Assessment of k-ε Turbulence Model in KIVA-II by In-Cylinder LDV Measurements 952385

In-cylinder measurements of turbulent integral length scales, carried out during the last 60 degrees of the compression stroke at 600 and 1,000 rpm by a two-probe volume LDV system, were used to assess the capability of the k-ε model used in KIVA-II code.
The objective of the paper is to address the following question: what is the most reasonable definition of turbulent length scale in the k-ε model for engine applications? The answer derived from the comparison between KIVA predictions and experiments that showed a fair agreement between the computed turbulent length scale and the measured lateral integral length scale. The agreement is a result of proper choice of the initial swirl ratio and turbulent kinetic energy at inlet valve closure (IVC) by taking into account the LDV measurements and the value of the constant Cμε in the k-ε model equations that relates the turbulent length scale to k and ε.
IN RECIPROCATING ENGINES the fuel evaporation and mixing processes are strongly influenced by the turbulent nature of the in-cylinder flows. The velocity gradient in the mean flow is the major source of energy for the turbulent velocity fluctuations. The air-jet created by flow during the intake stroke interacts with the cylinder wall and moving piston to generate large-scale rotating flow, both in the vertical and horizontal planes. The behavior of the in-cylinder turbulent flow can be characterized by monitoring the kinetic energy and the integral length scale evolution of the turbulent eddies that contribute to the turbulence production during intake and compression strokes [1].
The objectives of this work are to measure and model the in-cylinder flow field and compare the measurements to predictions, assessing the k-ε turbulence model within the 3-D KIVA-II code [2].
It is well known that the k-ε turbulence model [3] is widely used in many simulation codes for computing flows in internal combustion engines [4, 5]. However, the model was developed to simulate steady pipe flows and it is not yet understood if the turbulence model could predict the complex non-stationary turbulent flow in engines. For this reason, it is still important to evaluate the model capability using measurements of turbulence intensity and integral length scales carried out in real engines.
The present paper outlines measurements taken in a motored engine using a two-probe volume laser Doppler velocimetry (LDV) system. The lateral integral length scale has been computed from the spatial correlation coefficient of the ensemble fluctuating velocity for different separations [6, 7].
The mean tangential velocity and the turbulent kinetic energy measured within the cylinder in a point located at 5 min below the engine head and 26 min from the cylinder axis has been used as initial conditions for the code. Comparison between the measured integral length scales and those computed has been performed.


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