Air-Mist Heat Extraction and Visualization of Droplets–Surface Interactions From 60 to 1200 °C Under Steady-State Conditions

Abstract

Heat extraction and drop impact regimes occurring when a local portion of a horizontal flat-fan air mist impinges the active surface of a Pt disk hold at Tw from ∼60 to 1200 °C are investigated. Boiling curves comprise single-phase, nucleate boiling (NB), transition boiling (TB), and film boiling (FB). Mists are generated under wide ranges of water and air flow rates, and the disk is placed at center and off-center positions along the mist footprint major axis. Conditions generate a wide spectrum of water impact flux, w, droplet diameter, dd, droplet velocity, uzs, and impingement angle. Heat flux extracted, −q, along each boiling regime correlates very well with expressions involving Reynolds, Weber, and Jakob numbers evaluated in terms of local average characteristics of free nonimpinging mists—w, volume mean diameter, d30, normal volume weighted mean velocity, uz,v—and Tw; close estimation indicates that hydrodynamic and thermal forces are well accounted. During arrival of sparse parcels visualization of mist–wall interactions, using a high speed camera aided by laser illumination, allows determination of the predominance area diagram of droplet impact regimes in terms of normal impinging Weber number, Wez, and Tw. The regimes include stick, rebound, spread, and splash; the last subclassified as fine-, crown- and jet-atomization. Arrival of parcels in close succession is ubiquitous causing rapid surface flooding and leading to formation of discontinuous well agitated thick liquid films, which interacts longer with the surface than drops in sparse parcels, acting as heat sinks for longer periods of time.