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CLARA IACOVANO

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- Application of a zonal hybrid URANS/LES turbulence model to high and low-resolution grids for engine simulation [Articolo su rivista]
Iacovano, C.; D'Adamo, A.; Fontanesi, S.; Di Ilio, G.; Krastev, V. K.
abstract

A zonal hybrid unsteady Reynolds-averaged Navier–Stokes/large eddy simulation (URANS-LES) Zonal detached-eddy simulation (ZDES) model is applied to internal combustion engine (ICE) simulation and comparisons of predicted flow morphology and variability are carried out against on the transparent combustion chamber (TCC-III) particle image velocimetry (PIV) data set for motored conditions. To this aim, a previously developed model derived from a standard seamless-detached eddy simulation (DES) formulation is adopted for two different grid resolutions. In particular, two zonalization choices are evaluated based on previous single-grid results, in order to assess the model outcomes based on the joint turbulence treatment/grid density: the seamless-DES mode is applied (1) only to the cylinder (TCC-Z1) and (2) to the cylinder and intake port (TCC-Z2). Multi-cycle simulations (50 samples) are carried out and the results are compared to experimental data in terms of PIV images using multiple quality indices on multiple planes (Y = 0 and X = 0). Finally, comparison of predicted mean flow fields is extended to standard URANS mode. Results show that the use of a cylinder-only seamless-DES treatment on a relatively coarse grid results in a quantitative agreement between simulated and measured (PIV) flow fields, both in terms of average morphology and flow variability, whereas the extension of the DES mode to the intake port does not introduce relevant variations. Quality indicators seem to be moderately sensitive to the grid resolution, thus confirming the adaptive potential of a ZDES–like model and promoting the use of DES–type turbulence modelling even on relatively low-resolution grids. The analysis of average fields compared to URANS simulations highlights the benefit for both grids of a scale-resolving ZDES modelling when the same underlying turbulence model (k-ε RNG) is used. This study reinforces the recommendation in the use of hybrid URANS-LES models to simulate ICE flows. The adopted ZDES formulation based on the two-equation k-ε RNG model shows that high-quality results can be obtained even on engineering-grade grids, both in terms of average and cycle-to-cycle variation. The numerical results obtained using the two grids with variable resolution are consistent, and this further promotes a wider adoption of this class of models to simulate engine flows in industrial applications.

2021 - A Data-Driven Methodology for the Simulation of Turbulent Flame Speed across Engine-Relevant Combustion Regimes [Articolo su rivista]
D’Adamo, Alessandro; Iacovano, Clara; Fontanesi, Stefano
abstract

2021 - A wall-adapted zonal URANS/LES methodology for the scale-resolving simulation of engine flows [Articolo su rivista]
Iacovano, C.; d'Adamo, A.; Fontanesi, S.; Di Ilio, G.; Krastev, V. K.
abstract

In the present paper, a comprehensive, wall-adapted zonal URANS/LES methodology is shown for the multidimensional simulation of modern direct-injection engines. This work is the latest update of a zonal hybrid turbulence modeling approach, specifically developed by the authors for a flexible description of in-cylinder turbulent flow features with an optimal balance between computational costs and accuracy. Compared to the previous developments, a specific near-wall treatment is added, in order to preserve full-URANS behavior in the first near-wall cells, having in mind typically available mesh resolution in this part of the fluid domain. The updated methodology is applied to the multi-cycle simulation of a reference single-cylinder optical engine, which features a twin-cam, overhead-valve pent-roof cylinder head, and is representative of the current generation of spark-ignited direct-injection thermal power units. Results based on phase-specific flow field statistics and synthetic quality indices demonstrate the consistency and effectiveness of the proposed methodology, which is then qualified as a suitable candidate for affordable scale-resolving analyses of cycle to cycle variability (CCV) phenomena in direct-injection engines.

