# Flow Condensation of Low-GWP Zeotropic Mixtures Inside 5 mm OD Micro-Finned Tube
## 1 Introduction  
Due to novel regulations aiming to attenuate the greenhouse effect [1], implementation of refrigerants with low GWP is imminent. For instance, regarding new air conditioning systems, European F-gas regulation [1] sets a GWP limit of 750 for refrigerants placed on the market. The European Union also aims to accomplish an 80% reduction in HFCs placed on the market compared to a baseline period. [PERSON] [2], exploring the feasibility of the usage of HydroFluoroOlefins (HFOs) as a substitute for its contemporary counterpart HydroFluoroCarbons (HFCs), expressed that recent arc arcquing scientific endeavors encourage the use of HFO-1234 ze(E) and HFO-1234 yf as replacements for vastly utilized HFC-134a. Such a statement is a commonality in the literature as other authors [3; 4] claim similar plausibility. Despite a fair potential, the shortcomings of HFOs [5], which are their light flammability and slightly inferior thermal and hydraulic performances compared to HFCs, are a matter of indispensable concern. One solution to such drawback is regarding the utilization of HFO/HFC mixtures that not only boost the thermal performance [6] but rectify the flammability concern. An analysis [7] committed to foreseeing the feature demand of refrigerant emphasizes the use of such mixtures in the transitional phase. R450A has been designed to serve as a not-flammable solution to the widely used HFC-134a with a 60% reduced GWP. Among its practical applications, it is worth citing the use for heat pumps, air-cooled and water-cooled chillers, district heating and cooling, vending machines and beverage dispensers, high-stage CO\({}_{2}\) cascade systems, high-stage cascade CO\({}_{2}\) systems, and direct expansion medium temperature refrigeration systems [8]. R454B is a replacement candidate for the HFC-410A, with a 78% lower GWP. Thanks to its higher critical temperature and broader operating envelope in low evaporating temperatures, R454B could outperform other solutions such as R32 in reversible chillers and heat pump applications [9]. After a brief survey of the literature, there is a limited diversity of test sections dedicated to the experimental investigation of the two promisingzeotropic mixtures of R450A and R454B for flow condensation as opposed to the other candidates. The two refrigerants possess similar global warming potential categorizations, which are equal to 547 and 467 for R450A and R454B, respectively. Concerning R454B (31.1% HFO-1234 yf)--68.9% HFC-32 by mass), the attempts are confined to works of [10; 11; 12] for 9.52 mm and 7.2 mm cooper/aluminum smooth tubes of 4.7 mm inner diameter. The abovementioned investigations culminated in new correlations regarding the heat transfer coefficient (HTC) of HFC/HPO mixtures, a comparison against previous HTC and frictional pressure drop (FPD) models, and an inference regarding the dependency of the model's accuracies on tube material. Regarding R450A (58% HFO-1234 ze(E)-42% HFC-134a by mass), the literature is perhaps more enriched in terms of the geometrical diversity of the test sections. For instance, experimental campaigns [13; 14] on channels having diameters in the range of 0.9 to 3.3 mm and micro-finned tubes of 7 mm outer diameter (OD) by [PERSON] et al. [15] can be noted. It is important to note that the work of [PERSON] et al. [14] also visualized the flow patterns of R450A during condensation, aside from their thermal analysis. Overall, it is quite evident that neither refrigerant was tested inside the microscale (between 1 and 6 mm according to the definition of [16; 17]) micro-finned tubes. Most of the papers appearing in the literature are focused on the overall assessment of the vapor compression systems [18; 19; 20; 21; 22; 23]. Considering the absence of such research, it remains unclear whether the contemporary correlations for micro-finned tubes such as the model by [PERSON] et al. [24] could offer fairly accurate predictions.