Optimization of polymer-based PSP for cryogenic wind tunnels

In this paper, a new pressure-sensitive paint (PSP) formulation using a high polymer called poly(TMSP) is presented and its optimization for cryogenic wind tunnels is discussed. In our earlier work, we proved the feasibility of poly(TMSP)-PSP for pressure measurements at low temperature in low-oxygen environment of the cryogenic wind tunnel. However, we also found it difficult with this paint to achieve stable pressure measurements because its oxygen sensitivity was changed from a test to a test. In order to solve this problem, PSP samples with different polymer concentrations in solvent and/or dye concentrations were prepared and calibrated in air, low-oxygen, and cryogenic conditions. We found from these systematic tests that the characteristics of poly(TMSP)-PSP was sensitive to its preparation procedure, particularly to;polymer concentration in solvent. The PSP exhibited high sensitivity to oxygen when its polymer concentration was higher than a certain critical value. This dependency on polymer concentration was observed only at oxygen concentrations lower than 1000 ppm. The sample tests at cryogenic temperatures showed that Poly(TMSP)PSP had the maximum oxygen sensitivity when its polymer concentrations in solvent was from 8 to 10 mg/ml. It was also found that dye concentration in PSP had an influence on the shape of a Stern-Volmer curve. To validate the results obtained from the sample tests, poly(TMSP)-PSP with the optimized formulation was tested on a circular-arc bump model in the NAL 0.1-m Cryogenic Wind Tunnel. It was confirmed that the optimized poly(TMSP)-PSP could provide,a stable and high signal-to-noise pressure data at the temperature of 100 K and the oxygen concentration of 1000 ppm.