Roles of Boiling Surface Characterized by Micro-structures on Boiling Heat Transfer and Critical Heat Flux

摘要

As one of a key phenomenon in nuclear reactor thermal hydraulics, nucleate boiling has been widely studied by numerous researchers to improve plant design efficiency and safety. In general, the evaluation of the boiling performance mainly focus on two physical parameters: boiling heat transfer (BHT) and critical heat flux (CHF). In the nuclear power plants, since both BHT and CHF contribute the nuclear system efficiency and safety, respectively many new approaches to enhance the boiling performance compared to the conventional methodology have been investigated. Recently new progress in micro-nano manufacturing techniques has been made and enables to fabricate the precisely engineered boiling heating surface. The new techniques even allows to develop advanced boiling models by observing the fundamental phenomenology associated in boiling process by controlling the specific surface characteristics that includes cavity configuration, surface wetting conditions, etc. We recently focused on the implications and limitation of the microstructured surface on the enhancement of the boiling performance (BHT & CHF). We designed a set of experiments that prepare 13 samples; the 12 samples of microstructured surfaces and one bare surface has been fabricated by MEMS (Micro Electro Mechanical Systems) techniques. The samples was tested for the pool boiling experiments at saturated and atmospheric pressure conditions. For BHT, the experimental results show that BHT increases with the surface roughness defined as the ratio of the actual area to the projected area but the enhancement gradually slows. Overall, the heat transfer coefficient of the structured surfaces increases more than 300% compared to that of the bare surface. The increase of the heating surface area with the roughness ratio improves nucleate boiling heat transfer due to the enhancement of convective heat transfer. For CHF, the structured surfaces shows up to 350% enhancement with respect to that of the bare surface. However In the analyzing the capillary flow rate on the structured surface it is found that the critical size (gap) that limits the CHF enhancement exists. In this study, the critical size has been analytically discussed and compared with experimental data. Based on the above understanding role of the microstructured surface on both BHT and CHF found in our study, the optimal design of boiling performance (HTC & CHF) can be suggested.