Effect of electrohydrodynamics on thermal dissipation for high voltage thermosyphons

Abstract

Thermosyphon is a two-phase cooling approach that utilises coolant phase change to dissipate thermal energy. Selecting a suitable coolant is important to the thermosyphon design. In recent years, there has been considerable interest in selecting low global warming potential coolants to replace Hydrofluorocarbons (HFCs) and Perfluorocarbons (PFCs). Thermal properties of Hydrofluoroethers (HFE) and Fluorinated ketone (FK) are similar to HFCs, whilst they have low global warming potential (GWP). Consequently, they are considered as alternative fluids for the thermosyphon.
In order to improve overall high voltage plant efficiency, there is an increased use of thermosyphon technology to provide temperature control, replacing conventional pumps, fans and radiators. This two-phase cooling approach is not widely applied to high voltage devices due to the additional challenges that arise from electrical insulation and the introducing of electrohydrodynamics (EHD). The insulation design requires cooling fluid to have sufficient dielectric strength to avoid any electrical breakdown. Due to the presence of the electric field, electrohydrodynamics phenomena effect on thermal bubbles that further leads to the changing of the two-phase boiling behaviours and thermal dissipation. Hence, understanding of coolant dielectric behaviour and the EHD effect on boiling phenomena is integral to the development of an optimum thermosyphon design.
In this work, the AC and DC dielectric properties of HFE and FK have been characterised by dielectric spectroscopy, DC conductivity and AC breakdown measurements at different temperatures. Furthermore, in order to investigate the EHD effect on thermal bubble motion, a numerical approach has been developed by conducting force analysis upon a bubble within different electric fields, which is created by various electrode arrangements. Obtained results give predictions of the bubble trajectories for different applied fields. The pool boiling performances of both fluids are first outlined by showing the boiling curves. The effects of a DC field on the FK boiling heat transfer enhancement including bubble behaviour have been conducted under saturated temperatures.
The obtained data from this work is applicable to the design of HV thermosyphon for assessing both coolant dielectric behaviours and EHD heat transfer enhancements.

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ORCID for Paul Lewin: orcid.org/0000-0002-3299-2556

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