Classification of frost formation style interacting with mist formation on a flat plate with temperatures ranging from general-low to cryogenic using optical measurements

Cryogenic heat exchangers are important for vaporizing liquefied natural gas or liquefied hydrogen and utilizing their cryogenic energy, but frost formation causes severe problems that deteriorate performance. Frost formation on a cryogenic surface is characterized by simultaneous mist formation. However, the detailed frosting mechanisms, especially the interaction between frost and mist formation under forced convection, have not been revealed. In this study, we experimentally observed the frost and mist formation on a flat plate under forced convection over a wide range of cooling surface temperatures (Tw), from general-low to cryogenic. The frost and mist layer height was quantitatively measured with a white LED and a laser sheet light source in addition to the frosting amount. As a result, the frosting style was classified into three groups depending on the cooling surface temperature under the air temperature of 27°C, air humidity of 12 g/m3, and air velocity of 2.0 m/s. When Tw ≥ −75°C, the mist was hardly generated, and needle-shape or dendric frost formed by desublimation almost uniformly on the entire plate surface. On the other hand, frost is not uniformly formed in the flow direction at Tw ≤ −100°C. Frost hill is formed by desublimation at the leading edge and protrudes higher as the cooling surface temperature is lower. The high frost hill leads to the flow detachment behind it and the persistent mist generation, affecting the frost growth on the rear. Thus, under forced convection, the frost hill shape and the flow detachment behind it are important factors determining the frosting style.