Studies On Modified Algebraic Second Order Moment Model In Large Eddy Simulation Of Turbulent Combustion

Turbulent combustion exists in various kinds of industrial burners. Limited by the complex structures of actual equipment and immature measurement technology, the internal flow and combustion process cannot be fully revealed in the experiments. With the development of computer technology, numerical simulation can get detailed flow data with low time cost, thus becomes an important approach in the design process of combustion equipment. Large eddy simulation (LES) can capture the dynamic processes of combustion and turbulent vortexes, benefitting to promote the design progress. In this paper, LES of turbulent combustion in gas turbine combustor was investigated. After a long time development, the subgrid stress model in LES has been well developed. However, due to the combustion involves complex coupling of physical and chemical processes, the combustion model needs more study. Focusing on the combustion model suitable for LES of combustion chamber, this paper will develop the modified algebraic second-order moment model (MASOM). The model validations and applications will be discussed in this paper.The MASOM model is developed based on the second order moment model. Inheriting the consideration of the influence of subgrid scale vortexes on reaction, the MASOM model improves the method for reaction rate calculation, and eliminates the effect of given parameters. Furthermore, the MASOM model considers the influence of subgrid mixing process, expecting more exact and efficient prediction of the reaction rate.At first, the MASOM model was applied in LES of several typical laboratory flames with characteristics of real combustion chamber. The experimental data of these flames are sufficient to provide comparative data for model validation. In addition, the influence factors of these flames are simple, facilitating more concentration on specific combustion characteristic.The MASOM model was applied in the numerical simulation of turbulent jet diffusion flame. In order to validate the model rationality quickly, the MASOM model was applied to Reynolds Average Navier-Stokes method (RANS). Compared with the original second order moment model and finite rate/eddy dissipation model, the MASOM model obtained more consistent data with the experiment. Next, the MASOM model was applied in LES of the jet flame. Considering influence of subgrid turbulent fluctuation of temperature and composition, the MASOM model obtained more concentrated area of chemical reaction, inducing more consistent results with the experiment. The results confirmed the rationality of the subgrid reaction rate.The MASOM model was applied in LES of lifted flame with hot coflow. The hot coflow could represent the recirculation of combustion product in chamber, which is an important way to achieve combustion stabilization. Using the classic one-step mechanism of methane oxidation, the MASOM model could predict the liftoff phenomenon. Although the liftoff height obtained was lower, but compared with the result of eddy dissipation concept model, it was more conformed to the experiment. Furthermore, considering the influence of equivalence ratio, the global mechanism was improved and then the improved mechanism was applied in the MASOM model. The liftoff height and scalar distribution in Cabra flame were accurately predicted. Furthermore, using the MASOM model and improved mechanism, the variation trend of liftoff height with different conditions can be accurately simulated by LES.The MASOM model was applied in LES of swirling premixed combustion. Lean premixed combustion is often employed by the advanced gas turbine. In this part, the two-step methane oxidation mechanism was adopted. Compared with the partial stirred reactor model, the MASOM model obtained more consistent results with experiment. The MASOM model with multi-step simplified mechanism in LES of premixed flame was validated. Based on the detailed data obtained by LES, the combustion characteristics were investigated. It was shown the combustion process could take away the precession vortex core structure existing in the isothermal case, and the combustion could supply energy for turbulent flow. Under inflow equivalence ratio fluctuations, the premixed flame shows obvious response. The turbulence intensity in combustion area was significantly strengthened, and the flame surface wrinkled obviously. With lower disturb frequency and higher amplitude, more obvious influence was observed, but the variation in different amplitudes was inconspicuous compared with different frequencies.Finally, the MASOM model was applied in LES of a real combustion chamber. The Arrayed-Vanes Combustor was studied by LES. In order to reduce computation cost, the segmented method was proposed, in which the dynamic flow characteristics in arrayed-vanes nozzle could be transmitted to the combustion chamber. Using the MASOM model and two-step reaction mechanism, LES could predict accurate flame characteristic and outlet temperature in the chamber. The applicability of MASOM model in real gas turbine combustor was validated. Based on simulation results, the combustion characteristic under different equivalence ratio conditions was studied.