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**Figure 10.** Variation of (**a**) heat transfer coefficient and (**b**) heat flux as a function of vapor quality for *Gref* <sup>=</sup> 80 kg/(m2·s) and *Gref* <sup>=</sup> 100 kg/(m2·s) with *Dh* <sup>=</sup> 5.95 mm.

**Figure 11.** Variation of heat transfer coefficient and heat flux as a function of mass flux for (**a**) a hydraulic diameter, *Dh* = 5.95 mm with average vapor qualities of 0.35 and 0.5, (**b**) a hydraulic diameter, *Dh* = 6.95 mm with average vapor qualities of 0.45 and 0.5, and (**c**) hydraulic diameters, *Dh* = 5.95 mm and 6.95 mm.

The effect of mass velocities on the heat transfer performance was investigated with fixed average vapor qualities of 0.35 and 0.5; results are given in Figure 12. Figure 12a shows the heat transfer coefficient during flow boiling in the annulus of the 1EHT tube for *xave* = 0.35 (see Figure 12a); the heat transfer coefficient was enhanced by 11–36% when compared to the smooth tube. The heat transfer coefficient of the 1EHT tube increased with increasing mass velocities and heat flux. For *xave* = 0.5 (see Figure 12b), the performance of the 1EHT tube was not as good as the smooth tube at the tested mass velocities. The strange heat transfer performance of the 1EHT tube can be attributed to the enhanced external surface structure which consisted of depression and micro-pit arrays. The depression increases flow turbulence and enhances fluid mixing for single-phase flow; it would be reasonable to conclude that the droplet entrainment and interfacial phase mixing can be greatly enhanced by the depression. At a relatively high vapor quality, the liquid phase on the upper part of the annulus would accumulate in the large cavities; this would result in decreased film thickness for other parts of the surface without cavities. As mentioned previously, the stratified flow in the annulus may be less than the heat transfer performance of the upper part, with an accumulation of liquid phase in the large cavities. This may decrease the effective flow boiling heat transfer area for flow boiling, since nucleate flow boiling occurs only on the interface between the wall and liquid phase. A similar in-tube experimental investigation was conducted by Chen et al. [31], with their results indicating that the heat transfer coefficient decreased with increasing vapor qualities, especially when the vapor quality exceeded the transitional vapor quality between stratified-wavy flow and slug/stratified flow; the transitional vapor quality for the enhanced tubes was higher than that for the smooth tube. Similarly, the flow patterns in this study may change from slug/stratified flow to stratified-wavy flow for *xave* = 0.35 and *xave* = 0.5; the heat transfer performance can be seen to deteriorate. More experimental investigations are demanded to explore the complex interaction between enhanced structures, liquid distribution, and the flow pattern transition during flow boiling in the annulus.

**Figure 12.** Variation of heat transfer coefficient and heat flux as a function of mass flux for the smooth and 1EHT tubes at (**a**) *xave* = 0.35, and (**b**) *xave* = 0.5.
