Detection of boundary-layer transition in a cryogenic wind tunnel by using luminescent paint

A technique for boundary-layer transition detection in a cryogenic wind tunnel has been developed. This technique is based on the thermal quenching of luminescent molecules. Calibration results show that luminescence of the paint composed of ruthenium complex and silicone polymer is strongly sensitive to temperature over the range from 90 to 220 Kelvin. This capability allows one to visualize thermal signatures across the boundary-layer transition. A thin paint coating has been applied on three airfoil models with different thermal insulation properties. The paint was excited by a Xenon light and the luminescence image was acquired using a high resolution digital camera. To enhance the surface temperature signatures between laminar and turbulent regions, either the flow or the model substrate was cooled or heated in an active manner. Transition patterns have been successfully visualized by processing the luminescent images. Boundary-layer transition has been detected by using this technique over a cryogenic temperature range from 90 to 150 Kelvin in subsonic and transonic flows.