Conditional source-term estimation for diesel combustion

Thesis

The compression ignition engine will continue to be a significant factor in the transportation sector due to its unmatched efficiency and robustness. As emissions legislation becomes more stringent, pressure on the automotive industry will continue to increase as both government and consumers demand cleaner technologies. The detailed understanding of in-cylinder physics is paramount to improving efficiency and reducing engine-out emissions. Consequently, numerical analysis is playing a much more pivotal role in the engine design phase with the requirement to predict complex combustion events and pollutant formation driving the need for ever better models. In this investigation, current numerical tools are used to elucidate the effects of spray targetting and piston bowl geometry on combustion evolution and pollutant formation. Closed cycle computational fluid dynamics simulations are performed on a sector mesh at various load points using the 3 Zones Extended Coherent Flame Model coupled with adaptive mesh refinement. The computational fluid dynamics model is validated experimentally at the baseline conditions at each test point after-which, parametric sweeps of bowl geometry, exhaust gas recirculation rate and nozzle tip protrusion are conducted. Results indicate that appropriately pairing fuel injection strategy and piston geometry is essential in reducing engine-out emissions.

In addition, a novel chemical source term closure based on Conditional Moment Closure (CMC) is developed to simulate diesel combustion. Conditional Source-term Estimation (CSE) uses the conditional averages in evaluating the mean chemical source term. However, unlike Conditional Moment Closure where transport equations are solved for the conditional averages. CSE approximates the conditional averages through inversion of an integral. Previous studies have shown CSE is capable of accurately simulating non-premixed flames of light hydrocarbon fuels. In this study, CSE is extended to simulate spray flame combustion by coupling the CSE combustion model with Flamelet Generated Manifolds chemistry reduction methodology. The CSE-FGM model is applied to the Engine Combustion Network n-Dodecane Spray.A -- in a Large Eddy Simulation turbulence modelling framework. The model is successfully able to predict ignition delay and flame lift-off length with good agreement to experimental measurements. Additionally, the CSE-FGM model is validated further within a RANS framework to predict ignition delay and flame lift-off length over a wide range of ambient temperature and oxygen concentration conditions. The CSE-FGM model is successfully able to predict experimental trends and sensitivity with respect to ambient conditions.

Authors: Ismail, R

• DOI: 10.5287/ora-dewwo9and
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Combustion Modelling; Computational Fluid Dynamics; Conditional Source-term Estimation
• Subjects: Combustion Modelling; Computational Fluid Dynamics