# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 1 Introduction  
For a given heat transfer load, increasing the heat transfer surface area per unit volume makes condensers compact. A compact heat exchanger design means more light heat exchangers manufactured with less material. In addition to increasing the heat transfer surface per unit volume, increasing the total heat transfer coefficient also improves heat transfer performance. In a heat exchanger, under constant heat flux conditions, the heat transfer coefficient inside the channel increases as a result of reducing the channel hydraulic diameter. Achieving a high heat transfer rate in small volumes has, indeed, generally been one of the main challenges in heat exchanger design [1, 2]. In this scope, mini- and microchannel heat exchangers have been becoming promising designs to meet such challenges due to their outstanding advantages. Micro/minichannel condensers are good options for thermal systems because they have high heat transfer coefficients within the channel and a high ratio of surface area to volume.  
With the development of micro-production technologies in the last two decades, traditional condensers are being rapidly substituted with microchannel heat exchangers. The fact that the channel diameter in which the flow occurs in microchannels is very small creates different effects on the flow characteristics, which results in different flow regimes [3]. Condensation is the removal of thermal energy by producing a phase change from vapourto liquid in a system. Condensing flow inside mini and microchannels has been becoming a promising solution for refrigeration, HVAC, air-conditioning, heat pumps, heat pipes, and electronic cooling applications. In these applications, employing mini/microchannels in the condenser design results in the working fluid, generally refrigerant, undergoing a phase change inside the mini/microchannels [4].  
As the hydraulic diameter decreases, a different flow regime and heat transfer mechanism emerge in the micro/minichannel than in channels with large cross-sectional areas. During the flow in micro/minichannels, capillary effects become dominant while buoyancy forces lose their effect [3]. In other words, as the channel size decreases, the importance of surface tension relative to gravity increases. For this reason, condensation and evaporation models developed for large channels where gravity and inertial forces are dominant cannot be applied to micro/minichannels. Compared to boiling flows in microchannels, studies on condensation flows have been limited [5]. Therefore, in light of the studies in the literature, it is important to understand the intrachannel phase change mechanisms for micro/minichannel condensers. The first step in understanding condensation flow in micro/minichannels is the characterisation of the condensation flow and determination of the flow regime, which forms the basis of the heat transfer coefficient and pressure drop.  
In contrast to conventional condensers, the prevalence of surface tension forces over gravity forces in two-phase flow processes within micro/minichannel condensers has spurred significant growth in recent experimental, theoretical, and computational studies in this field. While a relatively small portion of these studies has been dedicated to understanding condensing flow regimes and heat transfer behaviour in micro/minichannels, especially for HVAC-R systems, they offer valuable insights. This review study includes published studies in the rapidly evolving domain of condensation in micro/minichannels, encompassing experimental, theoretical/empirical, and computational contributions. Therefore, the current study establishes a foundation for future research endeavours. Moreover, the selection of refrigerant, its thermodynamic performance, compatibility, and environmental effects significantly influence the design of thermal systems. Hence, this review study also addresses the environmental effects of refrigerants and the existing regulations governing them.