| OCR Text |
Show INTERACTION OF FUEL PROPERTIES AND BURNER PERFORMANCE The interactions of a fuel's chemical and physical properties with characteristics of importance in determining burner performance include those -v;hich affect burner performance from a combustion standpoint and those which apply to the usability of the.fuel and the durability of the burner. For example, thermal and oxidative stability are fuel characteristics which impact usability, while fouling and corrosion characteristics affect burner durability. Both of these can influence the combustion process as well: thermal and oxidative stability through alterations of fuel physical properties and composition, and fouling and corrosion through unintentional alteration of the burner flowfield. Liquid fuel combustion characteristics are those which relate to the preparation of the fuel-air mixture in the burner. Injection of the fuel in the burner produces a spray, and the characteristics of this spray are determined by the fuel properties, the type and geometry of the fuel injection system, and by the interaction of the injected fuel with the air stream. During and subsequent to the injection process, vaporization takes place, and burning of the vapor-phase fuel can also occur in the environment surrounding the droplets. The vaporization and burning process obviously affects the droplet lifetime, and droplet lifetime affects the burner length required to achieve complete combustion. Droplet lifetime also has an impact on flowfield aerodynamic features that are required to achieve complete combustion within a given burner length. Vapor or gas phase combustion phenomena fall into two subgroups: those which are involved in ignition and flame stabilization, and those involved in combustion completion and the production of the trace species and soot that constitute burner emissions. In the first category are ignition delay time and blowout characteristics, flame temperature, and laminar flame speed. Ignition delay time, which is affected by the rate of chemical reaction, and thus the ambient temperature and local heat transfer phenomena, clearly sets a lower limit on the residence time required for combustion to occur in the burner. In general, this time is small, but the residence time available in the recirculation regions that serve as flameholding locations in a burner may also be small. For a sufficiently small recirculation region, or a sufficiently high rate of mixing of cold unburned gas into the recirculation zone, residence time can fall below ignition delay time and blowout occurs. Thus there is a close relationship between ignition delay, blowout, and flame stabilization phenomena in general. Flame temperature is controlled by fuel thermodynamic 1.2.3 |
