Pierre Van Tiggelen

Kinetics in a Lean Formaldehyde Flame

Abstract The structure of a lean formaldehyde-oxygen flame burning at 22.5 Torr has been investigated experimentally using molecular beam sampling coupled with mass spectrometric analysis. Mole fraction profiles have been obtained for stable species as well as for atoms and radicals in a 17.9% CH2O-82.1% O2 flame. From data reduction it appears that about 80% of the formaldehyde is consumed in radical reactions and about 20% by bimolecular decomposition. The fast reactions of formaldehyde with atoms and hydroxyl radicals have the following rate constants: CH 2 +OH→CHO+HO 2 . k 3 =1.8×10 33 exp (−220/T) cm 3 mol −1 s −1 , CH 2 +OH→CHO+OH , xk 4 =6.0×10 13 exp (−1900/T) cm 3 mol −1 s −1 , CH 2 +OH→CHO+H 2 , k 5 =1.0×10 14 exp (−2480/T) cm 3 mol −1 s −1 , Besides these processes, the decomposition reaction of formaldehyde CH 2 +OH→CHO+H 2 +M , k 1 =2.5×10 14 exp (−14,250/T) cm 3 mol −1 s −1 , could play a role in the production of molecular hydrogen in this flame. Formyl radicals disappear mainly by CHO+O 2 +→CO 2 , k 7 =2.7×10 13 exp (−600/T) cm 3 mol −1 s −1 , and this reaction is the principal path for hydroperoxyl radical production. This last species reacts mainly with radicals and atoms (H, O, and OH). In the low temperature zone of the flame, CO2 is produced by CO+OH→CO 2 +H but also to some extent through CO+HO 2 →CO 2 +OH with a rate constant k10 = 3.5 × 1012 exp(−4125/T) cm3 mol−1 s−1

The influence of the heat capacity and diluent on detonation structure

In this article, we investigate the validity of certain common simplifications in the chemical and thermophysical models used as input to multidimensional detonation simulations, derive a more accurate model, and apply the model in two-dimensional studies of the structure detonations in hydrogen-oxygen mixtures diluted with argon and nitrogen. In a series of one-dimensional calculations, we examine the effects of (1) approximation of the temperature dependence of the ratio of specific heat, gamma, (2) varying the amount and rate of heat release, and (3) varying the chemical induction time, and we compare all of these approximations with a computation that uses a detailed model of the chemical kinetics and correct thermophysics. From these, we derive a simple form for the temperature dependence of gamma and show that this gives good results in comparison to the predictions of the detailed calculation for the detonation velocity and the thickness of the induction zone. In a series of two-dimensional calculations, we investigate the effects of using the more accurate simplified chemical models and varying the type of diluent while maintaining the same dilutions. In agreement with experiments, the mixture of hydrogen, oxygen, and argon mixture shows regular detonation structures and clearly formed detonation cells, whereas the mixture of hydrogen, oxygen, and nitrogen shows highly irregular cellular structure.

Pressure profiles in detonation cells with rectangular and diagonal structures

Abstract. Experimental results presented in this work enable us to classify the three-dimensional structure of the detonation into two fundamental types: a rectangular structure and a diagonal structure. The rectangular structure is well documented in the literature and consists of orthogonal waves travelling independently from each another. The soot record in this case shows the classical diamond detonation cell exhibiting ‘slapping waves’. The experiments indicate that the diagonal structure is a structure with the triple point intersections moving along the diagonal line of the tube cross section. The axes of the transverse waves are canted at 45 degrees to the wall, accounting for the lack of slapping waves. It is possible to reproduce these diagonal structures by appropriately controlling the experimental ignition procedure. The characteristics of the diagonal structure show some similarities with detonation structure in round tube. Pressure measurements recorded along the central axis of the cellular structure show a series of pressure peaks, depending on the type of structure and the position inside the detonation cell. Pressure profiles measured for the whole length of the two types of detonation cells show that the intensity of the shock front is higher and the length of the detonation cell is shorter for the diagonal structures.

