Measurement of Instantaneous Flamelet Surface Normals and the Burning Rate in a SI Engine 1999-01-3543

A recently developed technique, crossed-plane imaging, is extended to measure instantaneous flamelet surface normals in a single-cylinder, optical SI engine. Two simultaneous, orthogonal acetone PLIF images are used to measure the instantaneous flamelet orientation in three dimensions. The images are also used to measure contours of constant mean reaction progress variable < c> and the mean flamelet crossing density. Statistics of the flamelet surface normal are presented in spherical coordinates in terms of a polar angle, f, and an azimuthal angle,q; the pole is aligned with the normal to a <c> constant surface. The data are used to estimate marginal probability density functions (PDF's) in f and q. The estimated marginal PDF's are found to be well represented by the same functional forms applied previously to turbulent V-flames. The flamelet surface density and the mean fractional increase in flamelet surface area due to turbulence are also estimated. From the flamelet surface density estimates, the mass burning rate is calculated by integration of the volumetric mean rate of creation of products <w> over the cylinder volume. The burning rate is also estimated with a thermodynamic model based on the cylinder pressure/volume history. Burning rates estimated with the thermodynamic model are systematically lower than burning rates estimated by taking the volume integral of <w>, which suggests that stretch, strain, curvature and unsteadiness are important effects for these flames, and can substantially reduce the flamelet combustion rates.