Flame speed measurements

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. 

Flame velocity has been defined as the velocity at which the unbumed gases move through the combustion wave in a direction normal to the wave surface. 

FIGURE 4.14 Velocity and temperature variations through non-one-dimensional flame systems. 

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. 

It is apparent from Fig. 4.14 that it is difficult to select a particular linear flow rate of unbumed gas up to the flame and regard this velocity as the burning velocity. 

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Figure 15.4 A comparison of turbulent flame speeds measured in the TC apparatus with theoretical predictions and measurements by earlier investigators using other types of burners 1 — TC results, 2 — Abdel-Gayed et al. [3], 3 — Chang and Shepherd [1], 4 — Bedat and Cheng [2], 5 — Anand and Pope [15], and 6 — Yakhot [16]... 

At large values of the Zel dovich number, the chemical reaction is confined to a thin sheet in the flow. For all purposes except the analysis of the sheet structure, the sheet may be treated as a surface—for example, G(x, t) = 0—which in terms of the Cartesian coordinates (x, y, z) may be written locally as x = F y, z, t) if the x coordinate is not parallel to the sheet in its local orientation. When analyses are pursued in outer-scale variables, it is convenient to work in a coordinate system that moves with the sheet. For the undisturbed flow, let the x coordinate be normal to the planar flame, with the unburnt gas extending to x = — oo and the burnt gas to x = -1- oo. Employ the steady, adiabatic, laminar flame speed, measured in the burnt gas, = po Vq/p with Vq given by equation (5-78), and the thermal diffu-... 

Another aim is an examination of the influence of the Markstein number on the effect of the secondary pyroacoustic instability, associated with the development of transverse acoustic waves and pulsating cells on the flame [4, 5]. In addition, laminar-flame speeds measured during the stage of a nearly flat surface (Stage B in Fig. 7.1) are compared with those reported in the literature. 

Figure 7.3 Methane-air planar-flame speeds measured in the TC burner (J) compared with those obtained in earlier studies 2 — [15], 3 — [13], and 4 — [14].

In the reaction 2one, an increase in the intensity of the turbulence is related to the turbulent flame speed. It has been proposed that flame-generated turbulence results from shear forces within the burning gas (1,28). The existence of flame-generated turbulence is not, however, universally accepted, and in unconfined flames direct measurements of velocity indicate that there is no flame-generated turbulence (1,2). 

Hoff (1983) studied the effect of igniting natural gas after a simulated pipeline rupture by firing a bullet into the gas mixture. The tests were on a 10-cm diameter pipeline operating at an initial pressure of 60 bar and a gas throughput of 400,000 mVday. The openings created in the pipeline simulated full-bore ruptures. Maximum flame speeds of approximately 15 m/s, and maximum overpressures of 1.5 mbar were measured at a distance of 50 m. 

Several investigations were performed in channels (Table 4.5). In experiments in which the channel was completely confined, flame speed enhancements were similar to those observed in tubes. In experiments in which channels were open on top, thus allowing combustion products to vent, far lower flame speeds were measured. Partially opening one side of a channel permitted varying degrees of confinement. 

Flame speed The speed of a flame burning through a flammable mixture of gas and air measured relative to a fixed observer, that is, the sum of the burning and translational velocities of the unbumed gases. 

The chronology of the most remarkable contributions to combustion in the early stages of its development is as follows. In 1815, Sir Humphry Davy developed the miner s safety lamp. In 1826, Michael Faraday gave a series of lectures and wrote The Chemical History of Candle. In 1855, Robert Bunsen developed his premixed gas burner and measured flame temperatures and flame speed. Francois-Ernest Mallard and Emile Le Chatelier studied flame propagation and proposed the first flame structure theory in 1883. At the same time, the first evidence of detonation was discovered in 1879-1881 by Marcellin Berthelot and Paul Vieille this was immediately confirmed in 1881 by Mallard and Le Chatelier. In 1899-1905, David Chapman and Emile Jouguet developed the theory of deflagration and detonation and calculated the speed of detonation. In 1900, Paul Vieille provided the physical explanation of detonation... 

Figure 4.1.7 summarizes the measured laminar flame speeds of efhylene/air, n-heptane/air, fso-octane/air. 

Measured laminar flame speeds of (a) ethylene/air, (b) n-heptane/air, (c) iso-octane/air, and (d) n-decane/air mixtures as a function of the equivalence ratio for various unburned mixture temperatures. 

Further measurements on the flame speed have been obtained with the use of a rotating tube [11] and vortex ring combustion [12]. Figure 4.2.4 shows the flame speed in vortex rings [12]. The values of slope in the V( -plane is nearly equal to unity for the near stoichiometric methane/air mixtures. Thus, this value is much lower than the predictions of JPi/P, and flPn/P >. 

A theory, termed as the back-pressure drive flame propagation theory, has been proposed to account for the measured flame speeds [12]. This theory gives the momentum flux conservation on the axis of rotation in the form of... 

Ishizuka, S., Hamasaki, T., Koumura, K., and Hasegawa, R., Measurements of flame speeds in combustible vortex rings Validity of the back-pressure drive flame propagation mechanism, Proceedings of the Combustion Institute, 17, 727-734,1998. 

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. 

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