Ultra-Lean Premixed Turbulent Combustion: Challenges of RANS Modelling

Lorenzo Sforza, Suliman Abdelwahid, Tommaso Lucchini and Angelo Onorati
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Lorenzo Sforza: ICE Group—Energy Department, Politecnico di Milano, Via R. Lambruschini 4a, 20156 Milano, Italy
Suliman Abdelwahid: ICE Group—Energy Department, Politecnico di Milano, Via R. Lambruschini 4a, 20156 Milano, Italy
Tommaso Lucchini: ICE Group—Energy Department, Politecnico di Milano, Via R. Lambruschini 4a, 20156 Milano, Italy
Angelo Onorati: ICE Group—Energy Department, Politecnico di Milano, Via R. Lambruschini 4a, 20156 Milano, Italy

Energies, 2022, vol. 15, issue 16, 1-21

Abstract: The main challenge of improving spark ignition (SI) engines to achieve ever increasing thermal efficiencies and near-zero pollutant emissions today concerns developing turbulent combustion under homogeneous ultra-lean premixed mixtures (HULP). This continuous shift of the lean operation limit entails questions on the applicability limits of the combustion models used to date for SI engine design and optimization. In this work, an assessment of flamelet-based models, widely used in RANS SI engines simulations of premixed turbulent combustion, is carried out using an open-source 3D-CFD platform to clarify the applicability limits on HULP mixtures. Two different consolidated approaches are selected: the Coherent Flame Model (CFM) and the Flame Area Model (FAM). Both methodologies are embedded by the authors into the same numerical structure and compared against measurements over a simplified and controlled flame configuration, which is representative of engine-like conditions. The experimental steady-state flame of type “A” of the Darmstadt Turbulent Stratified Flame (TSF) burner is selected for the assessment. This configuration is characterized by flame measurements over a strong shear and mixing layer between the central high-speed CH 4 -air jet and the surrounding slow air co-flow, hence, it represents an interesting controlled condition to study turbulent HULP mixtures. A comparison between computed results and experimental data on trends of mean flow velocity, turbulence, temperature and mixture stratification was carried out. This enabled us to assess that the investigated flamelet-based combustion models failed in providing accurate and reliable results when the flame approaches turbulent HULP mixture conditions, demonstrating the urgency to develop models able to fill this gap.

Keywords: ultra-lean combustion; premixed turbulent combustion; flamelet; CFD; RANS; CFM; FAM (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2022
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