Development of Temperature Sensitive Paints for the Detection of Small Temperature Differences

Temperature sensitive paints (TSP’s) have recently been used to detect small temperature differences on aerodynamic model surfaces. These types of applications impose stringent performance requirements on a paint system. The TSP’s must operate over a broad temperature range, must be physically robust (cannot chip or peel), must be polishable to at least the smoothness of the model surface, and must have sufficient sensitivity to detect small temperature differences. TSP coatings based on the use of metal complexes in polymer binders were developed at NASA Langley Research Center which meet most of the requirements for detection of small temperature differences under severe environmental conditions. INTRODUCTION Sullivan et al. have recently tested numerous temperature sensitive paint (TSP) formulations for the measurement of model surface temperatures in wind tunnel tests (4,5,6,7,8,9). McLachlan et al. also demonstrated the use of TSP to derive boundary layer transition locations based on the change in heat transfer characteristics that accompany boundary layer transitions (10). The technique involves painting the model with a TSP (usually by spraying multiple coats), illuminating the model with light having the wavelength necessary to excite the luminescent compound in the paint, monitoring the luminescent intensity with a scientific grade CCD video camera, and enhancing the image through digital processing. This is represented by Figure I . This application of TSP requires paints which are sensitive to very small temperature differences (9). Sullivan et al. tested techniques for enhancing the temperature difference across the boundary (9). They showed that the temperature difference may be enhanced by either changing the temperature of the tunnel windstream (transient method) or by heating the model (steady method). Sullivan tested both non-metallic models and stainless steel models. When steel models are used it is necessary to apply a base coat under the TSP which serves as a thermal insulator. Although Sullivan et al. demonstrated that sensitive TSP can be used to visualize transition boundaries, some problems were encountered with physical defects developing in the paints. A TSP suitable for detecting small temperature differences in aerodynamic testing must meet the following performance requirements: . T .