Transient internal temperature distributions of vaporizing droplets have been carefully measured, using fine thermocouples at 1 atm. and 1000 K. Droplet diameters are fixed at 2000 ± 50 μm with Reynolds numbers being 17, 60 or 100. Fuels tested are JP-10, n-decane and JP-10 thickened with polystyrene. The effects of Reynolds number and liquid viscosity on internal temperature distribution and heating mechanism have been examined. Experimental results indicate that liquid viscosity or circulation intensity strongly affects the temperature distribution and heating mechanism. In contrast, the temperature distributions associated with the three different Reynolds numbers have shown little difference for both low- and high-viscosity cases. For the low-viscosity JP-10 droplets at Reynolds numbers up to 100, where the vortex model of Sirignano and coworkers (Prakash & Sirignano 1978; Tong & Sirignano 1983) has been claimed to be applicable, the vortex model appears qualitatively correct but quantitatively inaccurate. Physical reasons for the deviation have been discussed. Solutions of the full Navier-Stokes equations appear to accord better with the experimental temperature distributions. Circulative heat transport decreases progressively as liquid viscosity increases. A semi-empirical effective conductivity model for high-viscosity cases yields a very good simulation of the experimental temperature distributions at all the Reynolds numbers when proper effective conductivity factors are chosen. A discussion on internal droplet dynamics and heating mechanisms in physical terms has been provided.

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The disperse, structural, and electrophysical characteristics of fine alumina produced by combustion of metal droplet agglomerates were studied experimentally. Data were obtained by transmission electron microscopy and video recording of aerosol particles moving in a homogeneous electric field. The aerosol particles are aggregates with sizes ranging from a fraction of a micrometer to a few micrometers and a fractal dimension of 1.60 ± 0.04 which consist of primary particles with sizes of a few to hundred nanometers. Most of the aggregates have electric charges, both positive and negative. The characteristic charge of the aggregates is equal to a few units of elementary charge. Some large aggregates rotate when the electric field polarity changes, i.e., they are dipoles. According to current concepts [1, 2], fine alumina is formed in the combustion of liquid aluminum drops by "chemical" condensation in the zone of diffusion mi-croflame around the particle. The microflame is at a distance of the order of the particle size from the particle surface. Further coagulation growth of oxide particles occurs in the "smoke tail" formed by particle motion in the carrying gas flow. The size distribution and morphology of the aerosol particle microaggregates formed by condensation-coagulation growth can depend appreciably on the magnitude of the electric charges gained by primary particles during the physicochemical stages of combustion [3]. In the literature, little experimental information can be found on the charge (electrophysical) and morphological properties of fine alumina particles formed during combustion of aluminum droplets [4]. The present work is an attempt to fill this gap. Alumina aerosol was produced by combustion of aluminized solid propellant samples. The sample in the form of a parallelepiped 20-25 mm in length with a section of 1 × 1.5 mm was burnt at atmospheric pressure in a twenty-liter container in air filtered from aerosols. The mass of aluminum in the samples was ≈6 mg. The combustion surface of the sample generated several tens of aluminum droplet agglomerates with sizes of 100-500 µm, which burnt up while falling in the container within tenths of a second. This resulted in fine alumina aerosol accumulated in the container. Aerosol was sampled 6 min after burning, and then, its disperse , morphological, and charge characteristics were analyzed. The grain-size composition and morphology of aerosol particles were studied by transmission electron microscopy (JEM-100SX). Samples for electron mi-croscopy were collected by a thermal precipitator. To observe the motion, coagulation, and behavior of the aerosol in an external electric field, we used a facility for video recording in real time (25 frames per second), which consisted of the following basic units. 734