Competing Effects of Dielectrophoresis and Buoyancy on Nucleate Boiling and an Analogy With Variable Gravity Boiling Results

An experiment was performed that produced a controlled electrical body force (dielectrophoretic force or DEP force) over the length of a horizontal platinum wire heater during boiling. The DEP force was generated such that it either aided or opposed terrestrial gravity or acted nearly alone in microgravity. The net effect can be thought of as simply either “reducing,” or “increasing” buoyancy. To account for this change in force an “effective gravity ratio,” g′_{(b, e)} was defined that represents the ratio of the total DEP and buoyancy body forces to the constant terrestrial-gravity buoyancy force. Based on our own experimentation and on the published literature, it was argued that the nucleate boiling will be enhanced if the effective gravity acts to hold the vapor bubbles near the heater surface, while at the same time permitting access of the liquid to the surface in order to prevent dryout. However, a large electroconvection effect can dominate and reverse the trend. The nonboiling portion of the heat transfer coefficient was shown to be significant with an applied electric field, especially at high subcoolings. It was found that for 1 < g′_{(b, e)} < 3 a quarter power dependence appears to be a reasonable engineering approximation for the increase in critical heat flux with effective gravity.