1 Preliminary Investigations on Thermometry in Thermal Flows via Transient Grating Spectroscopy (TGS)

The laser-based diagnostic method Transient Grating Spectroscopy (TGS) is investigated as a noninvasive, local and time-resolved temperature measurement technique with respect to its application in thermal flow environments. TGS determines in a probe volume comparable to the Laser Doppler Anemometry (LDA) in a period of a few hundred nanoseconds the local speed of sound and thus the temperature of the flow medium by using the contrast modulation of a pulsed laser induced refraction index grating. The contrast modulation is detected by a continuous-wave probe laser beam. Figure 1 shows a typical TGS-Signal trace recorded on a digital oscilloscope. Compared to other resonant laser-diagnostic applications like Coherent Anti-Stokes Raman Spectroscopy (CARS) the Transient Grating Spectroscopy is experimentally less complex and less ext ensive. This work addresses the dependency of the signal strength and the signal-to-noise ratio on different parameters like ambient pressure, pulsed-laser energy and probe-laser power. The signal strength increases approximately quadratically with the ambient pressure and the pulsed-laser energy. The TGS signal amplitude is linearly related to th