Boundary conditions and SGS models for LES of wall-bounded separated flows: an application to engine-like geometries

The implementation and the combination of advanced boundary conditions and subgrid scale models for Large Eddy Simulations are presented. The goal is to perform reliable cold flow LES simulations in complex geometries, such as in the cylinders of internal combustion engines. The implementation of an inlet boundary condition for synthetic turbulence generation and of two subgrid scale models, the local Dynamic Smagorinsky and the Wall-Adapting Local Eddy-viscosity SGS model (WALE) is described. The WALE model is based on the square of the velocity gradient tensor and it accounts for the effects of both the strain and the rotation rate of the smallest resolved turbulent fluctuations and it recovers the proper y(3) near-wall scaling for the eddy viscosity without requiring dynamic pressure; hence, it is supposed to be a very reliable model for ICE simulation. Model validation has been performed separately on two steady state flow benches: a backward facing step geometry and a simple IC engine geometry with one axed central valve. A discussion on the completeness of the LES simulation (i.e. LES simulation quality) is given

Numerical Simulation of Diesel Injector Internal Flow Field

AbstractIn Diesel engines, fuel-air mixing process and spray evolution drastically affect combustion efficiency and pollutant formation. Within this context, a detailed study of the effects of injector geometry and internal flow is of great importance to understand the effects of turbulence and cavitation on the liquid jet atomization process. To this end, both numerical and experimental tools are widely employed.Objective of this work is to simulate the complex flow behavior inside the injector nozzle taking the most relevant physical phenomena into account. CFD simulations were carried out using a compressible solver with phase change modeling available in the OpenFOAM framework. In particular, cavitation was modeled by using an homogeneous equilibrium model based on a barotropic equation of state while the RANS k-ω SST model was used for turbulence. Experiments performed at Kobe University (Japan) on simplified nozzle geometries were used to validate the proposed approach in terms of velocity and vapor distributions. A rather good agreement between computed and experimental data was achieved in terms cavitation length, mass flow, momentum flux making possible to apply the proposed methodology also to real injector configurations in the near future

Experimental Validation of Combustion Models for Diesel Engines Based on Tabulated Kinetics in a Wide Range of Operating Conditions

Computational fluid dynamics represents a useful tool to support the design and development of Heavy Duty Engines, making possible to test the effects of injection strategies and combustion chamber design for a wide range of operating conditions. Predictive models are required to ensure accurate estimations of heat release and the main pollutant emissions within a limited amount of time. For this reason, both detailed chemistry and turbulence chemistry interaction need to be included. In this work, the authors intend to apply combustion models based on tabulated kinetics for the prediction of Diesel combustion in Heavy Duty Engines. Four different approaches were considered: well-mixed model, presumed PDF, representative interactive flamelets and flamelet progress variable. Tabulated kinetics was also used for the estimation of NOxemissions. The proposed numerical methodology was implemented into the Lib-ICE code, based on the OpenFOAM®technology, and validated against experimental data from a light-duty FPT engine. Ten points were considered at different loads and speeds where the engine operates under both conventional Diesel combustion and PCCI mode. A detailed comparison between computed and experimental data was performed in terms of in-cylinder pressure and NOxemissions

An investigation of the validity of a homogeneous equilibrium model for different diesel injector nozzles and flow conditions

In the present work, a methodology for modeling flow behavior inside the fuel injector holes is applied to a number of cases with different geometries and flow conditions. After assessment of the approach results through various experimental studies looking into the flows behavior inside the diesel nozzles, two series of analyses are defined. In the first study, the effect of inlet pressure is investigated by using a series of different rail pressures in both numerical and experimental tests in a single hole industrial injector. Results show a non-cavitating flow and an approximately linear increase of the velocity, turbulence kinetic energy, and turbulence dissipation energy with the increase of pressure difference and linear increase of the mass flow rate with the square root of the pressure difference in this nozzle. The second study is related to the effect of hole geometry on injector performance. The effects of entrance edge rounding and the tube conicity factor are investigated by changing these parameters in a series of geometries from an industrial diesel nozzle. Results show that cavitation occurs in the geometries with a sharper edge and low conicity. The role of the cavitation in emerging flow properties is emphasized in the values of the injector discharge factor and the turbulence properties. The results of this work can be used in the simulation of the primary breakup of fuel spray, and this approach is useful for design and optimization of the injectors for industrial sectors

