ENHANCEMENT OF EVAPORATION OF A LIQUID DROPLET USING EHD EFFECT - CRITERIA FOR INSTABILITY OF GAS-LIQUID INTERFACE UNDER ELECTRIC-FIELD

It was confirmed, in the preceding study, that an evaporation of a liq uid droplet on a heated surface was enhanced to a great extent by appl ying an electric field. Visual observation of the evaporation process indicated that small columns of the liquid were formed underneath the bottom of the droplet, causing direct contact between the liquid and t he solid surface. The direct contact underneath the bottom of the drop let was considered to be induced by the interfacial instability due to the electric field. In the present study, an experiment was carried o ut to clarify the mechanism that an electric field induced the instabi lity of a liquid surface. The static electric voltage was applied betw een the liquid surface and a horizontal planer electrode placed over t he surface. The applied voltage was raised gradually until the liquid surface became unstable. The threshold voltages were measured for diff erent distances between the electrode and the liquid surface and for d ifferent liquids. The test liquids used in the experiment were water, ethanol, refrigerant R113, carbon tetrachloride and cyclohexane. The v isual observation of the process leading to destabilization of the liq uid surfaces was performed using a high-speed video facility. Criteria for the onset of instability were derived analytically using a modifi ed Rayleigh-Taylor instability equation, finding that the theoretical results agreed very well with the experimental data. In addition, the temperature of the heat transfer surface above which the drop evaporat ion was enhanced was predicted using the result of the instability ana lysis.