*4.2. Heat Transfer Characteristics of Evaporation*

As shown in Figure 11a, the heat transfer coefficient for evaporation of R134A in the annulus, using the smaller outer tube (inside diameter *Di* = 25 mm), increased gradually as the mass flux increased. Different vapor quality ranges were employed for the evaporation experiments, and the average vapor quality was kept at 0.35 and 0.5. Little differences were found between the two series of results; the deviation fell within the range of experimental uncertainties. Generally speaking, the flow boiling heat transfer can be divided into two parts: (i) convective boiling and (ii) nucleate boiling heat transfer; nucleate boiling heat transfer increases with increasing heat fluxes (as detailed in Cooper [29]). For in-tube flow boiling in this study, the targeted mass velocity fell in (i) the stratified flow regimes at higher vapor qualities, and (ii) slug flow at lower vapor qualities. For relatively high

vapor qualities, gravity dominates the liquid distribution; this results in a thin liquid film or even only a gas phase on the upper part of the annulus, which decreases the effective flow boiling heat transfer area and causes heat transfer deterioration. Slug flow might exist at low vapor qualities; the transitional vapor quality between the two flow patterns is similar to the intermittent annular flow described by Wojtan et al. [30], and xI-A was determined to be 0.32. The heat transfer coefficient would drop rapidly near the transitional vapor quality due to the flow pattern change. Experiments were also conducted to explore the influence of vapor quality and heat flux on the flow boiling heat transfer coefficient with an annular hydraulic diameter of *Dh* = 5.95 mm. The results were obtained by keeping the vapor quality of the test section in the ranges of 0.2–0.8 and 0.1–0.8. As shown in Figure 11b, the heat transfer coefficients for *xave* = 0.5 were slightly higher than those for *xave* = 0.45. The effect of annulus width was also investigated under similar test conditions, with a fixed average vapor quality for the test section of 0.5. The heat transfer coefficient for the narrower annulus gap seems to be a little higher than that of the wider annulus gap; little difference was found, indicating that the annulus width may be less important for flow boiling heat transfer performance than for condensation heat transfer, due to the dominance of nucleate boiling for the test conditions.

**Figure 9.** Schematic diagram of the possible flow pattern during condensation in the annulus for (**a**) low mass flux rates, and (**b**) large mass flux rates [18].
