An objective of this research is to investigate transient transition boiling phenomena when a single ethanol droplet contacts on a hot nickel surface on which a special thermal sensor is mounted. The experiment was conducted for an extensive initial surface temperature ranged from 160 °C to 250 °C, including liquid superheat limit temperature of ethanol (197 °C), three different liquid subcoolings ranged from 53 K to 33 K, and three different droplet impact velocities ranged from 1.24 m ⁄ s to 2.04 m ⁄ s. The droplet diameter was 3.7 ± 0.2 mm. Our major interest was stable vapor film generation time required to change from initial wetted nucleation boiling to film boiling on the surface. The generation time may govern either the hot surface holding wetting (nucleation boiling) or dryout after liquid-solid contact. The typical generation time is within 10 milliseconds after the liquid has contacted solid. Therefore, a fast response Chromel-Nickel thermocouple was fabricated and placed at 3 μm beneath the surface to detect very short response time of 80 μs enough to measure transient transition boiling heat transfer. The surface temperature and surface heat flux were evaluated using an inverse heat conduction analysis and compared with observations using a high speed video camera. From the experimental result, it was clearly confirmed that the wet area can still exist for very short time even the surface temperature reached 250 °C.
The growth of mushy zone during solidification of supercooled alloy melt was studied to develop the structure control in material processing. Using the experimental evidence in the previous paper of this work, a theoretical model for primary arm spacing selection in centripetal solidification was presented. The model reflected the ununiformity of crystal arrays at the leading front in the mushy zone and the history dependence of crystal growth. Based on this model, a numerical simulation was carried out for changes of minimum, maximum, average primary arm spacing during the two-dimensional solidification of supercooled Pb-10wt%Sn alloy melt. The validity of this model was made clear by comparing with experimental results. The average primary arm spacing increased stepwisely with distance from the wall while oscillating up and down. Finally, it was clarified that the cycle of change was closely related with temperature gradient of supercooling.