The taylor wave configuration during boiling from a flat plate

Abstract

The Zuber pool-boiling hydrodynamic instability theory establishes that the critical heat flux qCHF,Z is based on rising surface vapor columns with liquid counterflow in a unit cell the of size of the critical wavelength λc determined by the Kelvin-Helmholtz and Rayleigh-Taylor interfacial instabilities. In the flow boiling, the forced velocity ul,o creates a leading-edge region complementing these phase-buoyant flows and breaking the periodicity of the Zuber unit cell. Here, with the direct numerical simulations (DNS), i.e., CFD-VOF-LES, of the Zuber unit cell for saturated water (one atm), the effect of ul,o on the hydrodynamic dryout qCHF,h is examined. The results show that the upstream liquid flow penetrates the boiling region, forming surface liquid tracks meandering between the vapor columns, and upon qCHF,h these tracks become unstable causing surface dryout. For ul,o as small as 5 cm/s, the axial liquid inertia deflects the vapor track and establishes the surface liquid track, raising qCHF,h noticeably over qCHF,Z and this is in good agreement with the available experiments. From the available theories, DNS-results and dimensional analysis, a relation is found between ul,o and λc establishing the wavelength modulation enhancement of the qCHF,h by increasing ul,o which decreases λc. In this new relation qCHF,h is proportional to ul,o1/6. The limit on this modulation enhancement is the capillarity limit qCHF,c= 3.3 MW/m2 compared to qCHF,Z= 1.1 MW/m2Next, the DNS results show that anisotropic arrangement of the vapor sites stabilizes the surface liquid track, thus increasing the qCHF,h. This geometric control of the vapor sites is achieved with a 3-D perforated porous coating, e.g., the flow-boiling canopy wick (FBCW). This geometric-modulated CHF enhancement is much larger than the wavelength-modulation enhancement and its vapor shear instability limit is overcome with the use of levees (geometric confinement) allowing for q> 10 MW/m2. However, the FBCW has an internal capillary-viscous limit qCHF,c−v (about 5.2 MW/m2 using sintered-powder wick) which favors small λc and currently is the bottleneck for achieving extremely large qCHF,h (which favors large λc).