The review deals with the analysis of the factors affecting the boundaries of two-phase regimes in the channels of different cross sections, whose minimal size is less than the capillary constant. The channels are classified by size. Data for two-phase flow regimes are systematized and summarized in tables for the round and rectangular tubes. It is indicated that the most studies identify the following two-phase flow regimes: bubble, slug and annular. The regimes found in some papers are described. The terminology used to describe the regimes is kept. Here we analyze the main factors affecting the structure of the two-phase flow, such as gas and liquid flow rates, parameters of the channel and input section, wettability of the inner surface of channels, liquid properties, and gravitational forces. It is shown that development of instability of the two-phase flow has a significant impact on formation, evolution, and change of the flow regimes. 266 the influence of capillary and gravitational forces. The relative influence of capillary and gravitational forces can be described using the Bond (Eötvös) criterion or the ratio of the channel diameter d to capillary constant l σ . In (Chinnov and Kabov, 2006), the following classification of the channel sizes is presented.1. The large-scale (convective) channels d > 5 l σ , characterized by the fact that there is no capillary force effect due to the channel curvature. Under these conditions, the capillary effects can occur at the level of other scales: bubbles, waves.2. The gravitational-capillary channels 0.5 l σ < d < 5 l σ , where the capillary and gravitational forces act together, but the gravitational forces dominate over the capillary ones. At d = 0.5 l σ , the stratified flow regime degenerates.3. The capillary-gravitational channels (minichannels) 0.1 l σ < d < 0.5 l σ with the joint effect of capillary and gravitational forces, when the capillary forces dominate over the gravitational ones. The channels of the second and third types can be called the mini-scale ones.4. The capillary channels (microchannels) d < 0.1 l σ , without the action of gravitational forces, and prevailing capillary effects. The channels of this type can be called the micro-scale ones.In (Rebrov, 2010), three typical groups of the flows in microchannels were distinguished: with prevailing surface tension forces, with prevailing inertial forces, and transient ones. The characteristic Eötvös number for the transition from macro-to microchannels is Eo = 0.84 (Bretherton, 1961), it is calculated by formula 2 diameter pipes, The Can. , 2011, Adiabatic two-phase flow in rectangular microchannels with different aspect rations. Part 1. Flow pattern, pressure drop and void fraction, Int. Flow pattern transition for horizontal air-water flow in capillary tubes. A microgravity equivalent system simulation, Int. Commun. Heat Mass Transfer, Vol. 21, No. 4, P. 461−468. I. Hassan, M. Vaillancourt, and K. Pehlivan, 2005, Two-phase flow regime transitions in microchannels: a comparative experimental stud...
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