2021 - Combustion modelling of turbulent jet ignition in a divided combustion chamber [Articolo su rivista]
Olcuire, M.; Iacovano, C.; D'Adamo, A.; Breda, S.; Lucchini, T.; Fontanesi, S.
abstract

Turbulent jet ignition is seen as one of the most promising strategies to achieve stable lean-burn operation in modern spark-ignition engines thanks to its ability to promote rapid combustion. A nearly stoichiometric mixture is ignited in a small-volume pre-chamber, following which multiple hot turbulent jets are discharged in the main chamber to initiate combustion. In the present work, a detailed computational investigation on the turbulent combustion regime of premixed rich propane/air mixture in a quiescent divided chamber vessel is carried out, to study the characteristics of the jet flame without the uncertainties in mixing and turbulent conditions typical of real-engine operations. In particular, the paper investigates the dependency of flame propagation on nozzle diameter (4, 6, 8, 12 and 14 mm) and pre-chamber/main-chamber volume ratio (10% and 20%); CFD results are compared to the experimental outcomes. Results show that the combustion regime in the quiescent pre-chamber follows a well-stirred reaction mode, rendering the limitation in using conventional flamelet combustion models. Furthermore, due to the very high turbulence levels generated by the outflowing reacting jets, also the main chamber combustion develops in a well-stirred reactor type, confirming the need for a kinetics-based approach to combustion modelling. However, the picture is complicated by thickened flamelet conditions possibly being verified for some geometrical variations (nozzle diameter and pre-chamber volume). The results show a general good alignment with the experimental data in terms of both jet phasing and combustion duration, offering a renewed guideline for combustion simulations under quiescent and low Damköhler number conditions.

2021 - Validation of a les Spark-Ignition Model (GLIM) for Highly-Diluted Mixtures in a Closed Volume Combustion Vessel [Relazione in Atti di Convegno]
Iacovano, C.; Zeng, Y.; Anbarasu, M.; Fontanesi, S.; D'Adamo, A.
abstract

The establishment of highly-diluted combustion strategies is one of the major challenges that the next generation of sustainable internal combustion engines must face. The desirable use of high EGR rates and of lean mixtures clashes with the tolerable combustion stability. To this aim, the development of numerical models able to reproduce the degree of combustion variability is crucial to allow the virtual exploration and optimization of a wide number of innovative combustion strategies. In this study ignition experiments using a conventional coil system are carried out in a closed volume combustion vessel with side-oriented flow generated by a speed-controlled fan. Acquisitions for four combinations of premixed propane/air mixture quality (φ=0.9,1.2), dilution rate (20%-30%) and lateral flow velocity (1-5 m/s) are used to assess the modelling capabilities of a newly developed spark-ignition model for large-eddy simulation (GLIM, GruMo-UniMORE LES Ignition Model). The model accounts for all the main physical phenomena governing flame kernel growth, including electric circuit and over-adiabatic thermal expansion, which are included in the LES formalism of ECFM-LES combustion model. In the first part of the study the agreement of the simulation results against measurements is carried out to assess a validated non-reacting condition and converged flow statistics. Then, combustion simulations are carried out, and ignition events are repeated at random timings to replicate flow variability at ignition. Finally, optical comparison is carried out for simulated enflamed volume against high-frequency Schlieren images for all the cases, and measurements of flame radius growth are presented. The agreement of the quantitative flame measurements, as well as the qualitative resemblance of flame development, indicate the use of the presented GLIM ignition model as a valuable model for multi-cycle engine simulations, with particular relevance on unstable and CCV-affected conditions

2020 - A Preliminary 1D-3D Analysis of the Darmstadt Research Engine under Motored Condition [Relazione in Atti di Convegno]
Iacovano, C.; Berni, F.; Barbato, A.; Fontanesi, S.
abstract

