead to non-physical values of the scalar dissipation rate of the mixture fraction, whereas the use of a mass-averaged diffusion coefficient of the mixture is a good approximation for the spray flames under investigation.

Absorption spectroscopy in the ultraviolet (UV) and mid-infrared (MIR) spectral region has been used in a comparative study for the detection of formaldehyde in laminar low pressure flames of dimethyl ether (DME) and methane. Both spectral regions were tested to explore respective advantages and limitations, especially for the detection of stable molecules in flames. In the UV, cavity ring-down spectroscopy (CRDS), a highly sensitive multi-pass absorption technique, has been used for the detection of formaldehyde in the A  −  X 4 1 0 hot band at 370 nm. For measurements in the mid-infrared, the fundamental CO-stretch vibration ν 2 of formaldehyde at 5.9 μm was used. MIR laser radiation was generated with a pulsed quantum cascade laser (QCL), a novel laser type which offers high output energies and excellent beam quality in the MIR. It was shown that MIR-QCL absorption spectroscopy and UV-CRDS have a comparable sensitivity, even though only single-pass absorption was used in the MIR. Additionally, MIR-QCL is clearly superior in terms of the simplicity of the experimental setup. This shows the large potential of QCL based laser absorption spectroscopy for the sensitive detection of trace species in the MIR.

Highly unsaturated hydrocarbons like diacetylene (C 4 H 2 ) or vinylacetylene (C 4 H 4 ) are important intermediates in combustion that can have impact on soot formation. One of their major loss channels is reaction with hydroxyl radicals (OH). We studied the reactions C 4 H 2  + OH → products (1) and C 4 H 4  + OH → products (2) in a quasi-static reactor with helium as bath gas. The hydroxyl radicals were produced by laser flash-photolysis of nitric acid at a wavelength of 248 nm and detected by laser-induced fluorescence with excitation at 282 nm. The rate coefficients were obtained from the intensity-time profiles under pseudo-first order conditions with respect to OH. We found a virtually temperature-independent rate coefficient for reaction (1): k 1  = (1.0 ± 0.3) × 10 −11  cm 3  s – 1 ( T  = 290–670 K, P  = 2.7–30.5 bar) and a weakly negative temperature-dependent rate coefficient for reaction (2): k 2 ( T ) = (6.4 ± 1.9) × 10 −12  exp (486 K/ T ) cm 3  s – 1 ( T  = 295–740 K, P  = 1.7–19.2 bar). For neither of the two reactions pressure dependence was observed. From comparisons with analogous reaction systems, we conclude that the dominating reaction pathway is OH addition, where in the case of C 4 H 4 the double bond is preferred over the triple bond.

This paper focuses on the combustion chemistry of 2-methylfuran (2-MF), a potential biofuel, and it is built on the previous work of Tran et al. [Combust. Flame 161 (2014) 766]. In their work, they had combined detailed flame chemistry modeling with flame speciation data based on flame-sampling molecular beam mass spectrometry (MBMS) with electron ionization and gas chromatography with MS detection. In this work, we significantly extend those previous studies by in-situ isomer-resolving species identification and quantification. Specifically, we have determined the detailed chemical structure of a premixed laminar 2-MF flame using flame-sampling high-resolution MBMS with synchrotron-generated vacuum-ultraviolet radiation. Mole fraction profiles of 60 intermediate, reactant, and product species were measured in order to assess the pollutant potential of this possible next-generation biofuel. Special emphasis is paid towards the fuel's ability to form aromatic and oxygenated intermediates during incomplete combustion processes, with the latter species representing a variety of different classes including alcohols, ethers, enols, ketones, aldehydes, acids, and ketenes. Whenever possible the experimental data are compared to the results of model calculations using the 2-MF combustion chemistry model of Tran et al. , but it should be noted that many newly detected species are not included in the calculations. The experimental data presented in this work provides guidance towards to development of a next-generation 2-MF combustion chemistry model.

