Experimental study on grid-patterned low thermal conductivity coatings for fast chill-down of flat plate in liquid nitrogen pool

Abstract

This study investigates a fast chill-down technology utilizing grid-patterned low-thermal-conductivity (low-k) coatings. Experiments were conducted on the transient pool boiling of liquid nitrogen on stainless steel plates under various conditions: bare, partially coated, and fully coated surfaces with coating thicknesses ranging from 50 to 200 mu m. Fully coated surfaces exhibited a 52-58 % shorter chill-down time owing to the rapid transition from film to transition boiling. However, these surfaces reduced thermal performance in the nucleate boiling regime owing to the added thermal resistance of the low-k coating. To address this, a partially coated surface with a grid-patterned design was proposed and examined for various pattern sizes of d = 2.5, 5, and 10 mm. The rapid regime transition was most effectively facilitated when the pattern size was comparable to the wavelength determined by the Taylor instability. In the nucleate boiling regime, thermal performance improved as the pattern size approached the capillary length. Consequently, the chill-down time for the partially coated surface with d = 2.5 mm was reduced by 79 %, owing to both the rapid regime transition and improved performance throughout the entire process. Finally, the grid-patterned coatings showed a twofold increase in coating effectiveness (similar to 4.8) compared to the fully coated surfaces (similar to 2.4). This study establishes a new design guideline for low-k coatings, optimizing cooling performance across all boiling regimes.