M.S. Thesis Defense by Joerg H. Stromberger

(Dr. Said Abdel-Khalik, advisor)

"An Experimental Investigation of Electrohydrodynamic (EHD) Enhancement of Boiling Heat Transfer"

The technology called electrohydrodynamic (EHD) enhancement utilizes the electric field to influence fluid flows and related processes, such as bubble growth and departure. Both, electric forces on free charges within a weakly conducting fluid and dielectric forces on a polarized fluid may be involved. While the interaction of an electric field with a fluid is difficult to predict or even interpret, the dielectric force should be most effective when both, the electrical permittivity of the fluid and the electric field vary strongly. In particular, a vapor bubble, which is a lump of lower permittivity in a higher permittivity medium, should be driven toward regions of weaker electric field gradient by the so-called dielectrophoretic force. This effect is known to enhance nucleate boiling; however, the process has not been thoroughly studied in practical designs. This thesis reports on an investigation of EHD enhancement in a realistic heat exchanger design, the tube-in-shell flooded evaporator.

In the experimental investigation, quantitative data defining the heat exchanger performance was collected and simultaneous high speed video images were recorded. The quantitative data revealed significant heat transfer enhancement particularly at lower heat rates when the fluid is not boiling vigorously. In addition, the high speed images confirm a simple descriptive model for boiling enhancement.