Cycle-Resolved LDV Integral Length Scale Measurements In an I.C. Engine 880381
Lateral integral length scales of the tangential velocity component were measured directly using a two-point, single probe—volume, Laser Doppler Velocimetry system in a motored, ported, single-cylinder I.C. engine with a pancake—shaped chamber. The measurements were made on the mid-plane of the TDC clearance height from 40 degrees before TDC to 25 degrees after TDC. The engine was operated at 600 rpm with a swirl ratio at TDC of approximately 4. Both an ensemble and a cycle-resolved statistical analysis were performed. Three compression ratios (5.7, 7.6, and 11.4) were used. They correspond to TDC clearance heights of 18.1 mm, 12.8 mm, and 8.2 mm, respectively. Contrary to expectations, both the lateral turbulence integral length scale (deduced from the cycle-resolved analysis) and the lateral fluctuation integral length scale (deduced from simple ensemble averaging) did not scale with TDC clearance height, but rather were almost independent of it around TDC. At TDC, the turbulence integral length scale was determined for the three cutoff frequencies of 150 Hz, 300 Hz, and 450 Hz to be about 1.1 mm, 0.75 mm, and 0.6 mm, respectively, while the corresponding fluctuation integral length scale was about 2.2 mm. Tills result demonstrates directly, for the first time, the correlation between the larger spatial scales and the lower frequencies of the fluid flow in an I.C. engine. Finally, the lateral turbulence integral length scale decreased monotonicaily over the crankangle window of the measurements, whereas the lateral fluctuation integral length scale reached a broad minimum around 10 degrees before TDC.