Analysis and Simulation of Non-Flamelet Turbulent Combustion in a Research Optical Engine

In recent years, the research community devoted many resources to define accurate methodologies to model the real physics behind turbulent combustion. Such effort aims at reducing the need for case-by-case calibration in internal combustion engine simulations. In the present work two of the most widespread combustion models in the engine modelling community are compared, namely ECFM-3Z and G-equation. The interaction of turbulent flows with combustion chemistry is investigated and understood. In particular, the heat release rate characterizing combustion, and therefore the identification of a flame front, is analysed based on flame surface density concept rather than algebraic correlations for turbulent burn rate. In the first part, spark-ignition (S.I.) combustion is simulated in an optically accessible GDI single-cylinder research engine in firing conditions. The turbulent combustion regime is mapped on the Borghi-Peters diagram for all the conditions experienced by the engine flame, and the consistency of the two combustion models is critically analysed. In the second part, a simple test case is defined to test the two combustion models in an ideally turbulence-controlled environment: this allows to fully understand the main differences between the two combustion models under well-monitored conditions. and results are compared against experimental databases of turbulent burn rate for wide ranges of Damkohler (Da) and Karlovitz (Ka) numbers. The joint experimental and numerical study presented in this paper evaluates different approaches within the unified flamelet/non-flamelet framework for modelling turbulent combustion in SI engines. It also indicates guidelines for reduced calibration effort in widespread combustion models

SVILUPPO DI UN MODELLO IBRIDO AVANZATO URANS/LES E TRATTAMENTO DI PARETE DEDICATO PER FLUSSI TURBOLENTI COMPLESSI E CONFINATI

La metodologia standard utilizzata per modellare flussi turbolenti multidimensionali in simulazioni di motori a combustione interna (ICE) si basa sull’approccio “Unsteady Reynolds Averaged Navier Stokes” (URANS). Esso garantisce sufficiente accuratezza con un costo computazionale limitato modellando l’intero spettro turbolento. Tuttavia, per diverse applicazioni tale approccio non fornisce il grado di dettaglio richiesto in fase di progettazione. La rapida diffusione di efficienti risorse di calcolo ha consentito l’uso di approcci “Large Eddies Simulations” (LES), soprattutto per analisi di flussi, combustione, spray e variabilità ciclica (CCV). L’approccio LES offre una descrizione spaziale e temporale dettagliata della turbolenza risolvendo le scale grandi (problema-dipendenti, anisotrope e ricche di energia) e modellando le più piccole (isotrope e universali). Tuttavia, il suo uso, per simulazioni ad alto numero di Reynolds (HR), è ancora limitato a causa dell’alto costo computazionale. In flussi lontani da pareti il numero di nodi di griglia stimato per simulazioni LES è legato al numero di Reynolds secondo la legge N=Re0.4. Per flussi intorno le pareti, la scala integrale della turbolenza è molto piccola, confrontabile con lo spessore dello strato limite e i forti gradienti in direzione normale alla parete vincolano l’altezza di cella in tale direzione. Inoltre, la risoluzione delle strutture più piccole richiede il raffinamento della griglia anche in direzione tangenziale ottenendo così la legge N=Re1.8. Tale approccio è noto come “Wall-Resolved LES” (WRLES) e esige uno sforzo computazionale eccessivo anche per applicazioni con numeri di Reynolds moderati, quali sono le simulazioni motoristiche. Una possibile alternativa all’approccio WRLES è fornita dai trattamenti ibridi URANS/LES, sempre più diffusi per simulazioni HR ed includono un gran numero di sottoclassi come Detached Eddy Simulation (DES) e sue varianti, Scale Resolving Simulation (SAS) e la più recente Stress-Blended Eddy Simulation (SBES). Soffermandosi sul modello DES, esso prevede la risoluzione delle scale grandi, lontane da parete, mentre quelle piccole, nello strato limite, sono modellata in ambiente RANS, che vanta decenni di studi nella descrizione dello strato limite. Tale approccio richiede griglie raffinate in direzione normale la parete, dove y+≈1 è limitato dall’uso del trattamento RANS Low Reynolds, ma meno raffinata parallelamente ad essa. Il trattamento DES, originariamente definito sul modello RANS ad una equazione di Spalart-Allmaras (S-A), può essere applicato a qualsiasi modello “eddy viscosity” in cui il criterio di switch agisce o sulla viscosità turbolenta o sull’energia cinetica turbolenta. In generale, in tutte le formulazioni DES il modello di background RANS emula il modello LES di Smagorinsky (o altri come Yoshizawa). In tale contesto, il presente lavoro di tesi mira a sviluppare una metodologia robusta e stabile per l’uso del trattamento turbolento Zonal-DES in simulazioni CFD-3D di motori a combustione interna. La metodologia proposta agisce su entrambi i termini di produzione e dissipazione dell’energia cinetica turbolenta e il criterio di switch è attivo su tutti nodi di calcolo, senza alcuna interfaccia tra i due approcci. Inoltre, è stato usato uno trattamento di parete dedicato, al fine di preservare l’approccio HR, basato sull’uso di wall functions. La metodologia è stata testata prima su un caso test di riferimento ampiamente riportato in letteratura e in seguito adottata in simulazioni multi-ciclo di due motori da ricerca ad accesso ottico. I risultati delle simulazioni motoristiche dimostrano l’affidabilità e l’efficienza della metodologia proposta, adatta ad l’analisi “Scale-Resolving” e volta a indagare fenomeni dominati dalla turbolenza nei motori ad iniezione diretta.The standard methodology to model turbulent flows in multidimensional internal combustion engine (ICE) simulations is still the Unsteady Reynolds Averaged Navier Stokes (URANS) approach. It allows both sufficient accuracy and limited computational cost modelling the whole turbulence spectrum. However, for several applications, RANS models cannot provide the degree of accuracy required during the design process (i.e. aero-acoustics simulations and massive separation phenomena). The rapid spread of cost-effective HPC resources has increased the use of Large Eddies Simulations (LES) turbulence treatment, especially for the analysis of cold flow, combustion, spray and cycle to cycle variability (CCV). LES approach offers accurate spatial and temporal descriptions of turbulence, resolving only the large scales (problem-dependent, anisotropic, and full of energy) and modelling the small ones (universal and isotropic). However, the application of LES models for High Reynolds (HR) number simulations, is still limited because of the high computational costs. For free shear flows, the number of grid points required for a LES simulation scales according to O(Re0.4). In wall-bounded flows, the turbulence length scale near the walls becomes very small compared to the boundary layer thickness and the high gradients impose a very small computational grid size in the wall-normal direction. Moreover, to resolve the small isotropic scales in the boundary layer, a high-resolution grid in the tangential direction is required as well, leading to more rigid scaling, i.e. O(Re1.8). This kind of LES approach is called Wall-Resolved LES (WRLES) and it is prohibitively expensive even for moderate HR number applications, such as ICEs simulations. A possible alternative to WRLES is represented by hybrid URANS/LES models, which are more and more diffused for HR number applications and include a great number of subclasses, such as Detached Eddy Simulation (DES) and its variants, Scale Resolving Simulation (SAS) and the more recent Stress-Blended Eddy Simulation (SBES) models. Focusing on DES, it is based on the principle that large eddies are resolved away from walls while the small ones, in the boundary layer, are modelled with RANS closures which have decades of validation in boundary layer description. It requires a fine grid at walls only in the wall-normal direction, where y+≈1 is required for the RANS Low Reynolds turbulence model, and a coarse grid in the tangential direction. It was originally proposed for the RANS Spalart-Allmaras (S-A) one equation model, but it can be applied to any others eddies viscosity models in which the switching criterion acts on either turbulent viscosity or turbulent kinetic energy. In general, all DES formulations performance to reduce the background RANS approach to a Smagorinsky-type SGS one-equation model (or similar such as the Yoshizawa one). In this framework, the present work aims at finding a stable and robust methodology to apply the Zonal-DES hybrid turbulence treatment in CFD-3D ICEs simulations. The proposed methodology acts on both the production and dissipation terms of the turbulent kinetic energy transport equation and the switching criteria applies to all grid nodes, both near-walls and bulk flows, without any kind of interface between the two approaches. A dedicated near-wall treatment is added to preserve the well-established HR approach in the first near-wall cell, based on the use of wall function. The methodology has been tested, at first, on a reference test case widely reported in literature and subsequently for multi-cycle simulations of two reference single-cylinder optical research engines. The engine simulations results demonstrate consistency and efficiency of the proposed methodology, which is a suitable candidate for affordable scale-resolving analyses aiming at evaluating turbulence-governed phenomena in direct-injection engine

