Experimental study on boiling instability and occurrence of cavitation in a two-phase square pipe subjected to natural convection

Unsteady boiling bubble behavior of water in a square pipe under a low pressure and subjected to natural convection was experimentally studied. The measurements included temperature and pressure, and the bubble behavior was visualized using a high-speed camera. The boiling phenomena in the pipe showed the three stages characterized by cavitation, intermediate, and fully developed boiling bubble in the Rayleigh number range (1.2-1.6) x 10(8). There is boiling bubble nucleation, growth and upward movement once heating starts and the explosion of boiling bubble occurs near the free surface, which results in the cyclic variation of superheat condition in the liquid column in the fully developed boiling instability condition. This leads to self-sustained oscillation of the free surface of the liquid column and a minor temperature oscillation. The cyclic period of boiling instability is 1-2 s in the natural convection, which is shorter than that of 2-100 s for the thermosyphon heat pipes in the literature. The cavitation phenomenon is detected by sound pressure measurement using a microphone in the initial stage of heating at the sub-cool condition. The simultaneous observation of bubble behavior in the liquid column shows that a large bubble in the liquid column collapses in a short time at the instant of cavitation noise detection.