Fuel-rich mixtures

Thermal energy in flame atomization is provided by the combustion of a fuel-oxidant mixture. Common fuels and oxidants and their normal temperature ranges are listed in Table 10.9. Of these, the air-acetylene and nitrous oxide-acetylene flames are used most frequently. Normally, the fuel and oxidant are mixed in an approximately stoichiometric ratio however, a fuel-rich mixture may be desirable for atoms that are easily oxidized. The most common design for the burner is the slot burner shown in Figure 10.38. This burner provides a long path length for monitoring absorbance and a stable flame. 

If ignition of fuel-rich mixture occurs, the release will bum as a fireball. Burning will occur primarily in the outer layer of the fuel-rich cloud. As the buoyancy of the hot gases increases, the burning cloud rises, expands, and assumes a spherical shape. Damage is again caused by direct flame contact and radiant heat. 

The stratified gaseous layer established over the liquid fuel surface varies from a fuel-rich mixture to within the lean flammability limits of the vaporized fuel and air mixture. At some point above the liquid surface, if the fuel temperature is high enough, a condition corresponds to a stoichiometric equivalence ratio. For most volatile fuels this stoichiometric condition develops. Experimental evidence indicates that the propagation rate of the curved flame front that develops is many times faster than the laminar flame speed discussed earlier. There are many less volatile fuels, however, that only progress at very low rates. 

Another non-equilibrium effect arises when the product composition contains a condensible substance. Solid propellant formulations based upon potassium perchlorate form solid potassium chloride and the acetylenic monopropellants upon decomposition form large quantities of carbon particles, as do very fuel-rich mixture ratios of hydrocarbon propellant systems. More recently metal and metal compounds have been used as fuels and form product oxides which are very high boiling point compounds that condense to varying degrees in the rocket chamber and nozzle. For example, estimates indicate that the normal boiling points of Li20, BeO,... 

The results of several rocket engine investigations are summarized as the variation of characteristic velocity with mixture ratio and are compared with the predicted values based on equilibrium combustion in figure m-A-1. Greater than theoretical performance is obtained at fuel rich mixture ratios while considerably less than theoretical performance is reported at oxidizer rich mixture ratios. The results cannot be dismissed as the consequences of poor injection technique, poor mixing, or insufficient reaction time (L ), especially with the observation of greater than theoretical performance. At near stoichiometric mixture ratios and at chamber pressures of about 300 psia, performance in terms of characteristic velocity is near the theoretically predicted value. 

The process of catalytic partial oxidation of methanol with air in a fuel-rich mixture has been commercialized to produce formaldehyde since about 1890. Various catalysts have been used but silver catalyst is by far most widely used. Song and Hwang [1991] used a packed-bed porous membrane tubular reactor wi the catalyst packed on the shell side. They used a model consmicted essentially from Equations (10-36) and (10-44) with the permeation terms replaced by some terms similar to Equation (10-56) and k- =0. The partial oxidation of methanol can be conveniently described by... 

This problem can be circumvented in a fuel-rich approach to catalytic combustion for gas turbines recently proposed. In this method fuel is mixed with air to form a fuel-rich mixture that is reacted over the catalyst to produce both partial and total oxidation products. The reaction products are then mixed with excess air and burned in a homogenous flame. Because the gases exiting the catalyst are fuel-rich, they cannot sustain combustion in the event of a homogenous flame backup. The promise of this method needs to be confirmed in full-scale turbine tests. 

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