A STUDY ON THE DEFORMATION AND BREAKUP OF SUPERCOOLED LARGE DROPLETS AT THE LEADING EDGE OF AN AIRFOIL

Open Access
Author:
Veras-Alba, Belen
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 09, 2017
Committee Members:
  • Jose Palacios, Thesis Advisor
Keywords:
  • Supercooled Large Droplets
  • SLD
Abstract:
Ice accretion is an issue that has affected aircraft since the early years of powered flight. Although it was a known problem, the full extent was not known. Both small and large droplets were of concern. The effects of both were countered with ice protection systems based initially on computer codes that predict the size, shape, and location of ice on aerodynamic surfaces for small droplets. The codes have been tested and validated for the conditions described in Federal Aviation Regulation Part 25 Appendix C (small droplets, up to 50 µm) and aircraft only had to be certified for those conditions. Supercooled large droplets (SLD) reach locations further aft on the surfaces than small droplets making the ice protection systems insufficient in SLD icing conditions. The protection systems remove ice but do not reach the limits of the SLD ice and ridges remain on the wing surfaces which continue to negatively impact the performance of the aircraft. Certification regulations regarding SLD have been implemented but the codes do not yet accurately predict ice accretion due to SLD. To validate the codes, experimental data on the behavior of larger droplets when impacting a lifting surface are necessary. The results of an experimental study on the deformation and breakup of supercooled droplets near the leading edge of an airfoil are presented. The experiment was conducted in the Adverse Environment Rotor Test Stand (AERTS) facility at The Pennsylvania State University with the intention of comparing the results to prior room temperature droplet deformation results. To collect the data, an airfoil model was placed on the tip of a rotor blade mounted onto the hub in the AERTS chamber. The model was moved at speeds between 50 and 80 m/s while a monosize droplet generator produced droplets of various sizes which fell from above, perpendicular to the path of the model. The temperature in the chamber was set to -20°C. The supercooled droplets were produced by maintaining the temperature of the water at the droplet generator under 5°C.The supercooled state of the droplets was determined by measurement of the temperature of the droplets at various distances below the tip of the droplet generator. A prediction code was also used to estimate the temperature of the droplets based on the size, vertical velocity, initial temperature, and distance traveled by the droplets. The droplets reached temperatures between -5 and 0°C. The deformation and breakup events were observed using a high-speed imaging system. A tracking software program processed the images captured and provided droplet deformation information along the path of the droplet as it approached the airfoil stagnation line. It was demonstrated that to compare the effects of water supercooling on droplet deformation, the slip velocity and the initial droplet velocity must be the same in the cases being compared. A case with a slip velocity of 40 m/s and an initial droplet velocity of 60 m/s was selected from both room temperature and supercooled droplet tests. In these cases, the deformation of the weakly supercooled and warm droplets did not present different trends when tested in room temperature and mild supercooling environments. The similar behavior for both environmental conditions indicates that water supercooling has no effect on particle deformation for the limited range of the weak supercooling of the droplets tested and the selected impact velocity.