Integral and Micro Time Scales Estimate in a D.I. Diesel Engine 971678
The present paper aims at developing a general method to estimate integral and microtime scales of turbulent in-cylinder flow field in reciprocating engines. The ensemble average technique was used to compute the integral time scale from the single point time autocorrelation function, whereas the microtime scale, representative of the most rapid changes that occur in the fluctuation, was computed as the intercept of the parabola that matches the autocorrelation function at the origin. Further, the microtime scale was also estimated by spectral analysis through the energy spectral density function of the ensemble turbulent fluctuation and the results obtained by the two methods were compared.
The procedures were applied to the tangential component of the instantaneous velocity data collected, at different engine speeds (1,000, 1,500, 2,000 rpm), within a motored d.i. diesel engine equipped with a re-entrant combustion chamber, using the Laser Doppler Anemometry (LDA) technique. Measurements of the tangential component of the instantaneous velocity were carried out on a horizontal plane at 5 mm below the engine head for different radii from 70° before top dead center (btdc) to 30° after top dead center (atdc) of both compression and expansion strokes.
The main results have shown that the integral time scale shows a global decrease along the compression stroke, reaching a minimum around top dead center (tdc), and has shown a slight increasing pattern during the first 20° of expansion. The micro time scales, obtained both by the time autocorrelation function and the energy spectral density function, have shown a similar trend with a minimum around tdc. Both integral and micro time scales have shown an approximately inverse scaling with engine speed.