**5. Conclusions**

An experimental investigation of the heat transfer characteristics during flow boiling and condensation of R134A in the annulus of the 1EHT tube and an equivalent smooth tube was carried out. The experiments were conducted at a constant saturation temperature of 318.15 K, and the effects of mass velocities, annulus width (gap), and vapor quality on the heat transfer coefficients were investigated and discussed. Several conclusions can be drawn, as outlined below.


**Author Contributions:** Conceptualization, W.T. and D.J.K.; methodology, W.T., and R.S.; software, W.T.; validation, W.T.; formal analysis, D.J.K.; investigation, W.T.; resources, D.J.K., and R.S.; data curation, W.T.; writing—original draft preparation, W.T.; writing—review and editing, D.J.K., and W.L.; visualization, W.T.; supervision, D.J.K., R.S. and W.L.; project administration, D.J.K., and W.L.; funding acquisition, D.J.K., R.S. and W.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Science Foundation of Zhejiang Province (LY19E060004).

**Conflicts of Interest:** The authors declare no conflicts of interest.
