Flame kinetic studies

In kinetics studies, as in mass spectrometry, data reduction can be helpful before starting a detailed analysis. A typical application in which data reduction is of value is high-temperature kinetics. Reactions in flames are complex, so study of these reactions is challenging not just experimentally but also... 

The Opposed-Flow Laminar Diffusion Flame Laminar diffusion flames can be more complicated chemically and physically than the corresponding premixed flames. This is especially so for a candlelike co-flowing situation. Because of the difficulties of adequately representing the two- or three-dimensional flow field, together with detailed chemistry, these flames are difficult to use as the basis for chemical-kinetic studies. How-... 

Compared to ordinary kinetic experiments the study of flame velocity has disadvantages we cannot simultaneously and independently vary the temperature and concentration of all the reacting components peculiar conditions interrelate, for example, the temperature and concentration of a deficient component. However these disadvantages are expiated by the tremendous expansion of the field of kinetic studies in a flame we study reactions which occur at 1500-3000°K and which are so rapid that the reaction time lies within the limits 10 3-10 6 sec. This expansion of the kinetic experimental region, undoubtedly, will yield a number of new laws of particular interest in connection with the fact that at high temperatures the equilibrium concentration of active centers of the reaction becomes significant. 

Kinetic studies in flames require a well defined environment, and particular attention was given to the design of the birnier (Figure 1). Soot was generated in a primary burner following Fenimore and Jones (3), and the soot-laden gases were cooled and injected into the base of a secondary burner. Both primary and secondary burners were constructed from spaced hypodermic tubing, and the tubes for the observa-... 

A flame may be defined as a localized reaction zone which is able to propagate itself sub-sonically through the material supporting it. Most flames are concerned with exothermic reactions of this type, in which typically reactants at near ambient temperatures are converted more or less adiabatically to combustion products at 1000 K or above. Detailed kinetic studies have principally been confined to premixed flames, in which a well-defined reactant mixture at a known initial temperature is converted into combustion products in full chemical equilibrium at the final flame temperature. Assuming adiabatic combustion, the final conditions may be calculated thermodynamically. 

X 10 cm ) and measured (1.5 x 10 cm ) peak CH concentrations was obtained. This further supports the findings of recent experimental reaction kinetics studies in which it was found that the CH + O2 reaction is significantly faster than the CEC recommendation. The present work indicates that a low rate for this reaction constitutes a possible major cause of errors in the computations of CH concentrations in premixed flames. 

Kinetic studies in a well-stirred flow reactor by Make and co-workers at the University of Washington [68-70] also apply to temperatures in excess of 1000 K. These studies provide information on the transition from the low-temperature combustion processes to those at normal flame temperatures, in the final stages of spontaneous ignition. These and related studies are summarized in the Appendix. A turbulent, high pressure, flow reactor has been developed by Koert et al. [14] for studies throughout the temperature range from 600 to 1000 K and at pressures up to 1.5 MPa. Results from it pertaining to propane oxidation are discussed in Section 6.4. 

Of the many detectors that are discussed in the literature, we shall consider three which are the most important for gas-phase kinetic studies. These are the katharometer, the hydrogen flame ionisation detector (fid), and the gas density balance. 

The existence of the hydroxyl radical as a separate. If transient, entity was first recognized in 1924 by Watson (19), who proposed that the water vapor bands emitted by flames and electric discharges In moist air were due to the OH radical, and not to excited H2O. In 1928 Bonhoeffer and Relchardt (20) obtained the absorption spectrum of the OH radical in partially dissociated water vapor at v>1873°K, and In 1935 Oldenberg (21) was able to follow the decay of the radical In the products flowing from an electric discharge through water vapor. This latter work was of Importance since the OH radical could then be monitored In a system applicable to kinetic studies. Early combustion studies showed that the hydroxyl radical Is also an Important constituent of flames, the most prominent feature of flame spectra (22) being emission from the (a2i + - X Il) band system of the OH radical. 

When the war was over most of the chemists returned to their pre-war interests, but some embarked on new ventures - Linnett in problems of molecular structure (although retaining his war-time interest in flames and combustion). Woodward in Raman spectroscopy, and Staveley in thermodynamics that he had studied before the War with Klaus Clusius in Munich but had not yet been able to follow up. Hinshelwood published some of the previously classified papers and continued work on the reactions on charcoal surfaces and other kinetic studies, but his heart was not in it as it had been earlier all that he was now really interested in was the kinetics of bacterial growth and adaptation. 

Kinetics in Nonshock Environments Flame Structure Studies. ... 

An important additional restriction on flame structure studies of kinetics is provided by the practical problem of maintaining one dimensional flame stability over a wide range of compositions, which limits the ranges of fuel/oxidizer ratios and percentage dilution by inert additives accessible to experimental study. The advantages to the kineticist of freedom in the selection of these quantities has been stressed above. The choice of experimental conditions in flames is also limited by the direct relationship between flame temperature and initial composition. In the shock tube where heating is provided by an outside source, i.e., the shock wave, these two variables can be selected independently of one another. The instability problem with exothermic reactions in shock waves is of a different character from that in flames. 

