Pool Boiling

surface rtds

Boiling systems are characterized by the Critical Heat Flux (CHF), a level of applied heat flux beyond which the near-surface liquid is displaced by a thin vapor film leading to a sharp rise in the surface temperature and heater failure. The highest heat transfer coefficients are observed near CHF, favoring operation close to the phenomena subject to a safety margin. However, real surfaces change their surface characteristics over time due to oxidation and/or particle deposition and alter the CHF. The effect of such changes cannot be predicted by current mechanistic models for CHF. Work in our lab investigates ways to predict CHF from temperature time series. We are attempting to characterize features of deterministic chaos in the local temperature measurements leading up to CHF and apply it in real time to operate close to the CHF while avoiding burnout.

Another way to operate at higher heat fluxes is by increasing the CHF itself. 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 burnout is delayed 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 burnout.