Study of the Effect of Thermal Properties of Various Metals on Leidenfrost Temperature

Abstract

Cooling using liquid droplets is applied to efficiently cool and dissipate heat from high-temperature materials in everyday life because of the latent heat associated with droplet evaporation. Despite applications across various areas, predictive models or Leidenfrost temperature, accounting for changes in thermal properties induced by material thickness and oxidation, are absent. The study aims to develop the predictive model and assess coolability by analyzing Leidenfrost temperature concerning thermal properties, thickness, and oxidation of material after controlling surface properties across materials. Inconel 718, AISI 304 stainless steel, brass, and aluminum were employed to investigate the influence of thermal properties and thickness on Leidenfrost temperature. Results from deionized water droplet evaporation tests indicated that Leidenfrost temperature decreased when the specimen thickness increased until reaching a critical thickness. When Leidenfrost temperature changed with thickness, it was called the transient Leidenfrost temperature in the thesis. When the thickness of the specimens exceeded the critical thickness and Leidenfrost temperature did not change, it was referred to as the asymptotic Leidenfrost temperature. The critical thickness which is the transition from transient Leidenfrost temperature to asymptotic Leidenfrost temperature was termed as the asymptotic thickness. Through a dimensional analysis of the one-dimensional unsteady heat conduction, the asymptotic thickness was considered as the thickness of the specimen which equals or surpasses heat penetration depth for contact time. The transient Leidenfrost temperature was simultaneously influenced by heat storage and heat propagation of the thermal properties which means that transient Leidenfrost temperature can be predicted through considerations of thermal conductivity as well as the heat capacity per unit area of materials. Asymptotic Leidenfrost temperature mainly reflected the heat propagation of the specimens. The explanation of Leidenfrost phenomenon in terms of thermal properties and thickness can be elucidated by concurrently accounting for heat capacity and heat transmission of materials. As a result, Leidenfrost phenomenon can be predicted through two different trends. The HANA™ and Zircaloy-4 were subjected to approximately 136 s to 1536 s of oxidation time in steam environment at 1200 °C to investigate the coolability during Loss-of-coolant Accident (LOCA). The results of oxidized zirconium alloys were characterized and the changes in Leidenfrost temperature using deionized water droplet evaporation tests were analyzed. The oxidation weight gain, oxidation thickness, and Equivalent Cladding Reacted (ECR) of zirconium alloys increased as oxidation time increased. In the initial oxidation step, HANA™ exhibited high oxidation resistance with lower oxidation weight of 5%-17% and lower oxidation thickness of 10%-23% compared to Zircaloy-4. However, after 836 s of oxidation time, the increase in oxidation weight and oxidation thickness became similar for both zirconium alloys, regardless of the type of zirconium alloys. When zirconium alloys underwent exposure to a steam oxidation environment at 1200℃ for more than 136 s of oxidation time, the contact angle reduced to 0° as nanostructure formed. Since the surface properties were similar regardless of the zirconium alloy type after 136 s of oxidation time, it can be affirmed that variations in coolability due to differences in thermal properties between HANA™ and Zircaloy-4 induced by oxidation. The reduction in thermal conductivity resulting from the formation of an oxide layer on the zirconium alloys played a major role in enhancing Leidenfrost temperature. However, upon reaching an ECR of over 24%, corresponding to the critical oxidation thickness, Leidenfrost temperature stabilized. Analysis of the one-dimensional unsteady heat conduction based on droplet behavior revealed that Leidenfrost temperature did not rise if oxidation thickness exceeds the asymptotic thickness. However, even if the oxide layer thickens and surpasses the asymptotic thickness, the asymptotic Leidenfrost temperature varied depending on the type of zirconium alloys. Thus, the asymptotic Leidenfrost temperature was not solely determined by the thermal properties of the oxide layer but was also influenced by the thermal properties of the zirconium alloy. As a result, above reaching an ECR of 24%, coolablity was no longer improved confirming that coolablity enhanced up to 17% of the regulatory limit of ECR.