Internal condensation heat transfer in a hydrophobic near-horizontal tube with high pressure and low mass flux

Abstract

Based on the potential of hydrophobic surface treatment leading to a significant increase in condensation heat transfer, in this study, condensation phenomena of water with hydrophobic coating in a nearly-horizontal tube were experimentally investigated whether increase by hydrophobic surface modifications is retained in extreme flow conditions. Various pressurized conditions (0.6 ≤ P ≤ 1.5 MPa) for different mass fluxes (7.0 ≤ G ≤ 27.5 kg/(m2·s)) were tested. The hydrophobically-coated tubes showed 8.95 times higher heat transfer coefficient h than non-coated tube at P = 0.6 MPa, but the increase became negligible as P approached 1.5 MPa. The result is consistent with the hypothesis that dropwise condensation is promoted by the hydrophobic coating at low P, where surface tension σ is high, but the effect is not sustained as P increases and σ decreases. Under a given P, h increased as G increased. The high G is expected to encourage droplet departure in the partial dropwise mode, and this increase in departure rate contributes to increase h. The experimental results are described with a newly-proposed dropwise condensation heat transfer model for hydrophobically-coated tubes that have dropwise condensation in the stratified flow regime.