The mechanisms of heat transfer during convective boiling under the influence of AC electric fields

In this investigation the local heat transfer coefficients along the top and bottom of a horizontal tube heat exchanger within which convective boiling of HFE7000 is occurring are measured. Of particular focus is the influence of Electrohydrodynamics (EHD) on the flow regimes and associated heat transfer. To achieve this, thermocouples are embedded into the walls of a transparent sapphire tube coated with a transparent yet electrically conductive layer of Indium Tin Oxide (ITO). The ITO layer allows visual access to the two phase flow while at the same time acts as an electrical heater element for the heat transfer and electrical ground for establishing the electric field. Tests have been performed for a fixed mass flux of G = 100 kg/m2 s, inlet quality of x = 3% and applied heat flux of q″=12.4kW/m2. An AC 60 Hz high voltage was applied across the fluid up to 8 kV in increments of 1 kV. The results show that, for the conditions tested, the application of EHD substantially increases the heat transfer coefficient at all measurement locations on the heat exchanger. Near the entrance, the top surface heat transfer enhancement reached over 7.2-fold and this decreased monotonically to 2.4-fold at the exit region. The bottom enhancement was more uniform along the heat exchanger ranging between approximately 3 and 4-fold at the highest applied voltage tested. The mechanisms responsible for the observed enhancement levels are discussed in the context of the flow regimes observed using high speed videography.