# Heat Transfer Enhancement of Falling Film Evaporation on a Horizontal Tube by Thermal Spray Coating
## 3 Experimental Results and Discussion  
### Effect of Liquid Flow rate on Falling Film Evaporation  
Figure 4 shows the heat transfer coefficients of the falling film evaporation against the film Reynolds number, _Ref._ The experimental results of [PERSON] and [PERSON] [2] for a smooth tube at a heat flux of 47 kW\(\cdot\)m\({}^{-2}\) were also plotted for reference. For the smooth tube, the heat transfer coefficient was improved by increasing the _Ref_ because a higher liquid flow rate led to an increase in the wetted area. The increasing gradient became larger with increasing heat flux. The deterioration in the heat transfer coefficient at a lower _Ref_ was caused by the formation of dry patches on the surface. Refrigerant flow behaviors on the smooth test tube are shown in Figure 5a-c. It was confirmed from the images that the liquid refrigerant flowed from the dummy tube to the test tube as a uniform and continuous liquid film. It could be also seen in Figure 5a that the entire heat transfer surface was covered by a falling film under the no heating condition with the _Ref_ of 323. Figure 5b,c show the falling film evaporation behaviors with a heat flux of 33.4 kW\(\cdot\)m\({}^{-2}\) for _Ref_ values of 430 and 643, respectively. A thicker liquid film led to larger heat resistance and a higher wall superheat. When the wall superheat became higher than that at the onset of nucleate boiling, nucleate boiling occurred in the liquid film. Liquid entrainment might increase with the detachment of vapor bubbles and subsequently cause a decrease in the liquid film flow rate. In Figure 5b,c, the nucleate boiling in the liquid film and dry patches were observed. The dry patch areas became smaller with increasing _Ref._ Because a higher heat flux led to higher bubble site density, the improvement in heat transfer by increasing the _Ref_ was greater under the high heat flux condition. In contrast, for the heat flux of 16.7 kW\(\cdot\)m\({}^{-2}\), the effect of _Ref_ was smaller because of the smaller effect of nucleate boiling. Compared with the results of a previous study by [PERSON] and [PERSON] [2], the results in this study at a heat flux of 50.1 kW\(\cdot\)m\({}^{-2}\) took almost the same values, especially at high _Ref_ values over 500. Furthermore, although this study did not cover high flow rate conditions, it could be said that the heat transfer coefficient saturated in the range of _Ref_ values over 1000.  
Figure 4: Heat transfer coefficients of falling film evaporation against the film [PERSON]’s number, _Ref._Heat transfer enhancement was clearly observed for the thermal spray coated tube. The heat transfer coefficients appeared to saturate for high \(Re_{f}\) values over 323. This tendency was clearly observed at a heat flux of 50.1 kW\(\cdot\)m\({}^{-2}\), where the heat transfer coefficient drastically increased with increasing \(Re_{f}\) up to 323 but became almost constant with \(Re_{f}\) values over 323. The \(Re_{f}\) value for saturation was lower than that in the smooth tube. Additionally, deterioration in the heat transfer coefficient at a lower \(Re_{f}\) was not observed for a heat flux of 16.7 and 33.4 kW\(\cdot\)m\({}^{-2}\) because here, the heat transfer surface was wetted, and no dry patches were observed. Figure 6 shows the evaporation behavior on the coated tube. The entire heat transfer surface of the coated tube was completely covered by the liquid film under the no-heating condition. Under the heating condition, dry patches were not observed, while nucleate boiling occurred in the liquid film, as shown in Figure 6b,c. The thermal spray coating surface had better wickability. Therefore, the liquid refrigerant spread well, preventing partial dry out. As nucleate boiling heat transfer on the coating was probably dominant compared to forced convection in the experimental conditions, deterioration in the heat transfer coefficient with decreasing \(Re_{f}\) was suppressed.