High resolution passive scalar dissipation measurements in an internal combustion engine

Petersen, B. R. High Resolution Passive Scalar Dissipation Measurements in an Internal Combustion Engine. University of Wisconsin-Madison, 2009.

High resolution laser-induced fluorescence (LIF) measurements were performed in an optically accessible internal combustion (IC) engine to investigate the in-cylinder turbulence field. This was accomplished by separating the intake system into two independent flow paths and adding a fluorescence tracer to one of the intake streams. Fluorobenzene was used as the fluorescence tracer and nitrogen was used as the carrier gas to permit high signal-to-noise ratio fluorescence measurements without oxygen quenching effects. The fluorescence data were used to examine the scalar energy and dissipation spectra as a function of two Reynolds numbers, one describing the intake flow and one describing the piston-induced flow. The measured scalar spectra were found to closely match turbulent kinetic energy model spectra over the full range of wavenumbers. The Batchelor length scale was measured at the 2% level in the scalar dissipation spectra and was found to range from 29 to 51 ?m. This is the first time, to the author’s knowledge, that direct Batchelor (dissipation) length scale measurements have been performed in an IC engine. Preliminary one-dimensional experiments determined that high fluorescence tracer number densities, high laser pulse energies, and a large spread in scalar values were required to resolve the 2% point in the scalar dissipation spectrum in order to make Batchelor scale measurements.

Two-dimensional experiments were performed to investigate the structures within the turbulent scalar field. Measurements of the Batchelor, integral, and Taylor length scales were made over a wide range of conditions. The integral scale ranged from 1.65 to 11.06 mm and Reynolds number estimates based on the measured Batchelor and integral scales revealed that the characteristic Reynolds numbers were relatively low, ranging from ?100 to ?1,000. The measured Batchelor and integral scales and estimated Reynolds numbers approached the values predicted by Kolmogorov scaling at later times in the compression stroke. The Taylor microscale measurements ranged from 0.54 to 3.18 mm over the range of conditions investigated in this study. Probability density functions (PDFs) of the measured scalar dissipation were also developed from the scalar measurements at all conditions.