The production of LG and OH shows little impact due to wind at strengths less than 0.01 (Fig. 5.11). While greater wind strengths result in a greater proportion of OH to LGthan at lower wind strengths, these winds speeds ultimately extinguish the reactions regardless.
If we consider the mass ofVandC(Fig. 5.12, p. 95), the effect of wind is to decrease the ratio of volatiles to charcoal. In all but the highest wind speed cases (Fig. 5.12a), the 1It is not unreasonable to suspect that no single critical value would be found to the precision of the 64-bit
Figure 5.11: Effect of varying wind speed,f, on the production ofLGandOH. At strengths less than 0.01, wind does not appear to influence the formation of these products.
amount ofVin the system in the initial stages of reaction is much greater (by an order of magnitude) than that ofC. As the wind speed increases so does the rate of increase ofC in the system. However, as the reactions unfold, the later stages of degradation become dominated by production ofV. At the higher wind strengths (Fig. 5.12b),Cdominates. Between wind strengths of 0.007 and 0.01 there appears to be a transition from where Vdominates to whereCdominates. This coincides with the critical region identified in Figure 5.10.
Figure 5.12c shows the coevolution ofV andCfor the same wind strengths investi- gated in Figure 5.10. In this regime of wind,Cdominates all but the very initial stage for all cases and the very final stages for those cases that eventually lead to ignition. In the case of f = 0.0076484375, Vactually declines before it recovers and increases, leading
to ignition. At wind strengths greater than this value, C dominates as V continues to decrease, leading to eventually to cessation of the reactions. However, because the mass of bothVandCare a result of both production and consumption reactions, there may be other aspects not apparent in just the mass of these quantities.
The effect of wind on the heat evolved in the formation of charcoal and volatiles is not straightforward. At the broad scale (Fig. 5.13a, p. 96), increasing wind results in a relative increase in the amount of energy involved due to the formation of volatiles, im- plying that volatile production increases (which is opposite to that suggested by the raw comparisons of masses). At the finer scale (Fig. 5.13b), the impact of wind is even more complex. In the early stage, increasing wind, regardless of wind strength, increases the relative energy associated with charcoal formation. At strengths greater than the near- critical value identified previously (but less than 0.008), there is then a relative increase in the heat involved in the formation of volatiles (forming a slight ‘s’-shaped bend in the curve) but then the energies associated with both volatilisation and charcoal for- mation decrease, resulting in a hook-turn as the reactions peter out. In the cases of
§5.3 Zero moisture, constant wind 95
(a) (b)
(c)
Figure 5.12:Effect of varying wind speed,f, on volatilesVand charcoalC. (a)f = 0→0.01. (b)f = 0.01→1.0. (c)f = 0.007→0.008. Increasing wind speed acts to decrease the ratio ofV:C.
3000 s, much greater in volatilisation than in the initial stages of the reaction. For greater wind strengths, the magnitude of the energy associated with volatilisation is less than the initial stage.
At wind strengths at or below the near-critical value—but greater thanf = 0.0075—a
dog-leg kink appears in the coevolution of the enthalpies of volatilisation and charcoal formation. In the most extreme case (f = 0.0076484375), there is a decrease in both the
formation of charcoal and volatiles before both recover and continue on to ignition. In the lesser cases (f = 0.007640625andf = 0.007625) there is essentially only a decrease
or a pause in the energy involved in the formation of charcoal. Subsequent evolution shows the greater volatilisation energy at higher wind speeds seen in Figure 5.13a.
Figure 5.14 (p. 97) shows the effect of wind speed on the consumption ofV andC through the heat released by the associated combustion reactions. At all wind speeds considered here at and around the critical wind speed value, the energy released due to volatile oxidation (i.e. flaming combustion) dominates that of the charcoal oxidation
(a)
(b)
Figure 5.13: Coevolution of the heat evolved from the formation of volatiles and charcoal (a) across a broad scale of energy, and (b) at the low energy end of scale. A greater magnitude of energy is released from the formation of charcoal. Increasing wind speed generally acts to increase the magnitude of energy from the formation of volatiles, up to the near-critical wind strength.
§5.3 Zero moisture, constant wind 97
Figure 5.14:Effect of varying wind speed,f, on the heat released through combustion of volatiles and charcoal. Increasing wind speed acts to decrease the ratio of heat released due to combustion of volatiles to the heat released due to combustion of charcoal.
(i.e. smouldering combustion) by an order of magnitude. This is due primarily to the rapid reaction rate of the volatile oxidation when compared to the charcoal oxidation rate, as the amounts of volatile and charcoal consumed are comparable. As wind speed increases, the amount of energy released through charcoal combustion increases, to the point at which the energy released through volatile combustion actually decreases (f =
0.00765625) while charcoal combustion continues. At wind speeds in excess of this, the
volatile combustion steadily decreases before the energy from charcoal combustion also decreases, leading eventually to cessation of the reactions as they peter out.