Laser-induced fluorescence imaging in free-piston shock tunnels

Abstract

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PLIF temperature measurements in a free-piston shock tunnel are presented. The viability of performing quantitative temperature measurements is established and application considerations are detailed. A single laser/camera system was used to measure rotational and vibrational temperatures in the freestream and in the shock layers on a wedge, a cylinder and a hemisphere. Two different laser sources were used to excite NO via the AΣ ← XΠ transition between 225 and 245 nm. The first was a Raman-shifted tunable excimer laser which was restricted to excitation of the (0,0) band near to 226 nm, and therefore was only utilised for rotational temperature measurements. The other was an excimer-pumped frequency-doubled dye laser which was used to access the first three vibrational bands (0,v=0–2) and to perform rotational and vibrational temperature measurements. Temperature measurements are compared with computations from CFD codes in an ongoing procedure of testing and validation. The calculations were performed with a quasi-one-dimensional nozzle flow code (STUBE) which has been modified to include vibrational freezing phenomena. The measured rotational temperature of 439 ± 20 K compares well with the calculated freestream rotational temperature of 396 ± 10 K. A vibrational temperature of 4500 ± 270 K was measured in the stagnation region on the hemisphere. This measurement was estimated to be systematically high by 160 K due to variations in the overlap integral between 225 and 245 nm. The temperature calculated using STUBE was 4146 K. The measured freestream and hemisphere temperatures are higher than those calculated, which questions the validity of the nozzle-reservoir temperature used in the CFD calculations and the method used to obtain it. The freestream vibrational temperature of NO was measured to be 890 ± 30K which is greater than the calculated value of 670 K. This was anticipated due to the limited vibrational relaxation data available for the calculation.