The boiling crisis phenomenon - Part II : dryout dynamics and burnout

• This is Part II of a two-part paper on the boiling crisis phenomenon. Here we report on burnout experiments conducted on fresh and aged heaters in pool boiling. Critical heat fluxes (CHFs) were found to vary from 50% to 140% of the hydrodynamic limit, previously thought to exist at well-wetting conditions. The burnout events were captured in action (for the first time), using highspeed, high-resolution infrared thermometry. Based on these observations and in conjunction with the levels of CHF reached, we are led to conclude that the phenomenon cannot be (macro)hydrodynamically limited, at east at normal pressure and gravity conditions. Based on infrared thermometry, and aided by X-ray radiography data on void fraction, the case for a scale separation phenomenon in high heat flux pool boiling is argued. This indicates that boiling crisis is controlled by the microhydrodynamics and rupture of an extended liquid microlayer, sitting and vaporizing autonomously on the heater surface. Further. the detailed dynamics of this microlayer, as revealed by our experiments. demonstrates that all previous thermally based models of boiling crisis are inappropriate.

Nyckelord

boiling crisis

burnout

critical heat flux

critical heat-flux

reduced gravity

pool

model

stability

liquids

water

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ref (ämneskategori)

art (ämneskategori)