Determination of surface heat-transfer coefficients of hypersonic wind tunnel aerodynamic models using emission characteristics of thermographic phosphor paints

Wind tunnel tests were recently conducted to verify the applicability of a thermographic phosphor paint technique for the thermal mapping of model surfaces in hypersonic flow. The tests provided data that supported the routine use of the thermographic method in thermal analysis, and demonstrated that the paint technique could be an effective alternative to other thermal mapping methods. Two 0.0175 scale models of the Space Shuttle Orbiter were used for the tests. The models were provided by Rockwell International and were constructed of a filled, high-temperature epoxy material of very low conductivity. These models were used because they had previously provided heat-transfer data with the more conventional thermal mapping approach of using thin-skin thermocouples [1].

The tests were conducted at the Arnold Engineering Development Center (AEDC), Arnold Air Force Base, Tennessee. The facility used was the AEDC von Kármán Facility Hypersonic Wind Tunnel B. Fifteen (15) runs were made at Mach 8, with Reynolds numbers (per foot) of 0.5 X 10⁶, 2.0 X 10⁶, and 3.7 x 10⁶, and a tunnel total temperature of 890° F (1350° R). Model angles of attack varied from 20 to 35 degrees. The models were painted with two different phosphor paints, unique in their range of temperature sensitivity, which allowed a large temperature variation to be examined. Exposed to ultraviolet (UV) light during the tests, the phosphor painted surface emitted light, the intensity of which was measured and related to surface temperatures.

Heat-transfer coefficients were computed by using a theoretical thermal response function of transient heating of a semi-infinite slab and experimental surface temperatures. The coefficients obtained were compared to coefficients obtained using thin-skin thermocouples. Agreement between the heat-transfer coefficients obtained by the two different methods was observed, though the accuracy desired by Rockwell International was not obtained. However, the thermographic phosphor technique is promising and further development is encouraged.