CHF ENHANCEMENT BY NANOPARTICLES-COATED SURFACE IN FC-72 FLOW BOILING

摘要

CHF is the essential limiting parameter in the boiling. Among two CHF types in the flow boiling, departure from nucleate boiling (DNB) was investigated. It is well known that CHF can be enhanced by nanoparticles deposited on the heated surface. Thus, the effect of Al_2O_3 nanoparticles-coated surface on flow boiling CHF was studied. The CHF experiments are conducted under the atmospheric pressure using a FC-72 flow boiling loop. Al_2O_3 nanoparticles were coated on heater surfaces through quenching of test tubes using alumina nanofluids or dispersions. The nanoparticles-coated tube showed CHF enhancement up to 30% at maximum under 16°C, 21°C, 25°C inlet subcooling and 2000 kg/m~2s, 2500 kg/m~2s, 3000 kg/m~2s mass fluxes compared to the bare tube. The causes enhancing CHF are related to the rewetting process derived from capillary action through porous structures built-up by nanoparticles without improved wettability for the nanoparticles-coated surface compared to the bare surface since highly wettable FC-72 refrigerant was used as a working fluid. The CHF enhancement mechanisms were investigated through infrared thermometry for the temperature distribution of each test tube near CHF condition. As the results, the reduction of bubble coalescence phenomena due to the porosity and roughness effects raised by deposited nanoparticles was observed.