بررسی عددی پایداری و خاموشی شعله پیش مخلوط v شکل

در مطالعه‌ی حاضر، احتراق پیش‌مخلوط پروپان هوا بررسی و بازه پایداری $v‌a‌r‌p‌h‌i=0.551.2$ تعیین شده که بازه پایداری تجربی را پوشش می‌دهد. علت اختلاف درنسبت هم‌ارزی‌های آستانه‌ی خاموشی، بالابودن شدت آشفتگی تجربی نسبت به مدل عددی است. میزان مونواکسید نیتروژن در احتراق رقیق در مقایسه با احتراق غلیظ بیشتر است که ناشی از عدم اختلاط کامل سوخت و هوا، لنگر انداختن شعله‌ی v شکل در یک طرف شعله‌نگهدار و ایجاد ناحیه‌ی گرم محلی است. در آستانه‌ی خاموشی در حالی که بخشی از شعله روی شعله‌نگهدار لنگر انداخته، بخش دیگر به‌صورت محلی به‌دلیل نرخ کرنش زیاد ناشی از برهم‌کنش گردابه‌ها و سطح شعله جدا شده و به پایین دست جریان منتقل می‌شود. سطح شعله در اثر اعواج‌های ناشی از آشفتگی باز شده و با کاهش بیشتر سوخت، گرمای آزاد شده احتراق برای پایداری شعله کافی نیست که با افت ناگهانی دما و رادیکال‌های o‌h شعله کاملاً خاموش می‌شود.

Numerical investigation of the premixed Vflame stabilization and blowout

Global warming and high fossil fuel consumption increase the application of the lowNox burner with leanpremixed combustion. However, this type of combustion is more susceptible to thermoacoustic instabilities. Then, a fundamental understanding of the leanpremixed combustion is essential. The aim of this work is the numerical study of a premixed propaneair Vflame 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 2step mechanism and EDC with CHEMKIN reduced kinetics with 28 species and 114 reactions are used for simulation. The lean and rich limits for Vflame 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 turbulenceflame interaction and transferred downstream with flow velocity. The Vflame surface was enhanced due to the vortex interaction with flame front. With a further reduction in the fuel amount, the heat release by the Vshaped 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.