Philip de Goey

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.

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.

Accuracy assessment of thermoacoustic instability models using binary classification

We apply binary classification theory to assess the (in)stability prediction accuracy of thermoacoustic models. It is shown that by applying such methods to compare a large set of stability predictions and experiments it is possible to gain valuable qualitative insight in different aspects of prediction quality. The approach is illustrated with a 2-port model and a large experimental data set. The presented framework provides an unified and practical tool to answer questions such as (i) What is the chance that a stable prediction will be correct? and (ii) How conservative is the model? It is shown that the most suitable quality indicator is strongly dependent on the actual purpose of the model. The method provides a solid starting point for model comparison and optimization.

Spray and failure analysis of porous injection nozzles

To improve the mixing of fuel and air in the combustion chamber of current diesel engines, research is carried out regarding injectors with a porous nozzle tip, replacing conventional nozzles with a limited number of holes. Preliminary tests with porous injectors showed that further research concerning spray distribution was necessary due to non-optimal spray shapes and low fuel velocities. Therefore, spray shapes and fuel velocities of porous injectors were examined at atmospheric pressures. These examinations show that the spray shapes can be adjusted by alternating the geometries. Geometrical influences have been studied and compared to conventional injectors, showing that the fuel velocity of the porous injectors has decreased with approximately a factor of 10. Subsequently, research concerning the lifetime of porous nozzles was necessary due to premature failure. Therefore, the micro structure of the porous material has been evaluated using an electron microscope, a micro CT-scanner and a hardness tester. These evaluations show that the current technique to shape the outside of the porous injectors has a negative influence on the lifetime. Furthermore, the relations between subjected materials and corresponding properties have been examined in order to improve the lifetime, resulting in lifetime extension possibilities up to a factor of 5.

Experimental Study on the Potential of Higher Octane Number Fuels for Low Load Partially Premixed Combustion

The optimal fuel for partially premixed combustion (PPC) is considered to be a gasoline boiling range fuel with an octane number around 70. Higher octane number fuels are considered problematic with low load and idle conditions. In previous studies mostly the intake air temperature did not exceed 30 °C. Possibly increasing intake air temperatures could extend the load range. In this study primary reference fuels (PRFs), blends of iso-octane and n-heptane, with octane numbers of 70, 80, and 90 are tested in an adapted commercial diesel engine under partially premixed combustion mode to investigate the potential of these higher octane number fuels in low load and idle conditions. During testing combustion phasing and intake air temperature are varied to investigate the combustion and emission characteristics under low load and idle conditions. The results show that PRF70, 80 and 90 present stable combustion when an intake temperature higher than 60 °C is used at low load and idle conditions. The coefficient of variations of the gross indicated mean effective pressure (COVIMEPgross) is below 3% and 4.5% at low load and idle condition respectively, which is well below the 10% limit. PRF90 has positive ignition dwells regardless of combustion phasing and intake temperature for all the measurements, and the oxygen concentration is sufficient at both low load and idle conditions to ensure near zero soot emissions. The difference in ignition delay between PRF 70 and 80 is smaller than that between PRF80 and 90, and PRF70 and 80 display similar emission behavior.

Spray analysis of the PFAMEN injector

In an earlier study, a novel type of diesel fuel injector was proposed. This prototype injects fuel via porous (sintered) micro pores instead of via the conventional 6-8 holes. The micro pores are typically 10-50 micrometer in diameter, versus 120-200 micrometer in the conventional case. The expected advantages of the so-called Porous Fuel Air Mixing Enhancing Nozzle (PFAMEN) injector are lower soot- and CO 2 emissions. However, from previous in-house measurements, it has been concluded that the emissions of the porous injector are still not satisfactory. Roughly, this may have multiple reasons. The first one is that the spray distribution is not good enough, the second one is that the droplet sizing is too big due to the lack of droplet breakup. Furthermore air entrainment into the fuel jets might be insufficient. All reasons lead to fuel rich zones and associated soot formation. To acquire more insight into the spray of the porous injector, several PFAMEN nozzles have been produced and investigated. The momentum of the spray was found to be an order of magnitude lower compared to conventional injectors. Afterwards, the porous injector was placed in an optically accessible engine, allowing the analysis of the spray development and combustion process. The main conclusion is that the spray penetration depth is relatively low. Finally, droplet size and velocities are presented using Phase Doppler Anemometry (PDA) and from these measurements it became clear that the droplets of the PFAMEN nozzle are larger compared to conventional injectors. This is believed to be caused by the low exit velocities.

