Abstract
Spray cooling of hot steel surfaces is an inherent part of continuous casting and heat treatment. When we consider the temperature interval between room temperature and for instance 1000 °C, different boiling regimes can be observed. Spray cooling intensity rapidly changes with the surface temperature. Secondary cooling in continuous casting starts when the surface temperature is well above a thousand degrees Celsius and a film boiling regime can be observed. The cooled surface is protected from the direct impact of droplets by the vapour layer. As the surface temperature decreases, the vapour layer is less stable and for certain temperatures the vapour layer collapses, droplets reach the hot surface and heat flux suddenly jumps enormously. It is obvious that the described effect has a great effect on control of cooling. The surface temperature which indicates the sudden change in the cooling intensity is the Leidenfrost temperature. The Leidenfrost temperature in spray cooling can occur anywhere between 150 °C and over 1000 °C and depends on the character of the spray. This paper presents an experimental study and shows function for prediction of the Leidenfrost temperature based on spray parameters. Water impingement density was found to be the most important parameter. This parameter must be combined with information about droplet size and velocity to produce a good prediction of the Leidenfrost temperature.
Highlights
The Leidenfrost temperature, TL, is of paramount importance to metal alloy quenching since it marks the transition from very poor heat transfer in film boiling to the far superior heat transfer associated with transition boiling [1]
Al-Ahmadi [3] proposes identification of the Leidenfrost point in the same way that is used in this study: The Leidenfrost point is identified from the surface temperature versus time
There is no change in the Leidenfrost temperature when we study the cooling of an oxide surface or steel surface with equal roughness
Summary
The Leidenfrost temperature, TL , is of paramount importance to metal alloy quenching since it marks the transition from very poor heat transfer in film boiling to the far superior heat transfer associated with transition boiling [1]. The Leidenfrost point is defined as the point where the film boiling curve experiences the minimum flux. This temperature, surface wetting increases heat flux rapidly. Even if the Leidenfrost point is clearly defined it is (in spray cooling cases) difficult to read it automatically from experiment data. The surface temperature records show a rapid change in the cooling rate 80 s into the experiment.
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