Coherent Stokes Raman Scattering for Simultaneous Measurement of CO2 and CH4 Temperatures

Abstract

Experimental measurements of fuel and flame temperatures in a methane/air co-flow diffusion flame are performed using coherent Stokes Raman scattering. A three-beam hybrid fs/ps configuration is employed such that the excitation bandwidth of the ultrafast pulses includes the rovibrational transitions in the Fermi-dyad of CO ${}_2$ and the $v_2$ mode of CH ${}_4$. Time-domain spectroscopic models were developed and employed to simultaneously extract the CH ${}_4$ and CO ${}_2$ temperatures separately from the same CSRS spectra. Axial and radial temperature profiles of CH ${}_4$ and CO ${}_2$ in the diffusion flame are presented. Methane temperatures ranged from 300 to 900 K while CO ${}_2$ temperatures ranged from 500 to 2500 K. At probe volume locations higher above the burner, the fuel and product temperatures were in strong agreement. Measurement locations near the methane injector revealed inhomogeneities between fuel and flame temperatures before significant mixing by diffusion. For the first time, a three-beam ultrafast CSRS configuration is demonstrated to simultaneously measure differing CH ${}_4$ and CO ${}_2$ temperatures.