Numerical Simulation for the Preliminary Design of Fuel Flexible Stationary Gas Turbine Combustors Using Conventional and Alternative Fuels

Currently, high efficiency and low emissions are most important requisites for the design of modern gas turbines due to the strong environmental restrictions around the world. In the past years, alternative fuels have been considered for application in industrial gas turbines. Therefore, combustor performance, pollutant emissions and the ability to burn several fuels became of much concern and high priority has been given to the combustor design.This paper describes a methodology focused on the design of stationary gas turbines combustion chambers with the ability to efficiently burn conventional and alternative fuels. A simplified methodology is used for the calculations of the equilibrium temperature and chemical species in the primary zone of a gas turbine combustor. Direct fuel injection and diffusion flames, together with numerical methods like Newton-Raphson, LU Factorization and Lagrange Polynomials, are used for the calculations. Diesel, ethanol and methanol fuels were chosen for the numerical study.A computer code sequentially calculates the main geometry of the combustor. From the numerical simulation it is concluded that the basic gas turbine combustor geometry, for some operating conditions and burning diesel, ethanol or methanol, are of similar sizes, because the development of aerodynamic characteristics predominate over the thermochemical properties.It is worth to note that the type of fuel has a marked effect on the stability and combustion advancement in the combustor. This can be seen when the primary zone is analyzed under a steady-state operating condition. At full power, the pressure is 1.8 MPa and the temperature 1,000 K at the combustor inlet. Then, the equivalence ratios in the primary zone are 1.3933 (diesel), 1.4352 (ethanol) and 1.3977 (methanol) and the equilibrium temperatures for the same operating conditions are 2,809 K (diesel), 2,754 K (ethanol) and 2,702 K (methanol). This means that the combustor can reach similar flame stability conditions, whereas the combustion efficiency will require richer fuel/air mixtures of ethanol or methanol are burnt instead of diesel.Another important result from the numerical study is that the concentration of the main pollutants (CO, CO2, NO, NO2) is reduced when ethanol or methanol are burnt, in place of diesel.Copyright

Numerical Study on the Autoignition of Biogas in Moderate or Intense Low Oxygen Dilution Nonpremixed Combustion Systems

The ignition delay of biogas in mixing layers is investigated using a one-dimensional combustion model, with its application in Moderate or Intense Low oxygen Dilution (MILD) combustion being the focus. The current study reveals the key aspects of the ignition of biogas in a nonpremixed, igniting mixing layer with a hot oxidizer of low oxygen content. The observed characteristics are contrasted against the existing studies on ignition in homogeneous mixtures under similar conditions. Biogas is considered here as a mixture of CH4 with variable amounts CO2. The influence of reactive, thermal, and transport properties of CO2 on the ignition is evaluated using artificial species to mimic the respective characteristics of CO2. While the ignition delay in homogeneous mixtures shows a strong dependence on CO2 content in the fuel, the ignition delay predictions from one-dimensional mixing layers show no significant influence of CO2 levels in biogas. In addition, the influence of oxidizer composition and temperature on ignition delay is determined for CO2 levels ranging from 0% to 90%. A sensitivity analysis of chemical reactions on the ignition delay shows a negligible effect of CO2 concentration in biogas. The current study emphasizes the role of oxidizer composition and temperature on the ignition characteristics of a MILD biogas flame.

FGM with REDx: chemically reactive dimensionality extension

We propose a new approach to improve the accuracy of flamelet-generated manifolds (FGMs) method by extending the manifolds with additional chemically reactive degrees of freedom. Following the ideas of intrinsic low-dimensional manifold, the dimensionality of the FGM is increased by performing a local time-scale analysis of the chemical source term. A few slow characteristic directions of the reaction kinetics are used to extend the FGM, while the remaining reaction groups, characterised by fast time-scales, are assumed in steady state. The introduced method for FGM REactive Dimensionality extension is abbreviated as FGM-REDx. It is tested in one-dimensional simulations reproducing an expansion of burnt gases in an aero-engine stator. This process is characterised by a rapid change of enthalpy and pressure, altering, among others, the chemistry of pollutants CO and NO. The primary focus was on the assessment of the FGMs capability to predict the pollutants emissions. The rates of physical/thermodynamic perturbations turned out to be severe enough for the chemical species composition to go off the flamelet. The FGM extended with one additional chemically reactive dimension has been generated and successfully applied to the test cases, yielding a high accuracy gain over the standard FGM.

Styrofoam Precursors as Drop-in Diesel Fuel

Styrene, or ethylbenzene, is mainly used as a monomer for the production of polymers, most notably Styrofoam. In the synthetis of styrene, the feedstock of benzene and ethylene is converted into aromatic oxygenates such as benzaldehyde, 2-phenyl ethanol and acetophenone. Benzaldehyde and phenyl ethanol are low value side streams, while acetophenone is a high value intermediate product. The side streams are now principally rejected from the process and burnt for process heat. Previous in-house research has shown that such aromatic oxygenates are suitable as diesel fuel additives and can in some cases improve the soot-NO x trade-off. In this study acetophenone, benzaldehyde and 2-phenyl ethanol are each added to commercial EN590 diesel at a ratio of 1:9, with the goal to ascertain whether or not the lower value benzaldehyde and 2-phenyl ethanol can perform on par with the higher value acetophenone. These compounds are now used in pure form. In future work, real streams, which are rich of these compounds, but contain various other chemicals as well, will be used. Experiments have been performed on a heavy duty (12.6L) diesel engine, of which one cylinder is a dedicated test cylinder. The results demonstrate that the emissions and efficiencies are more or less comparable for all aromatic oxygenates. Afterwards, the results are compared against neat diesel. It was found that, depending on operation conditions, either the efficiency of the oxygenates was higher, while the emissions where comparable to diesel or the emissions decreased dramatically with comparable efficiencies as diesel. Accordingly, compared to neat diesel, both the high- and low-value styrene streams yield overall positive engine behavior in all measured operating conditions.