Pressure and Temperature Sensitive Paints

The aim of this book is to provide a systematic description of pressure and temperature sensitive paints (PSP and TSP) developed since the 1980s for aerodynamics/fluid mechanics and heat transfer experiments. PSP is the first global optical technique that is able to give non-contact, quantitative surface pressure visualization for complex aerodynamic flows and provide tremendous information on flow structures that cannot be easily obtained using conventional pressure sensors. TSP is a valuable addition to other global temperature measurement techniques such as thermographic phosphors, thermochromic liquid crystals and infrared thermography. This book mainly covers research made in the United States, Japan, Germany, France, Great Britain and Canada. Excellent work on PSP in Russia has been described in the book “Luminescent Pressure Sensors in Aerodynamic Experiments” by V. E. Mosharov, V. N. Radchenko and

S. D. Fonov of the Central Aerohydrodynamic Institute (TsAGI).

We are truly grateful to our colleagues in the field of PSP and TSP for kindly providing their paper drafts, offering comments, and allowing us to use their published results. Without their helps, this book cannot be completed. Especially, we would like to thank the following individuals and organizations:

T. Amer, K. Asai, J. H. Bell, T. J. Bencic, O. C. Brown, G. Buck, A. W. Burner, S. Burns, B. Campbell, B. F. Carroll, L. N. Cattafesta, J. Crafton, R. C. Crites, G. Dale, R. H. Engler, R. G. Erausquin, W. Goad, L. G. Goss, J. W. Gregory, M. Gounterman, M. Guille, M. Hamner, J. M. Holmes, C. Y. Huang, J. P. Hubner, J. Ingram, H. Ji, R. Johnston, J. D. Jordan, M. Kameda, M. Kammeyer, J. T. Kegelman, N. Lachendro, J. Lepicovsky, Y. Le Sant, X. Lu, Y. Mebarki, R. D. Mehta, K. Nakakita, C. Obara, D. M. Oglesby, T. G. Popernack, W. M. Ruyten, H. Sakaue, E. T. Schairer, K. S. Schanze, M. E. Sellers, Y. Shimbo, K. Teduka, S. D. Torgerson, B. T. Upchurch, A. N. Watkins.

NASA, ONR, AFOSR, Boeing, Raytheon, Japanese NAL.

Quantitative measurements of surface pressure and temperature in wind tunnel and flight testing are essential to understanding of the aerodynamic performance and heat transfer characteristics of flight vehicles. Pressure data are required to determine the distribution of aerodynamic loads for the design of a flight vehicle, while temperature data are used to estimate heat transfer on the surface of the vehicle. Pressure and temperature measurements provide critical information on important flow phenomena such as shock, flow separation and boundary-layer transition. In addition, accurate pressure and temperature data play a key role in validation and verification of computational fluid dynamics (CFD) codes. Traditionally, surface pressure is measured by utilizing a pressure tap or orifice at a location of interest connected through a small tube to a pressure transducer (Barlow et al. 1999). Hundreds of pressure taps are needed to obtain an acceptable pressure field on a complex aircraft model. Manufacturing, tubing and preparing such a model for wind tunnel testing is very labor-intensive and costly. For thin models such as supersonic transports, military aircraft and small fan blades, installation of a large number of pressure taps is impossible. Furthermore, pressure measurements at discrete taps ultimately limit the spatial resolution of measurements such that some details of a complex flow field cannot be revealed. Similarly, a surface temperature field is traditionally measured using temperature sensors such as thermocouples and resistance thermometers distributed at discrete locations (Moffat 1990).

Since the 1980s, new optical sensors for measuring surface pressure and temperature have been developed based on the quenching mechanisms of luminescence. These luminescent molecule sensors are called pressure sensitive paint (PSP) and temperature sensitive paint (TSP). Compared with conventional techniques, they offer a unique capability for non-contact, full-field measurements of surface pressure and temperature on a complex aerodynamic model with a much higher spatial resolution and a lower cost. Therefore, they provide a powerful tool for experimental aerodynamicists to gain a deeper understanding of rich physical phenomena in complex flows around flight vehicles.

Both PSP and TSP use luminescent molecules as probes that are incorporated into a suitable polymer coating on an aerodynamic model surface. In general, the luminophore and polymer binder in PSP and TSP can be dissolved in a solvent; the resulting paint can be applied to a surface using a sprayer or brush. After the solvent evaporates, a solid polymer coating in which the luminescent molecules are immobilized remains on the surface. When a light of a proper wavelength illuminates the paint, the luminescent molecules are excited and the luminescent

light of a longer wavelength is emitted from the excited molecules. Figure 1.1 shows a schematic of a generic luminescent paint layer emitting radiation under excitation by an incident light.

The luminescent emission from a paint layer can be affected by certain physical processes. The main photophysical process in PSP is oxygen quenching that causes a decrease of the luminescent intensity as the partial pressure of oxygen or air pressure increases. The polymer binder for PSP is oxygen permeable, which allows oxygen molecules to interact with the luminescent molecules in the binder. For certain fast-responding PSP, a mixture of the luminophore and solvent is directly applied to a porous solid surface. In fact, PSP is an oxygen-sensitive sensor. By contrast, the major mechanism in TSP is thermal quenching that reduces the luminescent intensity as temperature increases. TSP is not sensitive to air pressure since the polymer binder used for TSP is oxygen impermeable, while due to the thermal quenching PSP is intrinsically temperature-sensitive. After PSP and TSP are appropriately calibrated, pressure and temperature can be remotely measured by detecting the luminescent emission. PSP and TSP are companion techniques because they not only utilize luminescent molecules as probes, but also use the same measurement systems and similar data processing methods.