*3.3. Experimental Validation*

Single-phase heat transfer experiments were conducted for the validation of test procedures. Figure 4 depicts the relationship between the heat transfer coefficient and the Reynolds number. The single-phase experiment was conducted using R134a and R410A in the turbulent regime to validate the present test procedure. The average temperature along the test section was kept in the range of 303–304 K, and the ambient temperature was controlled at 298 K by an air-conditioner. The results were compared against the Gneilinski [22] correlation and the Dittus–Bolter correlation. All the data points could be predicted within ±10% by the Gneilinski [22] correlation. The EHT tube could produce a single-phase heat transfer enhancement ratio of 1.45–1.76 for the present test conditions compared to the smooth tube. In addition, a series of repetitive experiments were conducted using R410A to validate the present test procedure, and the tube-in-tube heat exchanger with *Dh* = 6.95 mm was employed. The same tube-in-tube heat exchanger made of the smooth tube was used for the further experiments of R134a. Two experiments were carried out on two different days for the flow boiling heat transfer performance in the annulus of the smooth tube. The inlet and the outlet vapor quality were fixed at 0.2 and 0.8, respectively. Figure 5 demonstrates a comparison of the two results, and good agreement was found between them. Therefore, it is reasonable to conclude that the results in this study have good repeatability.


**Table 3.** Uncertainties of measured and calculated parameters.

**Figure 4.** Variation of the single-phase heat transfer coefficient as a function of Reynolds number, and comparison against Gneilinski [22] and Dittus–Bolter correlation using work fluids: (**a**) R134a; (**b**) R410A.

**Figure 5.** Repetitive experiments conducted for the flow boiling of R410A in the annulus with a hydraulic diameter of 6.95 mm.
