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 n-dodecane Spray Combustion with a Representative Interactive Linear Eddy Model

Many new combustion concepts are currently being investigated to further improve engines in terms of both efficiency and emissions. Examples include homogeneous charge compression ignition (HCCI), lean stratified premixed combustion, stratified charge compression ignition (SCCI), and high levels of exhaust gas recirculation (EGR) in diesel engines, known as low temperature combustion (LTC). All of these combustion concepts have in common that the temperatures are lower than in traditional spark ignition or diesel engines. To further improve and develop combustion concepts for clean and highly efficient engines, it is necessary to develop new computational tools that can be used to describe and optimize processes in nonstandard conditions, such as low temperature combustion. Thus, in the presented study a recently developed model (RILEM: Representative Interactive Linear Eddy Model [1]) for regime-independent modeling of turbulent non-premixed combustion is used to simulate the so called "Spray B' of the Engine Combustion Network (ECN), which is a heavy-duty optical engine experiment. RILEM directly resolves the interaction of turbulent mixing with the chemistry along a one-dimensional representative line of sight through the combustion chamber via stochastic sequences of statistically independent eddy events. RILEM in its present form consists of a single (one-dimensional) linear eddy model (LEM) instantiation that is coupled to an unsteady Reynolds-averaged Navier-Stokes solver within the OpenFOAM framework. The coupling is similar to unsteady flamelet concepts but features distinct and important differences, e.g. an intrinsic representation of the scalar dissipation rate distribution and its fluctuations. Cylinder pressure, heat release rates and ignition delay time from the computation are compared to experiments under parametric variation of temperature

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

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

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

Evaluation of wall heat flux calculation methods for CFD simulations of an internal combustion engine under both motored and HCCI operation

In the present work, a study of different numerical heat transfer models is presented used for Homogeneous Charge Compression Ignition (HCCI) internal combustion engine simulations. Since the heat loss through the walls of an engine is an important parameter during engine optimization, as it influences power, efficiency and emissions, accurate modeling techniques need to be available. In this work, the predictive capability of different Computational Fluid Dynamics (CFD) models has been assessed, by using data obtained from experiments on a Cooperative Fuel Research (CFR) engine, a simple single cylinder pancake engine, which has been probed with local heat flux sensors into the combustion chamber walls. The open-source software OpenFOAM (R) was used to perform simulations of this engine, under both motored and HCCI operation, with a specific focus on the performance of different heat flux models. Due to the simple engine geometry, more numerically demanding heat flux modeling methods could be used, maintaining an acceptable computation time. This allowed a full comparison between the equilibrium wall models as in standard use, an improved empirical heat flux correlation and a numerically intensive low Reynolds formulation. The numerical results considering all aspects of the heat flux - both its progress in time as well as quantitative aspects such as the peak heat flux or the total heat loss - have then been compared to an extensive experimental database. This allowed a full analysis of the performance of the different methods. It was found that the low Reynolds formulation described the physical behavior near the wall the best, while predicting acceptable results concerning the heat flux through the engine walls. The best heat flux prediction was however obtained with an improved empirical model, which additionally has a much shorter computation time. This is crucial when moving on to heat flux simulations of more complex production type engines. Lastly, the equilibrium models were never capable of accurately predicting the wall heat flux

Numerical investigation on the use of Dimethyl Ether (DME) as an alternative fuel for compression-ignition engines

Dimethyl Ether (DME) is an oxygenated fuel that could favour the transition of the heavy-duty transportation sector to carbon neutrality thanks to its similarities in terms of thermophysical properties with diesel fuel, which will facilitate the retrofitting of existing architectures, and the possibility to achieve good trade-offs between NOx emissions, soot formation and overall combustion efficiency. The possibility of producing it from a multitude of carbon-neutral sources and the low hydrogen-to-carbon ratio would allow for an overall lower CO2 output, making an attractive option in limiting the global warming impact of the heavy-duty transportation sector. In the present work, a numerical analysis of the combustion process of DME is carried out. First, the numerical setup is validated against experimental data available for a constant volume vessel with an initial density of 14.8 kg/m3, discussing the capabilities of a chemistry-based combustion model using tabulated kinetics of homogeneous reactors: the Tabulated Well Mixed (TWM) model. Ignition delay times (IDT) are compared for a wide range of temperatures, from 750 K to 1100 K, and oxygen concentrations, from 15% to 21%. The same setup is then applied in the simulation of a heavy-duty internal combustion engine (ICE). A first validation was done to assess the performance of the numerical methodology in a traditional Mixing Controlled Compression Ignition (MCCI) scenario. Then, two other points were simulated: an MCCI condition with 35% of EGR and a Late-Premixed Charge Compression Ignition (L-PCCI) one, with 35% of EGR and an SOIe of 4 CAD aTDC. Local temperature distributions were compared, analyzing the effect of these technologies in NOx emission mitigation and their impact on gross indicated efficiency (& eta;g), showing the advantages that using DME can have on a real-world application

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