Inscrição na biblioteca: Guest

ISSN Online: 2377-424X

ISBN Print: 978-1-56700-421-2

International Heat Transfer Conference 15
August, 10-15, 2014, Kyoto, Japan

Effect of the Vapor Flow on the Drop Spreading in the Leidenfrost Regime

Get access (open in a dialog) DOI: 10.1615/IHTC15.min.009574
pages 4869-4883

Resumo

Spray cooling is an attractive technique for its ability to dissipate high thermal fluxes with satisfactory spatial homogeneity and low coolant fluid consumption. This work presents an experimental investigation of the deformation process of liquid droplets impinging onto a hot substrate in the film boiling regime. The particular case of the bouncing regime is the main target of the paper. Droplets impact onto a smooth substrate (averaged roughness about 0.1?m) heated above 600°C, and they are observed by shadowgraphy using a high-speed camera. After image processing, the deformation of droplets in the 100 µm size range is characterized with a temporal resolution of about 0.2 µs and a spatial resolution in the order of 1-2 microns. Experiments are performed at moderate impact velocities (2-8 m/s). Under these conditions, bouncing of the droplets is observed without any fragmentation. Measurements include liquids of different viscosities (water, ethanol, 5 mixtures of water and glycerol), which allows covering a relatively wide range of Ohnesorge numbers (0.01 <Oh <0.25). Results reveal that Oh has a moderate influence on the maximum spreading time and resisdence time, while this parameter significantly modifies the spread radius which decreases with Oh for a fixed value of Weber number. Correlations based on the Reynolds, Weber and Ohnesorge numbers are considered to describe the main features of the deformation process. A theoretical model based on an analytical self-similar solution for the viscous flow in the spreading drop is used to describe the spreading of the drops. A comparison with binary collisions that have similarities with the studied phenomenon shows that the liquid in the lamella is accelerate by the vapor flow under the droplet in the Leidenfrost effect.  For the less viscous liquids, the velocity of the liquid entering into the rim at the edge of the droplet needs to be increased compared to the case of binary collisions where shear stress is null at the interface. A direct correlation between the velocity augmentation and the viscosity of the liquid of interest was determined.