RESUMEN
We reveal and justify, both theoretically and experimentally, the existence of a unifying criterion of the boiling crisis. This criterion emerges from an instability in the near-wall interactions of bubbles, which can be described as a percolation process driven by three fundamental boiling parameters: nucleation site density, average bubble footprint radius and product of average bubble growth time and detachment frequency. Our analysis suggests that the boiling crisis occurs on a well-defined critical surface in the multidimensional space of these parameters. Our experiments confirm the existence of this unifying criterion for a wide variety of boiling surface geometries and textures, two boiling regimes (pool and flow boiling) and two fluids (water and liquid nitrogen). This criterion constitutes a simple mechanistic rule to predict the boiling crisis, also providing a guiding principle for designing boiling surfaces that would maximize the nucleate boiling performance.
RESUMEN
Surface engineering has been leveraged by researchers to enhance boiling heat transfer performance, with benefits ranging from improved thermal management to more efficient power generation. While engineered surfaces fabricated using cleanroom processes have shown promising boiling results, scalable methods for surface engineering are still limited despite most real-world industry-scale applications involving large boiling areas. In this work, we investigate the use of sandblasting as a scalable surface engineering technique for the enhancement of pool boiling heat transfer. We vary the size of an abrasive Al2O3 sandblasting medium (25, 50, 100, and 150 µm) and quantify its effects on silicon surface conditions and boiling characteristics. The surface morphology and capillary wicking performance are characterized by optical profilometry and capillary rise tests, respectively. Pool boiling results and surface characterization reveal that surface roughness and volumetric wicking rate increase with the abrasive size, which results in improvements in the critical heat flux and the heat transfer coefficient of up to 192.6 and 434.3% compared to a smooth silicon surface, respectively. The significant enhancement achieved with sandblasted surfaces indicates that sandblasting is a promising option for improving boiling performance in industry-scale applications.
RESUMEN
We run pool boiling experiments with a dielectric fluid (FC-72) on Earth and on board an ESA parabolic flight aircraft able to cancel the effects of gravity, testing both highly wetting microstructured surfaces and plain surfaces and applying an external electric field that creates gravity-mimicking body forces. Our results reveal that microstructured surfaces, known to enhance the critical heat flux on Earth, are also useful in microgravity. An enhancement of the microgravity critical heat flux on a plain surface can also be obtained using the electric field. However, the best boiling performance is achieved when these techniques are used together. The effects created by microstructured surfaces and electric fields are synergistic. They enhance the critical heat flux in microgravity conditions up to 257 kW/m2, which is even higher than the value measured on Earth on a plain surface (i.e., 168 kW/m2). These results demonstrate the potential of this synergistic approach toward very compact and efficient two-phase heat transfer systems for microgravity applications.
RESUMEN
We present the first experimental observations of scale-free behavior in the bubble footprint distribution during the boiling crisis of water, in pool and flow boiling conditions. We formulate a continuum percolation model that elucidates how the scale-free behavior emerges from the near-wall stochastic interaction of bubbles and provides a criterion to predict the boiling crisis. It also offers useful insights on how to engineer surfaces that enhance the critical heat flux limit.