Analysis of Wavelength Modulation Spectroscopy for Water Vapour Measurements in Supersonic Combustion

Abstract

In this paper measurements of temperature and mole fraction of water vapour by wavelength modulation spectroscopy (WMS) are presented. This work is focused on hypersonic combustion diagnostics using tunable diode laser absorption spectroscopy (TDLAS) at 1995 nm. The experiments are performed with a vertical cavity surface emitting laser diode (VCSEL) which is tunable by current and temperature. Water vapour temperature and mole fraction are determined by matching the second harmonic (2f) peak height normalized by its first harmonic (1f) value to a database of simulated signals based on the characteristics of the laser. The measurements have been performed on flat flames for proof of concept. Two different flame conditions are investigated and the results are compared with direct absorption measurements using the same equipment and the application of a calibration free WMS method to the acquired data as presented by Rieker et al. The investigated flames have been extensively analysed using coherent anti-stokes Raman scattering (CARS) measurements and equilibrium calculations by Prucker et al. The results using direct absorption agree fairly well with the published CARS data. The applied WMS methods agree well with each other, however, there is a significant discrepancy to the direct absorption measurements at the high temperature condition. Since the resulting values for the two different flame conditions with a nominally large temperature difference are close to each other, independent from the applied WMS method, it is concluded that further research of the WMS technology is required with respect to the applied transitions and its sensitivity to the investigated flows.