Flow Boiling Characteristics Over Microheaters In PDMS Microchannels

The development of MEMS technologies has greatly facilitated the study ofmicroscale phase change and heat transfer mechanisms. The rapid controlledbubble nucleation, growth and collapse in microchannels and confined spacescan generate powerful driving forces. These forces have been used to designthermal ink-jet printers, micro-bubble pumps, micro-valves and micro-mixerdevices. These mechanisms also play very important roles in biologicalmicro-analysis systems (μ-TAS) and lab-on-a-chip devices with many possibleapplications.This project experimentally and theoretically studied the flow boilingcharacteristics of FC-72in PDMS-glass microchannels with constant or pulsedheating. The bubble behavior on a smooth platinum microheater was alsostudied. The study showed that high wall superheats were required fornucleation of FC-72bubbles on the smooth Pt microheater for both constantheating and rapid pulsed heating. A bubble first nucleated at the center of theheater at the maximum temperature location with the nucleation temperaturewell predicted by the classical kinetics of nucleation theory. The boiling curveon the Pt microheater for constant heating had only a film boiling region andpart of the transition boiling region. Boiling could be maintained at walltemperatures less than the nucleation temperature after an initial bubble formedon the heater.The growing vapor bubble on the microheater periodically departed andflowed down in the microchannel to generate bubbly flow, slug flow and annularflow in the microchannel. A special bubble departure mode with two vaporcolumns was observed in the500μm wide microchannel. The departure positionand frequency increased as the heating increased, with higher subcoolingscausing a higher departure frequencies and departures closer to the heater.Boiling started a short time after the heating pulse started. The boiling thencontinued after the end of the heating pulse until the surface superheat was lessthan about50.0oC. The bubble growth characteristics with pulsed heating in the microchannel can be divided into typical single bubble growth, a thin vapor filmconnected to the bubble, a very small bubble jet and an elongated bubble at theflow rates when the bubble was shed at half the frequency of the heating pulse.The rapid initial bubble growth at nucleation produced a strong shock wavewhich could be useful for the design of micro-actuators. The initial bubblegrowth was inertia control growth for about2.0ms with the growth rate thenfollowing a damped oscillation process. The bubble growth mode thentransitioned to heat transfer controlled growth.A3-D numerical model was developed in Fluent using the VOF two-phasemodel to simulate the bubble dynamics in the microchannel for pulsed heating.The numerical results agree reasonably well with the observations, but do notaccurately describe some of the complex special phenomena; thus, moresophisticated models are needed to accurately simulate the bubble dynamics.