Fuel spray combustion

Peters, J.E., and Mellor, A.M., An ignition model for quiescent fuel sprays, Combustion and Flame, 38, 65-74, 1980. 

The current status of prediction and modelling in the area of fuel spray combustion requires, among other parameters, the measurement of droplet or solid particle size distribution and the relative velocity between the fuel and the surrounding gas. Many optical techniques, based on laser light scattering, have been investigated to this purpose (Refs.1,2,2,]+,, 6 and j), but the only system able to simultaneously determine the size and the velocity is the dual-beam laser Doppler velocimeter shown in Figure 1. 

After the hydrogen combustion, the fuel was injected into the vessel and then combusted. Fuel spray combustion flame photographs were taken by ICCD camera. Light emission of flame was measured using two photo sensors a photo multiplier... 

Figure 23.6 Fuel spray combustion period at ambient of 3 MPa (Romphol and Wattanavichien, 2006).

Vei y small solid fuel particles such as sawdust, agricultural grains, or coal dust can sustain flames when they are suspended in air. In fact, very serious fires have occurred in grain storage towers and coal mines because of the flammability of suspended dusts. The combustion of the individual particles follows the usual pattern of solid particle burning— devolatization and char burning. The combustion of the whole cloud of particles is similar to spray combustion and its characteristics depend on the nature of the fuel, size of the particles, and the number of particles in a given volume. 

Figure 1.1. Schematic of spray combustion process (a) annular combustion chamber in a single spool turbojet with an axial flow compressor (b) fuel injection and droplet formation in combustion chamber.

Sirignano, W. A. 1983. Fuel droplet vaporization and spray combustion theory. Progress Energy Combustion Science 9 291-322. 

Stephens, J. R., S. Acharya, E. J. Gntmark, and D. C. Allgood. 1997. Active forcing of air and fuel feed in swirl stabilized spray combustion. ISOABE Proceedings. Chattanooga, TN. 

Experiments were designed to study the effects of porous media on spray combustion and resulting emissions. A number of factors could affect combustion performance with the presence of porous inserts, including the location, thickness, and pore size of the porous insert and operating conditions such as firing rate and fuel-air ratio. For different operating conditions, the baseline tests without porous inserts were completed. After the baseline tests, the same operating conditions were repeated with porous layers installed at various locations. More tests were then completed with different porous material properties. 

The shape of the fuel spray is related to viscosity. High viscosities cause poor atomization and a solid-stream jet spray pattern. Poor combustion and low power result. Low viscosities result in soft, nonpenetrating fuel spray, leakage of fuel past the injection plunger, and possible wear of fuel system components. 

In order for fuel to combust and bum efficiently, it must be atomized into extremely small droplets. Fuel injectors aid tremendously in performing this role. However, if fuel viscosity is high, atomization into small droplets becomes difficult. Highly viscous fuel will not disperse freely after being sprayed from the fuel injector. The fuel instead sprays as a stream or large drops rather than as a fine mist. The result is a decrease in fuel efficiency and power due to incomplete burning of larger fuel droplets. 

Experimental methods presented in the literature may prove of value in combustion studies of both solid and liquid suspensions. Such suspensions include the common liquid spray. Uniform droplets can be produced by aerosol generators, spinning disks, vibrating capillary tubes, and other techniques. Mechanical, physicochemical, optical, and electrical means are available for determination of droplet size and distribution. The size distribution, aggregation, and electrical properties of suspended particles are discussed as well as their flow and metering characteristics. The study of continuous fuel sprays includes both analytical and experimental procedures. Rayleigh s work on liquid jet breakup is reviewed and its subsequent verification and limitations are shown. 

Several papers give excellent introductions to the problems of spray combustion (5A, 9A, ISA, JHAy 19A). Lloyd (14A) includes a diagrammatic sketch of the physical and chemical factors influencing combustion. The fuel used, method, and degree of atomization, and the many combustion parameters all affect the final combustion efficiency. 

Hagerty (8C) presents an excellent summary of the work on continuous fuel spray conducted at the University of Michigan. Five major problems closely related in the field of spray research include the stability of the liquid phase, drop-size factors, spray distribution, metering characteristics, and the effect of all these on the resulting combustion. Hagerty discusses these factors in relation to theoretical and experimental studies of the process of spray formation. 

I he experimental determination of drop-size distributions of fuel sprays is important for all studies involving the atomization of liquid fuels. Investigations of the mechanism of atomization, influences of the many factors that determine fineness of the spray, and methods of atomization and nozzle design all require some means for determining the extent to which the liquid is broken up into droplets in preparation for combustion. 

Bolt, J. A., Boyle, T. A., Combustion of Liquid Fuel Spray, Trans. ASME 78, 609... 

Spalding, D. B., Combustion of a Single Droplet and of a Fuel Spray, Selected Com-... 

Reasonable correlations of combustion efficiency with fuel spray momentum and spray energy in two different combustors have been shown to hold over a range of altitude-engine idling conditions 133). As different curves were obtained with different injector nozzles, spray-cone angle was thought to be a factor. Further work showed that efficiency did correlate closely with expressions representing the spray momentum or... 

Godsave, G.A.E., Studies of the Combustion of Drops in a Fuel Spray--The Burning of Single Drops of Fuel. Fourth Symp. Int. on Combustion, Williams and Wilkens, Baltimore, 1953, pages 818-829. 

Black, C.H., "Review and Analysis of Spray Combustion as Related to Alternative Fuels," Argonne National Laboratory, ANL-79-77 1979. 

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