Upward flow boiling of HFE-7000 in high frequency AC electric fields

Abstract

An experimental investigation of upward flow boiling of HFE-7000 has been conducted in a vertical tube under a high frequency alternating current (AC) electric field with the aid of high speed photography. Importantly, the test section was coated with 600 nm transparent Indium Tin Oxide (ITO) layer allowing visual access to the boiling dynamics within the tube while simultaneously functioning as an electrical heater element for the heat transfer and electrode for establishing the electric field. Experiments were performed for a constant mass flux of G=100 kg/m2s, inlet subcooling of 2K, applied heat flux ranging between 5 to 50 kW/m2, applied voltages from 0 to 10 kVp-p AC and frequencies of 100 Hz, 1000 Hz and 10,000 Hz. For the 100 Hz applied frequency, the evidence supports the hypothesis that there are strong electrophoretic forces augmenting the heat and mass transfer phenomenon, with better than two-fold enhancement in the heat transfer at low heat fluxes. This tends to diminish with applied heat flux. Interestingly, the EHD enhancement increases for all heat flux levels when the frequency is increased to 1000 Hz, with an observed change to polarization-force dominated EHD augmentation. Increasing further to 10,000 Hz results in diminished enhancement, resulting in a peak enhancement at 1000 Hz.

Keywords

• Upward flow boiling
• Electrohydrodynamics (EHD), Flow regimes, Heat transfer enhancement, Flow visualization, Two-phase flow