Modeling the Effect of the Leidenfrost Vapor Coat Behavior on the Vitrification in Micro-Droplets Using the Cryogenic Droplet Levitation Time

The levitation (“hovering”) of a liquid droplet on the surface of the coolant such as liquid nitrogen (LN2) is a useful model for studying the Leidenfrost effecr (LFE), that is formation of a vapor film of boiling coolant around the surface of a relative,ly hotter sample, at cryogenic temperatures. Several models of the cryogenic droplet levitation (CDL) have been proposed but no experimental verifications had been proposed for this model in earlier papers. Utkan Demirci’s group has recently developed fast ice-free cooling (vitrification) of microdroplets formed by an ink-jet printer. The group proposed a combination of a theoretical model of film boiling on a hot sphere with the zone theory of non-isothermal kinetic ice propagation within an initially liquid levitating droplet, and they gave theoretical predictions and experimental evaluations of the CDL (Leidenfrost) time tLF of droplets hovering on the surface of LN2 [6]. Here, we report our own experiment results of verification of the data and predictions reported in [6] and describe a thermodynamical model that for elucidating the fate of the levitating droplets. This model adequately explains our experimental results on measuring tLF but almost predicts somewhat 4-fold departure from the numbers claimed by Demirci’s group. We also discuss possible flaws of the model and, especially, experimental claims presented in [6].

The Investigation of Shock Waves Forming by Disruption of Vessel With High-Enthalpy Coolant

The physical and mathematical model of the shock wave formation and evolution in the outflow of a boiling coolant from a vessel filled with the high pressure water after depressurization was investigated. For mathematical description of the pressure waves formation processes the half-empirical relaxation model and the model of maximum superheating was used that is correspond to the description of the boiling processes as vaporization initiated by fluctuation nucleation. The amplitudes of the shock waves and the time dependencies of the pressure acting on the obstacles situated on different distances from the tube rupture were calculated. The influence of the initial coolant pressure and temperature, time of disruption and diameter on the intensity of shock wave were revealed by a series of calculations. It was found that the pressure on obstacle after the normal shock wave falling is dramatically increasing and then decreasing. Relative growth of amplitude of the reflection shock wave is increasing with a growth of initial temperature of the coolant in vessel.