Cecile Pera

Application of Generalized RNG Turbulence Model to Flow in Motored Single-Cylinder PFI Engine

Abstract This paper describes a generalized renormalization group (RNG) turbulence model applied to simulate non-reacting flows in an optical single-cylinder PFI engine. A structured computational mesh of the combustion system with complex geometry was generated by ICEM-CFD in conjunction with KIVA-3V code. Turbulent flow in the 4-valve engine, including the exhaust, intake, compression and expansion strokes, was simulated with the standard k – ε and a generalized RNG turbulence model using the KIVA-3V code. Crank angle-resolved results from available experimental data were used as the boundary and initial conditions for the calculation setup. Pressure traces of the simulation results were compared to the phase-averaged measured pressure trace. Predicted radial and vertical velocities along a horizontal line at BDC and radial velocities along the cylinder axis at four crank angles were compared with the experimental measurements. In addition, the velocity field calculated by the generalized RNG turbulence model was compared with experimental data from Particle Image Velocimetry (PIV) measurements. Good agreement was found between the experiment results and simulation results with the generalized RNG turbulence model.

Evaluation of different turbulent combustion models based on tabulated chemistry using DNS of heterogeneous mixtures

This study used direct numerical simulations (DNSs) of combustion processes in turbulent heterogeneous mixtures for self-igniting partially-premixed configurations to assess the accuracy of partially-premixed turbulent combustion models that are based on the tabulation of chemistry progress in homogeneous reactors (HRs). DNS coupled with n-heptane/air detailed chemistry solving was considered as a reference result. Because the same detailed chemistry was used to generate the chemistry databases, the study was focused entirely on validating the modelling assumptions. Various HR-based tabulation models were tested: (1) the tabulated homogeneous reactor (THR) model, which is a direct exploitation of HR tabulation lacking any statistical information concerning mixture heterogeneity; (2) the presumed conditional moment (PCM) model, which includes a limited statistical description of the mixture and/or of the combustion advancement; (3) approximated diffusion flame (ADF) models, which consider the heterogeneous turbulent reactor as either a unique diffusion flame (simple ADF model formulation) or as a collection of flamelets with different strain rates (ADFχ model). The a priori response of the above mentioned models was compared with detailed chemistry DNS results. The main findings are as follows: (i) a direct use of HR tabulation (THR model) led to overly inaccurate results; (ii) an assumed independence between mixture fraction and progress variable was responsible for most PCM modelling failures in the context of turbulent heterogeneous self-ignited combustion; (iii) the presumed β-function of the progress variable distribution is likely to fail because of the complexity of autoignition kinetics; (iv) the best results were obtained with the ADF models; (v) a simple ADF formulation is preferable to ADFχ, which showed limitations in terms of accuracy concerning the distribution of the progress variable; (vi) all tested models provided an acceptable prediction of the autoignition delays, but only the ADF and ADFχ models are able to represent the temporal evolution of the progress variable.