# Heat Transfer Enhancement of Falling Film Evaporation on a Horizontal Tube by Thermal Spray Coating
## 1 Introduction
In a non-dry out condition, the effect of the flow rate becomes negligible, and that of nucleate boiling heat transfer becomes dominant, as reported by [PERSON] and [PERSON] [2], [PERSON] et al. [7], and [PERSON] et al. [8]. Therefore, nucleate boiling in the liquid film can be effective for heat transfer enhancement. An increase in the nucleation site density and a higher wettability can lead to enhanced nucleate boiling heat transfer with small vapor bubbles. Furthermore, the entrainment of liquid droplets can be suppressed when generated bubbles are small. [PERSON] and [PERSON] [9] observed boiling bubbles on glass plates with different surface roughness, heated by an indium tin oxide (ITO) layer on the surface: FC-72 at a saturation temperature of 57 \({}^{\circ}\)C was used as the working fluid, and the heat flux was varied from 0 to 400 kW-m\({}^{-2}\). The results showed that a rough surface produced a higher nucleation site density and that the boiling bubbles were small. [PERSON] et al. [10] experimentally investigated the effect of surface roughness on the heat transfer characteristics of falling film evaporation using HFC-134a as the working fluid. The arithmetic average surface roughness of the heat transfer surface polished by sandpaper was between 0.1 and 1.9 \(\upmu\)m. It was found that the heat transfer coefficient was improved by the surface roughness in falling film evaporation, as well as in pool boiling. However, the enhancement factor was higher for falling film evaporation. In our study, a rough and thin porous structure processed by a thermal spray coating was applied to the heat transfer surface. Previously, thermal spray coatings had been applied only to pool boiling. [PERSON] et al. [11] conducted their pool boiling experiments on a porous plasma-sprayed surface and reported that the heating procedure caused a large hysteresis effect on the heat transfer coefficient. [PERSON] et al. [12] evaluated the pool boiling heat transfer coefficients of HCFC-123 at a saturation temperature of 30 \({}^{\circ}\)C on copper tubes with thermal spray coatings of copper. Two types of thermal spraying methods, namely, wire flame spraying and vacuum plasma spraying, were applied. They reported that the heat transfer coefficient was improved by the thermal spray coatings. An enhancement factor of six was obtained by vacuum plasma spraying and was kept for heat flux up to the critical heat flux. [PERSON] et al. [13] evaluated the pool boiling heat transfer coefficients for R-134a and R600a on flame sprayed coated tubes with copper powder. Here, the heat flux was varied from 5 to 50 kW-m\({}^{-2}\) at a saturation temperature of 10 \({}^{\circ}\)C. They reported that the heat transfer coefficient increased with increasing heat flux, and an enhancement factor of 2.1 was obtainedby the thermal spray coating. For falling film evaporation, [PERSON] and [PERSON] [14] investigated the effect of a porous surface structure. An 800 \(\upmu\)m thick porous layer comprised of copper particles with a mean diameter of 75 \(\upmu\)m was sintered to the outer surface of a plain copper tube with an outer diameter of 15.88 mm. Distilled water at a saturation temperature of 60 \({}^{\circ}\)C was used as the working fluid, and both the sensible and evaporation heat transfers were evaluated. The results indicated that the evaporation heat transfer was higher on the porous tube, while sensible heat transfer was almost the same. Other than this, several studies on the porous structure surface, as a falling film heat transfer enhancement technique, have been conducted [15]. However, most of these studies are on water vaporization, and only a few reports have investigated the application of falling film evaporation in fluorocarbons, where nucleate boiling occurs more actively than in water. This study aimed to improve falling film evaporation heat transfer by a thermal spray coating. The outside of the tube was arc sprayed with copper.