Cavitation Corrosions in Capillary Pulsating Heat Pipe

When vapor bubbles are subjected to suddenly reduced pressure or immediate subcooling, it may rapidly condense, rupture, and generate cavitation corrosions. This phenomenon often occurs behind the blade of a rapidly rotating propeller or on any surface vibrating in liquid with sufficient amplitude and acceleration. In this article, we reported cavitation and its corrosive collapse occurring in capillary tubes, called pulsating heat pipes. Visualization images of cut and opened tubes show that internal copper surface was seriously etched after a certain period of operation. Sub-millimeter etching pits are observed on the tube internal surface. Copper particles in size of a few hundred micrometers are also found in the reclaimed operating fluid. Starting from this finding, the temperature effect of performance is analyzed to understand the cavitation occurrence and collapse. Pulsating heat pipe requires a certain temperature difference between the evaporator and condenser sections, typically > 10°C, to generate continuous two-phase oscillating movements. However, during the transient startup period, this temperature difference could reach as high as 50°C. Large saturation temperature difference, associated with highly turbulent two-phase flow, drives the saturated vapor bubbles from the hot evaporation region to the subcooled environment in less than 100ms. During the rapid condensation, the accelerated shrinking vapor bubbles create interface instability, followed by forming a strong impingement jet to etch the solid pipe wall. The collapse of cavitation is associated with the generation of acoustically tinkling signals that are often heard during most of the operating pulsating heat pipe.