It was noted in Chapter 2 (p. 23) that the rate constants have an Arrhenius form and thus are highly temperature sensitive. The preceding numerical experiments were all
§5.3 Zero moisture, constant wind 99
(a)
(b)
Figure 5.16: Coevolution of the amount of bound water in the substrate and (a) the mass of
C, and (b) the combined heat of formation and combustion of charcoal. Increasing wind speed increases the amount ofCin the system but decreases the total heat associated with the charcoal (i.e. formation and combustion) while increasing the amount of bound water.
(a)
(b)
Figure 5.17:Coevolution of the heat expended due to evaporation and the total heat in the system at (a) the large scale, and (b) the small scale. A complex interrelation between wind speeds, evaporation of moisture and the total heat in the system is apparent around the near-critical value of wind strength.
§5.3 Zero moisture, constant wind 101
(a)
(b)
Figure 5.18: (a) Evolution of substrate mass with varying ambient wind temperature (Ta =
300K → 700K of constant wind strength, f = 0.007. (b) Coevolution of substrate mass and substrate temperature. Increasing air flow temperature increases reaction rates and decreases time for complete consumption of the fuel substrate.
conducted with a fixed air flow temperature of 300 K, simulating the flow of ambient air over the combustion zone. This replicates the conditions that would be found on the upwind or windward side of a fire (i.e. that part of the fire that is exposed to the ambient air). This section explores, albeit in much less detail, the effect of the temperature of the air flow over the range from ambient to 700 K, well in excess of the 580 K initial substrate temperature.
Due to the many possible combinations and permutations of experimental condi- tions, the conditions explored here are limited to a wind strength of f = 0.007. As a
result, the potential for a critical wind speed is not determined.
Figure 5.18a shows the effect of the temperature of the ambient wind on substrate mass for the range of temperatures: Ta = 300K → 700K. As the temperature of the air flow increases, the rate at which the substrate is consumed increases as expected. The rate of increase of the substrate loss rate decreases exponentially with air flow tem-
perature. In each case, the initial substrate temperature (580 K) couples with the initial gas phase temperature (550 K), reaching approximately 567 K before the air tempera- ture begins to exert its effect (Fig. 5.18b). At air temperatures less than about 500 K, the substrate temperature continues to decrease before the exothermic reactions kick-in and lead toward ignition.
Figure 5.19 shows the effect of air flow temperature on reaction products. Increasing air temperature has the effect of increasing the ratio of LG to OH (Fig. 5.19a), V to C (Fig. 5.19c), heat released by combustion of volatiles to heat released by combustion of charcoal (Fig. 5.19d), and combined heat associated with (i.e. formation and combustion) volatiles to charcoal (Fig. 5.19e). As a result of the endothermic nature of the formation of levoglucosan, the effect of the increased mass of volatile causes a decrease in the ratio of the magnitude of the heat associated with volatile formation to that associated with charcoal formation. This decrease, however, is very minor and, like the evaporation of bound water formed through the oxidation reactions, makes little difference to the total heat of the system (Fig. 5.19f).
5.3.6 Discussion
The effect of wind can be seen to advance or retard the thermal degradation reactions and thus the rate of loss of substrate mass, depending on its temperature (Fig. 5.18). Typically, ambient air temperature (!290–310 K), which is in the order of 280 K below
the initial substrate temperature of 580 K for these simulations, acts to cool the reaction zone through the solid and gas phase coupling coefficients and slow the reaction rates. Elevated air temperature, in the order of the temperatures associated with the onset of ignition or greater (i.e.!580–700 K), act to promote the reaction rates, through the same
coupling coefficients.
The strength of the wind also affects the reaction rates, acting to magnify the effect of the air temperature (Fig. 5.5, p. 88); that is, a faster cool wind results in a slower reaction rate than that found with a lower strength wind of the same temperature, and vice versa with a hot wind. At a wind temperature of 300 K, a near-critical behaviour was identified with a wind strength between 0.007 and 0.008. Between these values, the system of reactions either progressed towards ignition of the volatiles and the rapid consumption of fuel or petered out and stopped.
A binary search of the wind strength space revealed an increasingly complicated sys- tem response as the near-critical wind strength was approached (investigated to 5 deci- mal places without any conclusive critical value reached) (Fig. 5.10, p. 93). The response consisted of an initial decrease in system temperatures (gas phase and solid substrate) followed by a steady increase, which either continued on until ignition or decreased again to reaction cessation. Analysis of the reaction products revealed little variation in the production of LG or OH, but a shift in the dominance in the production of V and CtoC. The exothermic charcoal formation reaction appears to support the endothermic volatilisation reaction around the near-critical wind strength values.
The critical behaviour is most evident in the sum of the enthalpies in the system (i.e. total heat), showing nearly opposite behaviour in a complicated manner in con- junction with the evaporation of bound water around the near-critical wind strength; reaction conditions that lead to ignition show a rapid increase in both the total heat and
§5.3 Zero moisture, constant wind 103
(a) (b)
(c) (d)
(e) (f)
Figure 5.19:Effect of air flow temperature on reaction products and heat: (a)LGandOH, (b) heat associated with the formation of volatiles and charcoal, (c)VandC, (d) heat associated with the combustion of volatiles and charcoal, (e) heat associated with the formation and combustion of volatiles and charcoal, and (f) total heat and energy associated with evaporation.
energy associated with evaporation (led by the production of water from the oxidation reactions), whereas the reaction conditions that lead to cessation show a corresponding decrease in the heat due to evaporation and total heat in the system (Fig. 5.17).