Microgravity experiments were performed on droplet evaporation of palm methyl ester (PME). Light oil and n-hexadecane were also examined as a reference fuel. Droplet temperature histories were obtained in addition to droplet diameter histories. Droplet temperature was measured with a K-type thermocouple (diameter: 13 μm). A small droplet (initial diameter: 0.30-0.40 mm) was employed for microgravity experiments in order to make effective use of short term microgravity. The droplet suspension system was improved to minimize the thermal effects of the suspension system on small droplet evaporation. Microgravity experiments at 773 K in the ambient temperature and from 0.1 to 2.0 MPa in the ambient pressure were performed with the CIT drop tower (microgravity duration: 1.1 s). It was found that the temporal variations of an evaporating droplet at normal and microgravity were almost identical when the time axis was normalized by the evaporation lifetime. The effect of natural convection on the droplet temperature at the end of evaporation was negligible.
Combustion behaviors of isolated fuel droplets for ethanol, n-decane and 1-butanol were investigated in high concentrations of carbon dioxide under microgravity. Fuel droplets were anchored at the tip of a quartz fiber with the diameter of 50μm. The ambience consisted of oxygen, nitrogen and carbon dioxide. Oxygen concentration was fixed at 21 % in volume, and the concentration of carbon dioxide was varied from 0 % to 60 % in volume. Detailed measurements of the droplet surface area were conducted using a high speed video camera, and instantaneous burning rates were calculated from histories of the surface area. Droplet flames were also observed using a video camera. An estimation of the droplet diameter using an ellipsoidal approximation had large error especially at the initial stage. The behavior of the instantaneous burning rate, which was measured from the change in surface area, showed droplet combustion events as with the thermal expansion, ignition and following combustions. Instantaneous burning rates of n-decane and 1-butanol showed an increasing trend, while that for ethanol was almost constant during the droplet life time. The burning rates for n-decane and 1-butanol were influenced by the initial droplet diameter. A stepwise increase in the instantaneous burning rate was observed for large n-decane droplet in air, which occured around the soot shell collation. However, this behavior was not observed in high concentration of carbon dioxide even for large droplets. In high concentration of carbon dioxide, soot production was suppressed, and this suppression was enhanced for smaller droplets.
This research conducted microgravity experiments of flame spread over fuel-droplet arrays at a low pressure in order to improve understanding of the flame spread in fuel sprays under high-altitude relight condition of jet engines. The results show that both the flame-spread rate and flame-spread limit distance at the low pressure are greater than those at atmospheric pressure. The pressure effect on the flame-spread rate was discussed considering some elementary processes, such as droplet heating and thermal diffusion. The thermal diffusion speed is inversely proportional to the pressure. The pressure effect on the flame-spread limit distance was discussed considering transient process of high-temperature region around a burning droplet. The maximum radius of the outer edge of the high-temperature region is proportional to -1/3 power of the ambient pressure. Group combustion occurrence was also demonstrated with a percolation model considering the flame-spread limit.
A model for flammability limits of thermally thin materials in microgravity environment has been developed by scale analysis. The flame spread over a thermally thin material with an opposed flow has two extinction limits; one is the blowoff limit and the other is radiation extinction. These limits are expressed by the following non-dimensional parameters, Da and Rrad as Da=1 and Rrad=1, respectively. In the previous model, the intersection of these limits corresponded to the minimum limiting oxygen concentration (MLOC). However, near the MLOC condition, due to the coupling of the effects of radiation and kinetics, the MLOC was far underestimated than the measured MLOC. In the new model, non-dimensional equation + R +1/ Da =1 η rad is proposed to take account of the coupling effect. The predicted extinction limit with the new model agrees with the actual limit obtained by flight experiments.
Superfluid helium (He II) is a unique fluid having many characteristic features different from those of ordinary fluids. Film boiling in saturated He II in earth gravity is strongly affected by the sub-cooling due to a small hydrodynamic pressure. In this study, microgravity experiments with He II were conducted using a drop tower, because zero sub-cooling can be realized in microgravity. The experimental study confirmed that the critical heat flux in He II in microgravity was subjected to the influence of the van der Waals force. Owing to the extremely high effective thermal conductivity of He II, the effect of the van der Waals force on the critical heat flux in microgravity became notable, although this effect did not appear at all in any ordinary fluids. Comparison of the critical heat flux in normal liquid helium (He I) with the previously reported results of Straub’s experiments, which is compared the extended Zuber’s relation, confirmed that the result of He I agreed with the correlation derived from the surface tension and buoyancy but the result of He II disagreed with it.
Microgravity circumstance is useful for measurement of thermal conductivity of molten metals, because thermal convection is suppressed. To measure the thermal conductivities of molten metals systematically under microgravity, it needs many chances for the microgravity experiments. The short-duration microgravity of drop tower experiments is suitable for this purpose. In this study, the thermal conductivities of several molten metals were measured by hot-disk method under short-duration microgravity of drop tower experiments to study the influence of convection, limitation of time and space. In addition, the thermal conductivities of molten metals without convection effect were compared with those derived from Wiedemann-Franz law. The thermal conductivities of Hg and Bi melts were good agreement with those derived from Wiedemann-Franz law, but those of Sn, Si and InSb melts were different from those derived from WiedemannFranz law.