Convective boiling with electrohydrodynamic enhancement: The influence of inlet quality

This work investigates the influence of alternating current electric fields on the flow patterns, associated heat transfer, and power penalty during convective boiling of HFE7000. A single-pass counter-flow heat exchanger is employed, whereby heated water flowing in the shell side transfers heat to the two-phase HFE7000 fluid flowing within the tube side. In a novel design feature, optical transparency is achieved by using a sapphire central tube surrounded by a Perspex water jacket containing the heated water. A stainless steel rod running concentrically through the tube acts as an electrode while the outer surface of the sapphire tube is coated with a thin layer of indium tin oxide forming an electrically conductive and optically transparent ground to establish an electric field across the working fluid. This unique test setup facilitates visualization of the flow patterns caused by the electrohydrodynamic (EHD) forces and allows high-speed videography of the HFE7000 while boiling. Tests were performed at a low mass flux (100 kg/m2s) and fixed average heat flux (12 kW/m2)for inlet qualities of 2%, 15%, 30%, and 45% and applied voltages of V = 0, 4, and 8 kV. The results show that the average heat transfer coefficient improves with applied voltage over the entire range of qualities tested. However, as inlet quality increases the heat transfer enhancement tends to decrease, as does the electrical power required for EHD (EHD penalty). Conversely, pumping losses were seen to increase as inlet quality increases.