High Resolution Scalar Dissipation and Turbulence Length Scale Measurements in an Internal Combustion Engine

High resolution planar laser-induced fluorescence (PLIF) measurements were performed in an optically accessible internal combustion (IC) engine to investigate the behavior of scalar dissipation and the fine-scale structures of the turbulent scalar field. The fluorescent tracer fluorobenzene was doped into one of the two intake streams and nitrogen was used as the carrier gas to permit high signal-to-noise ratio fluorescence measurements without oxygen quenching effects. The resulting two-dimensional images allowed for an analysis of the structural detail of the scalar and scalar dissipation fields defined by the mixing of the two adjacent intake streams. High levels of scalar dissipation were found to be located within convoluted, sheet-like structures in accordance with previous studies. The fluorescence data, which were acquired during the intake stroke, were also used to examine the scalar energy and dissipation spectra. The measured spectra were found to closely match model turbulent energy and dissipation spectra over the full range of wavenumbers for all conditions. The Batchelor length scale was directly measured using the point where the dissipation spectrum fell to 2% of its peak value and was found to range from 29 to 34 μm. The integral length scale and Taylor microscale, were determined from the spatial autocorrelation of the scalar field, and were found to range from 3.03 to 7.95 mm and from 0.54 to 1.30 mm, respectively. In general, the turbulence length scales were slightly different in the two dimensions captured in the fluorescence images, suggesting the turbulent scalar field is slightly anisotropic during the intake stroke. The length scales were found to vary only slightly with intake manifold pressure (60 - 109 kPa) and engine speed (600 - 1200 RPM).