Flame composition profiles

In combustion of liquid oils such as heavy diesel fuels, the fuel is sprayed through nozzles into air. After ignition, a flame forms around the evaporating drops, driven by the vaporization of the drop in the hot boundary layer. The latent heat of vaporization strongly affects the temperature of the drop so that the heat release by the reaction is compensated by the heat absorbed in vaporizing the fuel and the heat lost to the cooler gas in the air. The composition profiles we might expect are shown in Figure 12-11, but the temperature will also vary around the particle. 

Prescott, Hudson, Foner, and Avery (60) extended the mass-spectrographic technique to the study of composition profiles across a low-pressure, propane-air flame under somewhat lean conditions. The appearance and disappearance of hydrogen, carbon monoxide, ethylene, and acetylene in the flame were demonstrated clearly. The proportion of acetylene was not high. Nonetheless, it is evident that the formation of acetylene is not just a result of pyrolysis of excess hydrocarbon by heat released in combustion of part of the gas. It is a result of reactions which must occur to some extent in all hydrocarbon combustion, but which would not be observable except by special techniques, or under conditions—such as rich flames or cool flames—where the later reactions of acetylene can l>e minimized. 

Recent work on spatial stabilization has been directed towards the production of one-dimensional flames (Fll, P10). These may be either flat, cylindrical, or spherical. The primary purpose of such flames has been to measure velocities accurately and to provide a flame that can be described by a one-dimensional theory. The measurement of temperature and composition profiles is meaningful, of course, only in a flame in which the geometry is known. One-dimensional geometry greatly reduces the labor required to analyze such profiles in order to study kinetics. 

The present work involves measurement of k in a 0.1 atmosphere, stoichiometric CH -Air flame. All experiments were conducted using 3 inch diameter water-cooled sintered copper burners. Data obtained in our study include (a) temperature profiles obtained by coated miniature thermocouples calibrated by sodium line reversal, (b) NO and composition profiles obtained using molecular beam sampling mass spectrometry and microprobe sampling with chemiluminescent analysis and (c) OH profiles obtained by absorption spectroscopy using an OH resonance lamp. Several flame studies (4) have demonstrated the applicability of partial equilibrium in the post reaction zone of low pressure flames and therefore the (OH) profile can be used to obtain the (0) profile with high accuracy. 

Determine the composition profile of fatty acids as directed under Fatty Acid Composition, Appendix VII, with the following modifications (1) In the Sample Preparation, use about 55 mg of sample per 10 mL, and (2) in the Procedure, use a suitable capillary gas chromatograph (see Chromatography, Appendix IIA), equipped with a flame ionization detector, a 60-m x 0.25-mm (id) column, or equivalent, coated with a 0.20-p.m layer of 2-cyanopropylpolysiloxane (Supelco SP-2340, or equivalent), a capillary injection port (split mode, operated at a split ratio of 1 100), and an integrator. Set the initial column temperature at 150°, heat at a rate of 1.3°/min to 225°, and hold at 225° for 10 min. Set the injection port temperature to 210° and the detector to 230°. Set the carrier gas flow rate at 25 cm/s. 

Special situations exist for which this procedure simplifies considerably. If the intermediary under consideration is not a chain carrier but is merely produced and consumed through unimportant side reactions, then the burning velocity and the composition profiles of all other species in the flame are virtually unaffected by the presence of this intermediary. The structure of the flame (excluding the X profile) can therefore be determined completely by setting = 0 in the flame equations. After this structure is determined, a, b and the coefficients of the linear differential operator Si X ) are known functions of t. Therefore, equation (90) reduces to a linear nonhomogeneous differential equation with known variable coefficients,... 

The burning velocity, and the temperature and composition profiles in a low temperature, fuel-rich hydrogen—nitrogen—oxygen flame at atmospheric pressure having an unbumt gas composition 2, u = 0.1883, A N2,u 0.7657 and Aqj.u = 0.0460, with = 336 K, were measured by Dixon-Lewis et al. [156] while the burning velocities of a number of flames having compositions not too far from this were also examined by Dixon-Lewis and co-workers [158, 159]. In a number of these flames the main reaction zone extended from approximately 600—1060 K, and the predominantly recombination zone from about 1060—1080 K. The maxi-... 

Figures 25—27 show the temperature and composition profiles calculated for the standard flame by the refined treatment using set 2 of the rate coefficients of Table 30. Figure 25 also includes for comparison a number of points representing the observed temperature profile. Agreement is excellent. The composition profiles for the stable species in the flame were measured by means of a mass spectrometric probe, using the unbumt gas ratios of each species concentration to that of nitrogen as calibration standards. Realistic comparison is then in terms of these ratios, and is shown in Fig. 28. The relative intensities of sodium chemiluminescence in the recombination region of the low temperature flames are proportional to the square of the H atom concentrations. A comparison between theory and experiment on this basis, with intensities normalized with respect to the maximum H atom concentration and the...

