Effects of Graphene and SiC Nanofluids on Critical Heat Flux and Quenching for Advanced Nuclear Reactors

Abstract

My research purpose is the study on effects of graphene and SiC nanofluids for advanced nuclear reactors through the experiment on flow boiling Critical Heat Flux (CHF) enhancement and the quenching experiment. In here, nanofluids are nanotechnology-based fluids engineered for enhancing thermal conductivity by dispersing and stably suspending nanoparticles in traditional heat transfer fluids. In the present study, two kinds of works were conducted. First, the CHF is characterized by a sudden reduction of the local Heat Transfer Coefficient (HTC) that results from the replacement of liquid by vapor adjacent to the heat transfer surface and ordinarily, represents the thermal limitation in which a phase change happens during heating. When the CHF occurs, an inordinate decrease in the heat transfer rate for heat flux and temperature controlled system generates. Moreover, it is generally more important in applications such as power generation for heat flux controlled system because of maintenance of the integrity occurring in heated surface. So, it is very important to enhance the CHF to ensure the system safety and improve the efficiency. Many methods to enhance the CHF have been investigated and a new technique in recent years among these methods is nanofluids technology. The influences of 0.01 volume fraction (%) Al2O3, SiC and Graphene Oxide (GO)/water nanofluids and fluid thermal hydraulic conditions on CHF have been experimented. Experiments were performed using 1/2 inch SS 316L tube when the mass flux is 100, 150, 200, 250, 300 kg/m2s and inlet temperature is 25 and 50 °C. The maximum CHF enhancement of Al2O3/water nanofluid was 15 % at inlet temperature of 50 °C and mass flux of 200 kg/m2s. That of SiC/water nanofluid was 41 % at inlet temperature of 25 °C and mass flux of 150 kg/m2s. And, the maximum CHF enhancement of GO/water nanofluid was 100 % at inlet temperature of 25 °C and mass flux of 250 kg/m2s. The CHF enhancements of nanofluids were caused to enhanced wettability of the liquid film on the heater surface due to the deposition of nanoparticles. The enhanced wettability is due to the change of surface structure (porous structure). This is confirmed through macroscopic observation, SEM observation and contact angle measurement. Liquid film thickness affected by evaporation, entrainment and deposition mass transfer can be closely linked with wettability and nanoparticles properties. Also, the CHF enhancement of nanofluids is caused to increase of thermal activity related to thermal conductivity and thickness. Second, quenching experiments were conducted to investigate the effect of nanofluids on reflood heat transfer in a long vertical tube (1,300 mm in the heating length). When the potential application of nanofluids comes to Emergency Core Cooling System (ECCS), the situation of interest is quenching phenomena of fuel rods during reflood of emergency coolants. The reflood tests have been performed using SiC and GO/water nanofluids as a coolant, instead of water. We have observed a more enhanced cooling performance in the case of the nanofluids reflood. A cooling performance (quenching time) is enhanced more than 20 seconds for SiC/water and GO/water nanofluids. A more enhanced cooling performance is attributed to a high wettability of a thin layer formed on a heating surface by a deposition of nanoparticles. The enhanced wettability is due to the change of surface structure (porous structure). The enhancing cause of the cooling performance using the nanofluids were investigated through macroscopic observation, SEM, SEM-EDS and contact angles of the inner surface of the test section. Also, a more enhanced cooling performance is caused to increase of thermal activity related to thermal conductivity and thickness. Effects of graphene/SiC nanofluids show the enhancement of safety margin for advanced nuclear reactors in terms of CHF enhancement and an enhanced quenching performance.