Flow-boiling canopy wick capillary-viscous limit

Abstract

It was predicted that the perforated, leveed flow-boiling canopy wick (FBCW) can significantly increase the hydrodynamic instability limit of the flow boiling [1,2]. The perforations represent and modulate the vapor columns in the boiling crises hydrodynamic stability theory. The lower capillary-viscous limit, governed by the evaporator wick maximum capillary pressure and the viscous drag within the three-dimensional composite wick, may control the actual dryout performance. Saturated-water flow-boiling (one atm) experiment results are presented employing an FBCW, created from sintered-copper particles, with and without the hydrodynamic-stabilizing levees. The bimodal bilayer evaporator wick allows for sufficiently large maximum capillary pressure and permeability, compared to the monolayer wick. The experimental results indicate that the levees can increase the critical heat flux (CHF) over that without levees, whereas the existing maximum capillary pressure controls disallow achieving this larger hydrodynamic heat flux limit. This capillary-viscous dryout limit is predicted by employing the minimum surface energy principles, network models, and computational fluid dynamics (CFD), in good agreement with experiments. By employing both CFD and flow visualizations, good agreement is also established between the predicted and observed two-phase hydrodynamics above the canopy wick. They indicate that although the local dryout commences downstream; however, dryout is experimentally observed when a critical fraction of the liquid supply tracks dry downstream. The measured large thermal conductance (heat transfer coefficient) is beneficial for the predictions, verifying the vapor-occupied space between the canopy and evaporator wicks.