Extinction phenomena

Good heat transfer on the outside of the reactor tube is essential but not sufficient because the heat transfer is limited at low flow rates at the inside film coefficient in the reacting stream. The same holds between catalyst particles and the streaming fluid, as in the case between the fluid and inside tube wall. This is why these reactors frequently exhibit ignition-extinction phenomena and non-reproducibility of results. Laboratory research workers untrained in the field of reactor thermal stability usually observe that the rate is not a continuous function of the temperature, as the Arrhenius relationship predicts, but that a definite minimum temperature is required to start the reaction. This is not a property of the reaction but a characteristic of the given system consisting of a reaction and a particular reactor. 

D, W. Blair, CombustFlame 20 (1), 105—9 (1973) CA 78, 113515 (1973) A simple heat-transfer model is coupled with an Arrhenius-type pyrolysis law to study the effect of solid-state heat-transfer losses on burning rates of solid rocket-proplnt strands. Such heat-transfer losses materially affect the burning rates and also cause extinction phenomena similar to some that had been observed exptly. Strand diam and compn, adiabatic burning rate, and the heat-transfer film coeff at the strand surface are important variables. Results of theoretical analysis are applied to AP-based composite solid proplnts... 

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. 

In combustion research there is currently great interest in understanding how strained flames respond to fluctuations in the strain field [112,189-193,227,386]. This research is motivated by a need to understand how flames behave in turbulent flow fields, including flame stability, ignition and extinction phenomena, and pollutant formation. 

From these results and applying the theory of critical ignition and extinction phenomena of strongly exothermic heterogeneous reactions developed by Frank-Kamenetskil and Buben (9), it was found that the following form of kinetic equation is the most probable ... 

Experiments with the system H2-02 were performed also by Horak et al. (21-23) who observed pronounced ignition-extinction phenomena. They were able to construct a reliable mathematical model based on the heat and mass balances describing the gas-to-solid heat and mass transfer. Their general conclusion is that the multiplicity phenomena may be explained in terms of thermokinetic theory. However, on the other hand, because of the high thermal capacity of the pellet, the oscillations cannot be described by this mechanism (72). Obviously we should examine a more detailed kinetic mechanism to be able to analyze successfully this phenomenon (25). 

Flammability limits are limits of composition or pressure beyond which a fuel-oxidizer mixture cannot be made to burn. They are of practical interest especially in connection with safety considerations because mixtures outside the limits of flammability can be handled without concern about ignition. For this reason, extensive tabulations of limits of flammability have been prepared [1], [2]. Meanings of these tabulations and their relationships to ignition and extinction phenomena will be considered here in Section 8.2. 

Since we are not concerned presently with ignition and extinction phenomena (23,24, 25,26,27) caused by slow chemical heat release rates, the gas-phase chemical reactions can be assumed to occur at rates much faster than the gas-phase heat and mass transfer rates. This implies that the gas-phase consists of two (one in the case of pure vaporization) convective-diffusive regions separated by a fiame of infinitesimal thickness, at which the outwardly diffusing fuel vapor reacts stoichiometrically and completely with the inwardly diffusing oxidizer gas. 

O. Kalthoff and D. Vortmeyer, Ignition/Extinction Phenomena in a Wall-Cooled Fixed-Bed Reactor, Chem. Eng. Sci. 55 1637-1643 (1980). 

Hutchinson, R.A. Ray, W.H. Polymerization of olefins through hetereogenous catalysis. VII. Particle ignition and extinction phenomena. J. Appl. Polym. Sci. 1987, 34, 657-676. 

For the diffracted intensities from real crystals, there is absorption as well as some interaction between incident and scattered radiation within the crystal. In this paper, attention will be focused on those crystals where the extinction phenomena are accounted for such that the experimental intensities can be reduced to the intensities of Eq. (2) with a systematic error no larger than 5 percent. These diffraction data are called accurate. 

It is known that the catalytic combustion of hydrocarbons exhibits ignition-extinction phenomena which depend on feed composition [3]. The ignition experiments reported in this study were performed with inlet fuel concentration comprised in the region of surface flammability, in which hysteresis effects occur typical values were ycH4 0.04, yc3H8 0.02, y o 0.03. The superficial velocity (u) of the reactant gas was between 1 and 5 cm/s. 

In the present paper model experiments are reported under defined conditions for gas and solid flow by countercurrent operation. In this situation the energetic feedback by the effective heat conduction is increased through the countercurrent movement. Interesting stability problems arise concerning ignition/extinction phenomena. 

It must be stressed to the reader that the preceding discussion was based on the (simultaneous) steady state solution of the thermal and material transport equations. In a real physical situation the onset of ignition or extinction phenomena is inherently an unsteady state process it follows that the transition to ignition or extinction will take place at a finite rate. 

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