Interpretation of k-ε computed turbulence length-scale predictions for engine flows

Abstract

Three-dimensional computations of motored-diesel engine flows were made and compared with measured turbulence parameters. The tangential component of velocity, turbulence intensity, and integral length scale were measured in an engine featuring a re-entrant bowl combustion chamber (0.435 L. 21:1 compression ratio) using a laser doppler velocimetry (LDV) system. A two-probe-volume system was used to measure directly the lateral integral length scales of the tangential (swirl) velocity component. The measurements were made in a horizontal plane 5 mm below the engine head from 100° before top dead center (TDC) in the compression stroke to 60° after TDC in the expansion stroke. The engine was motored at 600, 1000, and 1500 rev/min. An ensemble-averaging technique was used, and the lateral integrallength scales were obtained from the correlation coefficients of the velocity fluctuations at two separated spatial points. The computations considered two turbulence models, namely, a standard k-ε and a modified RNG k-ε model. The results show that for nonequilibrium, rapidly distorted engine turbulence, the macro length seale ( k 3,2 /ε ) from two-equation turbulence models is not directly proportional to the integral length scale, as is the case in equilibrium turbulence. Instead, a new interpretation of the turbulence model length scale is given for rapidly distorting engine flows and is shown to be proportional to the product of the integral length scale and the turbulent Reynolds number.