Dendrite Growth during Freezing of Millimeter-Scale Eicosane Droplets

The freezing characteristics of small diameter eicosane (Tmelt = 37°C) droplets are studied here for their use in novel dry-cooling strategies based on spray freezing of recirculating phase change materials (PCM). PCM can be used to store thermal energy with relatively small changes in temperature (due to latent heat), as well as volume (due to small density changes). 4.2 mm diameter eicosane droplets are superheated to 40°C, placed on a cold stage at 10°C, and imaged during freezing (a). Similarly, liquid eicosane is enclosed within a custom-built experimental package creating a 5 mm diameter, 100 μm thick disc geometry with a temperature controlled boundary that is rapidly dropped from 40°C to 10°C (b). In both cases the liquid-solid interface is tracked, as well as the formation and growth of long dendrite structures which have been observed to play a critical role in the freezing process. (c) and (d) show the vertical position normalized by the droplet height , y/H, and the radial position (measured inward) normalized by the disc radius, r/R, of both the interface location and the average dendrite tip location. The total freezing time is observed visually, resulting in characteristic Fourier numbers of Fo = 0.55 ± 0.15 (droplet) and Fo = 3.5 ±0.15 (disc) at identical Stefan numbers of St = 0.3 ± 0.03, where the characteristic lengths are taken as the ratio of the eicosane volume to the cooled surface area.