http://www.tara.tcd.ie/bitstream/handle/2262/53273/Electric%20field%20effects%20during%20nucleate%20boiling%20from%20an%20artificial%20nucleation%20site.pdf;sequence=1

Electric field effects during nucleate boiling from an artificial nucleation site

The paper describes results from an experimental and theoretical study of the effect of an electric field on nucleate boiling and the critical heat flux (CHF) in pool boiling of R123 at atmospheric pressure on a horizontal wall with a smooth surface. Two designs of electrode (parallel rods and wire mesh) were used. The experimental data exhibit some differences from the data obtained by other researchers in similar experiments on a wall with a different surface finish and with a slightly different design of wire mesh electrode. The hydrodynamic model for EHD enhancement of CHF cannot reconcile the differences. A theoretical model has been developed for the growth of a single vapour bubble on a superheated wall in an electric field, leading to a numerical simulation based on the level-set method. The model includes matching of sub-models for the micro- and macro-regions, conduction in the wall, distortion of the electric field by the bubble, the temperature dependence of electrical properties and free-charge generation. In the present form of the model, some of these effects are realised in an approximate form. The capability to investigate dry-spot formation and wall temperature changes that might lead to CHF has been demonstrated.

/pdf/a-study-of-nucleate-boiling-and-critical-heat-flux-with-ehd-52oh7jt8c6.pdf

A study of nucleate boiling and critical heat flux with EHD enhancement

This article reviews and analyzes recent advancements on boiling and condensation heat transfer enhancement via the use of electric fields Historically, the majority of studies on phase change heat transfer enhancement have relied on passive approaches like surface engineering Electric fields provide distinct options to enhance and control the nano/micro/mesoscale thermofluidic phenomena associated with boiling and condensation This work focuses on the influence of electric fields on electrically conducting liquids like water and certain organic solvents After a brief review of past work on electric field–based heat transfer enhancement using electrically insulating liquids, we summarize and discuss recent studies involving electrically conducting liquids It is seen that electric fields can offer disruptive advancements and benefits in the control and enhancement of boiling and condensation Perspectives, future research directions, and applications of these novel concepts are also discussed

Electric Field–Based Control and Enhancement of Boiling and Condensation

EHD behavior of nitrogen bubbles in DC electric fields

We perform a quantitative characterization of a microbubble injector in conditions relevant to microgravity. The injector pregenerates a slug flow by using a capillary T-junction, whose operation is robust to changes in gravity level. We address questions regarding the performance under different injection conditions. In particular we focus on the variation of both gas and liquid flow rates. The injection performance is characterized by measuring bubble injection frequency and bubble sizes. We obtain two distinct working regimes of the injector and identify the optimal performance as the crossover region between them.

/pdf/experimental-study-of-a-microchannel-bubble-injector-for-zmivbm53xf.pdf

Experimental study of a microchannel bubble injector for microgravity applications

Visualization of bubble detachment and coalescence under the influence of a nonuniform electric field

Electric field effect on bubble detachment in reduced gravity environment

The effects of a nonuniform electric field on vapor bubble detachment and heat transfer in subcooled pool boiling from a microheater array are investigated. The heater array faced downward to simulate a -1 g gravity condition and to eliminate the dominant masking effect of the buoyancy force. Experiments were conducted at different subcooling levels for various wall temperatures and electric field magnitudes. A dielectric fluid, FC-72, was used as the working fluid at ambient pressure. The array of 3 x 3 independently controlled microheaters was maintained at constant temperature and the rate of heat transfer from each heater was measured. Bubble images were recorded using a high-speed camera. The electric field was applied between the horizontal downward-facing microheater array, which was grounded, and a spherical, off-axis electrode beneath it. Boiling heat transfer results with and without the electric field are presented in this study. In the absence of the nonuniform electric field, compared to the same bulk fluid temperature and wall superheat settings in the +1 g situation, a much larger primary bubble was formed on the heater array, due to the coalescence of the secondary bubbles that nucleated on the heater array. The vapor bubble remained on the heater array surface and no bubble detachment was observed. With the nonuniform electric field applied, bubbles were lifted and sheared off from the heater array surface. The electric field was able to break up the primary bubble into several smaller bubbles--considerably greater heat transfer enhancement was measured than under similar conditions in +1 g.