In the present paper, 1D and 3D CFD models of the Darmstadt research engine undergo a preliminary validation against the available experimental dataset at motored condition. The Darmstadt engine is a single-cylinder optical research unit and the chosen operating point is characterized by a revving speed equal to 800 rpm with intake temperature and pressure of 24 °C and 0.95 bar, respectively. Experimental data are available from the TU Darmstadt engine research group. Several aspects of the engine are analyzed, such as crevice modeling, blow-by, heat transfer and compression ratio, with the aim to minimize numerical uncertainties. On the one hand, a GT-Power model of the engine is used to investigate the impact of blow-by and crevices modeling during compression and expansion strokes. Moreover, it provides boundary conditions for the following 3D CFD simulations. On the other hand, the latter, carried out in a RANS framework with both highand low-Reynolds wall treatments, allow a deeper investigation of the boundary layer phenomena and, thus, of the gas-to-wall heat transfer. A detailed modeling of the crevice, along with an ad hoc tuning of both blow-by and heat fluxes lead to a remarkable improvement of the results. However, in order to adequately match the experimental mean in-cylinder pressure, a slight modification of the compression ratio from the nominal value is accounted for, based on the uncertainty which usually characterizes such geometrical parameter. The present preliminary study aims at providing reliable numerical setups for 1D and 3D models to be adopted in future detailed investigations on the Darmstadt research engine.

2020 - Comparison between Experimental and Simulated Knock Statistics Using an Advanced Fuel Surrogate Model [Relazione in Atti di Convegno]
Cicci, F.; Pessina, V.; Iacovano, C.; Sparacino, S.; Barbato, A.
abstract

The statistical tendency of a GDI spark-ignition engine to undergo knocking combustion as a consequence of spark timing variation is numerically investigated. In particular, attention is focused on the importance to match combustion-relevant and knock-relevant fuel properties to ensure consistency with the experimental evidence. An inhouse surrogate formulation methodology is used to emulate real gasoline properties, comparing fuel models of increasing complexity. Knock is investigated using a proprietary statistical knock model (GruMo Knock Model, GK-PDF). The model can infer a log-normal distribution of knock intensity within a RANS formalism, by means of transport equations for variances and turbulence-derived probability density functions (PDFs) for physical quantities. The calculated distributions are compared to measured statistical distributions. The proposed numerical/experimental comparison constitutes an advancement in synthetic chemistry integration into 3D-CFD combustion simulations.

2020 - Impact of Grid Density on the Analysis of the In-Cylinder Flow of an Optical Engine [Relazione in Atti di Convegno]
Barbato, A.; Iacovano, C.; Cicci, F.
abstract

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.

2020 - Large-Eddy simulation of lean and ultra-lean combustion using advanced ignition modelling in a transparent combustion chamber engine [Articolo su rivista]
D'Adamo, A.; Iacovano, C.; Fontanesi, S.
abstract

The need for Internal Combustion Engines (ICEs) to face near-future challenges of higher efficiency and reduced emissions is leading to a renewed interest towards lean-combustion. Several operational issues are associated to lean combustion, such as an abrupt increase of combustion cycle-by-cycle variability (CCV), leading to unbearable levels of Indicated Mean Effective Pressure (IMEP) variation and to misfiring cycles. Significant potential in the wide-scale establishment of lean combustion might come from Large Eddy Simulation (LES), which is able to elucidate the relationships between local physical processes (e.g. velocity magnitude, Air to Fuel Ratio (AFR), etc.) and early combustion progress (e.g. 1%) in unprecedented manners. To this aim, an improved ignition model for LES is proposed in the paper. Two premixed propane-air lean strategies are selected from the wide TCC-III database. A lean-stable (λ=1.10, also named lean) and lean-unstable (λ=1.43, also named ultra-lean) conditions are simulated, highlighting the model capability to well reproduce the sudden rise in CCV for increased mixture dilution. Explanations are given for the observed behaviour and a hierarchical quest for CCV dominant factors is presented. Finally, the different role of local flow field is highlighted for the two cases, and the comparison of optical acquisitions of OH* emission against simulated flame iso-surface up to 1% burn duration reinforce the simulation fidelity. The study shows the investigation possibilities of innovative combustion strategies given by advanced LES simulations. The understanding of turbulent combustion dynamics and the knowledge of the related lean-burn instabilities are key enabler for the exploration of new efficient lean-burn operations.

