Combustion wave steady propagation

When an explosive gas mixture is placed in a tube having one or both ends open, a combustion wave can propagate when the tube is ignited at an open end. This wave attains a steady velocity and does not accelerate to a detonation wave. 

The combustion wave of a premixed gas propagates with a certain velocity into the unburned region (with flow speed = 0). The velocity is sustained by virtue of thermodynamic and thermochemical characteristics of the premixed gas. Figure 3.1 illustrates a combustion wave that propagates into the unburned gas at velocity Mj, one-dimensionally under steady-state conditions. If one assumes that the observer of the combustion wave is moving at the same speed, Wj, then the combustion wave appears to be stationary and the unburned gas flows into the combustion wave at the velocity -Wj. The burned gas is expelled downstream at a velocity of-M2 with respect to the combustion wave. The thermodynamic characteristics of the combustion wave are described by the velocity (u), pressure (p), density (p), and temperature (T) of the unburned gas (denoted by the subscript 1) and of the burned gas (denoted by the subscript 2), as illustrated in Fig. 3.1. 

Explosion is a term that corresponds to rapid heat release (or pressure rise). An explosive gas or gas mixture is one that will permit rapid energy release, as compared to most steady, low-temperature reactions. Certain gas mixtures (fuel and oxidizer) will not propagate a burning zone or combustion wave. These gas mixtures are said to be outside the flammability limits of the explosive gas. 

No theoretical criterion for flammability limits is obtained from the steady-state equation of the combustion wave. On the basis of a model of the thermally propagating combustion wave it is shown that the limit is due to instability of the wave toward perturbation of the temperature profile. Such perturbation causes a transient increase of the volume of the medium reacting per unit wave area and decrease of the temperature levels throughout the wave. If the gain in over-all reaction rate due to this increase in volume exceeds the decrease in over-all reaction rate due to temperature decrease, the wave is stable otherwise, it degenerates to a temperature wave. Above some critical dilution of the mixture, the latter condition is always fulfilled. It is concluded that the existence of excess enthalpy in the wave is a prerequisite of all aspects of combustion wave propagation. 

It is found experimentally that limit mixtures, incapable of supporting combustion waves, nevertheless have theoretical thermodynamic flame temperatures of the order of 1000° C. or more. It is, therefore, not immediately clear why combustion waves, albeit slowly propagating, should not develop in mixtures possessing such substantial chemical enthalpy. The question arises whether the observed limits of flammability are true limits or whether such mixtures are actually capable of supporting combustion waves but are prevented from doing so by experimental limitations. Experimentalists believe that the limits are true. On the other hand, no theoretical criterion for the limit is obtained from the steady-state equations of the combustion wave. That is, the equations describe combustion waves without differentiating between mixtures that are known to be flammable and mixtures that are known to be nonflammable. Therefore, for nonflammable mixtures the combustion wave becomes unstable to perturbations and thus disappears (7). Conversely, for flammable mixtures the combustion wave can overcome perturbations—i.e., it returns to the steady state after being perturbed. 

A reaction at the initial temperature changes the characteristics of an explosive mixture before the flame front and introduces an element of nonsteadiness into the process of propagation of the combustion wave. The method proposed in [1] to describe this effect consists in replacing the original non-steady problem by a quasi-steady one with adiabatically increasing initial temperature Ta(f) and an effective source of heat release which takes this increase into account. We test this method below by comparing it directly with the results of a numerical solution of the original non-steady problem. 

Within the region of optimal experimental parameters, the combustion wave velocity remains constant and the temperature profile T(t) has the same form at each point of the reaction medium. This regime is called steady propagation of the combustion synthesis wave, or steady SHS process. As the reaction conditions move away from the optimum, where the heat evolution decreases and/or heat losses increase, different types of unsteady propagation regimes have been observed. These include the appearance of an oscillating combustion synthesis... 

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