Heat transfer and bubble detachment in subcooled pool boiling from a downward-facing microheater array in a nonuniform electric field.

The effects of a nonuniform electric field on vapor bubble detachment and heat transfer rate were studied in pool boiling at different subcooled conditions for various wall temperatures. Dielectric fluid (FC-72) was used as the working fluid at 1 atm at earth gravity with two extreme gas concentration levels. An array of 3×3 independently controlled microheaters each 0.7×0.7 mm2 in size were maintained at constant temperature using electronic feedback loops, enabling the heat transfer from each heater to be determined. An electric field was applied between the horizontal upward facing microheater array, which was grounded, and a spherical, off-axis top electrode. Boiling heat transfer results with and without the electric field are presented in this study. Without the electric field, a single large “primary” bubble was observed to form due to the coalescence of the individual “satellite” bubbles which nucleated directly from each single heater array. Before its detachment, a dry spot formed underneath this primary bubble resulted in a reduction in heat transfer. With the electric field applied, three or more small “secondary” bubbles that nucleated and grew more rapidly and detached more frequently were observed. Due to the nonuniformity of the electric field, bubbles moved away from the top electrode (into the weaker region of the electric field) during their development. Higher overall heat transfer rates were measured from the heater array. In addition, the bubble behavior showed agreement with our previous investigation of injecting air bubbles into a stagnant, isothermal liquid through orifices.Copyright © 2007 by ASME

Heat Transfer and Bubble Detachment in Subcooled Pool Boiling From a Microheater Array Under the Effect of a Nonuniform Electric Field

The formation and detachment of air bubbles injected into a stagnant, isothermal liquid through an orifice in a plane surface were studied at various reduced gravity levels under the influence of electric fields. Reduced gravity experiments were carried out in NASA’s reduced gravity aircraft, the KC-135. The objective of this study is to investigate and explore the possibility of substituting the buoyancy force with the electric field force by applying external electric fields in two phase flow and boiling. Two top electrode geometries were used: flat and off-axis spherical, to generate a uniform and non-uniform electric field, respectively. The bubble life cycle was first experimentally visualized, and then the characteristic dimensions of the bubble and its volume at the moment of detachment were measured using digital image processing. It is shown that both the level of gravity and the magnitude of the electric field significantly affect the bubble formation and detachment. Under microgravity with uniform electric fields, an increasing electric potential from U=0 to 20 kV decreases the detachment volume Vd by 51%. In a nonuniform electric field with U=20 kV, the detachment volume decreases by 80% as the gravity increased from 0.006g to 0.374g. The volume flow rates in variable gravity and terrestrial conditions are 2.57x10-7 m3/s and 3.33x10-7 m3/s, respectively. Acknowledgments: This research was supported by a NASA research grant. The experiments in the KC-135 aircraft were carried out by Cila Herman, Gorkem Suner, Steven Marra, and Ed Scheinerman. The support by the KC-135 crew and NASA Glenn Research Center was invaluable for the successful completion of the experiments. Variable gravity, nonuniform electric field U=20kV Microgravity, uniform electric field Microgravity, U=0V Electric field distribution

Zan Liu

Papers

Visualization of bubble detachment and coalescence under the influence of a nonuniform electric field

Electric field effect on bubble detachment in reduced gravity environment

Heat transfer and bubble detachment in subcooled pool boiling from a downward-facing microheater array in a nonuniform electric field.

Heat Transfer and Bubble Detachment in Subcooled Pool Boiling From a Microheater Array Under the Effect of a Nonuniform Electric Field

Bubble Formation and Detachment in Variable Gravity Environment Under the Influence of Electric Fields