Study of CHF phenomenon on micro and nano structures

Abstract

Consideration of the critical heat flux (CHF) requires difficult compromises between economy and safety in many types of thermal systems, including nuclear power plants. Much research has been directed towards enhancing the CHF, and many recent studies have revealed that the significant CHF enhancement in nanofluids is due to surface deposition of nanoparticles. The surface deposition of nanoparticles influenced various surface characteristics. This fact indicated that the surface wettability is a key parameter for CHF enhancement and so is the surface morphology. However, the nanoparticles on the heated surface could be indicated a possibility of the detachment problem of nanoparticles under flow boiling by Ahn et al. (2010). Therefore, the CHF enhancement on the nanoparticles coated heater could not let us an assurance to apply the real application. It would be needed the artificial surface modification which had same characteristics with the nanoparticles coated surface in order to increase CHF. In this study, surface wettability of zirconium alloy (Zirlo) used as cladding material of fuel rods in nuclear power plants was modified using the anodic oxidation method. At the certain condition of anodic oxidation solution (HF 0.5%), the zirconium alloys were modified as the anodic oxidation time increasing, from 49.3？ of contact angle to 0？. Pool boiling experiments of distilled water on the prepared surfaces was conducted at atmospheric and saturated conditions to examine effects of the surface modification on CHF. First, the experimental results showed that CHF of zirconium alloy can be significantly enhanced by the improvement in surface wettability using the surface modification, but only the wettability effect can？t explain the CHF increase on the modified zirconium alloy surfaces completely. It was found that below a critical value of contact angle (10？), micro/nanostructures created by the surface modification increased the liquid spreading ability on the surface, which could lead to further increase in CHF even beyond the prediction caused only by the wettability improvement. Second, the simple test was conducted in order to attain the quantitative heat flux gain due to the liquid spreading on the micro, nano, and micro/nano structures. Through the droplet falling test, a simple model to predict the CHF values with the effect of liquid spreading was established, and had a good agreement with the experimental CHF values. Third, the capillary wicking test was performed to attain the heat flux gain due to the capillary wicking of liquid on the micro, nano, and micro/nano structures. A simple model to predict the CHF values was also established, and had a good agreement with the experimental CHF values on the micro, nano, and micro/nano structures. Fourth, these results arise a question about the relation between the capillary wicking and the liquid spreading. Therefore, the porosity of the modified surfaces and the contact angle were considered to estimate the capillary pressure on the two dimensional surface. It was revealed that the capillary pressure on the micro, nano, micro/nano structures, which could represent the capillary wicking, generated the liquid spreading phenomenon. Finally, the CHF enhancement under flow boiling was considered on the modified zirconium ally tube, too. The liquid spreading and the good wettability of the heated surface strongly influenced the CHF enhancement under both the pool and flow boiling.