Bart Somers

Design and Operation of a High Pressure, High Temperature Cell for HD Diesel Spray Diagnostics: Guidelines and Results

This paper first compares strengths and weaknesses of different options for performing optical diagnostics on HD diesel sprays. Then, practical experiences are described with the design and operation of a constant volume test cell over a period of more than five years. In this test rig, pre-combustion of a lean gas mixture is used to generate realistic gas mixture conditions prior to fuel injection. Spray growth, vaporization are studied using Schlieren and Mie scattering experiments. The Schlieren set-up is also used for registration of light emitted by the combustion process; this can also provide information on ignition delay and on soot lift-off length. The paper further describes difficulties encountered with image processing and suggests methods on how to deal with them. Results are presented that illustrate the wide range of capabilities of this test-rig when combined with high speed video registration, in particular its potential for studying issues related to vaporizing fuel spray dynamics.

Spray impingement in the early direct injection premixed charge compression ignition regime

The main goal of this paper is to acquire more insight into the relationship between wall and piston impingement of liquid fuel and unburnt hydrocarbon emissions (UHC) emissions, under early direct injection (EDI) premixed charge compression ignition (PCCI) operating conditions. To this end, the vaporization process is modeled for various operating conditions using a commercial CFD code (StarCD). Predicted values for liquid core penetration, or liquid length LL , have been successfully checked against experimental data from literature over a wide range of operating conditions. Next, the correlation between the CFD results for wall and piston impingement and measured UHC emissions is studied. The diesel fuel used in the experiments is modeled as n-dodecane and n-heptadecane, representing the low and high end of the diesel boiling range, respectively. A distinction is made between liquid spray impingement on the piston surface and cylinder liner. For a conventional DI diesel nozzle, the high UHC emissions in the EDI PCCI regime correlate well with modeled cylinder wall impingement. Conversely, piston impingement is negligible in this regime. Accordingly, it may be assumed that the primary cause for high UHC emissions in the EDI PCCI regime, using conventional DI nozzles, is caused by liquid spray impingement against the cylinder liner. In this regime it was found that a higher intake and fuel temperature, as well as an elevated intake pressure have a positive effect on both UHC emissions and the spray impingement against the cylinder wall. This provides additional evidence that the two parameters (i.e. UHC and wall impingement) are linked. Lastly, the impact of nozzle cone angle is investigated. When adopting a narrow cone angle nozzle in the EDI PCCI regime, wall impingement is negligible and piston wetting becomes the dominant source of UHC emissions.

On the application of the Flamelet Generated Manifold (FGM) approach to the simulation of an igniting diesel spray

A study is presented on the modeling of fuel sprays in diesel engines. The objective of this study is in the first place to accurately and efficiently model non-reacting diesel spray formation, and secondly to include ignition and combustion. For that an efficient 1D Euler-Euler spray model [20] is implemented and applied in 3D CFD simulations. Concerning combustion, a detailed chemistry tabulation approach, called FGM (Flamelet Generated Manifold), is adopted. Results are compared with EHPC (Eindhoven High Pressure Cell) experiments, data from Sandia and IFP. The newly created combination of the 1D spray model with 3D CFD gives a good overall performance in terms of spray length and shape prediction, and also numerically it has advantages above Euler-Lagrange type models. Together with the FGM, also auto-ignition and a flame lift-off length is achieved.

Ignition Sensitivity Study of Partially Premixed Combustion by Using Shadowgraphy and OH* Chemiluminescence Methods

Partially Premixed Combustion (PPC) is a promising combustion concept for future IC engines. However, controllability of PPC is still a challenge and needs more investigation. The scope of the present study is to investigate the ignition sensitivity of PPC to the injection timing at different injection pressures. To better understand this, high-speed shadowgraphy is used to visualize fuel injection and evaporation at different Start of Injections (SOI). Spray penetration and injection targeting are derived from shadowgraphy movies. OH∗ chemiluminescence is used to comprehensively study the stratification level of combustion which is helpful for interpretation of ignition sensitivity behavior. Shadowgraphy results confirm that SOI strongly affects the spray penetration and evaporation of fuel. However, spray penetration and ignition sensitivity are barely affected by the injection pressure. There is a critical SOI range, in which a significant amount of fuel is trapped in the crevice volume. Injection in this critical range has a negative influence on the combustion efficiency and ignition sensitivity. Impingement of liquid fuel on the piston crown advances the combustion phasing by providing higher levels of stratification. Moreover, results of combustion stratification study show that stratification level has an inverse correlation with combustion phasing of PPC for late injections. (Less)

