## 1 Introduction  
The refrigeration and air-conditioning industry is calling for new pure fluids or mixtures to be used in the coming generation of HVAC equipment to tackle the Kigali Amendment to the Montreal Protocol, which states the gradually phase-out of the consumption and production of hydrofluorocarbons (HFCs). Whilst in the past years, just a few refrigerants were enough to cover different applications of the HVAC industry, a huge number of alternative refrigerants, both natural and synthetic, having lower values of global warming potentials (GWPs), are emerging nowadays. Hydrofluoroolefins (HFOs) are a promising category of working fluids having low values of the GWP, but they are generally classified as mildly flammable (A2L flammability class). To decrease the flammability class to A1, viz., not flammable, HFOs can be properly blended with a non-flammable HFC. R450A is a geotropic blend made of R1234 ze(E) and R134a (0.58 and 0.42 by mass, respectively), and it has been proposed as a replacement for R134a for medium- and high-temperature refrigeration applications. The presence of R134a in the mixture makes it not flammable, viz., it belongs to the A1 flammability class. On the contrary, R1234 ze(E) is an HFO, thus belonging to the A2L flammability class, but it has a GWP lower than 1, so R1234 ze(E) lowers the GWP of the mixture. Most of the works available in the literature considering R450A as a working fluid are related to its performance assessment in vapor cycle systems. [PERSON] et al. [1] experimentally tested R450A in a vapor compression system equipped with a variable speed compressor. The performance of R450A as a direct drop-in substitute of R134a was investigated: the cooling capacity of R450A was 6% lower on average, the COP was 1% higher, and the discharge temperature was 2 K lower. The lower cooling capacity of R450A compared to R134a in a vapor compression system was later confirmed by [PERSON] et al. [2], but a small refrigeration unit showed CO\({}_{2}\) equivalent emission saving when using R450A. Other works related to the experimental analysis of R450A vapor compression systems can be found in [PERSON] et al. [3], [PERSON] et al. [4], [PERSON] and [PERSON] [5], and [PERSON] et al. [6], whereas the mathematical modeling of refrigeration systems implementing R450A as a working fluid can be found in [PERSON] et al. [7], [PERSON] et al. [8], and [PERSON] et al. [9]. The two-phase heat transfer behavior of R450A, either during condensation or during vaporization, is still not widely studied, and just a few papers can be found in the literature. Among those, [PERSON] et al. [10] quantified the performance of four fluids, included R450A, during pool boiling on a re-entrant cavity surface. At the same wall-to-saturation temperature difference, the heat flux of R450A was, on average, 57% less than that of R134a, at heat fluxes between 30 and 110 kW m\({}^{-2}\). The flow boiling of R450A within a 9.5 mm OD microfin tube was studied by the same research group ([PERSON] and [PERSON] [11]); the results revealed that under the same working conditions, R450A had a 15% lower heat transfer performance compared to R134a. [PERSON] et al. [12] conducted experiments during the condensation of R450A inside a smooth tube with an inner diameter of 4.7 mm, obtaining heat transfer coefficients and pressure drops that were successfully compared against values predicted by correlations available in the literature. A numerical analysis of R450A, as well as of R134a and R1234 ze(E), condensing inside smooth tubes was performed by [PERSON] et al. [13], and they computed the heat transfer coefficient, liquid film distribution, local film thickness, and axial velocity. R515B is an azeotropic mixture made of R1234 ze(E) and R227 ea (0.911 and 0.089 by mass, respectively). Like R450A, R515B has been proposed as a substitute of R134a, and it is classified as non-flammable (A1 flammability class) due to the presence of R227 ea.