# Heat Transfer Enhancement of Falling Film Evaporation on a Horizontal Tube by Thermal Spray Coating
## 1 Introduction  
Recently, the charge amount of refrigerant is required to be reduced because the most widely used refrigerants have a high global warming potential (GWP). As a result, falling film evaporators are expected to act as substitutes for flooded heat exchangers. One important issue in improving the heat transfer performance of falling film evaporation is to keep a liquid film on the heat transfer tubes because dry out significantly deteriorates heat transfer [1]. A higher liquid circulation flow rate can improve the wall wetting condition. However, a thicker liquid film leads to higher heat resistance and wall superheat, which further results in nucleate boiling in the liquid film. When nucleate boiling occurs in the liquid film, vapor bubbles can cause liquid entrainment from the surface, leading to a decrease in the liquid film flow rate. If these entrained droplets flow into a compressor, they can cause erosion and deteriorate the adiabatic efficiency. The same problem exists in a binary power generation system that uses a geothermal source of waste heat. In refrigeration systems, machined tubes are usually used in the evaporator. However, in geothermal applications, because corrosion-resistant tubes, such as stainless steel and Inconel tubes, are used in the evaporator, it is difficult to apply a machining process. Therefore, other methods are required for heat transfer enhancement. [PERSON] and [PERSON] [2] investigated the heat transfer in falling film evaporation using HFC-134a on a smooth horizontal tube. The flow condition criteria for dry out occurrence were measured, and a correlative equation was developed considering dry out as an area fraction. [PERSON] and [PERSON] [3]evaluated the falling film evaporation and pool boiling heat transfer coefficients of HFC-134a and HFC-236 fa at a saturation temperature of 5 \({}^{\circ}\)C for two mechanically processed boiling heat transfer enhanced tubes, Turbo-B5 by [PERSON] and Gewa-B5 by [PERSON]. The film Reynolds numbers were varied from 0 to 3000, and the heat flux was varied from 35 to 95 kW-m\({}^{-2}\). It was observed that the heat transfer coefficient slightly decreased with increasing heat flux. To prevent partial dry out, [PERSON] and [PERSON] [4] developed a heat transfer tube bundle with liquid collectors and compared its heat transfer performance with pool boiling results using HCFC-141b. A liquid collector is a half tube that covers the bottom half of the heat transfer surface. Their study revealed that falling film evaporation on the tubes with the liquid collectors produced a higher heat transfer coefficient than those in pool boiling; further, the coefficient decreased without the collector because of partial dry out. [PERSON] and [PERSON] [5] investigated the effect of a mesh covering a low-finned tube on pool boiling and film evaporation heat transfer using HFC-245 fa and HFC-134a as working fluids. For this experiment, 60 fins per inch tube were used, which had a fin thickness of 0.21 mm and a fin height of 0.4 mm. The mesh had 100 mesh per inch with a wire thickness of 0.09 mm. For HFC-245 fa, the tube with the mesh produced a higher heat transfer coefficient than the tube without the mesh, while the opposite results were obtained for HFC-134a. Their study also reported that the effect of the mesh cover depended on the physical properties of the working fluids because the difference in the specific volume of vapor affected the bubble behavior under the mesh. [PERSON] et al. [6] investigated the effect of wettability of the heat transfer surface on falling film evaporation heat transfer. A super-hydrophilic surface processed by chemical etching was tested using water as the working fluid. Although the heat transfer coefficient was highest for the super-hydrophilic surface at a low liquid flow rate, it was only slightly different from that obtained for a smooth tube with a high liquid flow rate. As water, which has a higher surface tension, was used, the effect of nucleate boiling might be negligible in this case.