Ignition and extinction

CA 68,61103 (1968) Nonsteady burning of solid propints is being investigated both theoretically and exptly with attention to combustion instability, transient burning during motor ignition, and extinction by depressurization. 

J. Daou and A. Linan, The role of unequal diffusivities in ignition and extinction fronts in strained mixing layers. Combust. Theory Model. 2 449-477,1998. 

Chapter 3 deals with flarranability limits, ignition of a flarranable mixture, and extinction of limit flames. Both phenomena, ignition and extinction, are dependent on time. 

The problem of ignition and extinction of reactions is basic to that of controlling the process. It is interesting to consider this problem in terms of the variables used in the earlier discussion of stability. When multiple steady-state solutions exist, the transitions between the various stable operating points are essentially discontinuous, and hysteresis effects can be observed in these situations. 

Influence of operating conditions on ignition and extinction of reactions. 

Thus, the final product mixture will depend on the relative importance of mixing and reaction in determining (T )i(f). Finally, note that since the second environment was necessary to describe the ignition source, this simple description of ignition and extinction would not be possible with a one-environment model (e.g., the conditional moment closure). 

Surl iiec Icmpcraturc K Figure 26.1 The mole fraction of H2 just above the surface as a function of the surface temperature for 1 (a), 3 (6), and 4 atm (c), respectively. Gas-phase ignitions and extinctions are represented by arrows. The HB and VH points are indicated with circles and triangles, respectively. Stable and unstable branches are represented by solid and dashed curves, respectively. The mixture is 10% H2-air and the strain rate is 200 s ... 

Fig. 26.1a). At first, multistage ignitions and extinctions occur followed by a relaxation (long period) mode [7]. Oscillations die a few degrees below the ignition temperature at a saddle-loop infinite-period homoclinic orbit bifurcation point. This is an example where both ignition and extinction are oscillatory. 

Following the procedure of 1.7.2 we obtain the dimensionless residence times for ignition and extinction in terms of / 0 ... 

When P0 = i, the two roots of eqn (6.22) are exactly equal. The ignition and extinction points are coincident at ires = 64/27 multistability is lost. For larger inflow concentrations of B the stationary-state extent of reaction increases smoothly with the residence time and the distinction between the flow and thermodynamic branch is lost. 

We are again concerned with intersections of R and L on the flow diagram. The larger the value of k2, the steeper the minimum gradient of the flow line and hence L will not cut as far into R as tres varies. In particular we may lose the possibility of one or even both tangencies between the curves, and hence lose points of ignition and extinction. To illustrate the effect of the autocatalyst decay through if2 on the stationary-state response we can consider a CSTR which is fed only by the reactant A, so po = 0. 

Equation (7.28) only has real roots, and hence ignition and extinction can only occur if the discriminant under the square root sign is positive. This then makes a requirement on the size of the dimensionless adiabatic temperature excess 0ad. In particular, tangency only occurs if the reaction is sufficiently exothermic such that... 

For all p0 less than g, the ignition and extinction points in the x-rres locus are determined by the solutions of... 

Relaxation times near ignition and extinction points... 

Fig. 8.3. The approach to, or departure from, stationary-state solutions following small perturbations for simple cubic autocatalysis again showing the instability of the middle branch. The turning points (ignition and extinction) have one-sided stability as perturbations in one direction decay back to the saddle-node point, but those of the opposite sign depart for the other...

There is another type of time dependence possible in this system. If the inflow concentration of the autocatalyst is adjusted so that b0 - a0, then the ignition and extinction points merge at trcs = (k1ao) 1, with ass = Iu0 Under these special conditions, the coefficient of the term in (Aa)2 in the rate equation, and hence in the denominator of eqn (8.21), becomes zero as well as those of the lower powers in A a. Thus the inverse time dependence disappears, and the only non-zero term governing the decay of perturbation is that in (Aa)3 ... 

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. 

D.G. Vlachos. Reduction of Detailed Kinetic Mechanisms for Ignition and Extinction of Premixed Hydrogen/Air Flames. Chem. Eng. Sci., 51 3979-3993,1996. 

D.G. Vlachos, L.D. Schmidt, and R. Aris. Ignition and Extinction of Fames near Surfaces Combustion of H2 in Air. Combust. Flame, 95 313-335,1993. 

Effects of velocity on homogeneous-heterogeneous ignition and extinction (with RJ. Olsen and L.D. Schmidt). Combust. Sci. Tech., 99 (1995). 

Ignition and extinction of flames near surfaces Combustion of H2 in air (with D.G. Vlachos and L.D. Schmidt). Comb. Flame 95, 313-335... 

Ignition and extinction of homogeneous-heterogeneous combustion CH4 and C3H8 on Pt ... 

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