Fig. 34, Composition profile of the cool and second stage flames during acetaldehyde oxidation [56] (a) x, Acetaldehyde <, carbon monoxide o, methane , oxygen — temperature, (b) Carbon dioxide +, formaldehyde methanol.

This ester resembles its methyl homologue in possessing three modes of decomposition [131]. It also supports a self-decomposition flame, the multiple reaction zones of which are clearly separated at low pressures [122, 123, 125]. Temperature and composition profiles in the low-pressure decomposition flame have been measured [133]. The products include formaldehyde, acetaldehyde and ethanol with smaller amounts of methane and nitromethane. The activation energy derived from the variation of flame speed with final flame temperature was 38 kcal. mole", close to the dissociation energy of the RO—NO2 bond. The controlling reaction is believed to be unimolecular in its low pressure regime, and the rate coefficient calculated from the heat-release profile is... 

Direct on-line analytical techniques may also be used. Typical of these studies is that made by Bradley et al. of stabilised cool flames in the oxidation of acetaldehyde and propionaldehyde. A fine qu2utz probe was attached to an A.E.I. MS 10 mass spectrometer, and could be moved through the cool flame. In this way composition profiles were obtained for reactant aldehyde and oxygen and also for carbon dioxide, carbon monoxide, formaldehyde, methane and methanol. 

In oxy-burner testing, generally two types of measurements are involved (i) measurements inside flames and (ii) global measurements in the furnace. In-flame measurements include chemical composition, flame temperature profile, and optical properties. Global measurements include flame shape, emissions, heat flux, and measurements taken on the load. Figure 27.13 shows typical measurements and their locations in an oxy-pilot furnace. 

Seery and Zabielski (1980) utilized low-pressureCH4-air flames to infer k. Their data included NO and N2 composition profiles by molecular beam sampling mass spectrometry and microprobe sampling with chemiluminescent analysis, temperature profiles by thermocouples, and OH profiles by resonance absorption. Partial equilibrium was assumed in order to infer O-atom concentrations in the postflame region. The simplified expression (4) was used to infer at several flame locations. Although the scatter in the ki data show an average deviation of 28% about the mean, the mean agrees within 4% with the best-fit expression reported by Monat et al (1979). 

The basic approach taken in the analytical studies of composite-propellant combustion represents a modification of the studies of double-base propellants. For composite propellants, it has been assumed that the solid fuel and solid oxidizer decompose at the solid surface to yield gaseous fuel and oxidizing species. These gaseous species then intermix and react in the gas phase to yield the final products of combustion and to establish the flame temperature. Part of the gas-phase heat release is then transferred back to the solid phase to sustain the decomposition processes. The temperature profile is assumed to be similar to the situation associated with double-base combustion, and, in this sense, combustion is identical in the two different types of propellants. 

The simple physical approaches proposed by Mallard and Le Chatelier [3] and Mikhelson [14] offer significant insight into the laminar flame speed and factors affecting it. Modem computational approaches now permit not only the calculation of the flame speed, but also a determination of the temperature profile and composition changes throughout the wave. These computational approaches are only as good as the thermochemical and kinetic rate values that form their database. Since these approaches include simultaneous chemical rate processes and species diffusion, they are referred to as comprehensive theories, which is the topic of Section C3. 

FIGURE 4.6 Composition, temperature, and heat release rate profiles for a stoichiometric C3H8-air laminar flame at 1 atm and T0 = 298 K. 

Comparison of the silicone-epoxy ion profiles indicates that the presence of the flame retardant in sample E has little effect on the composition or formation rates of the major volatile species. The specific ion profiles characteristic of HBr and from the flame retardant in sample E... 

Fig. 17.4 Simulation of stoichiometric methane-air flames approaching a stagnation surface. The top panels show the axial velocity and temperature profiles. The lower panels show details of the species composition with the thin flame-front.

The theoretical and experimental results for a fuel-lean methane-air flame are given in Figures 5-7. These results include temperature and major species compositions. The experimental and theoretical results are compared by matching the abcissas of the temperature profiles. The model very accurately predicts the slope of the temperature profile but predicts a larger final flame temperature than is measured. This is a consequence of heat lost to the cooled, gold-coated burner wall that is 1.5 mm away from the positions where data were taken. 

We have developed several new measurement techniques ideally suited to such conditions. The first of these techniques is a High Pressure Sampling Mass Spectrometric method for the spatial and temporal analysis of flames containing inorganic additives (6, 7). The second method, known as Transpiration Mass Spectrometry (TMS) (8), allows for the analysis of bulk heterogeneous systems over a wide range of temperature, pressure and controlled gas composition. In addition, the now classical technique of Knudsen Effusion Mass Spectrometry (KMS) has been modified to allow external control of ambient gases in the reaction cell (9). Supplementary to these methods are the application, in our laboratory, of classical and novel optical spectroscopic methods for in situ measurement of temperature, flow and certain simple species concentration profiles (7). In combination, these measurement tools allow for a detailed fundamental examination of the vaporization and transport mechanisms of coal mineral components in a coal conversion or combustion environment. 

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