Comparative study of different Radiation, Turbulence and Chemistry models for the simulation of a Lab-scale MILD combustion furnace

Abstract

Flameless combustion, or MILD combustion is a technology that enables stable combustion with high efficiency while maintaining very low NOx and soot emissions. It is characterized by uniform heat flux and can be achieved when fuel and preheated oxidant jets are mixed into a strong recirculating flow. It is a complex process which combines multiple physical phenomena such as, heat transfer and reactions in a turbulent flow and is difficult to capture numerically via simple Computational Fluid Dynamics (CFD) simulations. Therefore, appropriate turbulence, chemistry, radiation and turbulence-chemistry interaction models are required to address this combustion regime via computational modeling. The present work tries to tackle this issue by conducting a comparative study of different radiation, turbulence and chemistry models for the simulation of a labscale MILD combustion furnace using RANS CFD simulations. This assessment was conducted in two steps. Firstly, the importance of radiative heat transfer in furnaces was investigated, by simulating an axisymmetric oxycombustion furnace using advanced radiation models. The SLW, Domain-based WSGG and Cell-based WSGG models were evaluated using simple chemistry and turbulence models. The results were validated based on the work of Webb et al and experimental results. It was concluded that detailed simulation of the radiative gas properties is important and directly influences the resulting temperature field, especially in the bulk flow region. Secondly, a detailed evaluation of a Dutch natural gas fired Delft lab-scale furnace that is operating in MILD combustion was conducted. For the turbulence modeling, the Standard, the Modified Standard, the Realizable k − ε and the k − ω SST Models were compared firstly qualitatively, based on velocity and reaction field contours, and secondly, quantitatively, based on available LDA velocity measurements. The influence of the chemistry modeling was evaluated in two parts. Firstly, the qualitative effect of changing the Cξ constant of the EDC model on temperature and reaction zones of the GRI-Mech 3.0, the DRM-19, the KEE58 and Chen mechanisms, was investigated. And secondly, the quantitative effect on the use of the GRI-Mech 3.0, the KEE58, Chen and Lu-30, based on CARS temperature measurements was researched. Lastly, the influence of the radiation model was investigated qualitatively and quantitatively using the standard 1-RTE domain based WSGG, a 4-RTE WSGG, a cell-based WSGG and the SLW model.
Out of the mechanisms coupled with the EDC, the GRI-Mech 3.0 performs well. Of the radiative properties models the domain based WSGG performs worse compared to the other models. The cell based WSGG performs similarly to the more advanced spectral and non-grey (4-RTE) models and has lower computational cost. For these reasons, the best overall combination for the studied flameless furnace is found to be the combination of Standard k−ε model, standard EDC with GRI-Mech 3.0 and cell based WSGG.