EXPERIMENTS AND MODELING OF A LIQUID DROPLET TRANSPORTED BY A GAS STREAM IMPINGING ON A HEATED SURFACE: SINGLE PHASE REGIME

摘要

Understanding the transport mechanisms involved in a single droplet impinging on a heated surface is imperative to the complete understanding of droplet and spray cooling. Evidence in the literature shows that gas assisted droplet sprays and mist flows are more efficient than sprays consisting only of liquid droplets. In both cases, understanding the transport physics due to the heat transfer from the surface to the droplet and then by convection and evaporation to the airflow is of fundamental importance. The current work focuses on the behavior of a single droplet as it is propelled to the target by a gas jet impinging on a heated surface. The study is restricted to the single-phase regime. High-speed photography was used to capture the droplet dynamics, including the droplet spreading and receding processes, over a range of jet Reynolds numbers. The instantaneous heat transfer coefficient from the surface to the liquid droplet was measured using a heated foil technique with a constant surface heat flux. It was found that the gas jet contributes to an increase in the maximum spreading diameter and in the instantaneous heat transfer coefficient, compared to a free falling droplet impinging onto a surface. The instantaneous, maximum heat transfer coefficient is achieved at intermediate times, apparently when an optimum liquid film thickness is achieved.