Mechanisms of electrohydrodynamic (EHD) flow and heat transfer in horizontal convective boiling channels

Experimental and numerical investigations have been conducted to study the mechanisms involved in the electrohydrodynamic (EHD) induced flow and heat transfer augmentation of two-phase systems. The experimental study involved tube-side boiling heat transfer of the environmentally friendly HFC-134a in a single-pass, counter-flow heat exchanger 1.5 m in length, 12.7 mm O.D., 10.92 mm I.D., with a 3.18 mm rod electrode. The electrode position was varied from a concentric geometry to an eccentric geometry offset vertically from the centerline by ±2.73 mm. The applied voltage was 0 kV to 8 kV DC or 0 kV to 24 kV peak to peak AC (60 Hz and 6.6 kHz).

Experiments conducted for the eccentric geometry have provided evidence that through the establishment of the appropriate electric field distribution, a desired change of flow regime will occur to augment the heat transfer rates at significantly lower voltage levels and pressure drop penalties. These results were based on the interpretation of the finite element results of the electric field distribution for the arrangement under investigation. The experiment has shown that when the electrode was positioned eccentrically +2.73 mm from the centerline, a 160% enhancement in heat transfer coefficient was observed under the application of a 2 kV DC voltage while the pressure drop increase was only 1.2 fold.

Through the evaluation of the dimensionless criterion for EHD induced effects, electric field distribution analysis, EHD flow regime transition criterion and local and overall parametric analysis, the present investigation has shown that the developed electric body forces lead to a reduction in the thermal boundary layer thickness, increased convection, enhanced boiling dynamics and interfacial instabilities that can result in a phase redistribution. The consequence of this flow regime transition is the potential for significantly enhanced heat transfer rates at the wall, with only minimal increases in pressure drop. (Abstract shortened by UMI.)