# Condensation Flow of Refrigerants Inside Mini and Microchannels: A Review
## 2 Micro and Mini-Channel Condensation  
### The Transition from the Macro to the Micro/Mini Dimension  
Due to the distinct flow regimes observed in micro/minichannels, there is a necessity to differentiate between conventional and small-sized channels. Consequently, multiple classifications have emerged in the literature; however, there remains a lack of consensus on the classification of these channel geometries. Small channel geometries, such as micro/minichannels, are typically categorised based on hydraulic diameter, which is a defining feature. In addition to classifications based on hydraulic diameter, there are also classifications in the literature based on flow phenomena within the channel. These classifications utilise dimensionless parameters that account for flow effects resulting from the thermophysical properties of the fluid within the channel. For instance, in condensation flow, if surface tension becomes dominant due to channel size, it may be classified as a microchannel as opposed to conventional channel sizes. Therefore, microchannel behaviour is characterised by phenomena absent in larger-scale channels but prevalent in small-scale channels. Since these phenomena are contingent upon the properties of the liquid and vapour phases of the relevant fluid, precise determination of fluid conditions is necessary for identifying a channel as a microchannel [3]. As a result, researchers have proposed numerous criteria for delineating the transition from macro to micro dimensions. The transition from the macro to the micro/mini dimension is summarised in Table 1.  
One of the widely recognised classifications was proposed by [PERSON] et al. [21]. In their research, [PERSON] et al. [21] categorised channels with hydraulic diameters between 1 \(\upmu\)m and 100 \(\upmu\)m as microchannels, those ranging from 100 \(\upmu\)m to 1 mm as mesochannels, from 1 mm to 6 mm as compact channels, and those larger than 6 mm as conventional channels. Another classification was put forth by [PERSON] and [PERSON] [22], who extensively studied small-sized channels. According to [PERSON] and [PERSON] [22], channels with hydraulic diameters ranging from 10 \(\upmu\)m to 200 \(\upmu\)m are classified as microchannels, those ranging from 200 \(\upmu\)m to 3 mm as minichannels, and channels with hydraulic diameters exceeding 3 mm as conventional channels. [PERSON] et al. [23] introduce the Laplace constant (also known as the capillary length scale) as a criterion for assessing whether a channel qualifies as a microchannel. The Laplace constant, given Equation (1), quantifies the ratio of surface tension to gravitational force. This constant delineates the point where the influence of surface tension starts to outweigh the effect of gravity, leading to stratified flow. \[L_{c}=\sqrt{\frac{\sigma}{g(\rho_{l}-\rho_{v})}} \tag{1}\]  
As an alternative classification, [PERSON] and [PERSON] [24] suggest the utilisation of the confinement number as a transitional criterion for microchannel effects. While initially developed for boiling flow, the confinement number can be extended to condensation applications as well. \[Co=\frac{\sqrt{\frac{\sigma}{g(\rho_{l}-\rho_{v})}}}{D_{h}} \tag{2}\]  
The confinement number is a ratio of the Laplace constant (Equation (1)) to the hydraulic diameter (\(D_{h}\)). The channel can be qualified as a microchannel when the confinement number exceeds 0.5. [PERSON] and [PERSON] [25] investigated the influence of gravity on condensation in meso/ microchannels, particularly focusing on the transition from symmetric flow (where the effect of gravity diminishes and becomes negligible) to asymmetric flow (resulting from gravity-induced stratification). In their study, [PERSON] and [PERSON] [25] identified the critical diameter and threshold value diameter for this transition criterion based on the capillary length scale. The critical diameter and threshold diameter are defined by Equations (3) and (4), respectively.