2020 - Standard and consistent Detached-Eddy Simulation for turbulent engine flow modeling: An application to the TCC-III engine [Relazione in Atti di Convegno]
Krastev, V. K.; Di Ilio, G.; Iacovano, C.; D'Adamo, A.; Fontanesi, S.
abstract

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.

2019 - Development of gasoline-ethanol blends laminar flame speed correlations at full-load Si engine conditions via 1D simulations [Relazione in Atti di Convegno]
Pecchia, M. D.; Pessina, V.; Iacovano, C.; Cantore, G.
abstract

Nowadays, most of the engineering development in the field of Spark-Ignited (SI) Internal Combustion Engines (ICEs) is supported by 3D-CFD simulations relying on flamelet combustion models. Such kind of models require laminar flame speed as an input to be specified by the user. While several laminar flame speed correlations are available in literature, for gasoline and pure ethanol at ambient conditions, there is a lack of correlations describing laminar flame speed of gasoline-ethanol blends, for different ethanol volume content, at conditions deemed to be representative of engine-like conditions. Toluene Reference Fuel surrogates with addition of ethanol (ETRF), suitable for representing gasoline-ethanol blends up to 85% vol. ethanol content are formulated. Thanks to these surrogates, 1D premixed laminar flame speed calculations are performed at selected engine-relevant conditions for a E5, E20 and E85 fuels. As a final outcome, three different laminar flame speed correlations based on the chemistry-based calculations are derived for E5, E20 and E85 gasoline-ethanol fuel blends focusing on typical full-load engine conditions. Such kind of correlations can be easily implemented in any 3D-CFD code to provide a chemistry-grounded estimation of laminar flame speed during combustion calculations. Such correlations are of practical use, since they might help in developing the next generation of bio-fuels powered internal combustion engines.

2019 - Numerical Simulation of Syngas Blends Combustion in a Research Single-Cylinder Engine [Relazione in Atti di Convegno]
Pessina, V.; D'Adamo, A.; Iacovano, C.; Fontanesi, S.; Martinez, S.; Lacava, P
abstract

Despite syngas is a promising alternative fuel for internal combustion engines (ICEs), its extensive adoption has not been adequately investigated so far. The dedicated literature offers several fundamental studies dealing with H2/CO blends burning at high pressure and room temperature, as well as preheated mixture at low pressure. However, these thermodynamic states are far from the operational conditions typical of ICEs. Therefore, it is essential to investigate the syngas combustion process at engine-like conditions to shed light on this fuel performance, in order to fully benefit from syngas characteristics in ICE application. One of the key properties to characterize a combustion process is laminar flame speed, which is also used by the most widespread turbulent combustion models. In the first part, a database of premixed laminar burning rates at engine-like conditions for different syngas (H2/CO) blends is created based on one-dimensional unstretched flame simulations using two validated chemical mechanisms. Then the resulting laminar flame speed values are fitted using a validated in-house method based on logarithmic correlations. In the second part of the paper, these are implemented in the G-equation combustion model and three-dimensional simulations of a four stroke Spark Ignition (SI) optical access engine fueled by syngas are carried out. The combustion characteristics of two H2/CO blends (50/50 and 75/25 volume fraction, respectively) are investigated and the simulation results are compared to the available experimental data for the same fuels. This joint numerical/experimental study allows to investigate and optimize the syngas combustion for ICEs and it provides general guidelines to further understand the feasibility of this alternative fuel in terms of ICE utilizations.

2018 - Analysis and Simulation of Non-Flamelet Turbulent Combustion in a Research Optical Engine [Relazione in Atti di Convegno]
Iacovano, Clara; D'Adamo, Alessandro; Cantore, Giuseppe
abstract

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.