What is Temperature-Sensitive Paint?
Traditional measurement techniques for acquiring surface temperature distributions on models have utilized embedded arrays of thermocouples and RTD’s. This requires significant construction and setup time while producing data with limited spatial resolution. An alternative approach is to use temperature sensitive paint (TSP) to measure surface temperature. The advantages of temperature sensitive paint include non-intrusive measurements and high spatial resolution when compared to conventional measurement techniques. Image based temperature measurements using TSP are accomplished by coating the model surface with the paint and illuminating the surface with light of the appropriate wavelength. The luminescence from the surface is recorded using a CCD camera through a long-pass filter to separate the luminescent signal from the excitation light. The luminescence from the TSP is a function of the local temperature, and therefore, each pixel on the camera acts as a thermocouple.
The photo-physical process governing the operation of a TSP is outlined here. We combine a luminescent molecule and a polymer binder. Generally, we prefer a binder that is impermeable to oxygen. The molecule is excited by the absorption of a photon. From the excited state the molecule has several competing relaxation paths. The path of interest for TSP is known as thermal quenching, a non-radiative decay mechanism. This deactivation results in a system where the luminescent intensity from the molecule is a function of the temperature to which the molecule is exposed. To determine this intensity versus temperature relationship, a sample of the TSP is placed in a calibration chamber. The sample is exposed to a series of temperatures and the luminescent intensity of the sample is recorded. For reasons that will be discussed later, each intensity is normalized by the intensity at a reference condition and plotted versus temperature. A plot of the calibration of Ru(bpy) in Shellac, the TSP used in this investigation, is shown here.
A typical TSP consists of the luminescent molecule and an oxygen impermeable binder. The basis of the temperature sensitive paint method is the sensitivity of the luminescent molecules to their thermal environment. The luminescent molecule is placed in an excited state by absorption of a photon. The excited molecule deactivates through the emission of a photon. A rise in temperature of the luminescent molecule will increase the probability that the molecule will return to the ground state by a radiationless process, this is known as thermal quenching. The temperature of the painted surface can be measured by detecting the fluorescence intensity of the luminescent paint.
At this point, we have luminescent paint that is sensitive to the local thermal environment. The goal is to build an image based temperature measurement system using this paint. First the surface is coated with the paint and the surface is illuminated with excitation of the appropriate wavelength. The surface is imaged through a long pass filter using a scientific grade CCD camera. The filter separates the illumination from the red shifted emission of the luminescent molecule. The luminescent intensity distribution is recorded and stored for conversion to temperature using a previously determined calibration.
Unfortunately the luminescent intensity distribution is not only a function of the temperature. In fact the luminescence from the painted surface varies with illumination intensity, paint layer thickness, and probe distribution. If we assume that these parameters don't vary in time however, they can be eliminated by taking the ratio of the image at the test condition or wind-on image to an image taken at a known reference condition or wind-off image. This wind-off to wind-on ratio is often referred to as radiometric TSP.
To determine this intensity versus temperature relationship, a sample of the TSP is placed in a calibration chamber. The sample is exposed to a series of temperatures and the luminescent intensity of the sample is recorded. To verify that the TSP is insensitive to pressure, the calibration often includes a series of pressures. Each intensity is normalized by the intensity at a reference condition and plotted versus temperature. A plot of the calibration of Ru(bpy) in Shellac, a commonly used TSP, is shown here.
One of the first uses of TSP is for the detection of boundary layer transition. In this application, the heat flux to the surface, and therefore the surface temperature, is a function of the local heat transfer coefficient. As the heat transfer coefficient is substantially larger for a turbulent boundary layer as compared to a laminar boundary layer, a sharp increment in the surface temperature is indicative of transition. The movie shows the evolution of the surface temperature of a 5 degree cone in the Mach 6 Ludwig tube at Purdue University. A small transition bump is located just upstream of the field of view. Note the wedge of high surface temperature located behind the transition bump.
An example of the use of TSP for aerodynamics measurements is the measurement of heat transfer. In this example, a sonic under-expanded jet is impinging on a flat plate at an inclined angle. The surface is illuminated using a pair of LM2-470 LED arrays and imaged using a CCD camera through a 570-nm long pass filter. The heat flux is generated by heating an aluminum plate with resistive heating. A thin layer of Mylar (0.1-mm) is applied to the surface of the plate and a TSP is applied over the film. The plate is heated and images of the TSP are acquired and converted to temperature using the calibration. The surface temperature distribution is converted to heat transfer using a heat transfer model. In this case, the simple 1D model conduction model shown in the figure was used.
An example of the use of TSP for aerodynamics measurements is the measurement of heat transfer. In this example, a sonic under-expanded jet is impinging on a flat plate at an inclined angle. The the measurement involves the heat transfer on the