Towards control-oriented modeling of natural gas-diesel RCCI combustion

For natural gas (NG)-diesel RCCI, a multi-zonal, detailed chemistry modeling approach is presented. This dual fuel combustion process requires further understanding of the ignition and combustion processes to maximize thermal efficiency and minimize (partially) unburned fuel emissions. The introduction of two fuels with different physical and chemical properties makes the combustion process complicated and challenging to model. In this study, a multi-zone approach is applied to NG-diesel RCCI combustion in a heavy-duty engine. Auto-ignition chemistry is believed to be the key process in RCCI. Starting from a multi-zone model that can describe auto-ignition dominated processes, such as HCCI and PCCI, this model is adapted by including reaction mechanisms for natural gas and NOx and by improving the incylinder pressure prediction. The model is validated using NG-diesel RCCI measurements that are performed on a 6 cylinder heavy-duty engine. For three different engine operating points, it is operated at various diesel injection timings and NG-diesel blend ratios. The validation is focused on variables that are relevant for engine control, such as CA50, peak cylinder pressure, and engine-out NOx emissions. The validation shows that the multi-zone method with detailed chemistry reproduces the correct trends for important control parameters. From this validated model, real-time, map-based RCCI models are derived, which are considered to be an important step towards model-based NG-diesel RCCI control development.

Combustion Stratification for Naphtha from CI Combustion to PPC

This study demonstrated the change in combustion homogeneity from conventional diesel combustion via partially premixed combustion towards HCCI. Experiments are performed in an optical diesel engine at a speed of 1200 rpm with diesel fuel. Single injection strategy is employed and the fuel is injected at a pressure of 800 bar. The cylinder pressure at TDC is maintained at 35 bar and a high-speed video of the combustion process is captured through optical piston. The high speed video is processed to study the combustion homogeneity based on an algorithm reported in previous studies. Starting from late fuel injection timings, the combustion homogeneity is investigated by advancing to early fuel injection timings. For late fuel injection timings, a direct link between fuel injection timing and combustion phasing is noticed. At advanced fuel injection timings, the start of combustion is independent of fuel injection timing. The combustion homogeneity for the transition from CI via PPC towards HCCI is also investigated for various levels of dilution by displacing oxygen with nitrogen in the inlet. The start of combustion was retarded with the increase in dilution, while the mixture homogeneity is enhanced due to longer ignition delay. To compensate for the retarded combustion phasing with dilution, the inlet air temperature is increased. The experimental results show that the high speed image is initially blue and then turned yellow, indicating soot oxidation. The images are processed to generate the level of stratification based on the image intensity. This study shows better combustion homogeneity for early fuel injection timing and higher level of dilution and temperature in the inlet.

Modeling fuel spray auto-ignition using the FGM approach : effect of tabulation method

The Flamelet Generated Manifold (FGM) method is a promising technique in engine combustion modeling to include tabulated chemistry. Different methodologies can be used for the generation of the manifold. Two approaches, based on igniting counterflow diffusion flamelets (ICDF) and homogeneous reactors (HR) are implemented and compared with Engine Combustion Network (ECN) experimental database for the baseline n-heptane case. Before analyzing the combustion results, the spray model is optimized after performing a sensitivity study with respect to turbulence models, cell sizes and time steps. The standard High Reynolds ( Re ) k-e model leads to the best match of all turbulence models with the experimental data. For the convergence of the mixture fraction field an appropriate cell size is found to be smaller than that for an adequate spray penetration length which appears to be less influenced by the cell size. With the optimized settings, auto-ignition and flame lift-off length are analyzed. In general, both techniques capture the qualitative trend of experimental results. However, typically, the HR tabulation method predicts shorter ignition delay and LOL results than the ICDF method. In a quantitative sense, the ICDF and HR methods give better results in LOL and auto-ignition predictions, respectively.

