Dynamic Leidenfrost Effects: Computational Modelling to Predict Transitions in Drop Impact #

Peter Lewin-Jones, James Sprittles, Duncan Lockerby

11:50 Monday in 4Q04.

Part of the Droplets and impact session.

Abstract #

When a liquid drop is placed gently on a sufficiently hot surface it is able to levitate on its own evaporative vapour cushion. This ‘Leidenfrost effect’ is well understood and has been the focus of much research, motivated by its importance for numerous industrial applications where the dramatic decrease in thermal conductivity caused by it is often detrimental. Notably, however, many technologies, such as spray cooling, actually involve the impact of droplets in Leidenfrost conditions.

Recently, experimental studies have probed the dynamic Leidenfrost effect, where droplets can be forced into contact when impact speeds are large enough, uncovering several interesting modes of contact and discovering new unexpected effects, such as an oscillating film height in certain regimes. To provide new insight into the physical mechanisms involved in such phenomena and as an important predictive tool, we have developed a novel computational model for the dynamic Leidenfrost process. This uses the finite element method to solve the Navier-Stokes equations in the drop, implemented in the open source library oomph-lib, and uses lubrication theory incorporating gas kinetic effects to model the evaporated vapour.

Our simulations have been benchmarked against experiments and simulations of isothermal impacts and drop-drop collisions. Our model enables us to explore the parameter space of impacting velocity and solid surface temperature, and probe different regimes of contact and vapour film behaviour, with the aim of predicting the minimum film thickness reached and the critical impact speed at which contact will occur.