Optimum coating thickness for the fastest chilldown of Teflon-coated stainless steel flat plate in liquid nitrogen pool

Abstract

In this study, a novel model was developed to accurately determine the optimum thickness of a low-thermal-conductivity coating layer applied on a stainless steel flat plate resulting in the fastest chilldown of the plate in a cryogenic liquid pool. A low-thermal-conductivity layer coated on a metal surface is known to significantly reduce the cryogenic chilldown time of the metal by facilitating the early transition from film to nucleate boiling. However, there is still a lack of models to estimate the optimum coating thickness. An attempt has been made to address this issue. To do so, the authors first investigated the transient pool boiling of liquid nitrogen on Teflon-coated stainless-steel plates with various coating thicknesses ranging from 0 to 300 μm. Both the pool mass and plate temperature over time were measured and used to determine the heat flux into the liquid pool. A coating layer that results in a lower thermal resistance during film boiling would lead to a higher thermal resistance in nucleate boiling. As the thickness of the coating layer increases, the thermal resistance in film boiling decreases and the transition from film to nucleate boiling occurs earlier. The early transition of the boiling regimes was successfully explained by the assumption of intermittent liquid-solid contact during the film-boiling regime. The coating thickness in the range of 100–150 μm was observed to be the optimum thickness that resulted in both the smallest thermal resistance and earliest transition of boiling regimes. Subsequently, a new model to estimate the optimum coating thickness was derived based on experimental observations and the assumption of intermittent liquid-solid contact. The findings of this study are expected to be beneficial in enhancing the chilldown efficiency of cryogenic systems.