EHD augmented convective boiling: Flow regimes and enhanced heat transfer

This work investigates the influence of electrohydrodynamics (EHD) on the flow and heat transfer during convective boiling of HFE7000. A unique tube-and-shell heat exchanger has been constructed with heated water flowing on the shell side and a saturated mixture of refrigerant flowing within the tube side. The heat exchanger is novel in that it allows full visual access to the flow in the inner tube while being both thermally and electrically conductive. This permits observation of the two-phase flow regimes, which is not possible with metallic test sections. In this work the influence of EHD on the flow regimes and subsequent overall heat transfer is investigated for fixed inlet refrigerant mass flux of 100 kg/m²-s, inlet quality of 3%, and wall superheat of approximately 11.5°C. For these conditions the applied voltage across a concentric inner electrode and the outer wall of the tube was varied between 0 kV and 10 kV at 60 Hz AC. In particular, this work focuses on quantifying the level of overall enhancement that is achievable with EHD for this heat exchanger. This is done in the context of the additional heat extracted by the working fluid in the heat exchanger compared with the field-free case and the additional power penalties required to do so. Heat transfer enhancements of up to 1.8 -fold were realized in this heat exchanger. Even so, there were hydraulic power increases as well as electrical power required to achieve the heat transfer enhancement. It was found that the electrical power was the dominant penalty and that an overall enhancement of 40 times more heat power extracted than input required was achieved. Finally, a proportional-integral-derivative (PID) control system has been utilized in conjunction with a high-voltage amplifier in order to accurately control the heat transfer rate of the heat exchanger. To our knowledge this is the first solid-state control system of this type for a two-phase heat exchanger.