Abstract
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.
Highlights
Thermal quenching in a liquid pool is a common and convenient laboratory and industrial process, spanning a wide range of applications
The thickness of the vapour layer reduces with the reduction in wall superheat (θ = T − Tf ) and breaks, when the wall temperature drops below the minimum film boiling temperature
In order to extend this to the coated cylinders we assume the coated surface in contact with the liquid will be at the above mentioned temperature when the critical heat flux (CHF) state is reached. This assumption can be verified from the results reported by Moreaux et al [6], where, a 2μm coating thickness of polymeric resin does not have much influence over the minimum film boiling temperature
Summary
Thermal quenching in a liquid pool is a common and convenient laboratory and industrial process, spanning a wide range of applications. The boiling heat transfer in saturated liquid nitrogen is classified into three regimes, namely the film, transition, and nucleate boiling [5] depending on the wall superheat temperature (θ = T − Tf ). At high wall superheat the vapour generated forms a layer between the wall and liquid, resulting in a limited heat transfer to the liquid. A transition to the nucleate boiling regime occurs, where due to increased liquid–solid contact, higher heat flux is obtained. These regimes are elucidated only to the materials with high thermal effusivity, while materials with a low thermal effusivity shows early transition from film to the nucleate boiling regime [6]. The early transition in the boiling regime is suspected due to an early drop in the outer wall temperature
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