Development of tunable diode laser spectroscopy sensor systems for line-of-sight measurement of water vapour in aero engine exhaust plumes

Measurements of exhaust plume water vapour concentration and temperature are acquired on the Rolls-Royce Environmentally Friendly Engine (EFE) demonstrator. Line-of-sight tunable diode laser spectroscopy (TDLS) measurements yield the absorption spectra of water vapour downstream of the engine exhaust, using direct measurement and wavelength modulation spectroscopy (WMS) to recover absorption from both the high temperature plume and the surrounding ambient airflow. The annular array of 24 exhaust nozzles has been identified as a suitable platform for future tests to create tomographic maps of exhaust products.
Initial field trials were performed to evaluate optical system performance in the harsh environments experienced downstream of aero engine exhaust nozzles. Beam steering in the exhaust plume and highly vibrating mounting hardware provide the greatest challenges to achieving adequate signal-to-noise ratios. The optical layout of the test bed installation and the overall experimental system is described. The recovered spectra are used to fit a theoretical signal, created using an assumed temperature and concentration distribution based on anticipated plume dimensions, showing plume temperatures up to 850 K. The presence of water vapour hot lines is also confirmed using high sensitivity WMS techniques and these are compared to lines predicted by the HITRAN 08 spectral database