Ultra-Lean Premixed Turbulent Combustion: Challenges of RANS Modelling

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 CH4-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.

A CFD ignition model to predict average-cycle combustion in SI engines with extreme EGR levels

“…This discrepancy can be ascribed, first, to limitations of the RANS modeling approaches for turbulent combustion, when operating too close to the flame extinction limit. In fact, most of the turbulent combustion models employed in literature for RANS simulations rely on the flamelet assumption (as the CFM model 11 ). The validity of this hypothesis can collapse in the case of high u ′ / s u ratios, namely when homogeneous ultra-diluted premixed mixtures (extreme EGR values with low s u ) are burned inside IC engines (high u ′ values).…”

“…The validity of this hypothesis can collapse in the case of high u ′ / s u ratios, namely when homogeneous ultra-diluted premixed mixtures (extreme EGR values with low s u ) are burned inside IC engines (high u ′ values). 11 The considered operating condition is characterized by an extreme level of cycle-to-cycle combustion variability (CoV of p max ≈ 25 % , see Table 1), with a significant portion of all cycles in which combustion is not sustained ( ~ 11 % misfire rate). This impacts the average-cycle s u value, which dramatically drops if misfire events are included in the calculus.…”

“…In particular, most of the research efforts are directed toward the control of low-temperature combustion strategies, in order to minimize thermal losses, cut N O x emissions, and avoid soot formation. 9–11 This would also have a positive impact on the after-treatment system, with a further reduction of size and costs. 12 Among the available low-temperature combustion strategies, air-fuel mixture dilution with exhaust gas re-circulation (EGR) is very attractive, thanks to the possibility of knock control, 13 minimal throttle losses at part loads, 14 and simplified after-treatment systems.…”

Improving the Efficiency of Fuel Combustion with the Use of Various Designs of Embrasures

“…Unlike conventional burners, porous ceramic radiant burners (PCRBs) have a high combustion rate, high thermal efficiency, and low pollutant emissions [20]. Innovative solutions such as pre-chamber turbulent jet ignition are currently being introduced, which provide longer burning time and increased lean burn limits when applied to homogeneous super lean premixes (HULPs) evaluated flame-based models used in RANS 3D Computational Fluid Dynamics (CFD) simulations for turbulent premixed combustion to assess their applicability limits for HULP mixtures [21]. Two different combined approaches are chosen: the flame zone model (FAM) and the coherent flame model (CFM).…”

Modelling a Turbulent Non-Premixed Combustion in a Full-Scale Rotary Cement Kiln Using reactingFoam

“…The introduction of OpenFoam in this work is motivated by the software's ability to simulate turbulent reactive flow in a full-scale three-dimensional geometry of industrial relevance. The simulation of turbulent combustion using OpenFoam has recently attracted considerable attention, e.g., [7][8][9]. Recent publications discussing the radiative heat and thermodynamics modeling in OpenFoam include [10][11][12][13].…”