STUDY ON PLASMA-ENHANCED CVD COATED MATERIAL TO PROMOTE DROPWISE CONDENSATION OF STEAM

The promoting properties of hard coatings with an amorphous hydrogenat ed carbon basis to attain dropwise condensation (DWC) of steam on coat ed copper surfaces were investigated. Using differently produced coati ngs, equilibrium contact angles of theta(eq) of 65, 74 and 90 degrees 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 ver tical wall is 11 times higher for the surface with theta(eq) = 90 degr ees than that measured for filmwise condensation (FWC), seven times hi gher for the surface with theta(eq) = 74 degrees and 3.5 times higher for the surface with theta(eq) = 65 degrees. In comparison to the heat transfer coefficient measured for a contact angle of 90 degrees for t he heat flux ranging From 0.4-0.9 MW m(-2) only 53-45% (for theta(eq) = 74 degrees) and 1-7.5% (for theta(eq) = 65 degrees) of the 90 degree s-values were determined. For theta(eq) = 90 degrees the observed DWC keeps very well stable up to a technically achievable maximum heat flu x of 1.54 MW m(-2). For theta(eq) = 74 degrees and for theta(eq) = 65 degrees, however, expanded condensation streams (mixed condensation) a ppeared 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 develope d in comparison to pure DWC, but still better than for pure FWC. (C) 1 998 Elsevier Science Ltd. Ail rights reserved.