Control and Enhancement of Saturated Flow-Boiling Thermal Conductance and Crisis with Porous Metasurfaces: Hydrodynamic-Stability and Capillary-Viscous Limit Theories, Direct Simulations, and Experimental Verification

Abstract

Flow-boiling heat transfer is used in high heat flux and thermal conductance applications and is bounded by the boiling crisis, i.e., the critical heat flux (CHF). This is when the vapor generation in the leading-edge region of the forced liquid flow over the heated surface intensifies, interrupting the liquid supply to the surface (dryout) resulting in rapid surface temperature rise. The thermal-hydraulic triggering this phenomenon and its control has been at the center of boiling heat transfer research. In this study, the flow-boiling crisis mechanism is investigated in the near-field region, as an extension to the Zuber pool-boiling hydrodynamic instability theory. This is the coupled interfacial Kelvin-Helmholtz and Rayleigh-Taylor instabilities of downward liquid flow countered by rising vapor columns arranged in a 2-D periodic unit cell based on the critical wavelength. We also analyze the far-field region, tracking the liquid-vapor turbulent two-phase flow using CFD with the large eddy simulations and down to the Kolmogorov scale. The control of the crisis is achieved with porous metasurfaces (periodic, 3-D multiscale capillary structures). The theoretical analyses are accompanied by direct numerical simulations and predict the flow-boiling crisis and lead to introduction of two CHF enhancement regimes, the wavelength modulation and the geometric modulation regimes, and these predictions are verified experimentally for saturated water flow boiling.

In the Zuber theory, the critical wavelength marks the onset of dryout and depends on the fluid properties. This wavelength was previously controlled using modulated porous surface coatings, increasing the CHF from the Zuber limit to the capillary-length limit. Here, this wavelength modulation is achieved with the liquid velocity and the relation among the liquid velocity, the critical wavelength and the CHF is developed and verified with the existing experimental data for plain surfaces. The leading-edge liquid track shear instability theory of flow-boiling crisis is introduced and verified by the direct numerical simulations.

To achieve yet higher CHF, the geometry of the Zuber unit cell is modified for flow boiling using aligned, rectangular and leveed vapor sites designed to reduce shearing of the leading-edge surface liquid track. This geometric modulation regime results in a very large hydrodynamic CHF. This unit-cell based surface is realized experimentally with a porous metasurface, i.e., the flow-boiling canopy wick (FBCW). The FBCW is a periodic, 3-D porous and perforated structure enabling capillary separation of the liquid and vapor and their direction along low resistance paths. The porous canopy separates the liquid channel flow from the vapor space beneath it and the canopy is connected to porous posts delivering liquid to a thin evaporator wick. The vapor generated on the meniscus flows through the vapor space between the posts and escapes into the channel through the canopy perforations (vapor flow sites) interacting with the flowing liquid. The FBCW results in large hydrodynamic CHF and thermal conductance, however, the FBCW internal wick flow resistance, the capillary-viscous limit, may become the bottleneck. This capillary-viscous CHF is predicted by point-wise (direct simulations), volume-averaged (using effective properties), and resistance-network treatments.

A collaborative-work fabrication (multimold, oven sintered copper particles), optimization and testing of the FBCW with saturated water at one atm, verifies the wavelength and geometric modulation regimes. The maximum measured CHF of 5.1 MW/m^2 is within the geometric modulation regime and controlled by the capillary-viscous limit. The maximum measured thermal conductance is 0.36 MW/m^2-K. Both record values.