A Tunable Diode Laser Absorption Spectrometer for temperature measurement in hypersonic flows

Abstract

Tunable diode laser absorption spectroscopy (TDLAS), as a time-resolved in situ and non-intrusive technique which allows combined measurement of selected gas concentration, temperature and velocity, has shown great potential and advantages in ground tests. Recent advances make it increasingly common to use multiple lasers for simultaneous measurements, this is to say, TDLAS have the capability of temperature tracking measurement of multiple flow field which suggest excellent potential for flow diagnostics. A Tunable Diode Laser Absorption Spectrometer with two specific line pairs near 1.4 μm has been developed for temperature measurement(230K-1000K) in the shock tunnel using two techniques (a scanned-wavelength direct absorption and a scanned-wavelength technique with wavelength modulation and 2f/1f detection). For DAS two DFB diode laser are used to scan over different H2O absorption transitions (7095.85cm-1 and 7168.437cm-1) at a 15kHz repetition rate. For WMS another line pair(7168.43cm-1 and 7185.597cm-1) was selected and the scanned rate is 1kHz with 250kHz modulated rate. In the shock tunnel test with Mach 6, the time-resolved static temperature of the nozzle exit flow is monitored using DAS and that of State 5 shock tube flow is measured by WMS. Based on temperature measurement the experimental investigation on air vitiation effects was carried out. Tunable diode laser absorption measurement is promising for development into routine diagnostic tools for quasi-one dimensional flow in the shock tunnel. And that is important for reliability judgment of aerodynamic/heat ground tests under hyper-sonic flow in the shock tunnel.