Abstract:
Nanofluids show great potential for conventional heat transfer fluids that could benefit industries and
save huge costs. Nanofluids are well known for enormously enhancing the thermal conductivity of a
base fluid. However, there is a lack of consensus in experimental and numerical results for the
natural convection heat transfer of nanofluids in a closed cavity. In this study, the cavity flow natural
convection of zinc oxide (ZnO)-water is investigated experimentally. The ZnO nanoparticles have an
average size of 20 nm and the nanofluids were prepared with different volume fractions of 0.09, 0.18,
0.36, 0.5 and 1 volume percentage (vol.%) (0.5, 1, 2, 3 and 5.67 weight percentage). The stability of
the ZnO nanofluid is verified using a spectrophotometer and zeta potential measurement at various
temperatures and concentrations of the nanofluids. Zeta potential values are measured within the
stable range, and no sedimentation of nanoparticles is indicated within 24 hours. The viscosity of
ZnO-water nanofluid is also measured experimentally, which is 20% higher than the use of the
traditional Einstein viscosity model at 1 vol.%. The heat transfer efficiency of natural convection of
ZnO-water nanofluid is examined experimentally in a closed square cavity at a Rayleigh number (Ra)
range between 7.9E+7 and 8.9E+8. The cavity is heated vertically from one vertical wall and cooled
from the opposite wall. Other sides, including top and bottom walls, are insulated to be adiabatic.
Consequently, the suspension of ZnO nanoparticles in water does not enhance the natural convection
heat transfer coefficient. The systematic deterioration of the natural convection heat transfer
coefficient is observed as increasing in the concentration of nanoparticles.
