Enhanced evaporative cooling heat transfer by bidirectional freeze-casting technique

Abstract

• Long-range ordered lamellar structure is made by bidirectional freeze casting. • Bidirectional freeze casting can significantly increase the wicking capability. • The dryout heat flux and the max evaporative heat transfer coefficient is tested. • The dryout heat flux increases with thickness. Capillary evaporation on porous structures enhances evaporation efficiency by increasing interfacial area and sustaining liquid supply for evaporation. Due to the counter interactions of flow resistance and capillary force, the heat transfer performance of evaporation is closely related to the nature of the porous structure. In this study, freeze casting, a promising technique to fabricate porous materials with complex pore shapes, is adopted to fabricate thin porous coating for evaporation cooling. Two different freezing methods, named unidirectional and bidirectional freezing, are used to fabricate porous coatings with unidirectional and bidirectional pores by freeze casting with one and two temperature gradients, respectively. Due to the continuous growth of ice crystals in the bidirectional freezing process, the porous coatings are characterized by long-range ordered lamellar structures. Capillary rise experiments show that the wickability of bidirectional freezing coating (BFC) is 3 times higher than that of unidirectional freezing coating (UFC). Capillary evaporation tests demonstrated that, compared to UFC, BFC is able to enhance evaporative heat transfer process significantly, which is attributed to the effective wicking of water into the entire heated area. Furthermore, effects of coating thickness on evaporation heat transfer are also investigated for UFC and BFC. It is found that the dryout heat flux increased as the coating thickness increased from 250 μm to 1000 μm for both UFC and BFC. For a coating thickness of 1000 μm, the dryout heat flux of UFC and BFC are 61.3 kW/m 2 and 92.4 kW/m 2 , respectively. And the corresponding evaporative heat transfer coefficient is measured to be 718 W/(m 2 ⋅K) and 1247 W/(m 2 ⋅K).