Final flame temperature

This is the dominant overall reaction for the decomposition on platinum or tungsten at 200 and 380 °C, respectively40. Most workers on hydrazine decomposition flames41-44, in which the reactions are homogeneous, report a stoichiometric equation similar to (b) for final flame temperatures up to 1900 °K. Measurements of MacLean and Wagner45 on decomposition flames and of Husain and Norrish37 on the flash photolysis of hydrazine indicate the contribution of the overall reaction... 

The first two reaction steps are endothermic however, the overall reaction is exothermic and the final flame temperature is 1800 K. The observed pressure dependence of the burning rate follows a second-order rate law the overall activation energy is consistent with the oxidation reaction by NO2 being the slowest and hence the rate-controlling step. 

The temperature profile in the combustion wave of a double-base propellant is altered when the initial propellant temperature Tq is increased to Tq -i- ATq, as shown in Fig. 6.15. The burning surface temperature is increased to -i- AT, and the temperatures of the succeeding gas-phase zones are likewise increased, that of the dark zone from Tgto Tg-t- ATg, and the final flame temperature from 7 to Tf-t- ATf If the burning pressure is low, below about 1 MPa, no luminous flame is formed above the dark zone. The final flame temperature is Tg at low pressures. The burning rate is determined by the heat flux transferred back from the fizz zone to the burning surface and the heat flux produced at the burning surface. The analysis of the temperature sensihvity of double-base propellants described in Section 3.5.4 applies here. 

Introducing the Mallard-Le Chatelier approximation (57) that the local temperature gradients in the conduction terms are equal to the average gradients and recognizing that the temperature gradient is zero when the final flame temperature T2 is reached, these energy equations reduce to ... 

The theoretical and experimental results for a fuel-lean methane-air flame are given in Figures 5-7. These results include temperature and major species compositions. The experimental and theoretical results are compared by matching the abcissas of the temperature profiles. The model very accurately predicts the slope of the temperature profile but predicts a larger final flame temperature than is measured. This is a consequence of heat lost to the cooled, gold-coated burner wall that is 1.5 mm away from the positions where data were taken. 

Consequently, it would appear that the flame temperature is determined not by the specific reactants, but only by the atomic ratios and the specific atoms that are introduced. It is the atoms that determine what products will form. Only ozone and acetylene have positive molar heats of formation high enough to cause a noticeable variation (rise) in flame temperature. Ammonia has a negative heat of formation low enough to lower the final flame temperature. One can normalize for the effects of total moles of products formed by considering the heats of formation per gram (Ahf) these values are given for some fuels and oxidizers in Table 1. 

A flame may be defined as a localized reaction zone which is able to propagate itself sub-sonically through the material supporting it. Most flames are concerned with exothermic reactions of this type, in which typically reactants at near ambient temperatures are converted more or less adiabatically to combustion products at 1000 K or above. Detailed kinetic studies have principally been confined to premixed flames, in which a well-defined reactant mixture at a known initial temperature is converted into combustion products in full chemical equilibrium at the final flame temperature. Assuming adiabatic combustion, the final conditions may be calculated thermodynamically. 

In flames with lower final flame temperatures where the thermal emission from added metal atoms is less, a chemiluminescent effect [134] may occur. Here, there is a rapid rise of intensity in the reaction zone followed by a steady decay towards the thermal level. The chemiluminescence is due to excitation of the metal (in this case sodium) by the reactions... 

This ester resembles its methyl homologue in possessing three modes of decomposition [131]. It also supports a self-decomposition flame, the multiple reaction zones of which are clearly separated at low pressures [122, 123, 125]. Temperature and composition profiles in the low-pressure decomposition flame have been measured [133]. The products include formaldehyde, acetaldehyde and ethanol with smaller amounts of methane and nitromethane. The activation energy derived from the variation of flame speed with final flame temperature was 38 kcal. mole", close to the dissociation energy of the RO—NO2 bond. The controlling reaction is believed to be unimolecular in its low pressure regime, and the rate coefficient calculated from the heat-release profile is... 

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