Mixture fuel-air

In practice, for motors, turbines or furnaces, the conditions of combustion are frequently far from those corresponding to stoichiometry and are characterized either by an excess or by an insufficiency of fuel with respect to oxygen. The composition of the fuel-air mixture is expressed by the equivalence ratio, (p, defined by the relation / 5 r)... 

In the applications where the compactness of the energy conversion system is the determining factor as in the case of engines, it is important to know the quantity of energy contained in a given volume of the fuel-air mixture to be burned. This information is used to establish the ultimate relations between the nature of the motor fuel and the power developed by the motor it is of prime consideration in the development of fuels for racing cars. 

The normal process is a rapid-but-smooth combustion of the fuel-air mixture in the engine due to the propagation of a flame front emanating from the spark created between the electrodes of the spark plug. 

Power output is controlled, not by adjusting the quantity of fuel/air mixture as in the case of induced spark ignition engines, but in changing the flow of diesel fuel introduced in a fixed volume of air. The work required to aspirate the air is therefore considerably reduced which contributes still more to improve the efficiency at low loads. 

An fuel-air mixture explosion can be initiated by a sudden discharge of static electricity. Yet, while flowing in systems, a fluid develops an electrical charge which will take as long to dissipate as the fluid is a poor conductor. The natural electrical conductivity of jet fuel is very low, on the order of a few picosiemens per meter, and it decreases further at low temperature. 

Figure 4 illustrates the trend in adiabatic flame temperatures with heat of combustion as described. Also indicated is the consequence of another statistical result, ie, flames extinguish at a roughly common low limit (1200°C). This corresponds to heat-release density of ca 1.9 MJ/m (50 Btu/ft ) of fuel—air mixtures, or half that for the stoichiometric ratio. It also corresponds to flame temperature, as indicated, of ca 1220°C. Because these are statistical quantities, the same numerical values of flame temperature, low limit excess air, and so forth, can be expected to apply to coal—air mixtures and to fuels derived from coal (see Fuels, synthetic). 

Vehicle Fa.ctors. Because knock is a chemical reaction, it is sensitive to temperature and reaction time. Temperature can in turn be affected either by external factors such as the wall temperature or by the amount of heat released in the combustion process itself, which is directiy related to the density of the fuel—air mixture. A vehicle factor which increases charge density, combustion chamber temperatures, or available reaction time promotes the tendency to knock. Engine operating and design factors which affect the tendency to produce knocking are... 

The prompt mechanism predominates at low temperatures under fuel-rich conditions, whereas the thermal mechanism becomes important at temperatures above 2732 °F (1500 °C). Due to the onset of the thermal mechanism the formation of NOx in the combustion of fuel/air mixtures increases... 

Flashback into a pre-mix duct occurs when the local flame speed is faster than the velocity of the fuel/air mixture leaving the duct. 

In eatalytie eombustion of a fuel/air mixture the fuel reaets on the surfaee of the eatalyst by a heterogeneous meehanism. The eatalyst ean stabilize the eombustion of ultra-lean fuel/air mixtures with adiabatie eombustion temperatures below 1500°C. Thus, the gas temperature will remain below 1500 °C and very little thermal NO will be formed, as ean be seen in Figure 10-21. However, the observed reduetion in NOx in eatalytie eombustors is mueh greater than that expeeted from the lower eombustion temperature. The reaetion on the eatalytie surfaee apparently produees no NOx direetly, although some NOx may be produeed by homogeneous reaetions in the gas phase initiated by the eatalyst. 

Surface Temperatures. At low temperatures, the oxidation reaetions on the eatalyst are kinetieally eontrolled, and the eatalyst aetivity is an important parameter. As the temperature inereases, the build-up of heat on the eatalyst surfaee due to the exothermie surfaee reaetions produees ignition and the eatalyst surfaee temperature jumps rapidly to the adiabatie flame temperature of the fuel/air mixture on ignition. Figure 10-26 shows a... 

Main fuel injector. This unit is designed to deliver a fuel-air mixture to the catalyst that is uniform in composition, temperature, and velocity. A multi-venturi tube (MVT) fuel injection system was developed by GE specifically for this purpose. It consists of 93 individual venturi tubes arrayed across the flow path, with four fuel injection orifices at the throat of each venturi. 

