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 were conducted for inlet qualities of 0% to 60%, mass fluxes from 100 kg/m²s to 500 kg/m's and heat fluxes of 10 kW/m² and 20 kW/m². Pressure drop, heat flux, quality and inlet void fraction were measured, the inlet and outlet flow regimes were observed and three different methods of estimating the heat transfer coefficients have been compared.

The EHD flow boiling experimental results show that the overall heat transfer coefficients can be increased up to 250% for an 8.0 kV DC applied voltage and up to approximately 300% for 24 kV (peak to peak) 60 Hz AC voltage in the concentric arrangement without a significant increase in pressure drop. Where the concentric electrode, the pressure drop across the heat exchanger increased 1.8 and 2.6 fold for the DC and 60 Hz AC maximum heat transfer enhancement tests, respectively. The EHD power consumption was less than 3 mW for the DC experiments and less than 0.2 W for the 60 Hz AC experiments. Locally, heat transfer coefficient enhancement in excess of 2000% has been observed. The results suggest that under the parameters investigated, augmentation increased with high electric fields, decreased with increased mass and heat fluxes and both increased and decreased with increasing quality, depending on the flow pattern. Finally, through the application of a low frequency (60 Hz) AC electric field, unlike any flow patten encountered in previous investigations, an entrained-droplet oscillatory flow regime developed. At the outlet of the test section the flow fluctuation occurred at approximately twice the frequency of the applied voltage as observed from this flow developed.

The onset threshold of EHD flow was studied through dimensional analysis. As is proposed, the influences of EHD induced flow on a system become significant when the Dielectric Rayleigh number is of the same order of magnitude as the square of the liquid Reynolds number, E_lε = Re_l². Flow visualization experiments have shown that when the proposed dimensionless criterion is satisfied, EHD body forces may have a strong influence on the flow pattern within the channel. The redistribution of the phases is observed at smaller mass flux levels and have been observed to induce a transition from stratified wavy flow to intermittent (plug or slug) or annular flow and in a transition from dispersed-annular flow to a symmetric annular flow. The various flow configurations clearly affect both the heat transfer and the pressure loss in the system.

The evaluation of the electric field plays an essential role in any attempt to determine the effect of the electrical forces on the liquid-vapour interface. Numerical calculations of the electric field distribution in two-phase flow with different vapour-liquid distributions and interfacial geometries in both concentric and eccentric electrode arrangements have been conducted both analytically and numerically using a finite element analysis. The results reveal qualitative evidence regarding the redistribution of phases and provide an estimate of the forces acting on the interface. Through the addition of the interfacial electric force to an established flow regime transition model (the Steiner map), a proposal is made to modify the flow mapping prediction method to account for the presence of the electrode (concentric) and electric field on the transition boundaries between stratified wavy and annular or intermittent flow. The flow regimes encountered in the convective boiling process have been reconstructed in an attempt to explain the pressure drop and heat transfer augmentation using the proposed EHD flow regime map and local surface temperature measurements.

In addition to the application of a DC voltage by previous investigators, high frequency AC voltages have been investigated to verify the dominant EHD forces, i.e. dielectrophoretic (E_lε) vs. electrophoretic (E_lσ). By applying a voltage frequency of 6.6 kHz, the flow pattern transition and heat transfer coefficient results coincide with those observed for the DC fields. As this frequency is expected to prevent the accumulation of a surface charge density on the liquid-vapour interface, the agreement between the DC and 6.6 kHz AC results suggests the electrophoretic component of the electric body force is not a significant factor in enhancing two-phase convective boiling.

Finally, 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.