LDV Measurements of Integral Length Scales in an IC Engine 961161
Tangential component of velocity and turbulence were measured in three locations in the re-entrant combustion chamber of a motored single-cylinder d.i. Diesel engine (0.435 liter, 21:1 compression ratio) using a Laser Doppler Velocimetry system. Moreover, a modified LDV system with two-probe volume was used to measure directly lateral integral length scales of the velocity tangential component at two engine speeds. The measurements were made on a horizontal plane at 5 mm below the engine head from 100 degrees before TDC to 60 degrees after TDC of both the compression and expansion strokes. The engine was motored at 1,000 and 1,500 rpm respectively. An ensemble-averaging technique was performed to analyze the instantaneous velocity information supplied by two Burst Spectrum Analyzers. The lateral integral length scale was obtained from the integral of the spatial correlation coefficient of the velocity fluctuation for different separation.
The results demonstrated, for all points investigated, that the turbulence increases during the last 30 degrees of the compression stroke deducting energy from the mean motion that consequently decreases. At the location closer to the combustion chamber wall the turbulence, after a first decrease at the beginning of the expansion stroke, reaches a relative maximum as consequence of the air coming out from the combustion bowl, then definitely decreases.
The lateral integral length scale decreases during the compression stroke reaching a broad minimum (about 1 mm) near TDC and increases during the expansion. It shows a dependence on the clearance height but seems to be not sensitive to engine speed.