Enhancement of heat transfer and mixing with two side-by-side freely rotatable cylinders in microchannel

Abstract

The flow disturbance technique is an effective method to enhance the heat and mass transfer efficiency in microchannels. This paper numerically investigates the influence of two side-by-side freely rotatable square cylinders on microchannel flow, heat transfer, and mass transfer under a Reynolds number (Re) range of 5‒300. Results indicate that the cylinders show five different vortex-induced rotation modes, namely the rotation mode (Re < ∼10), oscillation mode (∼10 < Re < ∼70), stationary mode (∼70 < Re < ∼90), random mode (∼90 < Re < ∼250), and reversal rotation mode (Re > ∼250). The vortices generated by rotations of the cylinders disturb the flow field significantly, thus promoting heat and mass transfer in the microchannel. As Re increases, the average Nusselt number (Nu) of the channel first rises slowly then sharply, specifically, from 3.50 when Re = 5 to 8.35 when Re = 75, and then to 42.45 when Re = 300. Under a small Re, the rotating cylinders only affect the local heat transfer near them, while only when Re > 100 can the cylinders significantly promote the heat transfer in a relatively long distance in the microchannel. This behavior should be attributed to the insufficient vortex disturbance under small Reynolds numbers. Mass transfer is scrutinized by releasing fluorescein at the upper half of the channel entrance, and mixing efficiency (η) is introduced to characterize the mixing situation. We show that when Re = 100, the substances in the channel are almost completely mixed (η = 0.98) compared with η = 0.84 for stationary cylinders; beside, as Re increases, the position where the mixture reaches a steady-state gradually moves forward in the channel, and when R > 100, the distribution of η along the channel seldom changes with Re. Considering fluid pressure loss, heat transfer enhancement index φ and mass transfer enhancement index ϕ are introduced to compare heat and mass transfer capability between microchannels set with rotatable cylinders and stationary cylinders, and phase diagrams relative to Re and x* are drawn. Analysis reveals that the freely rotatable cylinders better improve the heat and mass transfer with a maximum φ of 1.6 at Re ≈ 25, x* ≈ 12 or Re ≈ 100, and maximum ϕ of 1400 at Re ≈ 50, x* ≈ 10‒25.