Drop Impact on Dry Surfaces With Phase Change

• Main Author: Li, Hai
• Format: Others
• Language: English; en
• Published: tuprints 2013
• Online Access: https://tuprints.ulb.tu-darmstadt.de/3550/1/DissLi.pdf; Li, Hai <http://tuprints.ulb.tu-darmstadt.de/view/person/Li=3AHai=3A=3A.html> (2013): Drop Impact on Dry Surfaces With Phase Change.Darmstadt, tuprints, Technische Universität, [Ph.D. Thesis]

Airframe icing caused by the Supercooled Large Droplet (SLD) has been identified as a severe hazard of aviation. The impact of SLD in the in-flight icing condition remains unknown in multiple aspects. The impact velocity is very high, and most of the drop impacts are oblique. The accompanying drop splash invalidates the current engineering tools for design of the anti-icing system. Furthermore the involvement of supercooling in drop impact demands exploration. In the framework of the EU Project EXTICE and DFG project SFB - TRR 75, this thesis contributes to understanding of the impact of SLD by two experimental investigations, respectively on the effect of supercooling on drop impacts and the drop splash after high-speed impact. In the first experiment supercooled drops were created, and the drop impact with phase change was observed by both shadowgraph imaging and infrared imaging. The dynamic spreading diameter of the drop impact on aluminum surfaces was measured. Together with an analytical approach it was found that the phase change was negligible for drop impacts in typical icing conditions. The impact of supercooled drops on superhydrophobic surfaces revealed that the duration of the first stage of solidification in the drop impact was significantly shorter than that in a sessile liquid. Ice crystals formed in the supercooled water had a similar morphology to a snowflake. The drop receding on the hydrophobic surface was influenced by the contact temperature, which was measured by the infrared imaging. At low contact temperatures, asymmetrical receding was observed. In the second experiment high speed impacts of single drops with diameters ranging from 130μm to 200μm on dry surfaces of rapid motion were recorded by shadowgraph imaging up to 1Mfps. The target velocity varied from 10m/s to 63m/s. The impact surface had an inclination ranging from 0° to 75° in order to investigate the effects of oblique impact. Six outcomes of drop impact were identified: deposition, prompt splash, corona-corona splash, corona-prompt splash, single-side splash and the aerodynamic breakup. The aerodynamic breakup on a horizontal target was an interaction between the spreading lamella and the gas boundary layer. A qualitative force analysis made on the spreading lamella pointed out that in a corona splash the stabilizing factor is surface tension, and the destabilizing factors are aerodynamic force and inertial force. The lamella thickness and critical spreading velocity correlate with each other in a complementary manner, leading to non-monotonic threshold impact velocities at different impact angles. The velocity of the splashing jets and the asymmetric spreading radii were measured from video. The mass-loss coefficient was measured for the drop impact on horizontal targets.