Heat Transfer to Nitrogen Droplets Evaporating in a Heated Liquid Pool

N. Rebelo, H. Zhao, F. Nadal, C. Garner

Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Loughborough University, UK

Email: N.J.Rebelo@lboro.ac.uk; H.Zhao2@lboro.ac.uk

Liquid nitrogen is extensively used in electronics, metal processing and food industries where it is regasified using ambient air vaporisers before being used. The current interest is to utilize the cold thermal energy stored in liquid nitrogen that is otherwise lost to the ambient during the regasification process.  A spray of nitrogen droplets when directly injected into a heated pool of liquid will evaporate due to the heat transfer from the bulk liquid and the pressurized nitrogen gas could be used to produce some useful mechanical work. In this study, the evaporation characteristics of a single liquid nitrogen droplet in a heated liquid pool have been evaluated using high-speed imaging and the surrounding bubble growth was quantified from the post-processed experimental data.  The effect of the initial droplet size, bulk liquid temperature and the surface tension of the bulk liquid on the evaporation rate were determined by varying each of these parameters.  The experimental data suggest the bubble growth rate is mainly determined by the initial droplet size.  A scaling law based on the pure diffusion-controlled evaporation of droplet in open-air environment has been successfully implemented to scale the experimental data. The detailed bubble growth rates have been modelled by a heuristic one-dimensional, quasi-steady, conduction-based model where an effective thermal conductivity has been introduced to account for the complex dynamics of the droplet inside the bubble and the subsequent convective processes in the surrounding vapour.