Experimental study of delaying effect of the frost formation on cryogenic heat exchanger tube surfaces using super-hydrophobicity tube by anodic oxidation method

JAXA is developing the pre-cooled turbojet engine (PCTJ) operated from sea-level static to Mach 5 with the single mechanism. The key technology of PCTJ is the precooler, which is a heat exchanger to cool down the heated incoming air using liquid hydrogen fuel. Frosting on the precooler tube surface is a major problem and some countermeasures have been proposed such as the liquid oxygen injection, methanol injection, high speed air-jet injection, and so on. However, these methods require additional mechanisms increasing the engine weight with their complicated systems. Here, we focus on the surface treatment that does not need any additional mechanism. We made aluminum super-hydrophobicity tubes treated by anodic oxidation and fluorine addition. Our method attains the super-hydrophobicity with about 157 degrees contact angle. The frost formation test using liquid nitrogen was conducted in the wind tunnel. As the results, the total amount of the frost formation can be reduced by about 17% at maximum compared with the untreated tube. However, little improvement on the heat transfer rate was found because the frost density was reduced by super-hydrophobicity.