University of Minnesota
University of Minnesota: Department of Mechanical Engineering


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Roughness Effects on CHF in Pool Boiling

The Critical Heat Flux (CHF) is experimentally observed limit to the nucleate boiling heat flux, beyond which a vapour film blankets the heater surface, and thermal runaway occurs. Our experiments consistently refute the principles underlying the hydrodynamic theory of CHF occurrence, and strongly suggest that nanoscale roughness significantly enhances CHF, presumably through disjoining pressure effects. We also have demonstrated that nucleation site density alone does not increase CHF. These data lead towards a picture of dryout, in which thermal runaway is controlled by the ability of a wetting film to rewet a dry/hot spot on the surface under the combined action capillary suction, disjoining pressure, vapor shear, and local variations in evaporative mass flux. Our aim is to use precise control of the geometry of micro/nano-porous surfaces along with external forcing as a means of maximizing film speed, suppressing instabilities, and delaying dryout.

Design of Microstructured Wicks for Heat Pipes

Heat-pipe based thermal spreaders integrated into silicon and utilizing evaporative cooling are an attractive option to regulate device temperature. Designing high-performance heat spreaders requires an understanding of evaporation, meniscus motion, and fluid flow in the microporous medium that constitutes the heat pipe wick. Previous work demonstrated that hierarchical wick geometries can simultaneously extend the heat transport capacity and conductance. Recent experiments have identified several phenomena associated with high heat transport near the onset of dryout, such as periodic droplet ejection and bursting, followed by recession of the contact line, rewetting of dry spots, and renewed bursting. Understanding the surface morphology that leads to and enhances these phenomena is critical to enhancing wick designs.

Electrokinetic Flow with Phase Change

One of the challenges in realizing the potential of evaporation is to sustain a large area of thin film (thickness < 5 µm), and replenishing it with liquid as it evaporates, while suppressing long-wave instabilities. This requires an understanding of contact line motion on micro-textured surfaces in the presence of strong heating, disjoining pressure, and forcing. Electric fields have been used to propel liquid droplets through microfluidic devices; however the potential of electrowetting in phase change heat transfer remains unexplored. Our group is currently examining the feasibility of using electrowetting to enhance evaporation heat transfer from microstructured surfaces, through microfabrication of appropriate electrode geometries. In particular, a preliminary study will involve using a shallow (depth ~3 µm) microchannel array with sidewalls that serve as electrodes, and stretch the liquid in the streamwise direction.

Linear Stability Analysis of Globally Unstable Flows

Linear stability theory has been a surprisingly robust predictor of the absolute/convective instability of a flow with significant nonlinearity, such as bluff body wakes and low-density jets found in plasma torches. While the stability of a low-density jet has been assumed to be due to an inviscid mechanism, recent experimental work has shown that viscous effects are important. Current work examines the role of spatially varying viscosity and indicates that low-viscosity jets issuing into quiescent media of higher viscosity may be absolutely unstable. Experimental work is underway to characterize the onset of global modes in such flow systems. Linear theory is also used to examine the onset of absolute instability in core-annular flow in a pipe with counterflowing gas and liquid streams, to evaluate the possibility of enhanced mixing and use of such configurations for high performance combustion nozzles.