| This report presents a study on the Leidenfrost point and film boiling of single droplets and sprays. The first portion of the work includes fundamental studies of impinging droplet fluid and heat transfer characteristics presented in the form of droplet impact behavior regime maps. The maps, along with detailed photographs and heat transfer measurements, identify distinct heat transfer regimes for impinging water drops. Next, an experimental and a theoretical study of the Leidenfrost point (LFP) for sessile and impinging droplets are presented. The experimental results indicate weaknesses in several previous LFP models and serve as the basis for a new theoretically-based model. The new LFP model postulates that at liquid/solid interface temperatures at and above the LFP, a sufficient number of cavities are activated and the bubble growth rates are fast enough that a complete vapor layer is established nearly instantaneously between the liquid and solid. The LFP model is applicable for pools of liquid, sessile droplets, and impinging droplets and sprays and exhibits good agreement with experimental data. Finally, an experimental study of the film boiling heat transfer from a polished nickel surface to a continuous stream of monodispersed water drops is presented. Empirical correlations are derived for the film boiling heat transfer rate and droplet heat transfer efficiency over a wide range of parameters. These single droplet heat transfer characteristics are then employed to develop a film boiling heat transfer models for multiple droplet streams and sprays. |
