Cooldown of insulated metals in saturated and subcooled liquid nitrogen

摘要

The cooling rate of metallic objects quenched in liquid nitrogen can be enhanced by coating its surface with a material that has a low thermal effusivity. An early transition from film to nucleate boiling regime caused due to the formation of cold spots at the liquid-coating interface is reported as the reason for this enhanced cooling rate. However, untill now, optimization of the coating thickness to minimize the overall cooling time has only been an empirical proposition. Inspired by experimental data a phenomenological model is proposed. Using this model, an approximate insulation coating thickness that will approach the fastest cool down of an insulated metal quenched in liquid nitrogen can be predicted. This model is verified with experimental data of several copper cylinders coated with different thickness of epoxy quenched in saturated as well as subcooled liquid nitrogen. The optimum coating thickness reduces significantly with the degree of liquid sub-cooling.