شبیه‌سازی عددی اختلاط و احتراق در شرایط فوق ‌بحرانی در محفظه مدل

نوع مقاله : مقاله پژوهشی

نویسندگان

دانشگاه صنعتی شریف

چکیده

در این پژوهش، به مطالعه عددی اختلاط و احتراق در شرایط فوق بحرانی پرداخته شده است. رفتار سیال در شرایط فوق بحرانی بسیار پیچیده است. در این شرایط، کشش سطحی مایع صفر می‌شود و خواص ترمودینامیکی آن مانند ظرفیت حرارتی و چگالی به‌شدت دچار تغییر می‌شود. بدین منظور دو هندسه RCM01و RCM03انتخاب شده است که به­ترتیب جریان غیرواکنشی فواره فوق بحرانی نیتروژن در فشار حدود 60 بار و جریان واکنشی فوق بحرانی هیدروژن گازی-اکسیژن مایع، که در آن فشار محفظه 60 بار و بالاتر از فشار بحرانی هیدروژن و اکسیژن است، بررسی شده است. در مطالعه پاشش فواره  نیتروژن، و با گسسته­سازی مرتبه دوم معادلات حاکم، مدل‌های مختلف اغتشاشی بررسی شده و مشاهده شده است که مدل  نتایج بهتری درخصوص پیش‌بینی ناحیه لایه برشی، تشکیل گردابه‌های کناری و درنتیجه اختلاط ارائه می‌دهد. پیش‌بینی بهتر مدل  می‌تواند ناشی از تخمین بهتر جمله مربوط به گرانروی آشفتگی در فرض بوزینسک باشد. همچنین، مشاهده شده است که میزان بازشدگی فواره ورودی وابسته به نحوه پیش‌بینی گردابه مجاور دیواره در مدل‌های اغتشاشی مختلف است. هرچقدر میزان گردابه تخمین زده­شده بزرگ‌تر باشد، اختلاط در هسته مرکزی جریان با نرخ کمتری صورت می‌گیرد و نمودارهای مربوط به توزیع چگالی دیرتر یکنواخت خواهد شد. همچنین، در بررسی جریان واکنشی LOX-GH2، مدل‌های اغتشاشی مختلف و همچنین معادله حالت‌های مختلف برای بررسی این شرایط، مطالعه شده است. عملکرد مدل‌های مختلف اغتشاشی در پیش‌بینی شکل شعله و توزیع دما بررسی شده و دیده شده است که مدل  عملکرد بهتری در پیش‌بینی شکل شعله، در شرایطی که از گسسته­سازی مرتبه اول بالادست معادلات استفاده شد، دارد. اثر اعمال شرایط گاز حقیقی با شرایط گاز ایدئال در پیش‌بینی شکل شعله به‌خوبی نمایان می‌سازد که فرض گاز ایدئال در یک احتراق فوق بحرانی خطای زیادی در تخمین شکل و طول شعله به­همراه دارد. همچنین، معادلات مختلف پیشنهادشده برای رفتار گاز حقیقی در هر دو آزمایش بررسی شد که مدل SRKدارای نزدیک‌ترین نتایج به داده‌های تجربی موجود است.
 

کلیدواژه‌ها


عنوان مقاله [English]

Numerical Modeling of Mixing and Combustion at Supercritical Conditions for a Model Combustor

نویسندگان [English]

  • Ehsan Barani
  • Amir Mardani
چکیده [English]

This paper discusses numerical modeling of the mixing and combustion at supercritical conditions for a rocket model combustor. Fluid behavior is very complex at supercritical conditions. At these conditions, the surface tension of the liquid is zero and the thermodynamic properties such as heat capacity and density are dramatically changed. Therefore, two test cases of RCM01 and RCM03 were selected for modeling using a 2D-Axi-RANS approach. In primary test cases (i.e. RCM01), supercritical nitrogen jet at 59.8bar, and in the second test cases (i.e. RCM03), supercritical flow of gaseous hydrogen-liquid oxygen at a chamber pressure of 60bar and above the critical pressure of hydrogen and oxygen, were investigated. For the nitrogen jet, turbulence models have been studied and it was observed that the  predicts better results in the area of the shear layer and outer recirculation zone, and thus provides better mixing when the equations were discretized using a second order approach. Better predictions of the  model could be due to better estimation of turbulent kinematic eddy viscosity term on the Boussinesq eddy viscosity assumption. It has been observed that the spreading angle depends on the Outer Recirculation Zone (ORZ) predicted by different turbulence models. As the estimated size of ORZ is larger, mixing at core occurs in lower rates and density profile will be uniform posterior. Also, combustion of cryogenic propellants LOx/H2 at very high pressure were examined using the EDC turbulent combustion model and a detailed chemical mechanism. Different turbulence models and equations of state were studied while an upwind first order discretization method was used. The performance of the turbulence models in predicting the flame shape and temperature distribution were investigated and it was found that the  better estimates the flame shape. Checking the ideal gas and real gas EOS revealed that ideal gas assumptions suffer from large errors in estimating the shape and length of the flame. Different suggestions for the equations of real gas behavior were studied in both experiments and the results showed that SRK EOS yields the closest results to the experimental data.
 

کلیدواژه‌ها [English]

  • Combustion
  • Supercritical
  • Mixing
  • Real EOS
  • Turbulance models
  • Hydrogen-LOX
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