Extension of OpenFOAM Library for RANS Simulation of Premixed Turbulent Combustion

Licentiate thesis, 2013

Unsteady multi-dimensional numerical simulation of turbulent flames is a well recognized tool for research and development of future internal combustion engines capable for satisfying stringent requirements for ultra-low emission and highly efficient energy conversion. To attain success, such simulations need, in particular, well elaborated Computational Fluid Dynamics (CFD) software, as well as advanced predictive models of turbulent burning. As far as the software is concerned, a free, open source CFD software package called OpenFOAM (Open Field Operation And Manipulation) library has attracted increasing amounts of attention from both commercial and academic organizations over the past years. While the number of problems that have been studied using the package grows fast, applications of the code to Reynolds-Averaged Navier-Stokes (RANS) simulations of premixed turbulent flames are still rare and the standard version of OpenFOAM does not contain implementation of premixed turbulent combustion models with well documented predictive capabilities. Therefore, one goal of the present work was to further develop the code for multi-dimensional RANS simulations of premixed turbulent flames. As far as models are concerned, a number of models of turbulent burning have been proposed to be used, but they strongly need straightforward quantitative testing against a wide and representative set of experimental data obtained in well defined simple cases under substantially different conditions. Therefore, another goal of the present work was to further validate two advanced models of the influence of turbulence on premixed combustion, i.e. the so-called Turbulent Flame Closure (TFC) and Flame Speed Closure (FSC) models. The two models were implemented into OpenFOAM library and the so-extended code was successfully applied to simulate two widely recognized sets of experiments with two substantially different, well-defined, simple, laboratory premixed turbulent flames, i.e. (i) oblique, confined, preheated, highly turbulent, methane-air flames experimentally studied by Moreau and (ii) V-shaped, open, weakly turbulent, lean methane-air flames investigated by Dinkelacker and Hölzler under the room conditions. The obtained numerical results agree both qualitatively and quantitatively with the aforementioned experimental data, thus, validating both the implemented combustion models and the extended code. It is worth stressing that the influence of variations in the equivalence ratio on the measured data was quantitatively predicted without tuning. The capabilities of the TFC and FSC models and the extended code to well predict turbulent burning rates for various equivalence ratios make these two models and the code particularly interesting for multi-dimensional unsteady RANS simulations of turbulent combustion in Direct Injection Stratified Charge (DISC) Spark Ignition (SI) engines.