ADCOMB-CFD · Advanced combustion models for cfd applications

Final Activity Report Summary - ADCOMB-CFD (Advanced combustion models for cfd applications)

Combustion accounts for the major part of the energy conversion processes conducted throughout the world. An accurate modelling of combustion processes is thus essential if the current policy objectives of increased efficiency and reduction in emissions of combustion engines and devices are to be realised. Since there is a demand for reliable and accurate simulation tools for reactive flows, the Marie Curie Adcomb-CFD (ToK-IAP) technology transfer project was initiated. In this project, Numeca Int. and TU Delft (MSP), TU Darmstadt (EKT) and university of Heidelberg (IWR) have been collaborating in implementing advanced models for non-premixed combustion into the unstructured, multipurpose CFD software package FINE / HEXA. The FINE / HEXA integrated CFD software package that was used as a platform for the development of combustion models and to carry out numerical studies consists of HEXPRESS for the automatic generation of unstructured fully hexahedral meshes, the flow solver Hexstream, and CFView for post-processing and visualisation. The Navier-Stokes equations for compressible flows are solved on unstructured, hexahedral grids by means of an explicit time-marching finite volume scheme. In the present project, a special solution scheme was developed which ensures monotonous solutions and thus allows a robust and accurate resolution of both reactive and inert flow fields. By using agglomeration multigrid, implicit residual smoothing and parallelisation with automatised domain decomposition, the solution scheme is highly efficient. Non-premixed combustion is mainly controlled by the mixing process between the fuel and the oxidiser. This was exploited in the current project, in which the mixture fraction approach was implemented in FINE / HEXA for the simulation of non-premixed combustion. By using look-up tables based on the chemical equilibrium or the laminar flamelet concept, the flame structure, i.e. the thermo-chemical properties, can be calculated in a pre-processing step and the computational costs of a reactive simulation can thus be kept low. To accurately predict the pollutant formation a postprocessing-tool for the determination of the NOx concentrations was developed. Turbulence-chemistry interaction is accounted for by using presumed Probability density functions (PDF). This modelling concept forms also the basis for more advanced functionalities like a model for non-adiabatic combustion where radiative heat loss is accounted for. For non-adiabatic flow situations, the heat loss is determined via a transport equation for the enthalpy in conjunction with a radiation model. In the present project, a radiation model for Optically thin media (OTM) and a radiation model based on the first-order spherical harmonics method (P1 approximation) were developed for the simulation of the radiative heat transfer in the flow field. In addition to the models for purely non-premixed combustion, a framework for modelling partially premixed combustion has been implemented in FINE / HEXA. This framework is used in conjunction with combustion tables generated by the 'Intrinsic low dimensional manifolds' (ILDM) method where a detailed mechanism is automatically reduced and the combustion processes is parameterised by one (or several) progress variables characterising the chemical process and the mixture fraction. The combustion and radiation models have been verified and validated on a series of comprehensible test cases, ranging from simple verification test cases and elementary flames for validation through complex test cases like combustors and furnaces with strong heat loss. The developed models have been integrated in FINE / HEXA and can be accessed through a graphics user interface in a user-friendly manner. User manuals and tutorials have been written and both the developed module for non-premixed combustion and the module for radiative heat-transfer modelling have been released.

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