Exploitable results
The code is capable of simulating of soot volume fraction in engines and turbines, based on the soot model for size distribution in laminar and turbulent flames. The Lund code Chamble coded in Fortran90 for calculating laminar flames, and flamelets is prepared for including the additional transport equations to calculate the particle size distribution function. Within the actual version of the program new sub-models from the ongoing project have been implemented. These sub-models make it possible to simulate the soot mass interactively with CFD industrial codes. The formulations of the Sectional Method for turbulence-chemistry interaction models, PDF and Flamelet, has also been developed.
Combustion of a Diesel engine is the combination of many input data flow field, injection strategy, combustion chamber design all related in a non linear process. Furthermore, it is even more difficult to isolate the engine parameters that have a role in the soot formation and oxidation process.
In this sense, some guidelines in the role played by these parameters is very important in the development of new Diesel engines. A wide-data base of engine cases, simulated with a predictive CFD tool, will be generated and used in the identification of these guidelines that will support the engine designers.
CRF is using since 1990 the simulation software STAR form CD-Adapco for the analysis of in-cylinder flows. During the years a satisfactory agreement in the simulation of the spray and combustion processes have been achieved.
CFD results are compared with the experimental data in terms of pressure cycle. More than this, the CFD results of the 3D instantaneous evolution of the spray and temperature field within the combustion chamber are analysed in order to understand how the combustion chamber design and the flow-field characteristics could influence the combustion process.
This approach is useful for full load conditions but it is true that nowadays emissions (Nox and soot) are the key factors in the development of new Diesel engines.
All the available soot models need always some tuning and do not give any information about soot particle size, CRF is willing to include in STAR CD the new model developed by Prof. Mauss within the Partsize project.
A complete methodology to obtain bi-dimensional soot volume fraction, number density and temperature in Diesel flame has been developed combining the Planar Laser-Induced Incandescence (PLII) technique with Laser Elastic Scattering (LES) and two-colour pyrometry.
Planar LII can provide two-dimensional distribution of soot volume fraction, but combined with LES techniques can provide also relative particle number and number density. The methodology presented includes guidelines for a proper use of the combined technique, together with calibration and signal correction procedures, and calculation software to convert the incandescence and attenuation signals into quantitative information about soot structure and development in Diesel flames. This information is combined with flame temperature obtained with two-dimensional two-colour pyrometry.
The CFD code FIRE® is a multi purpose simulation software which can be used for different kind of applications. For the simulation of engines FIRE® provides special modules for the handling of the mesh movement as well as a number of mathematical models for the treatment of effects as fuel injection or combustion. The solution algorithm is based on the well-known SIMPLE (Semi IMplicit Pressure Linked Equations) procedure. The code can handle computational cells with an arbitrary topology. Within the actual version of the program new sub-models from the ongoing project have been implemented. These sub-models make it possible to simulate the soot mass on a higher level of accuracy then it has been done up to now.
A methodology for proper measurement of exhaust particulate matter characteristics with a Scanning Mobility Particle Sizer (SMPS) instrument have been developed, including recommendable settings and procedure for proper measurement in the exhaust ducts of Diesel engines. The methodology proposed provides not only guidelines on the proper use of the instrument, but also criteria on how to combine the results with those provided by more standard systems such as mini tunnels extensively used for R&D as well as in engine/vehicle homologation tests.
The procedure could possibly contribute to the background knowledge available for elaboration of future pre-normative regulations for engine emissions.
A data base of experimental results on soot volume fraction, number density, average particle size and temperature maps has been developed combining measurements obtained the Planar Laser-Induced Incandescence (PLII) technique with Laser Elastic Scattering (LES) and Two-colour pyrometry.
Experiments have been performed in a research facility capable of reproducing thermodynamic conditions as in a real Diesel engine, but provided with wide optical access for the application of the experimental techniques. Results on the database consist of the raw images, two-dimensional maps of soot volume fraction, relative number density and temperature, as well as numerical results on the evolution of measured values as a function of time.
