The effect of particle size on the effective thermal conductivity of Al2O3-water nanofluids

A steady-state method was used to evaluate the effective thermal conductivity of Al2O3/distilled water nanofluids with nanoparticle diameters of 36 and 47 nm. Tests were conducted over a temperature range of 27-37 degrees C for volume fractions ranging from 0.5% to 6.0%. The thermal conductivity enhancement of the two nanofluids demonstrated a nonlinear relationship with respect to temperature, volume fraction, and nanoparticle size, with increases in the volume fraction, temperature, and particle size all resulting in an increase in the measured enhancement. The most significant finding was the effect that variations in particle size had on the effective thermal conductivity of the Al2O3/distilled water nanofluids. The largest enhancement difference observed occurred at a temperature of approximately 32 degrees C and at a volume fraction of between 2% and 4%. The experimental results exhibited a peak in the enhancement factor in this range of volume fractions for the temperature range evaluated, which implies that an optimal size exists for different nanoparticle and base fluid combinations. This phenomenon can be neither predicted nor explained using the theoretical models currently available in the literature.(c) 2007 American Institute of Physics.