# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 4 Conclusions and Suggestions for Future Work  
The current review study presents an up-to-date, comprehensive review of studies on condensation in mini/microchannels. The transition from the macro to the micro/mini dimension as well as the void fraction and flow regimes of microchannel condensation flow were discussed as a basis for understanding small-scale condensation. The review includes experimental studies as well as correlation models, which are developed to predict condensation heat transfer coefficients and pressure drop. Predictive models for the condensation flow of refrigerants inside mini/microchannel were explored in detail. Moreover, numerical studies on micro/minichannel condensation were comprehensively presented. Environmental impacts of refrigerants were also discussed to encourage the usage of climate-friendly refrigerants together with micro/minichannels. Key conclusions of this study and future research needs are listed as follows:  
* HCs emerge as promising alternatives to HFCs and HCFCs for HVAC-R systems because they are harmless to the environment. However, there is a notable scarcity of studies focusing on crucial parameters essential for optimising systems employing them, such as heat transfer coefficients and pressure drop (Section 3.1 and Table 5). Experimental and numerical studies, particularly involving mixtures of HCs, are also lacking. Flow pattern and void fraction have an impact on the heat transfer coefficient. Mass flux and vapour quality during condensation flow are key phenomena in flow patterns. However, investigations into flow patterns and void fractions are notably lacking. Despite the apparent diversity in experimental conditions reported in the literature, the limited number of studies has resulted in sparse experimental conditions that lack complementarity with each other. * It is clear that conducting experimental studies on microchannels involves some difficulties. On the other hand, it is recommended to carry out experimental studies on the microchannel condensation flows of new refrigerants, especially those developed to be environmentally friendly. It has been concluded that the dissemination of these studies will make a significant contribution to the introduction of highly efficient, environmentally friendly, and ecological systems. This situation is compliant with sectoral needs as forced by regulations. * In addition to the manufacturing of microchannels and minichannels, the difficulty of experimental studies and the safety risks of environmentally friendly refrigerants emphasise the importance of numerical studies. In this context, it was concluded that validated numerical models are needed for various working fluids, geometries, and operating conditions. In particular, numerical models made with open-source CFD codes are quite limited. The spread of these studies will contribute to the emergence of new technologies. * Many prediction models that can be applied to micro/minichannels exist in the literature, but they are developed especially for HFCs, HCFCs, and mixtures of them. The prediction models for heat transfer coefficient were presented in Section 3.3.1, whereas pressure drop prediction models were provided in Section 3.3.2. Overall, researchers validated comparable qualitative behaviour in heat transfer coefficient and pressure drop when comparing pure and mixture HCs to HFCs and HCFCs across a range of parameters, including mass and heat flux, diameter, saturation temperature, vapour quality, etc., during condensation flow. Regarding the comparison between experimental data from the literature and prediction methods, it is concluded that reasonable agreements exist in the heat transfer coefficient of HC in micro/minichannels. * The numerical studies on flow in nanochannels, in addition to the microscale, may lead to enhanced nanochannel fabrication techniques. * In actual systems, refrigerants mostly flow together with lubricants.