Microscale Study of Nucleation Process in Boiling of Low-Surface-Tension Liquids
Kiger, Kenneth T.
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A novel MEMS device has been developed to study some of the fundamental issues surrounding the physics of the nucleation process intrinsic to boiling heat transfer. The study was focused on boiling of FC-72 liquid. Over the past 50 years, scientists have developed several competing mechanistic models to predict the boiling heat transfer coefficient. Although the developed models are intended to predict the heat transfer coefficient at macroscales, their fundamental assumptions lie on complex microscale sub-processes that remain to be experimentally verified. Two main unresolved issues regarding these sub-processes are: 1) bubble growth dynamics and the relative importance of different mechanisms of heat transfer into the bubble and 2) vapor/liquid/surface thermal interactions and the bubble's role in heat transfer enhancement during the nucleation process. The developed device generates bubbles from an artificial nucleation site centered within a radially distributed temperature sensor array (with 22-40 microns spatial resolution) while the surface temperature data and images of the bubbles are recorded. The temperature data enabled numerical calculation of the surface heat flux. Using the test results, the microlayer contribution to the bubble growth was determined to increase from 11.6% to 22% when surface temperature was increased from 80 C to 97 C. It was determined that the transient conduction process occurs predominantly at the bubble/surface contact area, and before the bubble departure, contrary to what has been commonly assumed in classical boiling models. For the first time, the convection heat transfer outside the contact area (often known as microconvection) and transient conduction within the contact area were differentiated. The microconvection heat flux was found to be relatively close to that of the equivalent natural convection produced by the same geometry, but becomes significantly stronger than natural convection at higher surface temperatures. Test results under saturation conditions showed that when surface temperature is increased from 80 C to 97 C, the contribution of the different mechanisms of heat transfer within a circular area of diameter equal to that of the bubble changes from: 1) 28.8% to 16.3% for microlayer, 2) 45.3% to 32.1% for transient conduction, and 3) 25.8% to 51.6% for microconvection.