Development of a K-k-ϵ phenomenological model to predict in-cylinder turbulence

Paper #:
  • 2017-01-0542

Published:
  • 2017-03-28
Abstract:
The turbulent flow field inside the cylinder plays a major role in Spark Ignition (SI) engines. Multiple phenomena that occur during the high pressure part of the engine cycle are influenced by in-cylinder turbulence. Early flame kernel development after ignition and flame propagation are strongly dependent on the turbulent intensity distribution inside the cylinder. Flames wrinkled by turbulence propagate at speeds which are much greater than laminar values. Gas-to-wall heat transfer is mainly dependent on forced convection, which is primarily controlled by turbulent charge movement. Turbulence inside the cylinder is primarily generated via high shear flows that occur during the intake process and also close to Top Dead Center (TDC) by the decay of macro-scale motions produced by tumbling and/or swirling structures. Understanding such complex flow phenomena typically requires detailed three dimensional CFD simulations. Such calculations are computationally very expensive and are typically carried out for a limited number of operating conditions. On the other hand, quasi-dimensional simulations, which provide a limited description of the in-cylinder processes, are computationally inexpensive (relative to CFD calculations) and can be carried out for the entire operational map. Such simulations typically use zero dimensional phenomenological sub-models to simulate the various in-cylinder phenomena such as heat transfer, combustion, flow variations etc. The current study presents a newly developed sub-model, available as a part of the software GT-SUITE, which governs the evolution of the mean and turbulent flow inside the cylinder. Within a 0-D context, the accurate knowledge of in-cylinder turbulence levels close to ignition is essential to reliably model turbulent flame propagation. Engines running with stratified charge and/or early spark timings also require the accurate estimation of turbulence levels close to Bottom Dead Center (BDC). The model presented utilizes a K-k-ϵ approach, which is a combination of the existing K-k and k-ϵ flow models in the literature. K is the mean kinetic energy of the flow, k is the turbulent kinetic energy and ϵ is the turbulent dissipation rate. The model is calibrated using results for a small number of cases obtained from motored in-cylinder flow simulations carried out using the CFD software CONVERGE. The calibrated flow model is then validated using further CFD results from a wide variety of cases that include sweeps of parameters such as engine speed, valve lifts, valve timings etc., without performing any case dependent tuning. The results indicate that the newly proposed model has the capability to predict the temporal evolution of in-cylinder flow quantities and responds well to changes in the operating conditions of the engine.
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