Water Droplet Impingement

Since 1984, the Icing Group at Wichita State University has been working with the Icing Technology Branch of NASA Glenn Research Center (GRC) and the FAA to develop an experimental droplet impingement database. The design of ice protection systems requires knowledge of the local and total impingement intensities in order to determine the heat inputs for removing ice buildup. A laser based dye-tracer technique (read more), which was improvised by the Icing group, was used to determine the impingement intensity from the amount of reflectance absorbed by the blotter strips placed on a test object. To date, droplet impingement measurements have been recorded on aircraft inlets, airfoil sections, 3-D wing, engine nacelle, empennage and simulated ice shapes. These were performed at the NASA Icing Research Tunnel (IRT) over the period between 1985 and 2003. Spray clouds representative of the Appendix C and Supercooled Large Droplets (SLD) clouds were used in the tests.

Listed below are a number of projects related to droplet impingement research:

Experimental Investigation of Water Droplet Impingement on Airfoils, Finite Wings, and Engine Inlets
In 1984, a research program was initiated to expand the initial experimental water droplet impingement database that was developed by NACA in the 1950’s. This program was sponsored by the NASA Glenn Research Center and the FAA Technical Center. The first series of impingement tests were conducted in September of 1985 in the NASA IRT for a period of four weeks. The geometries tested included a four-inch diameter cylinder, a NACA 65(2)-015 airfoil, an MS(1)-0317 supercritical airfoil, three simulated ice shapes mounted on a two-inch diameter cylinder, an axisymmetrical engine inlet model and a Boeing 737-300 engine inlet model. The second series of impingement tests were performed in the IRT facility during April of 1989 and also lasted for approximately four weeks. Models tested during this phase of the research program included two simulated ice shapes, a Natural Laminar Flow (NLF) airfoil section NLF-0414, an infinite span 30° swept MS(1)-0317 wing, a finite span 30° swept NACA 0012 wing, and a Boeing 737-300 engine inlet model. The droplet sizes used in the impingement tests were between MVD of 16.5 and 20.4 μm. In December of 1996, NASA awarded a second research program to the Icing Group of WSU to improve the experimental method developed during the previous research program, and to develop a more efficient data reduction method using a CCD camera. In addition, extensive impingement tests were also planned in the NASA Glenn IRT. The first series of the IRT impingement tests was conducted during the period between July and September 1997. The second series of impingement tests was conducted between January and March 1999. A total of eleven wind tunnel models were tested during these two IRT entries. Test models included six 2D airfoils, a 2D high-lift system, three swept horizontal tails and an S-duct engine inlet. In 1997, the MVDs tested were 11.5, 21 and 92 μm while in 1999 the MVDs were 11, 21 and 94. The 92-94 MVD case was selected to provide SLD impingement data.

Related Publications:
NASA/TM2002-211700 Experimental Investigation of Water Droplet Impingement on Airfoils, Finite Wings, and an S-Duct Engine Inlet

Experimental Study of SLD Impingement Effects
Recent developments in aviation rulemaking process to address aircraft operations in SLD conditions have highlighted the need for additional impingement research for large droplets. In the fall of 2000, the Icing Group conducted experimental SLD impingement tests for a range of airfoil geometries and compare the experimental results with analysis data from the NASA Glenn LEWICE ice accretion code. In addition to large MVD clouds, droplets from the Appendix C were also used in the tests that were conducted at the NASA IRT. The airfoil sections used include three 36-inch chord airfoils (MS(1)-0317, GLC-305, and NACA 65(2)-415), as well as a 57-inch chord Twin Otter horizontal tail section and a 22.5- and 45-minute LEWICE glaze ice shapes for the Twin Otter tail section. The impingement experiments were performed with spray clouds having MVD of 11, 21, 79, 137, and 168 microns and for a range of angles of attack. All the impingement experiments were conducted at airspeed of 175 mph corresponding to a Reynolds number of approximately 1.6 million per foot. In general, good agreement was observed between experiment and LEWICE analysis for small droplet sizes representative of the Appendix C cloud. For the large droplets, however, the LEWICE analyses were generally greater than the experimental impingement efficiencies. It is believed that the discrepancy was attributed to water mass loss due to splashing as experienced by the large droplets during impingement.

Related Publications:
DOT/FAA/AR-03/59 Experimental Study of Supercooled Large Droplet Impingement Effects
NASA/TM2007-213959 Large and Small Droplet Impingement Data on Airfoils and Two Simulated Ice Shapes

Droplet Impingement Visualization Experiments
In the fall of 2000, NASA and the FAA funded the Icing Group at WSU to conduct experiments to document large and small droplet impingement dynamics using advanced imaging techniques. Droplet splashing tests were performed in the Goodrich Icing Wind Tunnel (IWT) facility with a 21-inch chord NACA 0012 airfoil. The tests were carried out at a number of tunnel speeds between 50 and 175 mph and with cloud MVD in the range of 11 to 270 microns. The main objective of the investigation was to record phenomenon of droplet splashing on an airfoil surface and to assess the effect of MVD and airspeed on droplet splashing. The region of interest near the airfoil leading edge was illuminated with a thin light sheet that was created with a 100-mW red laser diode and a set of spherical lens. The droplet splash images were captured using a CCD camera capable of collecting at data rate of 1 MHz and 16-bit resolution. In general, the images of droplet splashing were clearly observed with large droplets, and the splashing intensity seems to increase with MVD and tunnel speed.


Water Droplet Impingement Experiments on Simulated Ice Shapes
Previous impingement studies had been conducted mainly with clean airfoils and the resulting data were used to validate the droplet impingement distribution computed by the LEWICE ice accretion code. However, as ice starts to accrete on the airfoils, these impingement characteristics are no longer applicable therefore new droplet impingement tests had to be obtained for airfoils with simulated ice shapes. The LEWICE code was used to generate the ice shape on the NACA-23012 airfoil for a number of icing conditions including droplets representative of the Appendix C and SLD clouds. The tests were performed at the NASA IRT with Appendix C and SLD clouds, and the research program was funded by the FAA and NASA Glenn Research Center. The impingement experiments were performed with spray clouds having MVD of 20, 52, 111, 154 and 236 μm and for an angle of attack of 2.5°. The comparison of the droplet impingement distribution between the LEWICE analysis and experiment show good agreement for Appendix C cloud. However, LEWICE predicted larger than expected impingement efficiencies for large MVD clouds hence poor agreement was found especially near the leading edge and regions behind the ice ridge. In previous comparison for the clean airfoil, droplet splashing was thought to be the main cause of the discrepancy but in iced airfoil, it is believed that droplet breakup (due to droplet interaction with viscous forces) may also have contributed to the discrepancy.

Related Publications:
NASA/TM2007-213961 Water Droplet Impingement on Simulated Glaze, Mixed, and Rime Ice Accretions