Single droplet combustion studies

Experimental techniques used for studying the combustion of single droplets can be divided into three groups suspended droplets, free droplets, and porous droplets, with ongoing research in all three areas (98). 

The study of the combustion of sprays of Hquid fuels can be divided into two primary areas for research purposes single-droplet combustion mechanisms and the interaction between different droplets in the spray during combustion with regard to droplet size and distribution in space (91—94). The wide variety of atomization methods used and the interaction of various physical parameters have made it difficult to give general expressions for the prediction of droplet size and distribution in sprays. The main fuel parameters affecting the quaHty of a spray are surface tension, viscosity, and density, with fuel viscosity being by far the most influential parameter (95). 

Experimental and theoretical information obtained from a study of the single droplet are of great value in understanding the behavior of sprays and the combustion of sprays in general. 

Some simplified analyses have been performed for the combustion (1-9) and pure vaporization (10,11,12) of sprays. Recently more elaborate numerical codes on the unsteady, two-dimensional, two-phase, chemically reacting flows are also being developed (13), In all these studies the spray is frequently assumed to be sufficiently dilute such that the mutual interferences between the motion and the vaporization processes of the individual droplets are either completely neglected or are manifested only through their collective modifications of the state of the bulk gas. (The term vaporization is used here to imply both combustion and pure vaporization.) Hence the vaporization and kinematic behavior of a single, isolated droplet in an infinite expanse of gas serve as fundamental inputs to the spray analysis. These single-droplet phenomena are discussed in this chapter. 

The feasibility of the dispersion fuels concept for applica--tion to gas turbine power plants is evaluated from disper-sion fuels formulation studies, from the results of single droplet tests directed toward demonstration of the droplet shattering process, and from the results of initial burner tests of dispersion fuels. Results demonstrate the existence of the microexplosion phenomenon in single-droplet combustion experiments. Gas turbine combustor tests indicate that fuel emulsification may alter favorably the efficiency of a practical gas turbine combustor without adversely affecting the turbine inlet temperature profile or CO, and smoke emissions. 

This chapter describes the results of research on evaluating dispersion fuels for application in gas turbine engines and includes studies of emulsion formulation, single-droplet combustion, and gas tiurbine combustor tests. 

The combustion of sprays in a high-temperature furnace is a complex physical and chemical process that involves simultaneous heat, mass and momentum transfer, phase transition, and chemical reactions. The droplet size, composition of the fuel, ambient temperature and pressure, and oxygen concentration are major factors that affect the combustion process. Owing to the complexity of the process, it is very difficult to obtain accurate information on the combustion of the spray. However, the evaporation and combustion of a single droplet of oil have been well studied since it is relative easy to carry out an experiment for the measurement of combustion. Furthermore, it has been theoretically investigated due to its simplicity. 

I he experimental investigation of the combustion of sprays is complicated by the many variables involved. Common sprays are composed of a wide range of droplet sizes distributed unevenly in the spray cone. Turbulence of the air and the relative motion of the droplets through the air are poorly defined. The burning of an isolated droplet itself presents a difficult problem, although much progress has been made in this field in the past few years. To study the effect of any single variable on the combustion characteristics of a spray, other variables must be held constant. This paper reviews those fields of effort in which work has been done to simplify the complex physical aspects of this problem. 

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