Flame-speed data

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

In open flames, the variation of ST with composition is generally much the same as for SL, and, S T has a well-defined maximum close to stoichiometric. Thus, many report turbulent flame speed data as the ratio of ST/SL. 

The TNT model is well established for high explosives, but when applied to flammable vapor clouds it requires the c3q>losion yield, T), determined from past incidents. There are several physical differences between TNT detonations and VCE deflagrations that limit the theoretical validity. The TNO multi-energy method is directly correlated to incidents and has a defined efficiency term, but the user is required to specify a relative blast strength from 1 to 10. The Baker-Strehlow method uses flame speed data correlated with relative reactivity, obstacle density and geometry to replace the relative blast strength in the TNO method. Both methods produce relatively close results in examples worked. 

The solid lines in Figure 4.5 represent extrapolations of experimental data to full-scale vessel bursts on the basis of dimensional arguments. Attendant overpressures were computed by the similarity solution for the gas dynamics generated by steady flames according to Kuhl et al. (1973). Overpressure effects in the environment were determined assuming acoustic decay. The dimensional arguments used to scale up the turbulent flame speed, based on an expression by Damkohler (1940), are, however, questionable. 

When representing the dependence of laminar flame speed (S°) on mixture preheat temperature (TJ in the form of S°(T, (Z>)/S°(To,(Z>) = (T /Tq)", where Tq is the lowest unburned mixture temperature investigated for a given fuel/air composition, the current experimental data can be correlated well with n in the... 

The counterflow configuration has been extensively utilized to provide benchmark experimental data for the study of stretched flame phenomena and the modeling of turbulent flames through the concept of laminar flamelets. Global flame properties of a fuel/oxidizer mixture obtained using this configuration, such as laminar flame speed and extinction stretch rate, have also been widely used as target responses for the development, validation, and optimization of a detailed reaction mechanism. In particular, extinction stretch rate represents a kinetics-affected phenomenon and characterizes the interaction between a characteristic flame time and a characteristic flow time. Furthermore, the study of extinction phenomena is of fundamental and practical importance in the field of combustion, and is closely related to the areas of safety, fire suppression, and control of combustion processes. 

Subsequently, the problem was investigated by Karpov and Severin [6]. They used closed vessels with a diameter of 10cm and 10, 5, and 2.5cm width, initially at atmospheric pressure. The vessels were filled with different lean hydrogen and methane/air mixtures and rotational speeds in the range of 130-4201/s were employed. They also included data from the study of Babkin et al. [3] in their analysis. Unfortunately, they did not observe the flame itself and measured only the pressure rise in the vessel, which was compared with pressure development in the vessel without rotahon, to draw a conclusion with respect to flame speeds and quenching. 

Very early, from the analysis of ignition, flame speed, and detonation velocity data, investigators realized that small concentrations of hydrogen-containing materials would appreciably catalyze the kinetics of CO—02. The H20-catalyzed reaction essentially proceeds in the following manner ... 

Many extensive models of the high-temperature oxidation process of methane have been published [20, 20a, 20b, 21], Such models are quite complex and include hundreds of reactions. The availability of sophisticated computers and computer programs such as those described in Appendix I permits the development of these models, which can be used to predict flow-reactor results, flame speeds, emissions, etc., and to compare these predictions with appropriate experimental data. Differences between model and experiment are used to modify the mechanisms and rate constants that are not firmly established. The purpose here is to point out the dominant reaction steps in these complex... 

For a long time there was no interest in flame speed measurements. Sufficient data and understanding were thought to be at hand. But as lean bum conditions became popular in spark ignition engines, the flame speed of lean limits became important. Thus, interest has been rekindled in measurement techniques. 

Reported flame speed results for most fuels vary somewhat with the measurement technique used. Most results, however, are internally consistent. Plotted in Fig. 4.21 are some typical flame speed results as a function of the stoichiometric mixture ratio. Detailed data, which were given in recent combustion symposia, are available in the extensive tabulations of Refs. [24-26], The flame speeds for many fuels in air have been summarized from these references and are listed in Appendix F. Since most paraffins, except methane, have approximately the same flame temperature in air, it is not surprising that their flame speeds are about the same (—45 cm/s). Methane has a somewhat lower speed (<40 cm/s). Attempts [24] have been made to correlate flame speed with hydrocarbon fuel structure and chain length, but these correlations... 

