Hossam Sadek

Visualization of Flow Regime Transitions in Two-Phase Flow Under High Voltage Electric Fields

The effects of applying DC high voltage electric fields on two-phase flow regime transitions for flowing refrigerant HFC-134a were visualized using a high speed camera. The viewing test section was made of 10 mm inner diameter quartz tube with a 3.18 mm diameter charged electrode placed along the center of the tube. The quartz tube was coated with an electrically grounded transparent conductive film of Tin Oxide. The experiments were performed for mass flux (55 kg/m2 s < G < 263 kg/m2 s), quality (20% < x < 80%) and applied voltage (0 kV < V < 8 kV). The flow regime transitions depend on the flow regime prior to applying the EHD. For stratified flow, EHD increases the interfacial instabilities and causes liquid extraction to the upper section of the tube. When the flow regime is initially annular flow, EHD increases the uniformity of the annular film by extracting liquid from the thicker liquid regions into the vapor core.Copyright

Horizontal Tube Side Convective Condensation Under an Applied DC Voltage

An experimental study was performed to investigate the tube side convective condensation under an applied DC high voltage. Experiments were performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed along the centre of the tube. A 8 kV DC voltage was applied across the annular gap between the central electrode and the pipe wall. The experiments were performed for mass flux in the range 45 to 156 kg/m2 s and average quality of 45 percent. The application of the high voltage electric field results in electric body forces at the liquid-vapor interface which extracts the liquid from the bottom stratum towards the vapour core. The application of the 8 KV DC voltage increased heat transfer and pressure drop by factor 3 and 4.5 respectively at the lowest mass flux of 45 kg/m2 s. The results show that liquid extraction from the bottom liquid stratum changed the mode of heat transfer at the bottom of the tube from convective condensation into film condensation.Copyright

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