An Experimental Studv Of The Mechanisms Of Regulated Transient Pool Boiling Heat Transfer By Surface Modification

The modification of microscopic characteristic on boiling surface has been considered the primary reason affecting the boiling behavior. In this work, the passive deposition of grapheme oxide sheets, the emerging carbon material with high thermal conductivity, and the active surface regulation have been conducted to investigate the mechanism of boiling by transient method.Firstly, we design and set up the visualized quenching apparatus. Both the boiling curves of successive quenching with one sample and different samples have good repeatability. The relative standard deviations of CHF and LFP with one sample are0.7%and0.56%, respectively. While the relative standard deviations of CHF and LFP with different samples are0.41%and1.38%, respectively.Then, we investigate the variation of the boiling behaviors with the concentrations (0.0001wt.%,0.001wt.%,0.005wt.%,0.01wt.%,0.05wt.%,0.1wt.%). We found that the graphene oxide nanofluids could enhance the heat transfer rate, and the linear fitting formula of the enhancement with concentration is y=14.76x+349.36, the correlation coefficient r=0.9831. At the concentration of0.1wt.%, the deposition of graphene oxide sheets on the boiling surface dramatically increase the local roughness, which enhance the short-lived solid-liquid contact, increase the LFP. And CHF, which is nothing to do with surface wettability in our experiment, is enhanced with the increase of concentration.At last, we achieve four surface with wettability of superhydrophilic(～0°), hydrophilic(～23°), hydrophobic(～119°), superhydrophobic(～165°). The result reveals that the LFP, MHF, CHF, and the corresponding superheat are all enhanced with improved surface wettability, and the linear fitting formula of the LFP with contact angle is y=65.45x+272.8, the correlation coefficient r=0.9782,and the linear fitting formula of MHF with contact angle is y=28.7+75.6, the correlation coefficient r=0.8840. Besides, we visualizes the vapor film features with high-speed camera. As to the superhydrophobic surface, the sample is in film boiling throughout the quenching without the collapse of stable vapor film. However, as to the superhydrophilic surface, the violent solid-liquid contact phenomenon occurs immediately the sample being plugged into the coolant, which strongly increase the heat transfer rate during film boiling.