Large eddy simulation of a fuel-rich turbulent non-premixed reacting flow with radiative heat transfer

摘要

The aims of this thesis are to apply the Large Eddy Simulation (LES) and beta Probability Density Function (β- PDF) for the simulation of turbulent non-premixed reacting flow, in particularly for the predictions of soot and NO production, and to investigate the radiative heat transfer during combustion process applying Discrete Ordinates Method (DOM). LES seeks the solution by separating the flow field into large-scale eddies, which carry the majority of the energy and are resolved directly, and small-scale eddies, which have been modelled via Smagorinsky model with constant Cs (Smagorinsky model constant) as well as its dynamic calibration. This separation has been made by applying a filtering approach to the governing equations describing the turbulent reacting flow. Firstly, LES technique is applied to investigate the turbulent flow, temperature and species concentrations during the combustion process within an axi-symmetric model cylindrical combustion chamber. Gaseous propane (C3H8) and preheated air of 773K are injected into this cylindrical combustion chamber. The non-premixed combustion process is modelled through the conserved scalar approach with the laminar flamelet model. A detailed chemical mechanism is taken into account to generate the flamelet. The turbulent combustion inside the chamber takes place under a fuel-rich condition for which the overall equivalence ratio of 1.6 is used, the same condition was used by Nishida and Mukohara [1] in their experiment. Secondly, the soot formation in the same flame is investigated by using the LES technique. In this thesis, the soot formation is included through the balance equations for soot mass fraction and soot particle number density with finite rate kinetic source terms to account for soot inception/nucleation, surface growth, agglomeration and oxidation. Thirdly, the NO formation in the flame is studied by applying the LES. The formation of NO is modelled via the extended Zeldovich (thermal) reaction mechanism. A transport equation for NO mass fraction is coupled with the flow and composition fields. Finaly, the radiative heat transfer in the flame is investigated. Both the luminous and non-luminous radiations are modelled through the Radiative Transfer Equation (RTE). The RTE is solved using the Discrete Ordinates Method (DOM/Sn) combining with the LES of the flow, temperature, combustion species and soot formation. The computed results are compared with the available experimental results and the level of agreement between measurements and computations is quite good.