Geometric-confinement suppression of flow-boiling instability using perforated wick: Part I CHF and conductance enhancement

Abstract

Bubbles formed in flow boiling merge and pickup momentum downstream and inhibit liquid supply to the heated surface. To investigate these competitions between the phases adjacent to the heated surface (hydrodynamic instabilities) and to establish leading-edge liquid supply tracks, the flow-boiling canopy wick (FBCW) was previously introduced. The FBCW is a periodic, 3-D porous (sintered, metallic powder) and perforated structure enabling capillary suction to optimally separate and direct the liquid and vapor paths adjacent to the heated surface, while the liquid track is formed between the periodic perforations.Here, addition of levees (geometric confinement) on the edge of the perforations ensures the hydrodynamic stability (by diverting the vapor stream and wall-stabilizing the liquid track), thus extending the liquid track between the levees. The irrigated canopy is connected to porous posts and monolayer-evaporator for liquid supply and the vapor generated over the monolayer wick escapes through the perforations. This thin evaporation wick results in large thermal conductance. The critical heat flux (CHF) or dryout limits (e.g., liquid-vapor hydrodynamic instability, capillary-viscous) of this leveed FBCW are examined using analytical and numerical (CFD, including the VOF technique) simulations. The CFD results for saturated water at 1 atm and for a horizontal, rectangular channel heated at the lower surface show the surface liquid track is stabilized by the levees and the flow-boiling CHF and thermal conductance are enhanced significantly beyond the plain surface, reaching the capillary-viscous dryout limit. The leading-edge liquid track irrigation of the wick allows for use of smaller liquid velocities. The results for the FC-72 fluid are also presented.