A series of experiments was conducted to observe nucleate boiling phenomena in horizontal tubes with inner diameters varying from 0.05 mm to 3.0 mm. Diverse behaviors of bubble growth were explored, identified by which tubes were classified into micro, mini and macro scales. In micro tubes (Di ≤ 200 μm), the liquid was emitted instantaneously with extremely fast liquid-vapor interfacial movement, referred as explosive emission boiling phenomenon. It is hard to record bubble growth process with high speed camera. In mini tubes (200 μm < Di < 2.5 mm), though liquid was also emitted outsides, the interface moves relative slow and the whole process of bubble growth can be observed. Two distinct stages, referred as spherical and oblate bubble growth stages, were divided. In macro tubes (Di ≥ 2.5 mm), only spherical bubble growth stage exists and the growth rate is much smaller than that in mini tubes. Furthermore, the mechanism of diverse bubble dynamics was analyzed. In mini/micro tubes, decreasing tube diameter can trigger a transition from spherical to oblate bubble growth and consequently establish a thin film between liquid-vapor interface and heated wall. The thin liquid film evaporates vigorously and accelerates the interfacial movement, which reversely enhances evaporation of thin film. A positive interaction between interfacial movement and thin film evaporation establishes, resulting in the interface moving faster and faster and consequently emitted liquid outsides instantaneously. In macro tubes, as tube diameter increasing, the transition and sequential positive interaction can not be raised. Hence, the bubble maintains growing spherically as that in pool boiling.

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