Now it is important to stress that, whereas the laminar flame speed is a unique thermochemical property of a fuel-oxidizer mixture ratio, a turbulent flame speed is a function not only of the fuel-oxidizer mixture ratio, but also of the flow characteristics and experimental configuration. Thus, one encounters great difficulty in correlating the experimental data of various investigators. In a sense, there is no flame speed in a turbulent stream. Essentially, as a flow field is made turbulent for a given experimental configuration, the mass consumption rate (and hence the rate of energy release) of the fuel-oxidizer mixture increases. Therefore, some researchers have found it convenient to define a turbulent flame speed, S T as the mean mass flux per unit area (in a... 

Although a laminar flame speed. S L is a physicochemical and chemical kinetic property of the unbumed gas mixture that can be assigned, a turbulent flame speed. S T is, in reality, a mass consumption rate per unit area divided by the unbumed gas mixture density. Thus,. S r must depend on the properties of the turbulent field in which it exists and the method by which the flame is stabilized. Of course, difficulty arises with this definition of. S T because the time-averaged turbulent flame is bushy (thick) and there is a large difference between the area on the unbumed gas side of the flame and that on the burned gas side. Nevertheless, many experimental data points are reported as. S T. 

You want to measure the laminar flame speed at 273 K of a homogeneous gas mixture by the Bunsen burner tube method. If the mixture to be measured is 9% natural gas in air, what size would you make the tube diameter Natural gas is mostly methane. The laminar flame speed of the mixture can be taken as 34cm/s at 298 K. Other required data can be found in standard reference books. 

The first measurements of flame speeds in turbulent, exothermic TC flow have been obtained and comparisons of these measurements with experimental and theoretical results from earlier studies have been made. Good agreement of present results with those of Cheng and Shepherd [1] and Bedat and Cheng [2] is found for u /U up to about 4, beyond which the bending effect exhibited in the data becomes significant. 

The trend observed in Fig. 17.5 illustrates the importance of swirl. The image sequence corresponds to data points using front swirl angles of 31°, 41°, and 68° with all other parameters fixed 4> = 0.68, RMS = 0.1, L = 12 in. (30.48 cm), total mass flow rate of 0.03 kg/s, and rear swirl angle of 85°). The chamber averaged swirl is defined as the sum of the front and rear drive angular momentum divided by the chamber radius and total mass flow. This provides a measure of the swirl experienced by the combustion chamber confined flow and allows comparison between different test conditions. The flame speed ratio... 

Bromine compounds reduced flame speeds in all cases, but chloroform increased them in most cases. Although the promoting effect of chloroform has not been reported before and the view generally is that chlorine compounds reduce flame speeds, a close examination of previous data (4, 6,7,9) reveals that under certain conditions, chlorine and methyl chloride had increased flame speeds. 

The previous intent has been to use kinetics simply as a tool to describe qualitatively the particular aspect of combustion under study. Numerical values of the kinetic constants were thus assumed for illustrative purposes or approximated from other types of data by making admittedly questionable major assumptions. Approximations include, for example, the extrapolation of low temperature hydrocarbon oxidation rates to high temperature hydrocarbon combustion rates. Other schemes involve application of semiempirical laminar flame speed theories or of flow patterns in the wake of a bluff body immersed in an air stream (43). 

The second is concerned with the need to have a complete and sensible chemical mechanism, valid over a wide range of temperature. Even a relatively simple combustion system will involve dozens of reactions, so that a well established reaction rate data base is essential. It is equivalently essential that the results be verified by comparison with detailed experimental data--such as that provided by laser probes. For example, in a study of the ozone decomposition flame (20). it was found that certain alternative but wrong choices of key input parameters were not discernible if flame speed were used as the sole predicted result for verification however, these choices did produce considerable differences in the profiles of the transient oxygen atom concentration and the temperature. 

By decreasing the percentage of nitrogen from that present in the atmosphere to nil, the flame speeds show enormously enhanced values. This is clear from the data shown graphically in fig. 24s.2... 

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