Effect of calcination temperature during synthesis of Al2O3 from local bauxite on stability and CHF of water-Al2O3 nanofluids

Nanofluid was introduced for replacing the conventional coolant such as water. It is predicted that the efficiency of heat transfer of some systems such as nuclear reactor and electronics will increase by this replacement. However, the stability of the nanofluid is still a problem. In this work, the stability of the nanofluids was engineered by controlling calcination temperature during synthesis of Al2O3 nanoparticles from Al(OH)(3) derived from local bauxite. The Al(OH)(3) was calcined at 750-1250 degrees C for 3 h. The nanoparticles were analyzed using XRD, TEM, and FTIR. Some nanofluids were prepared from the Al2O3 nanoparticles and characterized. According to the XRD data, the Al2O3 nanoparticles that were calcined at 750-1050 degrees C were gamma alumina with crystallite size of 4.04-6.04 nm, and those were calcined at 1200 and 1250 degrees C consisted of theta and alpha aluminas with crystallite size of 7.54 and 14.33 nm for theta alumina and 14.89 and 70.19 nm for alpha alumina. The FTIR data showed that the surface of the Al2O3 nanoparticles contained hydroxyl group. The stability of the nanofluids was affected by the amount of the hydroxyl group that existed on the surface of the nanoparticles. From the characteristics of the nanofluids, it was identified that the best nanofluids were that made of the Al2O3 nanoparticles calcined at 1200 degrees C for 3 h. The viscosity of the nanofluids was larger than that of water and increased with Al2O3 concentration, whereas the critical heat flux (CHF) enhancement of the nanofluids was 23-77% larger than that of water.