Effect of dimethoxymethane addition on the experimental structure of a rich ethylene/oxygen/argon flame

Structures of premixed ethylene/oxygen/argon-rich flat flames burning at 50 mbar have been established by using molecular beam mass spectrometry to investigate the effect of methylal (dimethoxymethane) addition on species concentration profiles. The aim of this experimental study is to examine eventual changes of the concentration profiles of detected hydrocarbon intermediates which could be considered as soot precursors (C2H2, C4H2, C5H4, C5H6, C5H8, C6H2, C6H4, C6H6, C6H8, C7H8, C6H6O, C8H6, C8H8, C9H8, and C10H8). The comparative study has been achieved on three flames with equivalence ratios (phi) of 2.25 and 2.50: two without any additive (F2.25 and F2.50) and one with 4.3% methylal in partial replacement of C2H4 (F2.50M). The three flat flames have similar final flame temperatures (congruent to1800 K). An increase of the flame equivalence ratio (0) from 2.25 to 2.50 leads to an increase of maximum mote fractions of most hydrocarbon intermediates, much larger than the initial fuel content difference. Methylal addition to the fresh gas inlet causes a slight shift downstream of the flame front. The replacement of 5.7% C2H4 by 4.3% C3H8O2, keeping the equivalence ratio equal to 2.50, is responsible for a decrease of the maximum mole fractions of most of the detected intermediate species. If this phenomenon is barely noticeable for C-2 to C-4 intermediates, it becomes more efficient for C-5 to C-10 species. Although the equivalence ratio is quite different in flames F2.25 and F.2.50M, most of the maximum mole fractions Of C-2 to C-10 intermediates are very similar but lower than those in the F2.50 flame. It seems to indicate that similar initial C/O ratio (F2.25, 0.75; F2.50M, 0.76) in the fresh gases mixtures better encapsulates the influence on maximum concentrations of hydrocarbon intermediates and soot precursors in rich flames.

Flame structure studies of several premixed ethylene-oxygen-argon flames at equivalence ratios from 1.00 to 2.00

A better understanding of combustion chemistry is needed to achieve the practical objective of reducing pollutant emissions from combustion and soot particles. Such an understanding requires the knowledge of the concentrations of the chemical species occurring in the combustion reaction zones and particularly of those which are closely connected with molecular growth phenomena. In this context, physical and chemical processes associated with soot formation in flames have been investigated extensively and several experimental studies as well as modelling are reported in the literature (Warnatz et al. 1982, Bastin et al. 1988, Westmoreland et al. 1989, Homann and Wagner 1967, Miller and Melius 1992, Seshadri et al. 1990, Miller et al. 1986) about the role of species wich could play some soot precursors.

Preliminary experimental investigation of the pressure evolution in detonation cells

Recently two types of three-dimensional (3D) structure of gaseous detonation have been documented: rectangular and diagonal modes easily distinguishable from soot records. This paper presents pressure measurements recorded along the central axis of the cellular structure. The pressure records are achieved rd by using piezoelectric gauges flush-mounted with respect to the surface of the soot-covered plate located in the detonation tube. The low pressure reactive mixture used (H-2, O-2, Ar; Equivalence ratio = 1) is ignited in a square cross-section tube. The detonation tube is operated in the shock tube mode. The time evolution of the local pressure exhibits several pressure peaks depending on the type of 3D structure and on the position in the detonation cell. The first peak characterizes the leading shock and the subsequent pulses correspond to the elaborate shock structure, The influence of the slapping waves (SW) is documented. The pressure profiles throughout the whole length of the detonation cell ale reported for the individual types of 3D structure. The second pressure jumps can be rationalized in terms of the classical transverse wave structure

