Electrohydrodynamic augmentation of a reflux thermosyphon

In this study, a reflux thermosyphon using HFE-7000 as the working fluid is fitted with a concentric electrode in order to investigate the influence of electrostatic forces on the evaporator thermal performance. Importantly, the thermosyphon is constructed from an ITO coated sapphire tube resulting in a thermosyphon enclosure with high thermal conductivity that is electrically conductive and transparent. This allows visualisation of the boiling dynamics within the thermosyphon for both the scenarios where there is no electric field and when electric fields of increasing intensity and frequency are imposed. Results are obtained for two heat fluxes, 8 kW/m2 and 15 kW/m2, for applied voltages ranging from 0 kV to 8 kV with AC frequencies between 20 Hz and 100 Hz. Fill ratios for the evaporator of 50% and 100% are considered. The main results show that the evaporator thermal resistance is fairly insensitive to applied electric field strength until it is strong enough to overcome local gravitational forces, and this occurs in the region of 3-4 kV. Subsequent to this the boiling dynamics and resulting heat transfer performance is notably augmented by the Electrohydrodynamic (EHD) forces. For low heat fluxes the evaporator heat transfer coefficient is enhanced up to ∼40% for the highest applied voltage. However, for the higher heat flux, the EHD forces result is a substantial deterioration of the heat transfer coefficient, being up to ∼70%. The mechanisms responsible for the heat transfer augmentation are discussed in context of the flow visualisation obtained using high speed videography. The opportunity for using an EHD thermosyphon as a thermal potentiometer is also discussed.