Toluene-based planar laser-induced fluorescence imaging of temperature in hypersonic flows

Abstract

Planar laser-induced fluorescence imaging is carried out in a hypersonic gun tunnel at a freestream Mach number of 8.9 and Reynolds number of $47.4 \times 10^6\,\hbox {m}^{-1}$ ($N_2$ is the test gas). The fluorescence of toluene $(C_7H_8)$ is correlated with the red shift of the emission spectra with increasing temperature. A two-colour approach is used where, following an excitation at 266 nm, emission spectra at two different bands are captured in separate runs using two different filters. Two different flow fields are investigated using this method: (i) hypersonic flow past a blunt nose, which is characterised by a bow shock with strong entropy effects, and (ii) an attached shock-wave/boundary-layer interaction induced by a flare located further downstream on the same blunt cylinder body. Measurements from as low as the freestream temperature of $68.3$ K all the way up to $380$ K $(T_{\infty }-5.6T_{\infty })$ are obtained. The uncertainty at the higher temperature level is approximately $\pm 15$ %, while at the low end of the temperature, an additional $\pm 15$ % uncertainty is expected. Application of the technique is further challenged at high temperatures due to the exponentially reduced fluorescence quantum yields and the occurrence of toluene pyrolysis near the stagnation region ($T_\mathrm{o}=1150$ K). Overall, results are found to be within $10$ % agreement with the expected distributions, thus demonstrating suitability of the technique for hypersonic flow thermometry applications in low-enthalpy facilities.