Unburned mixture

Reference stretch-affected flame speeds as a function of Karlovitz number for various (a) n-heptane/air and (b) iso-octane/air flames, showing how the reference stretch-affected flame speed is extrapolated to zero stretch to obtain the laminar flame speed. The unburned mixture temperature T is 360 K. Solid lines represent linear extrapolation, while dotted lines denote nonlinear extrapolation... 

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. 

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... 

Bunsen burner the conical flame angle (2) and the mean unburned mixture velocity (vu) in the tube can approximately give 5U ... 

The flame spread process in adiabatic. There is no heat transfer to the tube wall and the temperature of the burned products, Tb, and the unburned mixture, 7i are therefore... 

Far from being difficult to explain, the appearance of a shock is hard to prevent in view of the chemically reactive nature of the medium in which the wave is advancing. Heat released in the combustion makes the gaseous products expand,and they push against the unburned mixture ahead of the wave front. Thus they set up a pressure wave of velocity (pr), called a precompression or "precursor wave, which has been observed in schlieren photographs and streak camera records. The flame now advances into a mixture, still unburnt but "processed by precompression and heating which increases the reactivity, in a third region between the other two ... 

A combustion wave is established in an explosive medium by application of a local source of ignition. As heat and, possibly, chain carriers of various kinds flow from the source into the adjacent medium, reaction is initiated in the layer next to the source which in turn becomes a source for igniting the next layer, and so on. Let us consider a mass element of the unburned mixture being overrun by the combustion wave. The reaction rate is virtually zero at the initial temperature but increases with temperature at... 

Detonations. The magnitude of the decrease in vclocit r which occurs across a detonation wave may be more easily visualized in terms of the Mach number of the detonation wave, since the Mach number behind the wave is one, for the Chapman-Jouguct case, usually encountered in practice. For a given initial pressure and temperature, the velocity with which a supersonic detonation wave propagates itself through an unburned mixture is a function of the initial mixture composition. Figure 5 presents some experimental (32) Mach numbers of detonation waves as a function of initial mixture composition. Breton (7), Laffitte and Breton (23), Bone and Fraser (4), Bone, Fraser... 

The magnitude of the initial pressure of the unburned mixture also slightly affects the velocity of the detonation wave, as indicated in Figure 6 (32). 

When the volume uS of the original mixture is replaced by volume nuS of the hot reaction products, the unburned mixture must move so as to free a volume equal to (n — 1 )uS. Thus, the motion of the flame from the closed end at a velocity u results in motion of the original mixture with a velocity of (n — l)ii, i.e., 4-11 times greater. The flame acts as a moving piston causing the gas before it to move. 

We shall repeat the meaning and dimensions of all the notations Vg—linear velocity of flame propagation relative to the unburned mixture, cm/sec Ki—thermal conductivity at the theoretical temperature of combustion, cal/cm sec deg po—initial density of the mixture, g/cm3 n—concentration of the reacting substance, g/cm3 no—initial concentration of the reacting substance, g/cm3 L—calorific value of the mixture, cal/g To—initial temperature, deg Ti—theoretical temperature of combustion, T — To + L/c, deg E—heat of activation, cal/mole R—gas constant, cal/mole deg C—rate constant of a zero-order reaction, gr/cm3 sec C —rate constant of a first-order reaction, sec- 1 C"—rate constant of a second-order reaction, cm3/g sec. 

Can a detonation wave (C) push before it a shock wave (B) of high pressure That this is possible is confirmed by the example of deflagration (slow combustion) where the combustion products, expanding, push before them the still-unburned mixture, and here the pressure of the combustion products is lower than the pressure of the unburned mixture. 

A combustion aerosol differs from a premixed, combustible gaseous system in that it is not uniform in composition. The fuel is present in the form of discrete particles, which may have a range of sizes and may move in different directions with different velocities than the main stream of gas. This lack of uniformity in the unburned mixture results in irregularities in the propagation of the flame through the spray and, thus, the combustion zone is geometrically poorly defined. 

Fig. 4.16. Carbon monoxide-oxygen phase plane. The solid lines give the trajectories corresponding to a starting temperature of 1000 K and fuel-to-air ratios 0.5, 1.0 and 1.5, respectively, up to a simulation time of 0.01 s. The upper ends of the lines belong to the initial unburned mixture. For the preparation of the repro-model the initial composition of the mixture was uniformly distributed between = 0.5 and 1.5. Dots represent the values of CO and O2 concentrations used for the generation of the repro-model. Only a part of the 30,000 data points are plotted on the figure for clarity.

It was recognized by Zeldovich et al. [183,184] and Makhviladze and Rogatykh [185] that, if the conditions are such that the autoignition front moves into the unburned mixture at approximately the acoustic velocity, then the pressure wave generated by the combustion heat release can couple with the autoignition front, with mutual reinforcement of both fronts and very rapid reaction. When the autoignition wave moves much faster or slower than the acoustic velocity such coupling does not occur and the combustion is less intense. 

When a premixed gas-oxidant cloud is ignited, the flame can propagate in two different modes through the gas mixture—deflagration and detonation. Deflagrations propagate at subsonic speeds relative to the unburned mixture and the heat and mass are transported by conduction, diffusion, and convection. Gas mixture detonations propagate at speeds faster than the local sound speed of the unburned gas. In a gas mixture detonation, a shock wave is sustained... 

Tetraethyl lead is supposed to function by reducing the ionization of the unburned mixture directly ahead of the advancing flame front, and thus preventing the undue acceleration of combustion which leads to knocking. This reduction in ionization was by the suggested absorption of ions by the lead atoms and their discharge through recombination. 

The chapter contains the results of theoretical and experimental investigations of control of the deflagration-to-detonation transition (DDT) processes in hydrocarbon-air gaseous mixtures relative to propulsion applications. The influence of geometrical characteristics of the ignition chambers and flow turbulization on the onset of detonation and the influence of temperature and fuel concentration in the unburned mixture are discussed. 

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