نوع مقاله : مقاله پژوهشی
دانشکده ی مهندسی مکانیک و انرژی، دانشگاه شهید بهشتی
عنوان مقاله [English]
Global warming and high fossil fuel consumption increase the application of the low-Nox burner with lean-premixed combustion. However, this type of combustion is more susceptible to thermoacoustic instabilities. Then, a fundamental understanding of the lean-premixed combustion is essential. The aim of this work is the numerical study of a premixed propane-air V-flame stabilized on the designed flame holder with focus on the stability and blowout analysis using ANSYS Fluent. Different turbulent models in cold flow simulation are investigated and for steady flame with lower computational cost and for transient flame dynamics are selected. The combustion models including FR/ED, EDC with 2-step mechanism and EDC with CHEMKIN reduced kinetics with 28 species and 114 reactions are used for simulation. The lean and rich limits for V-flame are predicted and the combustion stability range is determined as using SAS turbulent model and EDC reduced mechanism. The numerical stability limit covers the experimental range. The experimental tests have higher turbulent intensity than numerical model, leading to the difference in the blowout threshold. The flame dynamics on the blowout limit is investigated by instantaneous temperature field and radicals. The NOx emission varies with the mean flame temperature and is higher in the lean combustion than the rich cases. The reason of this phenomenon is incomplete mixing of the fuel and air, leading to the anchoring of the flame on the one side of the flame holder more than the other side and the hot regions formation which results in higher amount of NOx. As the equivalence ratio decreases, the flame fragments are separated locally due to the high strain rate formed with turbulence-flame interaction and transferred downstream with flow velocity. The V-flame surface was enhanced due to the vortex interaction with flame front. With a further reduction in the fuel amount, the heat release by the V-shaped flame area is not sufficient to sustain the burning of the rest of the flame anchoring on the bluff body. Then, the mean temperature immediately reduces lower than ignition temperature, while the radicals are less than the stable combustion, leading to the global flame extinction.