Modeling Ignition and Premixed Combustion Including Flame Stretch Effects

Objective of this work is the incorporation of the flame stretch effects in an Eulerian-Lagrangian model for premixed SI combustion in order to describe ignition and flame propagation under highly inhomogeneous flow conditions. To this end, effects of energy transfer from electrical circuit and turbulent flame propagation were fully decoupled. The first ones are taken into account by Lagrangian particles whose main purpose is to generate an initial burned field in the computational domain. Turbulent flame development is instead considered only in the Eulerian gas phase for a better description of the local flow effects. To improve the model predictive capabilities, flame stretch effects were introduced in the turbulent combustion model by using formulations coming from the asymptotic theory and recently verified by means of DNS studies. Experiments carried out at Michigan Tech University in a pressurized, constant-volume vessel were used to validate the proposed approach. In the vessel, a shrouded fan blows fresh mixture directly at the spark-gap generating highly inhomogeneous flow and turbulence conditions close to the ignition zone. Experimental and computed data of gas flow velocity profiles and flame radius were compared under different turbulence, air/fuel ratio and pressure conditions

Gas Exchange and Injection Modeling of an Advanced Natural Gas Engine for Heavy Duty Applications

The scope of the work presented in this paper was to apply the latest open source CFD achievements to design a state of the art, direct-injection (DI), heavy-duty, natural gas-fueled engine. Within this context, an initial steady-state analysis of the in-cylinder flow was performed by simulating three different intake ducts geometries, each one with seven different valve lift values, chosen according to an estabilished methodology proposed by AVL. The discharge coefficient (Cd) and the Tumble Ratio (TR) were calculated in each case, and an optimal intake ports geometry configuration was assessed in terms of a compromise between the desired intensity of tumble in the chamber and the satisfaction of an adequate value of Cd. Subsequently, full-cycle, cold-flow simulations were performed for three different engine operating points, in order to evaluate the in-cylinder development of TR and turbulent kinetic energy (TKE) under transient conditions. The latest achievements in open source mesh generation and motions were applied, along with time-varying and case-fitted inizialization values for the fields of intake pressure and temperature. Finally, direct-injection of natural gas in the cylinder was incorporated in full-cycle simulations, to evaluate the effects of injection on charge motions and charge homogeneity at the estimated spark timing. Three specific engine operating points were simulated and different combinations of turbochargers and valve lift laws were tested. Results consistency was verified by means of validations with data from 1D simulations and literature

CFD modelling of flame stretch in SI engines

AbstractThe flame stretch is a phenomenon that affects the combustion initial stage in premixed charge Spark-Ignition (SI) engines with consequences on the laminar burning velocity, so its correct description is fundamental to predict the further turbulent flame development.In the context of a Computational Fluid Dynamics (CFD) investigation, two different flame stretch models were implemented. The first one is obtained starting from the equations and assumptions proposed by Bradley, Lau and Lawes. The result is a flame stretch expression that takes into account the influence of flame curvature, turbulence intensity, thermal and fresh mixture diffusivity (Lewis number), activation energy of the overall combustion reaction and flame thickness. The second one is already proposed by Herweg and Maly and takes into account the same parameters mentioned before. To assess the behaviour of these two models numerical simulations on combustion inside a simplified chamber were performed at different equivalence ratios and turbulence intensities. All simulations are carried out with the open-source platform OpenFOAM, involving a 3-D finite volume discretization using RANS turbulence modelling.Although no comparisons with experimental findings were performed, the achieved results show a good response of both stretch models with respect to theoretical considerations

Modeling Non-Premixed Combustion Using Tabulated Kinetics and Different Fame Structure Assumptions

Nowadays, detailed kinetics is necessary for a proper estimation of both flame structure and pollutant formation in compression ignition engines. However, large mechanisms and the need to include turbulence/chemistry interaction introduce significant computational overheads. For this reason, tabulated kinetics is employed as a possible solution to reduce the CPU time even if table discretization is generally limited by memory occupation. In this work the authors applied tabulated homogeneous reactors (HR) in combination with different turbulent-chemistry interaction approaches to model non-premixed turbulent combustion. The proposed methodologies represent good compromises between accuracy, required memory and computational time. The experimental validation was carried out by considering both constant-volume vessel and Diesel engine experiments. First, the ECN Spray A configuration was simulated at different operating conditions and results from different flame structures are compared with experimental data of ignition delay, flame lift-off, heat release rates, radicals and soot distributions. Afterwards, engine simulations were carried out and computed data are validated by cylinder pressure and heat release rate profiles

Numerical study of compressor fouling mechanism based on Eulerian-Eulerian approach

AbstractThe paper describes a fluid dynamic numerical model to study the fouling mechanism for turbomachines (such as large gas turbines and small turbochargers). For this purpose, the gas-particle behaviour over a compressor blade is modelled using an open source CFD tool, OpenFOAM®. Through this model, two-fluid implicit equations system, with Eulerian-Eulerian approach is considered. This approach uses RANS turbulent models and also takes into account the particle fluid interactions. Applying Johnson-Kendal-Robert (JKR) theory, the particle deposition over the blade surface is predicted. According to the simulation results, there are certain regions which are facing the huge amount of deposition. The surfaces of these regions are prone to form an almost thick layer of sticked particles and affect the boundary layers and blade aerodynamics significantly. In order to capture this effect, a model to predict the deformation of the boundary with respect to deposition rate is also introduced