Numerical Modeling of the Dynamics and Heat Transfer of Impacting Sprays for a Wide Range of Pressures.

摘要

A numerical model is developed to simulate the impingement of liquid sprays on surfaces heated at temperatures ranging from nucleate to film boiling. The droplets are modeled in the Lagrangian frame of reference, and are dispersed stochastically in the continuous gas phase. The model is based on the fundamental basics of single droplet impingements extended to full sprays, where the overall heat transfer process is broken down into its basic components: conduction associated with the droplet contact, bulk air convection, and surface radiation. Droplet dynamics at the wall are modeled based on an empirical correlation relating the droplet incoming to outgoing Weber number. Droplet contact heat transfer is modeled using an effectiveness parameter for the heat transfer that is a function of the droplet Weber number. This attempt of numerically modeling the droplet-wall dynamics with multiple wall collisions and the droplet contact heat transfer has not been addressed before in a numerical model. Simulations are presented for: single-stream droplet impactions, multiple-streams droplet impactions, and conical sprays.The model is tested at atmospheric pressure using experimental data for nozzles that dispense non-uniform droplets. Favorable comparison with the test data is demonstrated. The model capability is then extended to simulate high and sub-atmospheric ambient pressure conditions with a proper accounting of the droplet-wall interaction and air-mist heat transfer mechanism. At high and sub-atmospheric pressures, the model was tested against experiments for single stream impactions at various pressures.Spray simulation conducted for a wide range of pressures reveals the following important issues regarding to the droplet dynamics, heat transfer and vaporization: 1) At higher pressures, the larger the droplet size, the better is the droplet-wall impaction, while for sub-atmospheric pressures, larger droplets have a detrimental effect due to their ballistic impaction. 2) At higher pressures, the Leidenfrost point shifts to a higher temperature that leads to an increase in the droplet wetting capability, and to a higher heat transfer effectiveness. 3) At higher pressures, more vapor is generated from each droplet impaction on the surface, resulting also in an increase in the heat transfer effectiveness.