EHD augmented heat transfer during upward bubbly flow boiling for low to medium frequency AC electric fields

The effect of a non-uniform AC electric field on the flow regime and heat transfer performance of upward flow boiling has been investigated experimentally using HFE-7000 as the working fluid in a tubular transparent heat exchanger. The non-uniform electric field was established by a concentric electrode connected to a high-voltage power supply and an Indium Tin Oxide (ITO) film on the tube outer wall. High-speed videography was employed to visualize the flow regimes. The experiments were performed for the initially bubbly flow regime and inlet pressure of 1.2 bars, subcooling of 2 K, and mass flux of 100 kg/m2s. Of key interest is the influence of low to medium AC frequency, which was varied between 1 Hz ≤ f ≤ 100 Hz for a square waveform of 10 kV peak-to-peak amplitude. One very striking observation was the strong electrophoretic interaction with the phases. At lower frequencies, the electrophoretic force interaction was clear from bubbles being attracted to the electrode, opposite to the direction of the dielectrophoretic force. For heat fluxes up to 10 kW/m2, the initially bubbly flow heat transfer coefficient was enhanced up to 3.5-fold and optimal frequency of f = 10 Hz. This research underpins enhancement and potential solid-state control of vertical two-phase heat exchangers, as well as two-phase heat exchangers in microgravity conditions, where similar flow regimes exist.