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Applying an Interactively Coupled CFD-Multi-Zone Approach to Study the Effects of Piston Bowl Geometry Variations on PCCI Combustion
Abstract:
Recently, a consistent mixing model for the two-way coupling of a CFD code and a zero-dimensional multi-zone code was developed. This work allowed for building an interactively coupled CFD-multi-zone approach that can be used to model HCCI combustion.
In this study, the interactively coupled CFD-multi-zone approach is applied to PCCI combustion in a 1.9l FIAT GM Diesel engine. The physical domain in the CFD code is subdivided into multiple zones based on one phase variable (fuel mixture fraction). The fuel mixture fraction is the dominant quantity for the description of nonpremixed combustion. Each zone in the CFD code is represented by a corresponding zone in the zero-dimensional multi-zone code. The zero-dimensional multi-zone code solves the chemistry for each zone, and the heat release is fed back into the CFD code. The thermodynamic state of each zone, and thereby the phase variable, changes in time due to mixing and source terms (e.g., vaporization of fuel, wall heat transfer). An elaborated mixing model and an appropriate treatment of the source terms keep the thermodynamic state of the zones in the CFD code and the zero-dimensional multi-zone code identical.
The results obtained with the interactively coupled CFD-multi-zone approach are compared to experimental results for five selected operating conditions, showing very good agreement. In a following step, the engine geometry used in the numerical simulations is modified, yielding two different piston-bowl geometries which vary from the original shape in regard to bowl diameter and bowl depth. The volume of the bowl and thus the compression ratio are kept at the original value. Simulations of PCCI combustion using these modified geometries show the influence of the piston-bowl shape on mixture formation and pollutant emissions, leading the way to numerically optimise the piston-bowl design for PCCI combustion.