A series of experiments was conducted to visually observe the fundamental features of the scale effect on boiling nucleation in mini/micro tubes. Boiling nucleation and/or bubble generation was observed in mini tubes of 1.5 to 2.5 mm and micro tubes of 50 to 200 μm in inner diameter. As heat flux was increased, bubbles were normally observed generating and growing inside the tubes of 2.5 mm. As tube diameter decreased from 1.5 to 0.05 mm, or in mini and micro tubes, the liquid was emitted instantaneously rather than generating bubbles and bubble growth, which is referred as liquid exploding emission in this paper. In mini tubes of Di = 1.5 mm, violent jet flows with macro bubbles were formed, and the liquid was almost totally emitted out of the tube. While in micro tubes of Di = 50–200 μm, the emission was much stronger, and only fog-like jet flow was observed consisted of extremely tiny bubbles, which indicates the mechanism of nucleation is different from that in mini tubes. It is expected that the scale effects intensely influence on not only the onset of emission, but also the strength and nature of emissions.

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