Scalar dissipation measurements in the developing region of a jet

Abstract

Instantaneous planar images of CH 4 mass fraction in the developing region ( x d ≤ 17 ) of an isothermal jet were used to calculate instantaneous, mean, root mean square (rms), probability density functions (PDFs), and correlations of scalar dissipation and a scalar. The mass fraction images were derived from CH 4 concentrations obtained by simultaneously recording Raman-scattered light with two cameras oriented perpendicular to a thin sheet of laser light passed through the axis of the jet. Using two cameras enhanced the dynamic range by a factor of ten over a single-camera system and allowed concentration measurements down to a mass fraction of 0.003. Axial and radial components of the scalar dissipation were determined and, by assuming isotropy in the radial and azimuthal directions, total dissipation statistics were obtained. The results were used to evaluate modeling assumptions. The radial and axial dissipation components were found to be strongly nonisotropic in the mixing layer at x d ≤ 13 , but isotropic further downstream. At all locations along the centerline the dissipation was isotropic. The scalar and dissipation were found to be highly correlated in the mixing layer but uncorrelated on the jet axis. The PDF of dissipation on the axis, off axis, and at the stoichiometric surface were all found to be lognormally distributed. This property was used to collapse all of the PDFs onto one plot. The parameters that define the single plot, lognormal PDF mean, and standard deviation are reported. Instantaneous dissipation levels measured at the expected flame liftoff height were found to be consistent with flame extinction limits extrapolated from laminar opposed jet data. For the single condition measured, this data supports the hypothesis that excessive dissipation can extinguish flames in the developing region of jets.