Predicting solid surface molten droplet solidification

Gas turbine engines in airplanes provide required thrust by sucking in air, heating it to high temperatures in a combustion chamber, and exhausting it at high velocities. Small inorganic particles such as volcanic ash get sucked in along with the air. These particles melt in the high-temperature zones in the combustion chamber and solidify onto cooler zones in the engine, such as the turbine blades. The solidified droplets accumulate on the surface of the blades, deforming, and blocking cooling holes, deteriorating the performance and the life of the engine.

Predicting the process can help engineers develop and modify engine designs by determining how molten droplets solidify in contact with a cooler surface and an accurate simulation is fundamental to understanding the process.

In a study published in the International Journal of Heat and Mass Transfer, a group of scientists from Japan developed a model that can simulate the solidification of a single molten droplet on a flat surface. Their model doesn’t require any prior information and can be used to develop models that can predict the deposition process in jet engines.

The research term consisted of Dr. Koji Fukudome and Prof. Makoto Yamamoto from the Tokyo University of Science, Dr. Ken Yamamoto from Osaka University, and Dr. Hiroya Mamori from The University of Electro-Communications.

Previous models assumed the surface was at a constant temperature, but the new approach simulates the solidification process by considering the droplet behavior and the heat transfer between the hotter droplet and the cooler surface.

“We have been simulating droplet impact, but we could not ignore the difference from the experiment. In this study, we thought that considering the temperature change of the colliding wall surface would be consistent with the experiment,” Fukudome says.

The researchers opted for a meshless moving particle semi-implicit (MPS) method which didn’t require multiple calculations on each grid. The MPS method is based on fundamental equations of fluid flow (such as Navier-Stokes equations and mass balance conservation equations) and has been widely used to simulate complex flows. The temperature change was computed using the grid-based method, coupling particle-based and grid-based methods.

Researchers simulated the solidification of molten tin droplets on a stainless-steel substrate. The model replicated the solidification process observed in experiments. The simulations also provided an in-depth view into the solidification process, highlighting the spreading behavior and the temperature distribution of a droplet as it meets the solid surface.

The solidification is dependent on the thickness of the liquid film that was formed after the molten droplet met the surface. Solidification initiates as the liquid film expands on the surface and is observed at the edge of the liquid film near the surface. As the liquid film continues to spread and become thinner, solidification progresses until the entire film is turned into solid particles.