Experimental investigations of effects of nanoparticle size on force convective heat transfer characteristics of Al2O3 - MWCNT hybrid nanofluids in transitional flow regime

Abstract

This study reports an experimental investigation of nanoparticle sizes effects on the heat transfer characteristics of hybrid nanofluids along the transitional flow regime. Four different particle sizes were used to prepare hybrid nanofluids of Al2O3 and MWCNT (i.e., 5 nm and 20 nm for Al2O3 and <7 nm and 30–50 nm for MWCNT nanoparticles). Three hybrid nanofluids with different particle combinations (i.e., Al2O3(5 nm) – MWCNT (<7 nm), Al2O3(20 nm) – MWCNT(<7 nm) and Al2O3(20 nm) – MWCNT (30–50 nm)) at a percentage weight composition (PWC) of 60:40 and 0.3 volume concentration were prepared. Results showed that particle sizes significantly affected the convective heat transfer characteristics of the nanofluids. Along the transition region, all three fluids were found to have different critical Reynold numbers 1152, 1172, and 1898 for Al2O3(20 nm) – MWCNT (<7 nm), Al2O3(5 nm) – MWCNT (<7 nm) and Al2O3(20 nm) – MWCNT (30–50) respectively. Al2O3(20 nm) – MWCNT (<7 nm) have shown better heat transfer performances. Its Nusselt number shows an enhancement of about 48.86 % along the transition region. And its coefficient of thermal performance (COP) was better than that of Al2O3(20 nm) – MWCNT (30–50 nm) and Al2O3(5 nm) – MWCNT (<7 nm) with 43.53 % and 21.89 %, respectively. While its friction factor and pressure drop were lower than that of Al2O3(5 nm) – MWCNT (<7 nm) nanofluid by 5.2 % and 12.78 %. It was concluded that for a 60:40 hybrid nanofluid of Al2O3 and MWCNT, particle sizes have influenced heat transfer characteristics and affected other flow characteristics along the transition regime.