Crossed-plane laser tomography: Direct measurement of the flamelet surface normal

Abstract

Crossed-plane tomography, a new laser tomographic technique, is used to directly measure for the first time the instantaneous flamelet surface orientation in premixed turbulent flames. Simultaneous orthogonal tomographic images provide planar data that can be used to calculated the flamelet surface normal, N. The beam from a pulsed Nd:YAG laser is split and rendered into two orthogonal sheets that intersect along a horizontal line, z. above the burner face. The flame boundaries in the planes of the two laser sheets are recorded simultaneously using digital cameras. The technique relies on the use, of polarization of the laser illumination sheets to block, in the view of one plane, the light scattered from the other plane so that the flame boundary and boundary tangent vector can be evaluated without interference, from the other sheet, N is determined from the images by taking the cross-product of the flame-boundary tangent vectors at those points where the boundaries intersectz. The technique is evaluated by measurements on a laminar flame perturbed by a two-dimensional von Kármán vortex street. Data for six turbulent flame conditions are presented and discussed. The probability density function (PDF) of azimuthal angles of N with respect to rotation about the mean normal, <N>, is found to be uniform for all turbulent flames studied, while the surface-weighted PDF of the corresponding polar angles can be fit to the form Ps()=A exp[−(/σ)2] in all cases studied. The mean inverse direction cosine of N with respect to z is calculated, and the burning rate integral, BT, is estimated, BT results are compared with data obtained by an independent method and are found to be in good agreement with those data.