The initially opaque and milky-white droplet changes to the transparent one which contains a milky-white droplet inside it before disruption, that is, phase-separation occurs in the droplet. The dominant component of the internal droplet is assumed to be water, and water elements in the emulsion droplet would coalesce into the internal droplet [14]. On the other hand, continual puffing occurs right after ignition and disruption scarcely takes place till the end of droplet-burning for water-in-oil emulsions. The droplet remains opaque and milky-white. These differences between oil-in-water and water-in-oil emulsions may be attributed to their properties; As temperature increases, phase inversion occurs or water is drained for oil-in-water emulsions, while the emulsified condition holds for water-in-oil emulsions. It is, therefore, assumed that disruption occurs when a fairly large droplet of water is formed inside the primary droplet due to the phase-separation.
The primary objective of the present work is to study the combustion process and the secondary-atomization of an oil-in-water emulsion droplet. Photographic observation and temperature measurement were made of the emulsion droplet burning in quiescent air under microgravity conditions. The attention was focused on the phase-separation in the droplet including drainage, agglomeration and coalescence of water, and the time histories of droplet temperature and the amount of water in the droplet, during the period of time prior to the disruptive microexplosion. The relationship between the phase-separation and occurrence of the disruptive microexplosion were also examined by using the statistical analysis [13, 15].
2. EXPERIMENTAL PROCEDURE
Most of the experiments were conducted by using the drop shaft of JAMIC (Japan Microgravity Center) at Hokkaido, which provided 10 s of the effective time of microgravity experiments with high quality. Figure 2 shows the schematic diagram of the experimental apparatus. The experimental apparatus was integrated in the rack for the drop shaft. The combustion chamber was an acrylic-resin box, which was equipped with two windows for the observation of the droplet and for detecting droplet ignition. The well-known suspended droplet technique is adopted; An emulsion droplet was suspended at the spherical tip of a quartz fiber of the diameter 250 mm. A remote-controlled fuel supply system was provided to suspend the droplet right before the microgravity experiment. The fuel supply system consists of a hypodermic needle, a syringe, and two sets of a stepping motor and a rack-pinion mechanism. After the tip of the needle approached the fiber tip, the piston of the syringe was allowed to translate for squeezing the fuel inside it toward the needle. This resulted in formation of the droplet suspended at the fiber tip, being followed by the withdrawal of the needle away from it. When the microgravity experiment started, the electric current was turned on for heating a coil. This was followed by the approach of the coil to the droplet for ignition. The coil was removed immediately after droplet ignition, and was kept away from the droplet during combustion in order to prevent the droplet flame from being perturbed. A UV photodetector was provided to determine the instance of droplet ignition. The signal from the UV photodetector was also utilized to actuate the coil. The burning behavior of the droplet was recorded on a video movie. A CCD camera was available to take a magnified image of the droplet for observing the liquid-phase inside the droplet. The temperature at the droplet center was measured using a Pt-PtRh (13%) thermocouple of the diameter 50 mm, whose response time is shorter than 1.6 ms. The measured temperature was used as an indicator of the droplet temperature. Both the output of the thermocouple and the signal from the UV photodetector were recorded on a data recorder.
The oil-in-water emulsion tested in the present study consisted of distilled water, base fuel and small amount of surfactant. The base fuel was n-hexadecane. Its normal boiling temperature is 526 K. The surfactant, polyoxyethylene nonylpheny1 ether (Emulgen 906, Kao Corp., HLB: 10.8) was used as emulsifier. The volume concentration of the surfactant was kept to 1 %. The initial water content cw was varied from 0.1 to 0.3 in volume. From the microphotographs of the emulsion, microdroplets of fuel ranged from 5 to 15 mm were found to be surrounded by a thin water film with a thickness of less than 5 mm, as shown in Fig. 3. The emulsion was degassed by keeping it at about 5 kPa for 2 h.
The experiments were performed at the room temperature and the atmospheric pressure. The ambient gas was air. The initial droplet diameter d0 was kept constant at 2.5 mm. The droplet was assumed to be a spheroid and the major and minor diameters of the droplet were measured from the droplet image. The droplet diameter was defined as the diameter of the sphere which had the same volume of the spheroid.
Fig. 2 Schematic diagram of the experimental apparatus
Fig. 3 Microphotograph of n-hexadecane-in-water emulsion, cw = 0.2
