Effect of Frequency on Electrohydrodynamic Enhanced Tube-Side Condensation

The effect of high voltage AC electric fields on two-phase flow regime redistribution for flowing refrigerant HFC-134a has been investigated. In particular, the effect of the frequency of an applied square wave between 0 and 8 kV was studied. The flow visualization test section at the exit of the heat exchanger was made of quartz tube coated with an electrically grounded transparent conductive film of tin oxide. The flow regime transitions under the applied AC electric fields were recorded using a high speed camera at frames rates of 2000 frames per second for frequencies in the range of 4 Hz to 2 kHz. The experiments were performed at a mass flux of 55 kg/m2s, inlet and outlet qualities of 45% and 30% respectively, which correspond to stratified flow with the liquid level below the electrode (without EHD). At the low frequency range, both the heat transfer and pressure drop increased with an increase of frequency due to the periodic extraction of the liquid stratum. In the intermediate range of frequencies, the time period of the applied signals was less than the time needed to complete the extraction cycle, and therefore the heat transfer decreased while the pressure increased with an increase of frequency. In the high frequency range, the flow regimes approach those for the DC case where the effect of frequency is negligible on both heat transfer and pressure drop. This is mainly because the fluid medium cannot respond to the high frequency of the applied signals