Rotating Blade Stall Maps Measured by Differential Infrared Thermography

Abstract

RAFFEL and Merz [1] demonstrated the feasibility of unsteady boundary-layer transition detection by differential infrared thermography (DIT), a method whereby the difference of two sequentially recorded infrared images is analyzed to extract the point of greatest difference between the flows, which, for attached flows, is equivalent to the transition position. For flows with dynamic flow separation, the strongest feature in the infrared difference images is no longer the transition, but the separation of the flow. Gardner and Richter [2] showed that the standard deviation of pressure measurements at the transition point has a small peak, but that separated flow results in an even larger peak. Similarly, the infrared images can be analyzed to extract the presence and extent of separated flowover an airfoil. DIT offers a great advantage over using pressure sensors as indicators for separated flow because DIT does not require any special equipment to be attached to or built into the model observed. The major aim of the investigations presented here is the generation of “stall maps” in which stalled areas in the rotor plane are geometrically described. Currently, these maps are often produced through the investigation of pressure sensor data from sensors integrated into the rotor blades [3]. Alternatives are the use of tuft visualization [4] or pressure-sensitive paint [5]. However, these techniques require installations or coatings on the rotor blades, which can be avoided by the technique described in the following