A series of experimental investigations were conducted to visually observe the fundamental features of boiling nucleation of water in micro capillary tubes covered with transparent metal film and ethanol in mini tubes winded with a heating wire as a heater. Normally, boiling nucleation and/or bubble generation were not observed even at very high heat fluxes compared with usual nucleate boiling situations. In mini tubes violent jet flows formed and played an important role of triggering the emission, while in micro tubes the liquid and/or vapor was instantaneously emitted as the applied heat flux reached a certain high value, rather than initialing nucleate boiling and jet flows which was much weaker than exploding emission. After the exploding emission, an ultra thin liquid film formed on the tube wall and evaporated intensely, causing a perceptible wall temperature drop.

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