An Experimental And Theoretical Investigation On Meniscus Evaporation And Heat Transfer On Rib/Smooth Surface In An Electric Field

Heat transfer enhancement by electric fields is an active heat transfer enhancement technology. The enhancement is mainly caused by the electrical convection effect and the bubble dynamics under high voltage electric fields. Bubble growth and departure behaviors are important elements of bubble dynamics. The applied electric force causes bubble deformation and changes bubble dynamic behavior, which can directly affect the nucleate boiling heat transfer. The heat transfer enhancement mechanism of boiling heat transfer can be reflected by the dynamic characteristics of a bubble. In this paper, changes of microlayer thicknes, EHD effects on an evaporating meniscus, a single boiling bubble and boiling phenomenon on a rib surface under electric fields are investigated in details.Microlayer beneath Nucleation Bubble: The laser interferometric method and high-speed camera techniques were used to study dynamic characteristics of the microlayer beneath an ethanol vapor bubble during nucleation process.(i) In combination with other researchers’ experimental data correlation equations for gas-liquid-solid triple contact line, micro-contact angle, bubble root radius, and volume of the microlayer during the bubble nucleation process before and after dried out are abtained.(ii) Dimensionless bubble radius of dry area meet polynomial change rule. The micro-contact angle with respect to bubble growth time presents a linear relationship.(iii) Before appearance of a dry spot, the bubble root radius increases rapidly during initial bubble growth period, and the increases become smaller subsequently.(iv) In the beginning of an ethanol vapor bubble growth, the microlayer volume increases with time before a dry spot appears. Due to the evaporation of the microlayer, its volume begins to decrease with time after the appearance of a dry spot.EHD effects in an evaporating meniscus: The laser interferometric method was used to study EHD effects in the thin film region of an evaporating meniscus of DI water in direct contact of air and formed adjacent to a vertical heated wall.(i) When an electric field is applied, the meniscus climbs up on the vertical wall and then keeps on fluctuates up and down along the wall because of interactions of the electric force, gravitational force, surface tension and the inertial force.(ii) When an electric field is applied, the contact angle decreases and the film thickness become thinner. These phenomena enhance heat transfer from the wall to the meniscus.(iii) At a given heat flux, the evaporation mass flow rate from the meniscus to air and the meniscus length increase at a stronger electric field. At a given voltage, more liquid in the meniscus evaporates at higher heat flux, resulting in the decrease of the meniscus length at higher heat fluxes. These mean that an imposed electric field enhances mass transfer from the meniscus to the adjacent air.EHD effects on single bubble growth: we have studied EHD effects on a single R113 vapor bubble growth on and departure from a small ITO film heater in a non-uniform electric field.(i) With increasing imposed electric field strength, the bubble departure diameter(and volume) and departure frequency decreased, while the bubble growth time and bubble waiting time increased.(ii)The electric field decreases the latent heat transfer while electroconvection increases the sensible heat transfer. This results in a net increase in boiling heat transfer.(iii) At the beginning period of bubble growth, the dynamic contact angle decreases rapidly, but the decrease rate of contact angle is slow near the end of the bubble growth period. The dynamic contact angle at bubble departure increases with the increasing strength of imposed electric field.(iv) The circular triple line(i.e., the bubble base radius) moves outward rapidly along the radial direction at the beginning of bubble growth period and then increases slowly near the end of the bubble growth period. Subsequently, the circular triple line moves inward along the radial direction during the rewetting period. With the increase of electric field strength, the time of bubble growth period increases.EHD effects on a single bubble growth: We studied the boiling phenomena on rib surface at different electric field strengths and various heat fluxes. In pool boiling experiments from surface with different rib heights, we found that when rib heights were 0 mm, 0.2 mm and 0.5 mm, in low superheat region(△T<20℃), strengthening heat transfer effect was caused by the electroconvection; In medium superheat region(20℃<△T<30℃), due to the suppression effect of electric fields on bubbles the boiling heat transfer was deteriorated as wall temperature rising after applying an electric field; In high superheat region(△T>30℃), bubbles merge into vapor column or block bubbles, the electric field force made them broken, the liquid flew back to the heat transfer surface, thus enhanced boiling heat transfer. When the rib height was 2 mm, there were only two regions, electric field enhanced boiling heat transfer in low superheat region, while in medium and high superheat regions, electric field deteriorated boiling heat transfer. We simulated the electric field force acting on the bubbles which were placed at different positions of the rib surface. The simulation results were in good agreement with the experimental results. Finally, based on the force balance principle on a bubble, we established bubble departure diameter model and critical heat flux model on smooth surface and under an uniform electric field. The model predicted values and experimental data agreed well.