Temperature control is omnipresent in today’s life: from keeping your fridge cold, maintaining a room at a pleasant temperature or preventing your computer from overheating. Efficient ways of heat transfer are often based on phase change, making use of the high latent heat of evaporation. In the context of spray cooling, liquid drops are impacting a hot plate to ensure a rapid cooling. At some temperature however, no contact occurs between the liquid and the plate, and the heat transfer rate is strongly reduced. The origins of this so-called Leidenfrost phenomenon are investigated in this thesis from an experimental point. By investigating the wetting behaviour and bubble generation at the interface between a heated plate and a liquid drop, new light is shed on the onset of this undesirable phenomenon. Rather than a sudden change between contact boiling and Leidenfrost boiling, a new regime is found and described where the drop is only partially in contact with the wall. The lower temperature boundary of this new regime is found to correlate strongly with that of a deposited gently Leidenfrost drop. The upper temperature boundary depends strongly on the impact dynamics, forcing the drop onto the wall in the early stages of the impact process. The identification of the relevant time- and length scales give insight on how the both liquid and solid properties influence the onset of the Leidenfrost effect. Depending on the impact parameters and plate properties, a second touch-down mechanism is found in which the heat transfer towards the evaporative front is limited in the case of a poor thermal conducting plate. These findings allow for a better prediction of the heat transfer between drops and a heated plate, aiding the optimization of efficient cooling processes.
|Award date||20 Jan 2017|
|Place of Publication||Enschede|
|Publication status||Published - 20 Jan 2017|