Impact dynamics and heat transfer characteristics of liquid nitrogen drops on a sapphire prism

M. A. J.  van Limbeekab, T. Nesa and S. Vanapallia 
a) Energy Materials and Systems – University of Twente
b) Max Planck Institute for Dynamics and Self Organization

Many application utilize the evaporation of liquids to control the heat transfer from a hot wall. In the context of spray cooling, drops impact the hot wall and depending on the wall temperature the drop either touches-down (contact boiling) or levitates (film boiling). The latter case results in a great reduction in heat transfer by the insulating vapour layer under the drop. Here, we study this phenomena for cryogenic systems, where we investigate the impact of liquid nitrogen drops on a sapphire prism. In these cryogenic conditions, the prism behaves as a perfect thermal conductor, while its transparency enables us to study the contact behaviour during the impact and the spreading phase of the drop. By varying the prism temperature and impact velocity of the drops we obtain a phase diagram of the impact characteristics.  The phase diagram is then compared with a second type of experiment in which a stream of drops cools the prism over time. The results of the two different type of measurements agree well, from which we conclude that the cooling power of a drop is strongly related to the wetting behaviour of the impacting drops. Finally, by comparing the wetted area with the contact line length we show that heat transfer in contact and transition boiling is dominated by conduction rather than evaporation. Our study gives insight how the current literature established for  non-cryogenic conditions can be applied to the present case of liquid nitrogen drops.