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Steady and Transient CFD Approach for Port Optimization
Abstract:
The intake and exhaust port design plays a substantial role in performance of combustion systems. The port design determines the volumetric efficiency and in-cylinder charge motion of the spark-ignited engine which influences the thermodynamic properties directly related to the power output, emissions, fuel consumption and NVH properties. Thus intake port has to be appropriately designed to fulfill the required charge motion and high flow performance.
While turbulence intensity and air-mixture quality affect dilution tolerance and fuel economy as a result, breathing ability affects wide open throttle performance. Traditional approaches require experimental techniques to reach a target balance between the charge motion and breathing capacity. Such techniques do not necessarily result in an optimized solution. Progress in development of Computational F luid Dynamics (CFD) tools, Design of Experiment (DOE) and optimization techniques combined with increased computational power led to the development of new methodologies over the past decade. Such advancements have the potential to deliver optimized solutions.
Recent releases of engineering CAD packages, CATIA V5 and Pro-Engineer, enable both parametric modeling and associative design update. This paper demonstrates a coupling procedure of CFD with engineering CAD software using process integration and design optimization software (PIDO). CATIA V5, ICEM-CFD meshing tool and FLUENT-UNS CFD code were integrated to run through many port designs using ISIGHT. The automatic coupling was aimed at optimizing the port layout for a certain cost function such as flow restriction or charge motion, subject to manufacturing and packaging constraints. Accomplishing this task necessitates running the executables of various software using macros and scripts. This integrating methodology utilized best design practices for an intake port, and numerous numerical experiments were attempted.
Based on the above mentioned DOE approach, a few designs were selected along with designs optimized based on charge flow and tumble (turning ability of flow). Further, a steady state analysis at full lift was performed with finer mesh to confirm the coarse mesh results. A sensitivity chart of flow and tumble with respect to each design parameter was derived so as to better understand the design parameter response on the combustion system.
But the steady state approach has its own limitations. It has been shown in the past, high tumble during intake stroke necessarily guarantee high tumble/turbulence at ignition. After the intake valve closing, the chamber and piston shape play bigger role in preserving the tumble. To determine the effectiveness of optimized shape, it is essential to do transient analysis. The transient analysis allows the detailed analysis of the in-cylinder system with more realistic assumptions and boundary conditions. This leads to better understanding of the flow characteristic at various valve lifts as well as evaluation of significant quantities like turbulent kinetic energy (TKE) and tumble values at the ignition.
This paper is a continuation of the effort initiated by Korivi et. al. [1,5] in terms of extending the optimization approach to transient analysis. In the present study, all the selected designs were further meshed using ES-ICE for the transient simulations. ES-ICE coupled with STAR-CD provides very prominent tool for moving mesh transient in-cylinder simulation. A novel approach was employed for faster turn around time in which chamber along with exhaust port was meshed only once and intake ports were meshed separately. Various parameters like turbulent kinetic energy, tumble ratio, valve curtain utilization area along with flow field inside the chamber were visualized.