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Abstract
An exploratory study of phenomena of spray combustion is performed. A unique multi-orifice impulsed spray generator is designed and developed. This generator is capable of producing uniform size arrays of droplets, where the droplet size, and droplets spatial distribution can be changed. Precise control of fuel flow rate and droplet size is obtained. The applicability of this spray generator for parametric study of the spray combustion is demonstrated through combustion studies of iso-hexane and some other commercially important fuels. Three regimes of spray combustion are identified. Completely pre-vaporized spray combustion which is similar to the pre-mixed gas combustion with blue flame front character; the isolated droplet or droplet cluster burning and propagation; and partially pre-vaporized spray which has the mixed feature of premixed gas combustion and droplet burning while passing through the gas flame front. Effect of fuel initial temperature and the fuel type on the flame structure and flame speed of sprays with average droplet size of 300 (mu)m are studied. Both cases reveal that the dominant factor which is effecting the spray combustion phenomena and mechanism of the flame propagation in sprays, is the vapor fuel fraction at the flame front. The amount of vapor fuel is dependent upon other factors such as liquid fuel initial temperature, fuel type, and radiation heat from the flame. Two mechanisms of flame propagation in droplet/air flow combustion are identified. The premixed gas type mechanism and relay type mechanism.
A semi-empirical model is developed to calculate the flame speeds of sprays for premixed gas type of flame propagation mechanism. Effect of initial droplet diameter and the fuel vapor fraction in the fresh mixture on the flame speed is studied analytically. Flame speed reduces with increase in initial droplet diameter in sprays with overall lean equivalence ratio. For overall rich equivalence ratio sprays the flame speed might increase with increase in droplet diameter. In rich sprays the local droplet diameter and the fuel vapor fraction would determine the increase or decrease in flame speed with an increase in the droplet diameter.
The heating and evaporation of the fuel droplet at the upstream of the flame front is analyzed considering the effect of the thermal radiation from the hot zone of the combustion system. The radiative transport in a typical cylindrical combustion chamber is calculated considering the hot wall, the emitting and absorbing combustion products, and the absorption of the two phase droplet flow. Analytical solution is obtained. The heating and evaporation of a single droplet by thermal radiation in cold air has also been studied. (Abstract shortened with permission of author.)
