- The effect of high-voltage AC electric fields on two-phase flow regime redistribution for flowing refrigerant HFC-134a in a circular tube has been investigated. In particular, the effect of the frequency of an applied square wave between 0 and 8kV was studied for frequencies in the range 4Hz to 1kHz. The electric field was applied through a central electrode along the centerline of the tube. The flow regime redistributions under the applied electric field were visualized using a high-speed camera at frame rates up to 2000fps. The experiments were performed for a constant mass flux of 55kg/m2s and test-section outlet vapor quality of 30%. These flow conditions correspond to stratified flow with the liquid level below the electrode (without EHD). At the low-frequency range (4Hz⩽f⩽10Hz), the two-phase flow was able to respond to the repeated on/off action of the applied square-wave signals causing liquid extraction from the bottom stratum to the vapor phase. In the intermediate range of frequencies (10Hz\textlessf\textless100Hz), the time period of the applied signals was less than the time needed to complete the extraction cycle, and only liquid–vapor interactions below the electrode were observed. In the high-frequency range (100Hz⩽f), the flow regimes approach those for the DC case where the liquid–vapor interactions are less dynamic compared to the lower frequency range. This is mainly because the fluid medium cannot respond to the high frequency of the applied electric field.