Behavior of diffusion flames under the low and high gravity was experimentally studied for the better understanding of the convective air inflow into the diffusion flame. Also how the gravity affects the stability and blow off of the flame was investigated. The gravity level was artificially controlled by using a drop tower and a spin tester in the range from G=0.02 to 15 where G= 1 means the normal gravity on the earth. The laminar diffusion flame of butane was made on the tip of the small vertical pipe. The results showed that the length and the width of the flame were reduced with an increase of the gravity level. The oscillation and blow off of the flame were observed in the high gravity level. The blue flame region observed at the anchor portion of the diffusion flame became more intense with increasing gravity level. It can be explained that the phenomena of the natural convective air inflow is enhanced by the high gravity.

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Normal- and microgravity experiments examined spontaneous ignition of isolated fuel droplets. Suspended fuel droplets of either n-dodecane or n-heptane is employed. A Michelson interferometer visualized cool and hot flames and discovered two-stage ignition process. According to the observation, induction times to cool flame and hot flame occurrences are determined. Significantly deformed vapor layer preceding ignition and elongated induction times under normal gravity conditions are found out in comparison with microgravity data. Influence of initial diameter, that is a measure of transport time scale, appeared only on the induction time to cool flame occurrence. These gravity and initial diameter effects on induction times clarified the roles of natural convection and the rela­tive importance of transport and chemical characteristics on the ignition process during each induction time.

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Experiments on ignition of droplet matrix are conducted by use of the drop shaft in the Microgravi­ty Laboratory of Japan (MGLAB), whose test duration is about 5 seconds. When ignition time is very short, the change of droplet diameter is quite small and so quasi-droplets which are made of porous ceramics soaked with decane or hexadecane are available. Here we used quasi-droplets instead of real droplets, because making many droplets with an equal small size is quite difficult. At the commencement of the drop, the droplet matrix is moved into a chamber heated electrically in advance. The matrix is suddenly exposed to strong radiation and its ignition is induced. Ignition time depended on the droplet size and the spacing. When the size was large, ignition time decreased monotonously as the spacing became large, because the larger the spacing the less the ther­mal interaction between droplets . As the droplet size was small, however, ignition time took a mini­mum at a certain spacing and after that ignition time approached the ignition time of single droplet, in similar to the case of droplet array in the previous paper. The tendency of ignition time in normal gravity was very complex because of natural convection. OH emission images taken by CCD camera with an imageintensifier showed a group ignition in microgravity rather than ignition of an individual droplet in normal gravity.

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Ignition and flame spread phenomena of paper sheet in slow external air flow were observed in microgravity to prevent the effect of natural convection on the flow field. Two ignition methods, with wire heater (two-dimensional spread) and lamp heater (three dimensional spread), were examined to know the effect of flame front contour on the spread phenomena. The experimental parameters were air flow velocity (0-10 cm/ s) and oxygen concentration (21-50%). The ignition delay time and flame spread rate were measured for various experimental conditions based on the video images taken in microgravity. The experimental results showed that the flame spread rate increased with in­ crease in air flow velocity and oxygen concentration, which implied that the condition covered in the experiments were in the region of oxygen supply control. When oxygen concentration is high, the tendency to increase in flame spread rate for air flow velocity saturated over 5 cm/s. This revealed that it was easy for flame spread in high oxygen case to transfer from oxygen supply control to heat conduction control. Ignition delay time showed the negative dependency on oxygen concentration and weak dependency on external air flow velocity.

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Marangoni convection (thermocapillary convection) in a liquid bridge at high Marangoni numbers (above 1 x 105) was measured under microgravity by means of three-dimensional particle tracking velocimetry. Four cameras were installed; one for an end-view and three others for side-views. The camera images were recorded on 8 mm video tapes and were digi­tized, and the three-dimensional positions of the tracer particles reconstructed. The three­dimensional positions and velocities were then determined from the light reflection due to the deformation of the liquid bridge. A numerical simulation was also done for comparison. The velocity profiles obtained through the simulation and the experiment agreed well.

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In order to obtain the -true- diffusion coefficient of liquid Pb1- xSnxTe, a microgravity ex­periment was conducted using a TR-IA-4 rocket launched on August 25, 1995. A diffusion couple composed of Pb0.8Sn0.2Te and Pb0.7Sn0.3Te cylinders with 2 mm in diameter s kept at )277 K for 231 seconds and quenched by injecting helium gas. All processes were realized under microgravity. Cooling was controlled so as to avoid axial directional solidification, which might spoil the diffusion experiments due to a large degree of segregation. The diffu­ sion during the heating and cooling periods was included in the analysis, based on an analyti­cal solution of Fick's second law with an assumption of D=A(T/ Tm)n (A and n are constants and Tm is the melting point). A diffusion coefficient of 7.9 x 10- 9 m2s- 1 at 1277 K was obtained from the microgravity experiment. Convection in the liquid diffusion couple spoiled the diffusion experiment in the 1 g reference experiment, however.

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The solidification mechanisms of three different compositions of Sn-Pb eutectic system alloys near and at the eutectic point were examined in space using sounding rocket TR-IA-4 on August 25, 1995. Samples were melted and solidified in an isothermal furnace. The samples were 10 mm in diameter and 10 mm in length. Three samples were melted before launch and solidified under microgravity conditions to examine whether gravitational segregation oc­cured. The other three were melted and solidified under microgravity conditions. The primary crystals distributed themselves evenly throughout the samples during flight so that homogeneously solidified structures were obtained. The solidification mechanisms of these alloys in space are considered. The results are applied to confirm the formation mechanisms of gravity segregation. It is concluded that segregation is caused by the movement of primary crystals in the liquid eutectic due to their density differences.

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Temperature fluctuation measurements in a half-zone bridge of molten silicon are conducted under microgravity on board the TR-IA-4 rocket launched on 25 Aug. 1995. Two types of temperature fluctuations are observed during the melting process of silicon and after the formation of cylindrical half-zone melt. The former fluctuation with a frequency of about 0.1 Hz has an antiphase correlation of temperatures measured in thermocouples separated by 90- degree azimuthal angles. The latter fluctuation in the cylindrical liquid bridge has no remarka­ble frequency; however, it tends to have the antiphase correlation in between thermocouples with 180-degree azimuthal angles. Keyword: Marangoni convection, Temperature fluctuation, Floating zone silicon crystal growth, TR-IA rocket

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This paper describes scientific meaning of Diffusion experiments, Japanese diffusion experiments in MSL-1 which is space shuttle mission scheduled in April, 1997, and issues in diffusion experiments under analysis.

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