Sensing in Sooting Flames: THz Time-Domain Spectroscopy and Tomography

We present line-of-sight measurements with broadband THz radiation through a methane diffusion flame, with the motivation to develop a modality for sensing and imaging within sooty flames. We present the design details of a bespoke high-pressure burner suitable for THz line-of-sight measurements, as well as hard-field tomography imaging from 4 projections. Further we introduce the setup for THz time-domain spectroscopy (TDS) and show the results of TDS measurements of a methane-air diffusion flame at different values of peripheral air co-flow at 3, 5, 10 and 12 l/min. The time-domain waveforms of the THz electric field transmitted through the flame are processed to extract the values of the time delay for each case. The dependence of these values on the air co-flow reveals that after an initial linear increase, the optical density along the sensing path levels out after 12 l/min. This is interpreted in terms of the radial temperature distribution of the gas flow. The effect of these parameters on the correct measurement setup for flame sensing and imaging at various geometries is briefly discussed.