Impact of Grid Density on the Analysis of the In-Cylinder Flow of an Optical Engine

The evaluation of Internal Combustion Engine (ICE) flows by 3D-CFD strongly depends on a combination of mutually interacting factors, among which grid resolution, closure model, numerics. A careful choice should be made in order to limit the extremely high computational cost and numerical problems arising from the combination of refined grids, high-order numeric schemes and complex geometries typical of ICEs. The paper focuses on the comparison between different grid strategies: in particular, attention is focused firstly on near-wall grid through the comparison between multi-layer and single-layer grids, and secondly on core grid density. The performance of each grid strategy is assessed in terms of accuracy and computational efficiency. A detailed comparison is presented against PIV flow measurements of the Spray Guided Darmstadt Engine available at the Darmstadt University of Technology. As many research groups are simultaneously working on the Darmstadt engine using different CFD codes and meshing approaches, it constitutes a perfect environment for both method validation and scientific cooperation. A motored engine condition is chosen and the flow evolution throughout the engine cycle is evaluated on two different section planes. Pros and cons of each grid strategy are highlighted and motivated

A Data-Driven Methodology for the Simulation of Turbulent Flame Speed across Engine-Relevant Combustion Regimes

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

Standard and consistent Detached-Eddy Simulation for turbulent engine flow modeling: an application to the TCC-III engine

Multidimensional modeling of Cycle-to-Cycle Variability (CCV) has become a crucial support for the development and optimization of modern direct-injection turbocharged engines. In that sense, the only viable modeling options is represented by scale-resolving approaches such as Large Eddy Simulation (LES) or hybrid URANS/LES methods. Among other hybrid approaches, Detached-Eddy Simulation (DES) has the longest development story and is therefore commonly regarded as the most reliable choice for engineering-grade simulation. As such, in the last decade DESbased methods have found their way through the engine modeling community, showing a good potential in describing turbulence-related CCV in realistic engine configurations and at reasonable computational costs. In the present work we investigate the in-cylinder modeling capabilites of a standard two-equation DES formulation, compared to a more recent one which we call DESx. The DESx form differs from standard DES in the turbulent viscosity switch from URANS to LES-like behavior, which for DESx is fully consistent with Yoshizawa’s one-equation sub-grid scale model. The two formulations are part of a more general Zonal-DES (ZDES) methodology, developed and validated by the authors in a series of previous publications. Both variants are applied to the multi-cycle simulation of the TCC-III experimental engine setup, using sub-optimal grid refinement levels in order to stress the model limitations in URANS-like numerical resolution scenarios. Outcomes from this study show that, although both alternatives are able to ouperform URANS even in coarse grid arrangements, DESx emerges as sligthly superior and thus it can be recommended as the default option for in-cylinder flow simulation