Parameters such as injection conditions (pressure, nozzle orifice size) and thermodynamic conditions of the intake air have been varied in the parametric studies whose results are included in the database.
A data base of experimental results on particulates size distribution and mass emitted is available as a result of a series of parametric studies performed in state-of-the-art Direct Injection Diesel engines, modifying engine operation conditions, and combining the information provided by a Scanning Mobility Particle Sizer (SMPS) instrument and more standard exhaust gases, smoke and particulate matter measurement devices such as gas analysers, mini tunnel and opacimeters.
Parametric studies performed consider the effect of injection strategy (Start of Injection, Injection Pressure and Split injection) and Exhaust Gas Re-circulation.
Soot formation and oxidation inside modern diesel engines still are poorly understood. So it is important to have a methodology to get accurate information of these processes. The laser-induced incandescence is a measurement technique that is highly specific for carbonaceous particles. With its high spatial and temporal resolution it enables exact qualitative statements about the occurrence of soot inside the combustion bowl. These results can afterwards for example be used for a soot formation model in CFD simulations. There it is really necessary to know where and when the soot formation and oxidation takes place and at the end the LII measurements inside the engine are at the same time validation and optimisation for the CFD simulations.
Soot Particle Size Distributions in Diesel Engines - Detailed Kinetic Modelling and Experimental Investigations
P. Priesching, AVL List GmbH, Graz, Austria
F. Mauss, Lund University, Lund, Sweden
W. Bauer, MAN AG, Nürnberg, Germany
M. Schmid, LTT, Nürnberg, Germany
The Diesel engine is known to be the most efficient power source for transportation today. The high efficiency leads to low green house gas emission and there is still the potential to increase fuel efficiency of Diesel engines with the corresponding further reduction of CO2 emissions. The improvements of the recent years, not only in the respect of economical and ecological characteristics, but also in respect of drivability and comfort, made Diesel engine powered cars more and more popular especially in Europe. Therefore special care has to be taken of the specific emission problems of the Diesel engine.
The ongoing discussion on the health risk related to soot particle emissions from high speed and commercial Diesel engines is a great challenge to engine manufacturers world-wide. Increases in engine efficiency have to be achieved in face of increasingly more stringent legislation on soot emissions and particle sizes.
However, the most important issue of diesel soot emissions is neither the total number density, nor the total mass. The most important issue is the toxicology of soot particles. The size of the particles, the particle surface properties and the shape of the particles determine the toxic levels of soot. The size of the particles determines how deep a particle may enter a human lung. The surface properties and the shape determine if the particle can cause chemical activity.
The intention of the presented work was to develop a new simulation tool that fits into a CFD workflow and provides information about the soot particle size distribution. Additionally it was necessary to improve and use state of the art measurement techniques in order to be able gain more knowledge about the behaviour of the soot particles and to validate the archived simulation results.
Nowadays, diesel particulate mass concentrations at exhaust are decreasing much more, whereas a great number concentration ultrafine particle (less than 100 nm in diameter) was observed.. A complete characterization of soot particle formation during diesel combustion must be furnished by non intrusive technique.
More details an "in situ" and real time chemical and physical characterization of soot precursors and soot particles is necessary. In order to perform this characterization, extinction and scattering flame spectroscopy, must be carried out. This technique is based on broadband ultravioletvisible extinction and scattering spectroscopy. Multi-wavelength technique overcomes single wavelength technique, allowing using the data at several wavelengths to retrieve particle number size distribution with better accuracy. Moreover, the analysis of spectral scattering and extinction features gives information about particulate chemical nature and allows characterising the particle agglomeration degree.
The results of the test bed of heavy duty truck engines in five very different operating points with a big amount of soot and nearly no soot will be recorded and calculated with 1D-software, so that all boundaries for 3D-CFD-simulation are well known. These five operating points should be varied in the level of injection pressure and the amount of exhaust gas re-circulation. For all this operating points its is planed to calculate the soot emission with the standard software and the new parts from FIRE. Both parts of results will be compared.
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