Effects of Injection Timing on Fluid Flow Characteristics of Partially Premixed Combustion Based on High-Speed Particle Image Velocimetry

Partially Premixed Combustion (PPC) is a promising combustion concept ,based on judicious tuning of the charge stratification, to meet the increasing demands of emission legislation and to improve fuel efficiency. Longer ignition delays of PPC in comparison with conventional diesel combustion provide better fuel/air mixture which decreases soot and NOx emissions. Moreover, a proper injection timing and strategy for PPC can improve the combustion stability as a result of a higher level of fuel stratification in comparison with the Homogeneous Charge Compression Ignition (HCCI) concept. Injection timing is the major parameter with which to affect the level of fuel and combustion stratification and to control the combustion phasing and the heat release behavior. The scope of the present study is to investigate the fluid flow characteristics of PPC at different injection timings. To this end, high-speed Particle Image Velocimetry (PIV) is implemented in a light-duty optical engine to measure fluid flow characteristics, including the flow fields, mean velocity and cycle-resolved turbulence, inside the piston bowl as well as the squish region with a temporal resolution of 1 crank angle degree at 800 rpm. Two injectors, having 5 and 7 holes, were compared to see their effects on fluid flow and heat release behavior for different injection timings. Reactive and non-reactive measurements were performed to distinguish injection-driven and combustion-driven turbulence. Formation of vortices and higher turbulence levels enhance the air/fuel interaction, changing the level of fuel stratification and combustion duration. Results demonstrate clearly how turbulence level correlates with heat release behavior, and provide a quantitative dataset for validation of numerical simulations.

Experimental and numerical analyses of liquid and spray penetration under heavy-duty diesel engine conditions

The modeling of fuel sprays under well-characterized conditions relevant for heavy-duty Diesel engine applications, allows for detailed analyses of individual phenomena aimed at improving emission formation and fuel consumption. However, the complexity of a reacting fuel spray under heavy-duty conditions currently prohibits direct simulation. Using a systematic approach, we extrapolate available spray models to the desired conditions without inclusion of chemical reactions. For validation, experimental techniques are utilized to characterize inert sprays of n-dodecane in a high-pressure, high-temperature (900 K) constant volume vessel with full optical access. The liquid fuel spray is studied using high-speed diffused back-illumination for conditions with different densities (22.8 and 40 kg/m3) and injection pressures (150, 80 and 160 MPa), using a 0.205-mm orifice diameter nozzle. High-speed Schlieren imaging is used to analyze the influence of these boundary conditions on the spray penetration. Simulations of the fuel spray are performed using a dedicated computational mesh with refinements at the known location of the jet to capture the smallest scales of interest. Using a blob injection model refined with a primary atomization and secondary breakup model, correct trends and good agreement are achieved for both liquid and spray penetration. The capability of capturing the trends at largely varying boundary conditions with a single computational approach provides a solid base for future work.

Investigation of the Livengood–Wu integral for modelling autoignition in a high-pressure bomb

The reaction progress variable, which is widely used in premixed and diffusion combustion studies, comprises a set of pre-selected intermediate species to denote reaction progress. Progress towards autoignition can also be traced by the Livengood–Wu (LW) integral. Autoignition occurs when the LW integral attains a value of unity. This concept is further explored by applying it to an inhomogeneous mixture scenario, to determine the time and place of autoignition occurrence. A semidetailed mechanism (137 species and 633 reactions) for n-heptane/iso-octane/toluene is used in this study. Two numerical schemes based on the LW integral are proposed and incorporated into a computational fluid dynamics platform, to model autoignition in a 3D configuration, when a spray is injected into a constant volume bomb under diesel engine conditions. Tabulated chemistry, a traditional method of modelling autoignition using information from pre-calculated igniting diffusion flames, is also used for comparison purposes. The associated predicted pressure profiles are compared with experimental measurements.