Enhancement mechanism of convective heat transfer via nanofluid:An analysis by means of synchrotron radiation imaging

•The enhancement mechanism of heat transfer for the nanofluid consisted of 50 wt% ethylene glycol (EG) aqueous solution and SiO2 nanoparticles were investigated at 300.15 K.•The enhancement effect of heat transfer depends on the diameter of the original particles.•The heat transfer efficiency of the nanofluid is by up to 10% higher than that of EG solution at the same fluid velocity.•The particle concentration of the nanofluids near the wall (ranging within about 20 μm) is lower than that in the center of flow channel.•The thickness of turbulent velocity boundary layer for the nanofluid is thinner than that of EG solution.