Thermometry and transient flow evolution in the FL-63 wind tunnel using toluene two-color PLIF

Accurate measurement of aerothermodynamic states in pulsed supersonic wind tunnels is essential for evaluating hypersonic vehicle environments. This study investigates quantitative, non-intrusive thermometry based on toluene two-color planar laser-induced fluorescence (PLIF) in the FL-63 pulsed combined high-enthalpy wind tunnel operated in Ludwieg-tube mode at Ma 4.0. A static calibration system consisting of a tube furnace and a 266 nm Nd:YAG laser was first established to measure toluene fluorescence spectra from room temperature to 473 K. By coupling the measured spectra with the transmittances of the BP280 and WG295 filters, a temperature inversion model based on the dual-band intensity ratio was obtained (R² = 0.982). The model was then applied to flow-visualization and thermometry experiments over a 20-degree wedge, with and without a transition band. The extrapolated freestream static temperature agrees well with theoretical and numerical values, and the measurements clearly resolve the shock-induced local temperature rise associated with the transition-band configuration. Furthermore, a time-series evolution analysis of the absolute fluorescence intensities and their corresponding ratio in the shock wave and freestream regions revealed a step-like signal decay law corresponding to the step-like pressure drops characteristic of the Ludwieg tube. Based on this physical law, the flow field evolution was precisely demarcated into four stages, identifying a structurally stable and effective test window lasting approximately 570 ms. The results demonstrate that toluene two-color PLIF can provide simultaneous flow-structure visualization, quantitative 2D thermometry, and test-window assessment in pulsed supersonic facilities.