There is a voluminous hterature concerned with the study of flame spectra, but the application of spectroscopy to the study of flame kinetics followed the introduction of flame photometry as a general analytical tool. The chief interest before this was in the spectra of the flames, which could serve to demonstrate the presence of intermediates in the combustion process. These were in general detected by the emission spectra of excited species and therefore were not necessarily indicative of the concentrations of ground state species. The difficulties of constructing burners which were sufficiently large and uniform to allow the study of absorption spectra prohibited a measurement of the species in their ground states, until the development of the multiple pass technique. ... 

Soundy and Williams designed the low duty cycle rotating probe which has been the basis for most of the more recent kinetic studies in flames. In this technique, the probe, properly insulated, is swept through the flame at the end of a rotating arm, and the cmrent is recorded on an oscilloscope, with the time base triggered just before the probe enters the flame. As a variant, the stabbing probe which is driven into and out of the flame by a crank has also been used. 

The measurement of the concentration of the minor species in the burnt gases—H atoms, OH radicals, etc—can be achieved in several ways. The majority of the studies of flame kinetics have been carried out in flames for which the minor constituent composition has been determined by a photometric technique, and the first and most important of these is the sodium/litUum comparison technique of Bulewicz, James and Sugden. ... 

Most flame photometric studies reveal the presence of a faint background continuum due to radical recombination, and the continuum may be enhanced by additives— the pale lilac colour of the potassium flame test in elementary analysis is due in large measure to the K—OH recombination continuum. V le not strictly a problem in chemical kinetics, it is interesting to survey some of the results obtained from the study of continua. 

An increasing amount of work is devoted to processes in two-phase systems—and a reasonably complete account of the transformation of a droplet or particle, whether of fuel or additive, into the final state of uniformly dispersed vapour is as yet far away. In these studies many of the techniques used in flame kinetics are applied to follow physical rather than chemical changes. 

The evaluation of rate constants by flame studies is particularly important because the temperature regime involved is higher than that attained in the usual kinetic studies and this extended range greatly improves the reliability of the determinations of activation energies and frequency factors. Flame studies have also been used to evaluate diffusion coefficients and thermal conductivities at high temperatures. 

Only the hydrogen-bromine flame has been studied by flame structure techniques (Frazier et al., 1965). Two important conclusions were reached by this preliminary study (a) the simple mechanism used to describe low-temperature kinetic studies was... 

Wilk, R.D., Cernansky, N.P., Pilz, W.J. and Westbrook, C.K. (1989) Piropene oxidation at low and intermediate temperatur a detailed chemical kinetic study. Combust. Flame 77,145-170... 

Special interfaces are used for highly specialized tasks. For example, Sloane and Rat-cliffe [207] devised an instrument for molecular beam sampling of transient combustion phenomena by TRMS. It was possible to detect various flame components with good spatial and temporal resolution [207]. Reaction kinetics and the chemistry of low-pressure flames was studied with aid of a photoionization method [208]. 

Fluoro-olefins have proved popular substrates for kinetic studies during the period of this Report. A study of the relative rates of reaction of active nitrogen, using diffusion flame techniques, has suggested the following order of reactivity ... 

Radiative Association Reactions The study of radiative association reactions, (Eq. 2.2), has been of considerable interest [6-8] in chemical kinetics, planetary and interstellar chemistry, flames, and a variety of other areas. The kinetic study makes it possible to model the formation of complex molecular species in the interstellar science. At the very low molecular number densities in interstellar environments, the probability of formation of the products of association reactions by collisional stabilization is very low. Therefore, the radiative association process becomes an extremely important one for the production of the complex molecular species observed by astronomical physicist. The methodology is either flowing afterglow (FA) or Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. For the study of the apparent bimolecular rate constant for formation of association products as a function of pressme of a third body (N), the pressure should be set up to be sufficiently high in order to release the energy in the associated complex. Under the high pressure conditions collisional stabilization has competed with and usually dominated over radiative associatioiL As a result, the radiative association rate was then extrapolated from the intercept of a plot of apparent rate constant versus pressure of a third body, N. 

Carja lonela-Daniela, Serbezeanu Diana, Lisa Gabriela, Vlad-Bubulac Tachita, and Hamciuc Corneliu. Thermal degradation and kinetic studies of new flame-retardant phosphorus-containing polymers with liquid crystalline properties. Int. J. Polym. Anal. Charact. 19 no. 4 (2014a) 372-382. 

Laser techniques have a great potential for studies of microscopic as well as macroscopic combustion in flames and engines. Combustion diagnostics with lasers was discussed in several reviews [10.8-13]. We will here give examples of measurements of concentrations and temperature (flame kinetics) using fluorescence, Raman and coherent Raman techniques. In practical combustion systems turbulence is extremely important and we will also briefly discuss laser techniques for flow and turbulence measurements. 

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