Homogeneous nucleation boiling during jet impingement quench of hot surfaces above thermodynamic limiting temperature

What happens in the early stages of jet impingement quenching, when the surface temperature remains well above the thermodynamic limiting temperature for stable solid-liquid contact, is not clearly understood yet. The footage of the boiling and flow phenomena of this period of quenching reveals that the jet impacts on the surface due to hydrodynamic force and immediately splashes away violently, because of explosive boiling or boiling explosion that leaves the surface dry again. It is speculated that, a repetition of wetting and drying of the surface continues until the surface is cooled enough down to allow stable solid-liquid contact. However, neither high speed visual observation nor temperature measurement can help us to find out any concrete and quantitative explanation of the above mentioned wetting and drying phenomena of the surface as it takes place within a very short time. In the present study, we focused on the event of explosive boiling due to homogeneous nucleation that occurs when a liquid jet comes in contact with a very hot surface during jet impingement quenching. Assuming the liquid jet as 1-D semi-infinite solid during its brief contact with the solid, the authors proposed a model based on the concept of 1-D semi-infinite heat conduction and that of molecular theory of homogeneous nucleation boiling. In this model, a characteristic liquid cluster at the boundary is considered and the corresponding energy balance is obtained by accounting two parallel competing processes that take place inside the cluster, namely, transient external heat deposition and internal heat consumption due to bubble nucleation and associated growth. Results obtained are presented in terms of the liquid temperature escalation within the cluster, the limit of maximum attainable liquid temperature and the time necessary to reach the temperature limit at the boiling explosion. From the present model, the temperature at which homogeneous nucleate boiling takes place during quenching of carbon steel with water (20 degrees C) at atmospheric pressure can be obtained to be higher than 335 degrees C which might be regarded as the lower limit of surface temperature for homogeneous nucleate boiling. This lower limit of surface temperature for homogeneous nucleate boiling depends on the ratio of thermo-physical properties of the solid and the liquid, namely, beta = root(pc lambda)(5)/(pc lambda)1. As the precontact temperatures of the hot surface increases above the lower limit, the interface temperature between the solid and the liquid increases and the time at which homogeneous nucleation boiling appears is greatly shortened. (C) 2011 Elsevier Ltd. All rights reserved.