Implementation of the Semi Empirical Kinetic Soot Model within Chemistry Tabulation Framework for Efficient Emissions Predictions in Diesel Engines

Soot prediction for diesel engines is a very important aspect of internal combustion engine emissions research, especially nowadays with very strict emission norms. Computational Fluid Dynamics (CFD) is often used in this research and optimisation of CFD models in terms of a trade-off between accuracy and computational efficiency is essential. This is especially true in the industrial environment where good predictivity is necessary for engine optimisation, but computational power is limited. To investigate soot emissions for Diesel engines, in this work CFD is coupled with chemistry tabulation framework and semi-empirical soot model. The Flamelet Generated Manifold (FGM) combustion model precomputes chemistry using detailed calculations of the 0D homogeneous reactor and then stores the species mass fractions in the table, based on six look-up variables: pressure, temperature, mixture fraction, mixture fraction variance, progress variable and progress variable variance. Data is then retrieved during online CFD simulation, enabling fast execution times while keeping the accuracy of the direct chemistry calculation. In this work, the theory behind the model is discussed as well as implementation in commercial CFD code. Also, soot modelling in the framework of tabulated chemistry is investigated: mathematical model and implementation of the kinetic soot model on the tabulation side is described, and 0D simulation results are used for verification. Then, the model is validated using real-life engine geometry under different operating conditions, where better agreement with experimental measurements is achieved, compared to the standard implementation of the kinetic soot model on the CFD side

Applying a Tabulated Chemistry Approach for the Calculation of Combustion and Emissions in Diesel Engines

It is generally acknowledged, that more details of the chemical reactions occurring in the flame front should be accounted for in the CFD simulations, but with increasing the number of species and reactions involved the associated CPU cost grows quickly beyond practical engineering time limits. Aim of this work is to increase computation efficiency by using a tabulation technique, without losing any accuracy. In order to achieve these goals, dedicated software solution for the generation of CFD look-up tables for advanced combustion models, is applied. Simulations were run for real life Diesel engine, for 5 different EGR levels. FGM results are showing very good match with measurements and direct calculation of the chemical reactions. The runtime for CFD simulations, including chemistry pre-processing, does only mildly increase with the number of species used in the reaction mechanism; simulations with 1000+ species have been realized within 20 hrs on 8 CPU cores

Multidimensional simulation of combustion and knock onset in gas engines

Natural gas fuelled internal combustion engines enable efficient energy conversion with relatively low environmental impact. Depending on the specific application, the available fuel quality, and the emission regulations to be fulfilled, different types of gas-engine combustion systems are in use. The major performance and hence efficiency limiting factors in gas fuelled engines are related to the lower ignitability of natural gas at part load and the appearance of abnormal combustion (knock) at high load conditions. This article provides an overview of the multidimensional CFD simulation workflow for the investigation and assessment of flame propagation and knock onset characteristics in different types of natural gas fuelled internal combustion engines. The most common approaches for simulating flame propagation/combustion under engine conditions are presented together with selected models for describing the pre-flame reactions finally leading to knock onset in the unburned in-cylinder charge ahead of the flame. Based on selected application examples, the models’ performance and capabilities with respect to reflecting the essential characteristics of flame propagation and knock onset are presented

Metoda LES jako narzędzie do analizy fluktuacji ciśnienia dla kolejnych cykli pracy w silnikach benzynowych o wtrysku bezpośrednim

The Large Eddy Simulation method (LES) has become a powerful computational tool for the application to turbulent flows. It links the classical Reynolds Averaged Navier–Stokes (RANS) approach and Direct Numerical Simulation (DNS). This means that the large eddies are computed explicitly in a time-dependent simulation using the filtered Navier-Stokes equations. The LES resolves the large flow scales that depend directly on the geometry where the small scales are modelled by the subgrid-scale models. LES is expected to improve the description of the aerodynamic and combustion processes in Internal Combustion Engines. This paper addresses the topic of developing the combustion model GCM (Gradient Combustion model) for the Large Eddy Simulation (LES) method. Another part of this paper presents numerical investigations of cycle-to-cycle combustion pressure variability with comparison to experimental data. The Gradient Combustion model (GCM) based on the Turbulent Flame Speed Closure Model (TFSCM) is validated against the experimental data for a multi-cycle gasoline direct injection research engine (SCRE). It is shown that the introduced combustion model is stable and capable of proper representation of the experimental results which is one of the assets of the LES method.Metoda LES jest obecnie zaawansowanym narzędziem numerycznym do analizy przepływów turbulentnych. Metoda LES opiera się na połączeniu klasyczej metody uśredniania równań Naviera-Stokes (RANS) z bezpośrednią analizą numeryczną (DNS). Oznacza to, że duże struktury wirowe są rozwiązywane niejawnie poprzez filtrowanie równań Naviera-Stokesa. W metodzie LES oznacza to obliczanie przepływu dużej skali, który zależy od geometrii, podczas gdy przepływ w małej skali jest modelowany modelem podsiatkowym (ang. Sub-grid-scale models, SGS). Uważa się, że metoda LES pozwoli na poprawienie numerycznego opisu aerodynamiki i procesów spalania w silnikach tłokowych. Artykuł przedstawia wyniki prac rozwojowych nad modelem spalania w metodzie LES. Model GCM (model spalania oparty na metodzie gradientu) został zastosowany do obliczeń wielocyklicznych i ich weryfikacji z wynikami eksperymentalnymi. Wyniki eksperymentalne pozyskano z badań na jednocylindrowym silniku badawczym (SCRE) o wtrysku bezpośrednim. W pracy pokazano, że model spalania jest stabilny numerycznie oraz otrzymane wyniki są zgodne z wynikami eksperymentalnymi, co jest jedną z ważniejszych zalet metody LES

Optimisation of diesel autoignition tabulation procedures for AVL code "FIRE"

The modifications of existing Diesel auto-ignition tabulation for CFD code AVL "FIRE" will be presented in this paper. Current n-heptane tabulation (used to simulate Diesel behaviour in IC engines) did not include the phenomenon of cool flame ignition. This phenomenon is important since the temperature of the air/fuel mixture is significantly higher after its occurrence and the simulation results could be improved if this is also taken into consideration when simulating combustion in Diesel engines. Current methods of auto-ignition computation in AVL FIRE are based on the extraction of ignition delay times from tabulated data dependent on four parameters: temperature, pressure, air excess ratio and EGR mass fraction. The new tabulation procedure was developed using the same parameters as starting points for two-step chemical combustion. Temperature changes were observed and a compilation of several criteria was used to determine the start of both cool flame and main ignition. The above parameters were varied and the calculations were performed for each parameter set. Chemical software was used for two-step combustion calculations, using reduced then complex chemical mechanisms, and the results of calculations were stored in a binary file. Results included the values for cool flame ignition delay, main ignition delay, and cool flame and main ignition heat releases. This paper will present the methods used to determine the ignition delays (cool flame and main ignition), as well as results comparing the data acquired using three different chemical mechanisms (three levels of complexity). The data acquired from the calculations would be used to optimize the tabulation procedure using more complex (accurate) chemical mechanisms in a way which will also be described