نویسندگان

1 دانشکده مهندسی مکانیک، دانشگاه گیلان، رشت

2 دانشکده مهندسی مکانیک، دانشگاه صنعتی اصفهان، اصفهان

چکیده

مدل‌سازی درست پدیده‌ شناوری و انتقال حرارت در بستر سیال‌های گاز-جامد، صرفاً به نوع مدل عددی انتخابی و الگوریتم‌های درگیر وابسته نیست. در واقع انتخاب مدل مناسب برای هر شرایط عملکردی خاص، اجرای صحیح هر مدل، انتخاب درست پارامتر‌ها و شروط مرزی مناسب عوامل تعیین‌کننده‌ صحت نتایج حاصل در بررسی عملکرد بستر سیال‌ها است. در تحقیق حاضر، به منظور شبیه‌سازی دقیق فرآیند انتقال حرارت در بستر سیال‌ها، پارامتر‌های مهم و تأثیرگذار بر روند حل عددی به روش اویلری دو سیالی با به‌کارگیری تئوری انرژی جنبشی جریان دانه‌ای بررسی شد. به این منظور، اثرات ضریب بازگردانی ذره-ذره و ذره-دیواره، ضریب انعکاس و شیوه‌ تعیین دمای دانه‌ای و ضریب هدایت گرمایی مؤثر بر روند حل عددی ارزیابی شد. این بررسی‌ها نخست در فرآیند گرم کردن ذرات با هوای داغ ورودی در یک بستر سیال آدیاباتیک و سپس در بستر سیالی با دیواره‌های دما ثابت و برای دو رژیم جریان پرکاربرد حبابی و آشفته انجام گرفت. نتایج نشان داد که ضریب انعکاس و هدایت گرمایی مؤثر پارامتر‌هایی تأثیرگذار در فرآیند انتقال حرارت از دیواره به بستر هستند. به گونه­ای که در این شرایط مقدار صفر ضریب انعکاس سبب می‌شود که دمای هوا در رژیم حبابی حدود 7 درجه و در رژیم آشفته حدود 5 درجه بالاتر رود و مقدار واحد آن نتایج یکسانی با شرط عدم لغزش به‌دست می‌دهد. علاوه بر این در نظر گرفتن ضرایب هدایت گرمایی ماده جامد و گاز سبب می‌شود که دمای هوای خروجی حدود 26 درجه بیشتر از دمای به‌دست آمده با در نظر گرفتن ضرایب هدایت گرمایی مؤثر با رویکرد استاندارد باشد. فرم مشتق جزئی و جبری معادله‌ بقای انرژی نوسانی فاز جامد در بستر سیال‌های متراکم جواب‌های یکسان به‌دست می‌دهند، هرچند در نظر گرفتن یک دمای دانه‌ای ثابت می‌تواند سبب بروز خطاهای محاسباتی شود

کلیدواژه‌ها

عنوان مقاله [English]

Evaluation of Eulerian Two-Fluid Numerical Method for the Simulation of Heat Transfer in Fluidized Beds

نویسندگان [English]

  • S. Torfeh 1
  • Ramin Kouhikamali 2

چکیده [English]

Accurate modeling of fluidization and heat transfer phenomena in gas-solid fluidized beds is not solely dependent  on the particular selected numerical model and involved algorithms. In fact, choosing the right model for each specific operating condition, the correct implementation of each model, and the right choice of parameters and boundary conditions, determine the accuracy of the results in the evaluation of the performance of fluidized beds. In this research, in order to accurately simulate heat transfer in fluidized beds, important and effective parameters on two-fluid Eulerian model that incorporate the kinetic theory of granular flow were investigated. For this purpose, effects of particle-particle and particle-wall restitution coefficient, specularity coefficient, granular temperature and effective thermal conductivity coefficients determination methods on the numerical solution were evaluated. These investigations were first carried out on heat transfer from hot air to solid particles in an adiabatic fluidized bed, and then on a fluidized bed with constant temperature walls for bubbling and turbulent regimes. Results showed that specularity coefficient and effective thermal conductivity are important parameters in heat transfer process from wall to bed. In this case, the zero value of the specularity coefficient causes the air temperature to increase by about 7 degrees in the bubbling regime and about 5 degrees in the turbulent regime, and its unit value gives the same results with the no-slip condition. In addition, considering the solid and gas material thermal conductivities causes the outlet air temperature to be about 26 degrees higher than the temperature that is obtained by considering the effective thermal conductivity coefficients with standard approach. The partial differential and algebraic form of the conservation equation for the particles kinetic energy show identical results in dense fluidized beds, although considering a constant granular temperature can cause computational errors.

کلیدواژه‌ها [English]

  • Eulerian Method
  • Two-Fluid Model
  • Kinetic Theory of Granular Flow
  • Gas-Solid Fluidized Bed
  • Heat transfer
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