The pyrolysis of 2,5-dimethylfuran has been studied in a single pulse shock tube equipped with fast probing device at temperatures between 1175 K and 1450 K and pressures of 8.0±0.5 bar. The initial concentration of 2,5-dimethylfuran diluted in argon (500 ppm) was much lower than in previous studies reported in the literature. Sixteen different product species were quantified by gas chromatography. The product distribution pattern was compared with the prediction of two comprehensive chemical kinetic reaction mechanisms taken from the literature. In general, the predictions of the mechanisms fit the results of the experiments; however, the comparison reveals some differences between the two mechanisms as well as between simulations and experiments.

Progress in understanding turbulent combustion requires access to information that is derived from instantaneous 3D images showing the contours of the flame front, the flow field, and other quantities. The experimental effort to acquire such information is typically extraordinarily high and therefore not as widely used as needed. The introduction of commercially available plenoptic cameras now provides a means to simplify some of these measurements as is described in this study. A single 29 mega-pixel plenoptic camera was used to record the Mie scattering signals from volumetrically illuminated fuel sprays. Instantaneous 3D images were acquired from within the cylinder of an optically accessible engine for three automotive fuel injectors of differing nominal spray angles. Current reconstruction algorithms do not yet take translucent imaging conditions into account and therefore a quantitative analysis of the full 3D structure of the sprays was not possible. However, the plenoptic system did prove capable of accurately reconstructing bulk spray features such as spray angles. The 3D spray images were validated with data from a conventional 2D imaging system. This study demonstrates the feasibility of using a single plenoptic camera to view spray geometry in 3D, even from within the challenging in-cylinder engine environment.

Correct detailed chemical-kinetic descriptions of the consequences of the additions of oxygen molecules to alkyl radicals involve large numbers of elementary steps. The resulting complexity of the system of equations needed to describe combustion processes often taxes computational capabilities. Simplified descriptions of the chemical kinetics that retain essential elements of the results therefore can be useful for a number of purposes, even though they fail to include many of the chemical processes. Such simplified descriptions for combustion processes are presented here for certain normal alkanes. Their potential utility and prospects for future developments are discussed.

A gas turbine model combustor (Turbomeca Burner) for premixed methane/air flames has been operated at atmospheric pressure in two different modes of premixing. In the partially premixed mode, fuel was injected into the air flow within the swirl generator shortly upstream of the combustion chamber while in the perfectly premixed mode fuel and air were mixed far upstream. The main objective of this work is the study of the influence of the mode of premixing on the combustion behavior. Stereoscopic particle image velocimetry has been applied for the measurement of the flow field, OH chemiluminescence imaging for the visualization of the flame shapes and single-shot laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature. The mixing and reaction progress and effects of turbulence-chemistry interactions are characterized by scatterplots showing the correlations between different quantities. To isolate effects of mixing from combustion instabilities that were frequently observed in this combustor, operating conditions without thermo-acoustic oscillations or coherent flow structures were chosen. While the mode of premixing had no major influence on the general flame behavior characteristic differences were observed with respect to flame anchoring, the flow field in the inner recirculation zone and the CO concentration level. The results further extend the data base of previous experimental and numerical investigations with this burner.

New simulations of graphene growth in flame environments are presented. The simulations employed a kinetic Monte Carlo (KMC) algorithm coupled to molecular mechanics (MM) geometry optimization to track individual graphenic species as they evolve. Focus was given to incorporation of five-member rings and resulting curvature and edge defects. The model code was re-written to be more computationally efficient enabling a larger set of simulations to be run, decreasing stochastic fluctuations in the averaged results. The model also included updated rate coefficients for graphene edge reactions recently published in the literature. Sensitivity analysis, enabled by the more efficient coding, confirmed the importance of the updated rates as well as providing further insight on key reactions controlling layer growth and morphology. The new KMC code was applied to simulate graphene-layer growth in the environments of laminar premixed flames. In these flame simulations, C-H ratios of evolving structures were computed and compared to those from experiment and from an alternate model.