The pool boiling heat transfer and critical vapor column coalescence mechanism of block-divided microstructured surfaces

In this paper, the pool boiling heat transfer performance of cylindrical microstructured surfaces that were fabricated using the dry etching technique was studied. The working fluid was FC-72, and the experimental conditions included four different liquid subcooling temperatures (0, 15, 25, and 35 K). The heated microstructured surfaces consist of smooth parts and circular micropillar blocks, which are classified as single-block type (MP-1), four-block type (MP-2), 16-block type (MP-3, MP-4, MP-5) or composite-block type (MP-6). The material that was used as the substrate was P doped silicon chip. The experimental results showed that the heat transfer coefficient (HTC) and the critical heat flux (CHF) of all of the microstructured surfaces are greatly enhanced compared with the smooth surface because the block divisions and blank area could effectively prevent vapor columns from coalescing. Among the microstructured surface types, surface MP-3 has the largest CHF with different subcooling, while its actual heat transfer area of microstructured surface is relatively small. Furthermore, the mechanism and behavior of vapor column coalescence under critical heat flux conditions were analyzed. The prediction of CHF by using the critical vapor column radius (r_gc) was compared and analyzed with the experimental data. Finally, the critical metastability phenomenon was demonstrated, and its occurrence mechanism was explored and explained. The experimental results show that restricting the coalescence of the vapor column is an effective method to augment CHF, and a high CHF can be obtained even when the surface area enhancement ratio is relatively low.