Abstract
Turbulent combustion modelling in internal combustion engines (ICEs) is a challenging task. It is commonly synthetized by incorporating the interaction between chemical reactions and turbulent eddies into a unique term, namely turbulent flame speed sT. The task is very complex considering the variety of turbulent and chemical scales resulting from engine load/speed variations. In this scenario, advanced turbulent combustion models are asked to predict accurate burn rates under a wide range of turbulence–flame interaction regimes. The framework is further complicated by the difficulty in unambiguously evaluating in-cylinder turbulence and by the poor coherence of turbulent flame speed (sT) measurements in the literature. Finally, the simulated sT from combustion models is found to be rarely assessed in a rigorous manner. A methodology is presented to objectively measure the simulated sT by a generic combustion model over a range of engine-relevant combustion regimes, from Da = 0.5 to Da = 75 (i.e., from the thin reaction regime to wrinkled flamelets). A test case is proposed to assess steady-state burn rates under specified turbulence in a RANS modelling framework. The methodology is applied to a widely adopted combustion model (ECFM-3Z) and the comparison of the simulated sT with experimental datasets allows to identify modelling improvement areas. Dynamic functions are proposed based on turbulence intensity and Damköhler number. Finally, simulations using the improved flame speed are carried out and a satisfactory agreement of the simulation results with the experimental/theoretical correlations is found. This confirms the effectiveness and the general applicability of the methodology to any model. The use of grid/time resolution typical of ICE combustion simulations strengthens the relevance of the proposed dynamic functions. The presented analysis allows to improve the adherence of the simulated burn rate to that of literature turbulent flames, and it unfolds the innovative possibility to objectively test combustion models under any prescribed turbulence/flame interaction regime. The solid data-driven representation of turbulent combustion physics is expected to reduce the tuning effort in ICE combustion simulations, providing modelling robustness in a very critical area for virtual design of innovative combustion systems.
Highlights
IntroductionThe progress in CFD models witnessed in the recent past has led to remarkable advances in the field of fluid-dynamic simulation of internal combustion engines (ICEs). Among the complex processes present in modern combustion systems, the simulation of turbulent combustion requires a solid description of both flow (e.g., large-scale structures, local turbulence) and fluid properties (e.g., laminar flame speed, dilution rate)
The separation of turbulent and chemical scales is a fundamental assumption of the so-called flamelet combustion models, and it traces back to the pioneering Damköhler theoretical expressions for turbulent flame speed [7,8], its validity is an open discussion as non-flamelet conditions are likely to be present in modern combustion systems
The methodology is applied to the extended coherent flamelet model-3 zones combustion model [5,35], whose application for engine combustion simulation has been presented in conjunction with advanced ignition models [36,37,38], knock models [39,40,41,42], alternative fuels [43,44,45,46], and in conjugate heat transfer analyses [47,48,49], despite that undesirable case-to-case tuning is often required to match the experimental burn rate [50,51]
Summary
The progress in CFD models witnessed in the recent past has led to remarkable advances in the field of fluid-dynamic simulation of ICEs. Among the complex processes present in modern combustion systems, the simulation of turbulent combustion requires a solid description of both flow (e.g., large-scale structures, local turbulence) and fluid properties (e.g., laminar flame speed, dilution rate). The separation of turbulent and chemical scales is a fundamental assumption of the so-called flamelet combustion models (including ECFM3Z [5] and G-equation [1,6]), and it traces back to the pioneering Damköhler theoretical expressions for turbulent flame speed [7,8], its validity is an open discussion as non-flamelet conditions are likely to be present in modern combustion systems. Each and every of these situations contributes to the overall burn rate, and it is required that the combustion model accurately reproduces all the concurring regimes
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