Direct Numerical Simulation and Advanced Modelling of Turbulent Flame Kernels for High-Efficiency Low-Emission Spark Ignition Engine

1 Introduction
Road transportation contributes to carbon emissions and the climate emergency. The transition to electrified vehicles will require intermediate solutions, especially for heavy goods vehicles. Developing new engines with high efficiency and low emissions is expensive and time intensive, hence fast and accurate computational modelling is necessary. In practise, fast industrial software sacrifices much of the detailed small-scale physics, which has to be replaced by engineering models. Developing such models is challenging and relies upon high-fidelity simulations that include fundamental physics based on first principles; this cannot be done in an industrial setting.

2 Overview
Ricardo was founded on expertise in engine efficiency, and remains a pioneer in engine technology for improving efficiency and reducing emissions from gasoline, diesel, emerging bio-fuel and gas engines. Current state-of-the-art applied spark-ignition engine modelling frequently relies upon empirical closures for turbulent flame speed. This suffers from substantial uncertainties associated with validity and applicability across the range of the engine operation parameters and the effect of the tuning coefficients inherent in the model. At the moment, available experimental data (with the majority focusing on planar flames) does not provide sufficient information to reduce this uncertainty. This project will use high-fidelity simulation of flame kernels under typical engine conditions to improve our fundamental understanding, enhance accuracy and robustness, and to reduce uncertainty associated with turbulent flame speed prediction.

3 Research Question
The aim of this project is to improve predictive capabilities of turbulent flame models for engineering applications. The project is sponsored by Ricardo UK, whose software will provide the test bed for model development and evaluation.
Objectives:

* Create a high detail and high accuracy direct numerical simulation database using PeleLM consisting of turbulent flame kernels and flame wall interactions.
* Exploit the database to develop fundamental understanding of the behaviour of turbulent combustion and how the design of an IC engine can affect the flame behaviour, and therefore engine performance.
* Using the database, compare the performance and accuracy of the turbulent-flame models used by Vectis, and develop and implement modifications as necessary.

4 Project Plan
A database of flame kernel high-fidelity simulations will be constructed that span a range of conditions relevant to SI engines; Ricardo will supply flow and turbulent field conditions representative for natural gas and gasoline engines, based on tuned RANS simulations that can be used for DNS initialisation. Fundamental understanding will be developed through detailed analysis of turbulent flame response and assessment of individual terms in the transport equations for turbulent kinetic energy and energy dissipation rate. Such analysis is a distinct advantage only possible through analysis of DNS, and will enable identification of important terms for developing turbulent flame models that incorporate effects such as integral length scale, heat release and curvature.

The resulting turbulent flame models can then be implemented in the VECTIS CFD suite and evaluated in real engine simulations, as a part of the student's time at Ricardo UK. The DNS calculations will be carried out using PeleLM, which was developed at the Center for Computational Sciences and Engineering at the Lawrence Berkeley National Laboratory. Simulations increasing in size will be carried out in stages, starting from a high-specification workstation, through Newcastle's HPC facility "Rocket", up to the national supercomputer "Archer" (compute time application to UKCTRF).