The impact of secondary flow intensity on heat transfer efficiency of the wire-to-plate electrohydrodynamics devices

The performance optimization of electrohydrodynamics (EHD) induced heat transfer enhancement has attracted much interest in recent decades. Although various EHD device designs have been proposed, coupling optimization based on comprehensive parameters, including Reynolds number, voltage, and electrode spacing, is still absent, and the overall heat transfer efficiency is rarely considered. In this study, the heat transfer efficiency of a wire-to-plate EHD device in a wide range of secondary flow intensity NEHD = 0.4–5 is investigated. Here, NEHD is a dimensionless parameter that integrates Reynolds number, voltage, electrode radius, etc. The average Nusselt number Nu rather than the enhancement rate ER is selected for optimization. It is demonstrated that NEHD = 2 is the optimal secondary flow intensity in both single-electrode and multiple-electrode configurations. The too-weak or too-strong secondary flow will lead to a decrease in the heat transfer efficiency. The underlying physics is revealed by the barrier effect and oversize vortex. An optimal electrode spacing of l > 0.014 m is proposed in the multiple-electrode configuration. A strong interaction between adjacent vortices will significantly decrease the heat transfer efficiency when l < 0.0014 m. A new design criterion for EHD devices is proposed: make sure that the secondary flow intensity NEHD = 2.0 and the electrode spacing l > 0.014 m, then arrange as many electrodes as possible in the channel.