Laminar flames experiments with

Flame dynamics is intimately related to combustion instability and noise radiation. In this chapter, relationships between these different processes are described by making use of systematic experiments in which laminar flames respond to incident perturbations. The response to incoming disturbances is examined and expressions of the radiated pressure are compared with the measurements of heat release rate in the flame. The data indicate that flame dynamics determines the radiation of sound from flames. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects, usually treated separately, appear as manifestations of the same dynamical process. 

Experiments in closed vessels by Abdel-Gayed and Bradley [19], left Ka.Le = 0.003 continuous laminar flame sheet, right Ka.Le = 0.238 breakup of the continuous flame sheet. (Reprinted from Lewis, B. and Von Elbe, G., Combustion, Flames and Explosions of Gases, Academic Press, New York, 1961. With permission. Figure 204, p. 401, copyright New York Academic Press (Elsevier editions).)... 

With representative values for A, Cp, and po with Vq 50 cm/s, equation (4) gives S 10 cm. Therefore 5 is large compared with a molecular mean free path (about 10 cm), and the continum equations of fluid dynamics are valid within the deflagration wave but 3 is small compared with typical dimensions of experimental equipment (for example, the diameter of the burner mouth, and hence the radius of curvature of the flame cone, for experiments with Bunsen-type burners), and laminar deflagration waves may be approximated as discontinuities in many experiments. Since equations (3) and (4) imply that 3 at constant temperature, experimental... 

For flames that exhibit the parametric instability, the velocity at which the exponential growth of velocity fluctuations started for each experiment was noted. These critical velocities are shown in Fig. 7.5, normalized by the laminar-flame speeds reported in [13]. All points shown on this plot represent the ensemble average of measurements from five experiments, and the error bars indicate the standard deviation about the mean value. The other curve on this plot was calculated using the analytical model of a premixed flame under the influence of an oscillating gravitational field by Bychkov [17], ris described above. Each point represents the smallest normalized acoustic velocity at the most unstable reduced wave number that resulted in the parametric instability. The experimental results show the same trend as the theoretical model mixtures with an equivalence ratio of 0.9, which require the smallest normalized acoustic velocity to trigger the parametric instability while flames on either side require larger values. 

Cuoci, A., Frassoldati, A., Faravelli, T., Ranzi, E. Numerical modeling of laminar flames with detailed kinetics based on the operator-splitting method. Energy Fuels 27,7730-7753 (2013b) Daescu, D., Sandu, A., Carmichael, G.R. Direct and adjoim sensitivity analysis of chemical kinetic systems with KPP Part n—Validation and numerical experiments. Atmos. Environ. 37, 5097-5114 (2003)... 

The pressure effect on a laminar flame velocity cannot always be reliably determined in experiments with hydrogen-containing mixtures. Contradictory conclusions for normal flame velocity dependence on pressure can be found in the literature. In technical applications, power functions for a flame velocity dependence approximation on temperature and pressure are used. Such power functions are called thermal and baric indexes respectively. 

Figure 2.26 presents the measured results (the open circles) and the calculated data (the curves) for the laminar flame velocity. The calculation and the experiments have been performed in the mixtures with various water steam content. In fact, the mixture with 0 = 0.39 cannot be diluted with more than 32% H2O, and the mixture with over-fuel factor 0 = 0.26 cannot contain more than 12.5% H2O. 

Triple and edge flames [12, 13] can be considered as a development of a partially pre-mixed mixture combustion occurring as the result of a non-mixed structure breakdown. They can be observed at the local extinction of a diffusion flame. The triple flame phenomenon was observed in experiments with diffusion combustion of a methane layer located under a horizontal ceiling simulating mine boring conditions [14]. The detailed investigation of such a phenomenon started after a triple flame configuration was discovered in a laminar flame stabilized in an elevated position over the nozzle section. 

If, in an infinite plane flame, the flame is regarded as stationary and a particular flow tube of gas is considered, the area of the flame enclosed by the tube does not depend on how the term flame surface or wave surface in which the area is measured is defined. The areas of all parallel surfaces are the same, whatever property (particularly temperature) is chosen to define the surface and these areas are all equal to each other and to that of the inner surface of the luminous part of the flame. The definition is more difficult in any other geometric system. Consider, for example, an experiment in which gas is supplied at the center of a sphere and flows radially outward in a laminar manner to a stationary spherical flame. The inward movement of the flame is balanced by the outward flow of gas. The experiment takes place in an infinite volume at constant pressure. The area of the surface of the wave will depend on where the surface is located. The area of the sphere for which T = 500°C will be less than that of one for which T = 1500°C. So if the burning velocity is defined as the volume of unbumed gas consumed per second divided by the surface area of the flame, the result obtained will depend on the particular surface selected. The only quantity that does remain constant in this system is the product u,fj,An where ur is the velocity of flow at the radius r, where the surface area is An and the gas density is (>,. This product equals mr, the mass flowing through the layer at r per unit time, and must be constant for all values of r. Thus, u, varies with r the distance from the center in the manner shown in Fig. 4.14. 

Currently, one of the most developed, hence most illustrative, examples of practical application of SM is provided by the GRI-Mech project [1]. In its latest release, the GRI-Mech 3.0 dataset is comprised of 53 chemical species and 325 chemical reactions (with a combined set of 102 active variables), and 77 peer-reviewed, well-documented, widely trusted experimental observations obtained in high-quality laboratory measurements, carried out under different physical manifestations and different conditions (such as temperature, pressure, mixture composition, and reactor conhguration). The experiments have relatively simple geometry, leading to reliably modeled transport of mass, energy, and momentum. Typical experiments involve flow-tube reactors, stirred reactors, shock tubes, and laminar premixed flames, with outcomes such as ignition delay, flame speed, and various species concentration properties (location of a peak, peak value, relative peaks, etc.). 

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