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Multiphase Science and Technology
SJR: 0.183 SNIP: 0.483 CiteScore™: 0.5

ISSN Druckformat: 0276-1459
ISSN Online: 1943-6181

Multiphase Science and Technology

DOI: 10.1615/MultScienTechn.v18.i3.30
pages 251-276

HIGH HEAT FLUX BOILING AND BURNOUT AS MICROPHYSICAL PHENOMENA: MOUNTING EVIDENCE AND OPPORTUNITIES

Theo G. Theofanous
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA
True-Nam Dinh
Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA-93117, California, USA; and Royal Institute of Technology, Stockholm S-10044, Sweden USA

ABSTRAKT

In our recent works [Theofanous et al., 2002a-b], we have demonstrated that burnout in pool boiling is not hydrodynamically limited, at least not in the sense that has been perceived in the past. In this paper, we discuss the opportunities created by the new understanding of mechanisms that govern the boiling crisis. This understanding is built upon a scales separation phenomenon, referring to a vapor blanket separating the liquid film on the heater surface from the chaotic, churning flow in the two-phase pool. In essence, the scales separation suggests that mechanisms of boiling crisis should be sought within the micro-hydrodynamics of the evaporating liquid microlayer rather than in the pool thermal-hydraulics. Detail analysis of surface temperature patterns obtained by infrared imaging at high heat fluxes points to a nearly static picture of boiling heat transfer, with intense cooling at locations which were nucleation sites activated at lower heat fluxes. Furthermore, we show that control of the surface and coolant chemistry offers the potential to enhance resistance to burnout and achieve critical heat fluxes (CHF) exceeding those defined by the so-called hydrodynamic limit. Our more recent experiments show an improved resilience of the heater to burnout when a high-solubility salt or nanoparticles are added to the coolant (water). We explain the observed phenomenon through the increase in disjoining pressure at the meniscus contact line that promotes liquid spreading towards the dry area. It is noteworthy that the scales separation phenomenon provides a basis to suggest that mechanisms of enhancement to burnout in pool boiling are also active, even to a larger extent, in spray cooling and flow boiling.


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