(An) experimental study on the CHF enhancement in flow boiling using $Al_2O_3$ nano-fluid

Abstract

The critical heat flux (CHF) is one of the most important thermal hydraulic parameter in thermal hydraulic design and safety analysis. The CHF enhancement allows higher limit of operation condition so that heat transfer equipments can be operated safely with more margins and can have more economics. Nano-fluid is one of the most potential methods which can enhance the CHF and many researchers are working on. This study consists of two parts. First part is dispersion stability of $Al_2O_3$ Nano-fluids and second part is flow boiling CHF experiment with $Al_2O_3$ Nano-fluids. In the first part, zeta potentials of $Al_2O_3$ Nano-fluids with varying time and concentration were measured by ELS-Z2 of Otsuka Electronics to find out the effect of time and concentration on the zeta potentials. The experiments were conducted at concentration of 0.001 vol% ~ 0.5 vol% and time of 1hour ~ 3days. All measurements of zeta potentials were in the range of 30 mV ~ 60 mV. The zeta potentials of $Al_2O_3$ Nano-fluids slightly increased as time increased within 1day. The zeta potentials of $Al_2O_3$ Nano-fluids slightly increased as particle concentration increased from 0.001 vol% to 0.5 vol%. The pHs of $Al_2O_3$ Nano-fluids were also measured. In the second part, flow boiling CHF enhancement experiments using $Al_2O_3$ Nano-fluids with three different concentrations (0.001 vol%, 0.01 vol%, and 0.1 vol%) were conducted under atmospheric pressure, at mass flux of 100~300 $kg/m^2 s$, at inlet temperature of 50~75℃. The CHFs of $Al_2O_3$ Nano-fluids were enhanced in flow boiling for all experiment conditions up to about 70%. Maximum CHF enhancement (70.24%) was shown at 0.01 vol% concentration, 50℃ inlet temperature and 100 $kg/m^2 s$ of mass flux. Inner surfaces of test section tube were observed by FE-SEM and Zeta potentials of $Al_2O_3$ Nano-fluid were measured before and after the CHF experiments.