# Flow Condensation of Low-GWP Zeotropic Mixtures Inside 5 mm OD Micro-Finned Tube
## 4 Conclusions  
Amid current and prospective regulations mandating the use of low-GWP refrigerants, it is posited that mixtures of HFC/HFO could prove effective in short-term transition. Since solid information was not established on the flow condensation of micro-finned tubes for two zeotropic mixtures of R450A and R454B, the following paper conducted an experimental campaign to explore their heat transfer coefficient (HTC) and frictional pressure drop (FPD). The accuracy of previously constructed empirical models was also assessed with the help of experimental results. The new experimental measurements allowed for the enlargement of the worldwide database of heat transfer coefficients and frictional pressure gradients during condensation of new lower-GWP refrigerants inside enhanced tubes. These new data play a fundamental role in the validation and, eventually, in the update, of empirical models for the design of the next generation of condensers for HVAC&R equipment with lower carbon emissions. Furthermore, new techniques such as machine or deep learning models are in great demand of new data for their training and validation.  
Overall, the outcome of the research is recapitulated and given as the following:  
1. The higher the mass flux, the higher the slope by which the values of HTC and FPD per unit length increase. This occurrence is related to prevailing flow patterns.
2. R450A possesses higher HTC and FPD values as a result of inherent thermophysical properties that cause the vapor velocity to be considerably higher. The higher surface tension also plays a role in higher values of HTC for R450A.
3. Higher saturation temperature entails higher vapor densities, which attenuates the vapor velocity and, consequently, the shear stress forces, therefore causing lower HTC values.
4. The correction of [PERSON] and [PERSON] [25] can result in the amelioration of the HTC model developed by [PERSON] et al. [24], especially for R454B due to its higher temperature glide. The coefficient \(C\), implemented by [PERSON] et al. [24] for an optimal number of fins, does not necessarily offer improved accuracies. Overall, the model of [PERSON] et al. [24] offered fair predictions of HTC values.
5. Both the FPD models of [PERSON] [35] and [PERSON] et al. [36] offer fair predictions for both refrigerants.  
Conceptualization, A.D.; validation, N.I. and A.D.; investigation, N.I. and A.D.; writing--original draft preparation, N.I.; writing--review and editing, L.R. and A.D.; supervision, L.R. and A.D.; funding acquisition, L.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Italian Ministry of Education, Universities and Research-MUR, PRIN Project number 2017F/KZWS.  
Not applicable.  
Data Availability Statement: Data are available in the figures. If readers want more information, more data will be available on request.  
The support of Wieland-Werke AG and [PERSON], from Wieland-Werke AG, for this research activity is also gratefully acknowledged.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.