Experimental and Theoretical Studies of Burning Drops of Hydrocarbon Mixtures

Abstract

This paper presents an experimental and theoretical study of the self-ignition and burning behaviour of drops of hydrocarbon mixtures. In the experimental work, the mass histories, as well as temperature, shape, and flame histories, of drops of heavy hydrocarbon mixtures, suspended on fine thermocouple beads and subjected to heated air streams, were obtained. Due to thermal decomposition and irregular burning, the masses could not be determined from temperature and size and were measured by a liquid-nitrogen quenching technique. Temperature, flame, and shape histories were obtained in the usual manner by thermocouple and photographic means. Drops of grade 6 fuel oil and grade 6 fuel oil minus its asphaltene constituent, of 1·2 and 1·7 mg initial mass, subjected to 1450 and 1600°F air-stream temperatures, were studied. The drop histories can be divided into four phases: (1) pre-ignition, (2) self-ignition and combustion, (3) thermal decomposition, and (4) carbon residue, or cenosphere, burning. The asphaltenes contributed a great deal to burning irregularities but not to burning rates or temperatures. The latter were higher the higher the air temperature, but were affected less by changes in air velocity. In the theoretical work, a generalized treatment predicting the histories of drops undergoing unsteady vaporization, burning, thermal decomposition, or combinations of these was formulated. Based on a spherically symmetric model, governing equations of state, continuity, chemical species conservation, and energy conservation were solved with the aid of simplifying assumptions. A computer program was developed covering a wide range of operating conditions. The theoretical model showed reasonable agreement with the experimental results. A universal plot estimating drop histories of heavy residual fuels was prepared. The distribution of the total heat input into sensible heat, latent heat of vaporization, and endothermic heat of decomposition was also plotted versus dimensionless time.