A temperature correction has been developed to enable the extraction of pressure images and corresponding pressure data from images of photoluminescence of pressure-sensitive paints (PSPs). These paints are used on wind-tunnel models for mapping surface pressures associated with supersonic flow fields. PSP has been successfully used in wind-tunnel test ranging from 60 mi/h (97 km/h) to the supersonic range >3. The photoluminescence of an ideal PSP would depend on pressure only, but the photoluminescence of a real PSP depends on temperature also. In order to extract a pressure image, one must be able to invert the luminescence-image data on the basis, not only of the pressure dependence but also of the temperature dependence and of the distribution of temperature on the painted surface; in other words, one needs to incorporate a temperature correction into the pressure calibration of the luminescence of the paint.
A PSP contains luminophores (basically, dye molecules), that luminesce in a suitable wavelength range in response to photoexcitation in a shorter wavelength range. The photoluminescence is quenched by oxygen at a rate proportional to the partial pressure of oxygen and thus proportional to the pressure of air. As a result, the intensity of photoluminescence varies inversely with the pressure of air, and the basic equation for calibrating a photoluminescence image is the following:
P/PREF = A + BIREF/I,
where P is the unknown pressure that one seeks to determine under the test condition (e.g., in the presence of wind), PREF is the pressure under a reference condition (e.g., in the absence of wind), IREF is the intensity of luminescence under the reference condition, and I is the intensity of luminescence under the test condition. The need for temperature correction arises because A and B depend on temperature.
The temperature correction is based on the experimental observation that in the above equation for P/PREF, the combined effects of pressure and temperature can be expressed by use of a corrected intensity ratio; that is,
P/PREF = A + B[IREF/I]CORR
The corrected intensity ratio is given by
[IREF/I]CORR = (IREF/I)(CT2+DT+E),
where T is the absolute temperature and C, D, and E are constants obtained via a least-squares best fit to calibration data acquired by use of a PSP test rig. Alternatively, calibrations usually use a combination of PSP calibration cell data and conventional pressure taps. The taps are used as truth points to help set an absolute level since this is a delta (change) pressure measurement and the largest error is typically the absolute pressure level. The figure presents calibration plots for a PSP, as they appear before and after the application of the temperature correction.
Of course, in order to be able to apply the temperature correction to PSP images of a model in a wind-tunnel test, one must know the temperature distribution on the model during the test. A temperature image can be acquired by coating the model with a temperature-sensitive paint (TSP) and testing it under the same conditions as those used when it is coated with PSP. Alternatively, this can help for a uniform temperature change but does not help for correcting for temperature gradients on the test article. Large temperature gradients exist on thermally conductive models that need full field temperature measurements to fully compensate for the temperature sensitivity of PSP.
This work was done by Timothy J. Bencic of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category.
Inquiries concerning rights for the commercial use of this invention should be addressed to:
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Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 – 8
21000 Brookpark Road
Cleveland, Ohio 44135.
Refer to LEW-16915.