Modelling of Spray Combustion, Emission Formation and Heat Transfer in Medium Speed Diesel Engine
Taskinen, Pertti (2005)
Tampere University of Technology
Konetekniikan osasto - Department of Mechanical Engineering
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The thesis deals with the spray combustion, emissions formation and heat transfer theories of phenomena and their modelling related to medium speed diesel engines. The aim of the work has been to develop a simulation tool for medium speed diesel engines that can be applied later in the optimisation process of the engine economy with the allowed pollution level. In developing work of the KIVA-II code main attention was focused on the following phenomena: the drop vaporisation under a high-pressure envinronment, the soot formation modelling by the Hiroyasu or TM models and the oxidation by the NSC model, the soot radiation modelling by the simplified model(pure emission) or the DOM, the convective heat transfer modelling and the spray turbulence modelling by the RNG/STD k-e turbulence models.According to the results obtained the high-pressure drop vaporisation model is necessary in the code instead of the corresponding low-pressure model in order to get a more realistic ignition of the fuel vapour/air mixture and combustion. Also the Hiroyasu soot formation and the NSC soot oxidation model were added into the code and formulated into the source term form using either the computational cell average or the EDC-weighted values of the cell quantities in the soot transport equation. The soot emissions after modifications were a more realistic level than in the case of the original formulation and the models. Also the lack of a NSC soot oxidation model able to predict the soot oxidation rate correctly was taken into account by the extra constant in the model. The soot radiation was taken into account in the internal energy equation by simplified model(optically thin radiant media) from the radiant media or the RTE solved by the DOM. The radiant heat flux to piston top becomes the more realistic level with the DOM than with the simplified model compared to the experimental values of the slightly other type diesel engine than the modelled medium speed diesel engine. This shows that the absorption of soot radiation in the radiant region must also be taken into consideration. Effect of the soot radiation on temperature of the gas appears only in the soot region, not in the fuel vapour reaction zone where the soot is not found. Therefore the soot radiation does not reduce maximum temperatures of the gas in the fuel vapour reaction zone or in the nitrogen oxide formation regions near the reaction zone and so influence in the NOx emissions from the engine. Further the original temperature wall function of the KIVA-II based on the modified Reynolds analogy under-predicts the heat flux to wall considerably. The model was replaced by the model which was based on the use of a one-dimensional energy equation and the correlation of the dimensionless temperature including an increasing turbulent Prandtl number near the wall. The heat flux to piston top with the new model was a more realistic level than with the original model of the code compared to the experimental values of the other type diesel engine.Finally in the work the modified RNG k-e model was improved based on the results obtained with the STD and the basic RNG k-e models. According to the results mentioned above the STD model is too diffusive while the basic RNG is too less diffusive in the high rate of the strain region(spray region) and therefore the fuel vapour mixing(combustion) occurs in an un-satisfactorily way. In the turbulence model developed the additional term of the epsilon equation was modified suitably and therefore the spray spreading and the combustion occur more realistically compared to either the basic RNG or the STD k-e turbulence model cases. The gas turbulence intensity was reduced in the early phase of combustion and emphasized in the later phase of combustion compared to the situation with the STD model. The cylinder pressure curve becomes by far the closest with the new turbulence model than either of both the models mentioned above. In the work the failure of the basic RNG turbulence model of the KIVA-3V was found and rectified.
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