Process design for the laser functionalization of the inner surface of metal pipes for superhydrophobic wetting and enhanced heat transfer

Abstract

Dropwise condensation in the condenser section of passive heat transport devices provides the potential for an improved heat transfer. In two-phased closed thermosiphons dropwise condensation can be achieved with a water-repellent surface on the inside of the pipes. However, the surface functionalization of the inner surface of the pipes is difficult due to the limited access to the inside of these pipes.Thus, the pipes with a length of up to 1 m were first separated into half-pipes by wire erosion, then functionalized by structuring with ultrashort laser pulses, subsequently stored in a solution of long-chained hydrocarbons, and finally rejoined by electron beam welding.As a result, superhydrophobic and water-repellent inner pipe surfaces with a contact angle of 153° were achieved. In the low temperature range at 45°C, the optimized thermosiphon allows for a heat transfer rate, which is three times the heat transfer rate of an untreated pipe surface.