Experimental and numerical study on the flow and heat transfer characteristic of nanofluid in the recirculation zone of backward-facing step microchannels

Highlights: • The application of nanofluids in the backward-facing step was studied. • The performance of nanofluids in laminar and transition flows were studied. • A combination of experiment and numerical simulation was used. • Step height has an effect on heat transfer and resistance. With the development of electronic devices towards high integration and high heat flux, the heat dissipation of electronic products has become one of the bottlenecks restricting the development of electronic devices. Nanofluids, as a flow medium to enhance heat transfer, have been widely concerned and their applications in the field of microelectronic heat transfer have broad prospects. The backward-facing step model is widely used in fields requiring heating or cooling because of its simple and easy processing, the sudden expansion of the flow cross-section and the separation, which shifts the stable flow state to the unstable flow state. In order to study the application characteristics of nanofluids in backward step structure channels, in this paper, the flow and heat transfer characteristics of TiO₂-water, SiO₂-water and Al₂O₃-water nanofluids with volume fractions of 0.1%, 0.2% and 0.3% were experimentally and numerically studied under laminar and transitional flow in the backward-facing step microchannel. The height of microchannel steps is 450 μm, 550 μm and 650 μm respectively. The upstream length is 80 mm and the downstream length is 180 mm. The results showed that: (1) nanofluids are more effective in reducing the temperature of the bottom surface downstream of the microchannel, the downstream bottom temperature tends to decrease; (2) the position of the reattachment point moves backward with the increase of Re number and step height, and the Nu number of the bottom of the step increases with the increase of Re as well as the peak value of Nu appears at the reattachment point and increase with step height increasing for laminar flow; (3) For transition flow, when Re is constant, the pressure drop of nanofluid increases with the increased concentration, the flow resistance coefficient increases with the increase of step height and decreases with the increase of Re. The volume fraction of nanofluid is an important factor affecting heat transfer performance. The larger the volume fraction is, the greater the heat transfer coefficient will be. At the same time, increasing the step height can enhance the heat transfer effect, but also increase the flow resistance.