The intake valve is now closed as the piston moves from the bottom dead center (BDC) to top dead center (TDC), compressing the fuel/air mixture. At Point 3, just prior to TDC, a spark ignites the fuel/air mixture and the resulting combustion causes the pressure and temperature to begin a very rapid rise within the cylinder. 

The single 30-mesh stainless steel flame arrester was effective in arresting flashback flames from all eight fuel-air mixtures tested. 

Howard, W. B., Rodehorst, C. W., and Small, G. E. 1975. Flame Arresters for High-Hydrogen Fuel-Air Mixtures. CEP Loss Prevention Manual, 9, 46-53. 

Langford, B., Palmer, K. N., and Tonkin, P. S. 1961. The Performance of Flame Arrestors Against Flame Propagating m Various Fuel/Air Mixtures. Fire Research Station Note No. 486. Fire Research Station, Borehamwood, Herts., England. 

Equivalence Ratio The ratio of fuel concentration in the actual fuel-air mixture divided by the fuel concentration in a stoichiometric mixture. 

Generally, at any moment of time the concentration of components within a vapor cloud is highly nonhomogeneous and fluctuates considerably. The degree of homogeneity of a fuel-air mixture largely determines whether the fuel-air mixture is able to maintain a detonative combustion process. This factor is a primary determinant of possible blast effects produced by a vapor cloud explosion upon ignition. It is, therefore, important to understand the basic mechanism of turbulent dispersion. 

The mechanism of flame propagation into a stagnant fuel-air mixture is determined largely by conduction and molecular diffusion of heat and species. Figure 3.1 shows the change in temperature across a laminar flame, whose thickness is on the order of one millimeter. 

A slightly more realistic concept is the Zel dovich-Von Neumann-Dohring (ZND) model. In this model, the fuel-air mixture does not react on shock compression beyond autoignition conditions before a certain induction period has elapsed (Figure 3.4). 

TABLE 3.2. Characteristic Detonation Celi Size for Some Stoichiometric Fuel-Air Mixtures... 

In relatively low-reactive fuel-air mixtures, a detonation may only arise as a consequence of the presence of appropriate boundary conditions to the combustion process. These boundary conditions induce a turbulent structure in the flow ahead of the flame front. This turbulent structure is a basic element in the feedback coupling in the process by which combustion rate can grow more or less exponentially with time. This fundamental mechanism of a gas explosion has been described in Section 3.2. 

A deflagration-detonation transition was first observed in 1985 in a large-scale experiment with an acetylene-air mixture (Moen et al. 1985). More recent investigations (McKay et al. 1988 and Moen et al. 1989) showing that initiation of detonation in a fuel-air mixture by a burning, turbulent, gas jet is possible, provided the jet is large enough. Early indications are that the diameter of the jet must exceed five times the critical tube diameter, that is approximately 65 times the cell size. 

As with a high explosive, a fuel-air mixture requires a minimum charge thickness to be able to sustain a detonation wave. Hence, a fully unconfined fuel-air charge should be at least 10 to 13 characteristic-cell sizes thick in order to be detonable. If the charge is bounded by a rigid plane (e.g., the earth s surface) the minimum charge thickness is equal to 5 to 6.5 characteristic-cell sizes (Lee 1983). 

The characteristic magnitudes of detonation cells for various fuel-air mixtures (Table 3.2) show that these restrictive boundary conditions for detonation play only a minor role in full-scale vapor cloud explosion incidents. Only pure methane-air may be an exception in this regard, because its characteristic cell size is so large (approximately 0.3 m) that the restrictive conditions, summarized above, may become significant. In practice, however, methane is often mixed with higher hydrocarbons which substantially augment the reactivity of the mixture and reduce its characteristic-cell size. 

A fuel-air mixture is detonable only if its composition is between the detonabil-ity limits. The detonation limits for fuel-air mixtures are substantially narrower than their range of flammability (Benedick et al. 1970). However, the question of whether a nonhomogeneous mixture can sustain a detonation wave is more relevant to the vapor cloud detonation problem because, as described in Section 3.1, the composition of a vapor cloud dispersing in the atmosphere is, in general, far from homogeneous. 

In the experiments described in Section 4.1, no explosive blast-generating combustion was observed if initially quiescent and fully unconhned fuel-air mixtures were ignited by low-energy ignition sources. Experimental data also indicate that turbulence is the governing factor in blast generation and that it may intensify combustion to the level that will result in an explosion. 

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