Spatial-averaging effects in Raman / Rayleigh measurements in a turbulent flame

Spatial-averaging effects on laser-based measurements of scalar quantities in turbulent flames are investigated. Raman/Rayleigh measurements are made in the shear layers of a turbulent reverse flow reactor (RFR) with two different probe volumes to study the effects on the measured mean, rms, and probability density functions (pdfs). In addition, the errors in the measured variance are estimated by scalar spectrum analysis for an isotropic flow, using the Pao-Corrsin scalar spectrum. The data show that spatial averaging reduces the rms fluctuations of the species concentrations and Rayleigh signal. Also, the form of the pdf is affected in such a way that for a monomodal distribution, it may reduce any sharp peak, whereas for a bimodal distribution it reduces the peaks and fills the gap in between, i.e, reduces its bimodality. These errors are slightly higher for reacting species and the Rayleigh signal data than for nonreacting species. For the measurements reported here the effects are not large enough to change the overall shape of the pdf greatly. The analytical estimates agree quite well with the experimental data, and the theory shows that the effect of the length of the sensor on the variance depends on both the integral scale of the turbulence and the microscale of the scalar, which are related by the turbulence Reynolds number.