Study on plasma enhanced CVD coated material to promote dropwise condensation of steam

Abstract

The promoting properties of hard coatings with an amorphous hydrogenated carbon basis to attain dropwise condensation (DWC) of steam on coated copper surfaces were investigated. Using differently produced coatings, equilibrium contact angles of θ eq of 65, 74 and 90° could be reached for water. Stable and well reproducible heat transfer measurements could be performed. For a subcooling temperature of the condensor surface of 5 K, the DWC heat transfer coefficient at the vertical wall is 11 times higher for the surface with θ eq = 90° than that measured for filmwise condensation (FWC), seven times higher for the surface with θ eq = 74° and 3.5 times higher for the surface with θ eq = 65°. In comparison to the heat transfer coefficient measured for a contact angle of 90° for the heat flux ranging from 0.4–0.9 MW m -2 only 53-45% (for θ eq = 74°) and 1–7.5% (for θ eq = 65°) of the 90°-values were determined. For θ eq = 90° the observed DWC keeps very well stable up to a technically achievable maximum heat flux of 1.54 MW m -2.For θ eq = 74° and for θ eq = 65°, however, expanded condensation streams (mixed condensation) appeared on the surface at heat fluxes of 1.03 MW m −2 and 0.7 MW m −2. In these situations the performance characteristic is less developed in comparison to pure DWC, but still better than for pure FWC.