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

•Research gap in hybrid nanofluid research is compiled and discussed.•Particle size effects of Al2O3–MWCNT hybrid nanofluids are experimentally investigated in the transition region.•Hybrid nanofluids of Al2O3–MWCNT, (60:40) were prepared using four different particle sizes, and their properties were evaluated.•Effects of using different particle sizes on the start and end of the transition regime are discussed.•Effects of particle size variation in the hybrid nanofluids on the thermal efficiency were discussed. 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