Peculiar features in lean butane flames

Lean isobutane and n-butane hat lean flames burning at low pressure have been investigated by means of molecular beam mass spectrometry. Profiles of stable, atomic and radical species have been measured as well as temperature. Acetone and butene occur as intermediates in significant amount. - In isobutane flames the production of both acetone (C3H6O) and butene (C4H8) is ascribed to direct reactions of butyl radicals t-C4H9 + O --> C3H6O + CH3 C4H9 + H, OH --> C4H8 + H-2, H2O t-C4H9 --> C4H8 + H - In n-butane flames, acetone is not produced and butene is accounted for along radical reactions C4H9 + H, OH --> C4H8 + H-2, H2O The occurrence of radical-radical reactions in higher hydrocarbons flames cannot be neglected and supplements the usual thermal decomposition reaction (beta-scission) in the detailed combustion mechanisms.

Modeling of Gas-phase Detonation in Complex Reactive Systems

A numerical study of detonations in hydrogen-oxygen-argon mixtures containing CF4 or CF3H is presented. Experiments have established the promoting effect of these additives on the detonation velocity. The Chapman-Jouguet model fails to explain the observed behavior and a numerical approach solving the steady equations of the fluid dynamics provides a first grasp of such an unexpected behavior. In this paper, we use a numerical model that solves the unsteady equations of the fluid dynamics to simulate the detonation wave and to predict the stabilized detonation velocity. The chemical model used is a parametric one that takes into account a temperature and composition dependence of the heat capacity. In a serie of one-dimensional calculations, we describe first the numerical ignition of the detonation wave. In particular, we examine the effect of the pressure in the driving gas section of the numerical domain. Then, we examine the influence of the additives on the detonation wave propagating in a mixture of H2/O2/Ar. We compare successfully the results of the modeling to experimental data. The promoting behavior of both fluorocarbons is numerically observed up to about 10% of the additives. Our conclusion is that it is possible to model the overall description of a detonation wave in complex reactive system. Prerequisites are the knowledge of the chemical kinetics to within a reasonable accuracy, robust algorithm for computing the fluid dynamics and attention to coupling.

Inhibition of detonation wave with halogenated compounds

The influence of CF3Br, CF2HBr, CF2HCl and CF3H on a benchmark mixture composed of stoichiometric H2−CO−O2−Ar is experimentally investigated. Several ratios hydrogen/carbon monoxide are studied. For each benchmark mixture, the initial pressure is adjusted in such a way that the detonation cell sizes are quasi identical. The effect of the additives on the detonation velocity and the detonation cellular structure is analyzed. The experiments show that CF3Br is the best inhibitor and CF2HBr might be substituted for CF3Br. CF3H does not inhibit the detonation wave. Simple chemical kinetics analysis gives us a better understanding of the inhibiting and promoting effect of the halocarbons.

MASS SPECTROMETRY, GAS CHROMATOGRAPHY, AND COUPLING GC/MS AS COMPLEMENTARY TECHNIQUES FOR FLAME STRUCTURE ANALYSIS

Molecular beam mass spectrometry (MBMS) and gas chromatography (GC) are complementary methods that provide a detailed description of flame structures. MBMS can measure most stable and reactive species but mass overlapping (isomers, species at same m/e), isotopic, and ionic fragmentation interferences can be solved by using GC. To improve species identification, an experimental technique coupling both mass spectrometry and GC is developed. Rich flat premixed ethylene/oxygen/argon flames (φ = 2.25 and 2.50) have been investigated by both methods. After adequate calibrations, mole fraction profiles of several species measured by both techniques agree very well, but for methane, allene, propyne, and benzene, concentrations in burnt gases are somewhat larger when using GC than when using MBMS. C2H6, C2H4O, C3H6, and C3H8, which have similar masses as CH2O, CO2 or C3H8, CH2CO, and CO2, respectively, have been identified, separated, and calibrated by GC, which confirms that GC and MBMS are complementary techniques.