Experimental and numerical studies on the forced and mixed convections of water in eccentric micro annular channels

Microchannels are frequently employed to enhance heat transfer in the compact and highly efficient heat transfer devices. However, studies on the smooth eccentric micro annular channel remain scarce. The flow and heat transfer performance of horizontal eccentric micro annular channels with a hydraulic diameter of 6.0 mm and an eccentricity of 0.5–1.5 mm subjected to constant heat flux are investigated using experimental and numerical methods. For laminar and turbulent flows, the laminar model and the realizable k-ε model with the Menter-Lechner near-wall treatment in Fluent are adopted, respectively. Results indicate that the critical Reynolds number for laminar-to-turbulent transition is 992. The friction factor, Nusselt number and entropy generation are significantly influenced by the operational and structure parameters. The Nusselt number under forced laminar flow conditions exhibits turbulent-like characteristics. The critical buoyancy number for forced-to-mixed convection transition decreases with increasing the eccentricity: for laminar flow, the values are 0.0107, 0.0082, and 0.0035 at the eccentricities of 0.5 mm, 1.0 mm, and 1.5 mm, respectively; for turbulent flow, the values are 0.0099, 0.0092, and 0.0026 at the same eccentricities. Compared to the forced convection, the buoyancy force enhances the Nusselt number and reduces the entropy generation, while the friction factor remains independent of the buoyancy force. Finally, the correlations for the friction factor and Nusselt number under both the laminar and turbulent flow regimes are proposed.