Pool Boiling Study on Designed Microstructure Surface

Abstract

The effectiveness of microstructured surfaces in enhancing boiling heat transfer (BHT) and critical heat flux (CHF) was investigated with an experimental setup and fundamental bubble growth analysis. A set of pool boiling experiments was designed with microstructured surfaces and a bare surface. Samples were fabricated using microelectromechanical systems (MEMS) techniques. The samples were tested using pool boiling experiments under saturated and atmospheric pressure conditions. The bubble growth characteristics on the structured surface were visualized with high temporal and spatial resolution optical and infrared cameras. Boiling performance results showed that BHT increased with surface roughness, defined as the ratio of the rough surface area to the projected area, but this enhancement gradually slowed. The heat transfer coefficient of the structured surface was more than 300% greater than that of the bare surface. The increase in the heating surface area due to the roughness ratio improved nucleate BHT, and the enhancement was analyzed in terms of the fin effect of the microstructured surface. In terms of CHF, the structured surface showed a 350% improvement in CHF over the bare surface. However, through an analysis of the capillary flow rate on the structured surface, a critical gap size that limits CHF enhancement was found. The critical gap size is discussed analytically and compared with experimental data. Designs for optimal boiling performance are proposed by studying the role of microstructured surfaces in both BHT and CHF. Furthermore, a fundamental analysis of bubble growth on the structured surface was performed. Through a visualization technique, characteristics of bubble growth on the structured surface were analyzed, and the physical role of the designed microstructures on boiling performance enhancement was examined. Finally, the boiling performance of the designed microstructures was evaluated by comparison with other pool boiling research that showed performance enhancement by surface modification. The comparative analysis not only gave a reasonable explanation for the performance trend but also illustrated the excellent performance of the structures designed in this study.