Rewetting and boiling in jet impingement on high temperature steel surface

Abstract

Interface topologies and boiling phenomena are observed within the water jet impingement zone during quenching of a high temperature (300 °C–900 °C) steel plate by direct optical observations. Stable film boiling may occur, but surface asperities may easily penetrate the vapor film and interact with the flowing liquid. By cooling down more rapidly than the remaining solid surface, such asperities act as a kind of micro-fin. Rather surprisingly, non-coalescing bubbles on top of the thin vapor film have been observed, probably formed on top of penetrating surface asperities. After establishing contact between water and solid, so after rewetting, an intense bubble activity is normally seen of bubbles that are footed on a dry plate area with a contact line reflecting the light-emitting diode lights and condensing at later times. These are vapor bubbles. A high surface roughness promotes rewetting that may be initiated at several places simultaneously. For an initial plate temperature of 300 °C and a total height roughness of 5 μm, rewetting took place without the occurrence of a vapor film; surfaces at or above 450 °C exhibited vapor film formation, even for a high jet subcooling of 80 °C. Temperature and time of rewetting are strongly affected by the initial surface temperature and jet subcooling and less by the jet velocity. New correlations are provided to predict these dependencies.

The nature of boiling during rewetting of surfaces at temperatures exceeding the thermodynamic limit for water superheat

“…The observations reported in the above are typical for sandblasted surfaces with initial plate temperature in the range 300-650 • C. The complete recording of rewetting on a sandblasted surface and a detailed video of the flashing pattern are available on the online version of this paper as movie 1. Figure 4 corresponds to a series of snapshots taken from the high-speed recordings during the quenching process of a smooth plate at 650 • C. As reported before by Leocadio, van der Geld & Passos (2018), an initial film boiling period is observed (figure 4a). The film boiling regime is characterized by the presence of small moving white spots, which do not condense or coalesce.…”

The Run Out Table in the Lab: Quenching of Fast Moving Steel Plates

“…The water jet system has been upgraded to allow for installation of both a circular jet nozzle of 9 mm diameter and a planar slit jet nozzle of 2x50 mm. As reported elsewhere [1], [7], a borescope and high speed camera arrangement enables the direct observation of the boiling activity in the jet stagnation zone. Additionally, a side view high speed camera is installed to record the jet and flow development on the complete test plate.…”

“…For this reason, quenching by water jet impingement has been widely studied in literature. However, most of the experimental studies reported in literature have focused on static surfaces [1]- [3] or surfaces moving at maximum speeds of 1.5 m/s [4]- [6], much lower than the real ROT conditions. In order to obtain heat transfer predictions that are applicable and representative of the ROT process, the effect of high surface speeds on the thermal boundary layer must be considered and studied experimentally.…”

Heat Transfer Mechanism for Roller Quenching with an Ultrathick Plate

“…), [4,5] plate parameters (roughness, initial cooling temperature, etc. ), [6,7] and parameters (shape, number, etc.) [8,9] of the thin plates used.…”