Enhanced boiling of saturated water over 2×3 AISI 304 tube bundles coated with DLC for two-phase heat exchanger applications

The present study aims to address the endeavor of the research community to improve two-phase heat transfer (HT) characteristics of AISI 304 tubes with low thermal conductivity. This is achieved by applying DLC coating over smooth tube using PVD techniques. This article specifically examines the pool and flow boiling HT phenomena of distilled water over tube bundle (TB) coated with DLC and is compared with conventional TB. The experimental wall superheat and heat transfer coefficients (HTC) of both smooth and coated TBs are analyzed for a range of mass flux [20-100kg/(m2s)], heat fluxes [15-75kW/m2], and P/D [1.25, 1.6 & 1.95]. The findings show that DLC-coated TB results in higher HTC than the smooth TB. The DLC-coated tubes have higher wettability, which facilitates the flooding of Nano and Micro pores leading to enhanced HT performance of TB. In particular, DLC coated TB showed a 40% increase in HTC at lowest P/D in pool boiling mode and a 34% increase in flow boiling at highest P/D. This clearly shows a distinctive feature of DLC coated TB where enhanced HT performance in pool and flow boiling modes occurred in reverse arrangement of P/D. The qualitative study behind the better HT ability of DLC coated TB is done through high-speed image visualization. The study can help to implement DLC coated TB in two-phase heat exchangers as its excellent corrosion resistance and anti-fouling properties are well established.