Considerations When Performing Icing Wind Tunnel Testing to Determine Critical Temperature 2003-01-2120

Critical temperature is defined as the threshold free stream air temperature at which ice will no longer accrete on a structure even though supercooled water is present in the air stream. Many factors determine the critical temperature, including airspeed, geometry of the accreting body, attitude of the body relative to airflow direction, thermal history of the body, liquid water content, and evaporative, convective, conductive and radiative heat transfer mechanisms operating at the accreting surface. A recent Certification Review Item issued by the Joint Aviation Authorities has highlighted the importance of understanding critical temperature characteristics of accretion-based probe-type ice detectors and the monitored surfaces they protect. Icing wind tunnels are perhaps the best available tool for characterizing critical temperature.

If critical temperatures of structures are being compared under like icing conditions, there is a high likelihood that differences will be small - perhaps on the order of 1° to 2°C or even less. Unfortunately, even the best icing wind tunnels can introduce errors of this order. In more conventional tunnel experiments run at colder temperatures to assess ice shapes, this level of uncertainty would usually be acceptable, whereas it typically would not be when assessing critical temperature. It is therefore especially important when characterizing critical temperature to recognize and address to the greatest degree practical the multiple contributors to uncertainty. Even when all reasonable steps are taken, uncertainty at the test article location(s) may still be greater than desired. This paper notes the major sources of uncertainty within the icing wind tunnel environment, and addresses considerations and practices to minimize their impact.