Experimental investigation of spray cooling on steel plates with periodic micropillar structures: Two-stage quenching phenomenon

• Transient spray cooling of micropillared surfaces showed novel two-stage quenching. • Temperature drops within a pillar induced a first quench at the tip. • Secondary quench occurred when the wetting front reached the bottom of the pillars. In this study, water spray cooling tests were conducted on stainless steel samples with micropillars fabricated using a laser texturing method. The micropillars had a fixed pitch of 100 µm and varied in width (10–80 µm) and height (30–150 µm). The experiments revealed two distinct stages of quenching, observed as abrupt increases in the heat transfer rate during cooling from 700 °C to 100 °C on the micropillared samples. The first quench occurred at temperatures above 400 °C, as evaluated at the base of the pillars based on thermocouple readings, and was followed by a second quench at approximately 280 °C. The first quench was attributed to the onset of stable liquid–solid contact on the tops of the micropillars, enabled by significant temperature drops within the pillars. The wetting front subsequently advanced from the pillar tips to their bases, eventually covering the entire surface and resulting in the second quenching event. Taller micropillars were found to increase the “apparent” first quench temperature due to the longer heat conduction path through the pillars, which allowed stable wetting at the tips while maintaining higher temperatures at the bases. In contrast, shorter micropillars exhibited a gradual increase in cooling rate without a distinct first quench point. Increasing the micropillar width up to 30 µm (for a height of 50 µm) improved cooling performance due to the enhanced heat transfer area at the pillar tips. However, further increasing the width led to diminished performance, with heat transfer characteristics resembling those of a smooth sample. The second quench consistently occurred at approximately 280 °C, similar to the smooth sample, regardless of the pillar geometry.