دراسة تأثير درجة الحرارة الابتدائية على سرعة الاحتراق الطباقي باستخدام غرف احتراق مختلفة ثابتة الحجم

الباحث

أحمد عبد الأمير عبد الرحيم

المشرف

عادل محمود صالح - هارون عبد الكاظم شهد

الجامعة

الجامعة التكنولوجية

الكلية / القسم

قسم الهندسة الميكانيكية / الهندسة الميكانيكية

الشهادة

دكتوراه

السنة

2022

لغة الرسالة

انكليزي

الكلمات المفتاحية

Laminar burning velocity;Initial temperature;Constant volume chamber;Flame speed

This study investigates experimentally and numerically the laminar flame speed and burning velocity of premixed flames for different fuels. A centrally ignited constant volume chamber (CVC) has been designed and constructed to investigate experimentally the stretched and the un-stretched flame speeds (Sn, and Sl), laminar burning velocity (Su), Markstein length (Lb), minimum ignition energy (MIE), and the maximum explosion pressure (Pmax). The combustion process is recorded by a high- speed camera using schlieren technique. The un-stretched flame speed was obtained using both linear and nonlinear extraction methodologies. The experiments have been conducted using two chambers with different internal radii (150 mm and 100 mm), different pure fuels (methane, propane, methanol, ethanol and n-propanol), and different propane-methanol blends with blending ratio varies from 0-100% by volume. The tests were done at different initial temperatures (306 K, 348 K, 373 K, and 398 K), different initial pressures (0.5 bar, 1.0 bar, and 1.5 bar), and different equivalence ratios of air/fuel mixture range from 0.8 to 1.3. The adiabatic flame temperature and the density ratio (ratio of burned to the unburned gases densities) were computed using CHEMKIN of ANSYS19.2 which were used for predicting the burning velocity. The laminar burning velocity of premixed methane/air was first measured for validating the experimental setup and the measurement methodologies. The comparison showed good agreement with the results published previously with average percentage difference of 2.6 %, verifying the high accuracy of the present rig. The experimental results showed that the linear extrapolated flame speed results in an insignificant overestimation of the un-stretched flame speed compared with the two nonlinear extrapolation methods for all conditions examined. For stoichiometric methanol/air mixture at 373 K and 1 bar the percentage error was 1.7% related to the first nonlinear equation (NL1), and 2.5% related to the second nonlinear equation (NL2). This percentage error decreases with increasing equivalence ratio, initial temperature, initial pressure, methanol ratio in propane-methanol blends, and vice versa. It is also noticed that both the un-stretched flame speed and the laminar burning velocity are directly proportion with initial temperature and inversely proportional with initial pressure for all types of fuels used and conditions examined. For stoichiometric M60 mixture, increasing the initial temperature from 348 K to 398 K led to increase Sl from 332 cm/s to 363.3 cm/s and to increase Su from 48 cm/s to 59.4 cm/s, while increasing the initial pressure from 0.5 bar to 1.5 bar for stoichiometric methanol/air mixture at 373 K caused a reduction in Sl from 390.7 cm/s to 329.9 cm/s and a reduction in Su from 60.7 cm/s to 50.7 cm/s. Furthermore, it is found that the stoichiometric mixtures have the highest un-stretched flame speeds and laminar burning velocities than that of the lean or the rich mixtures except that for the methanol where the maximum value of both of them shifted to the rich mixture (Ø=1.3). Also, these two parameters (Sl and Su) for the propane-methanol blends increase with increasing methanol blends in the mixture at all initial temperatures examined, and the maximum values occurred at the stoichiometric mixture. However, when the methanol blending ratio is greater than 80% these maximum values move towards the rich mixture side (φ=1.3). For M60 at Ti=398 K the value of these two parameters increased by about 9% compared with that of pure propane at the stoichiometry. The results also showed that the effect of combustion chamber size on Sl and Su was insignificant with a difference of about 3% for methane at atmospheric conditions. It was also noticed that the ignition energy has a significant effect on flame trajectory at flame radius less than 6-8 mm, and this critical radius increases with increasing the equivalence ratio. An empirical relationship describing the effect of equivalence ratio, initial temperature, initial pressure, blending ratio on laminar burning velocity has been derived which exhibit good agreement with the experimental data. The experimental part also investigates the minimum ignition energy (MIE) that can burn the combustible mixture. The results indicated that MIE is significantly affected by equivalence ratio. The variation of MIE is a U shape like when plotted against equivalence ratio. This U-shape extends between the two flammability limits with the minimum located around the stoichiometric mixture. The MIE for methane occurs at Ø=0.95 while it is shifted to the rich mixture (Ø=1.15) for propane fuel. The theoretical part included numerical predicting the laminar burning velocity for methane and propane by CHEMKIN of ANSYS19.2 using three mechanisms (GRI 3.0, USC 2.0, and San Diego). This section also investigated the sensitive analysis, reaction paths, and molar distribution for the combustion of these fuels. Different initial temperatures (306 K, 348 K, 373 K, and 398 K), initial pressures (0.5 bar, 1.0 bar, 1.5 bar, and 2.0 bar), and wide range of equivalence ratios (0.8 - 1.4) are used in this section. The theoretical results showed that the agreement between experimental and simulation Su under variable initial pressure is better than that under variable initial temperature. Non-uniform temperature distribution and non-ideal adiabatic environment is a cause of large error percentage. Whereas the percentage difference increases with increasing the initial temperatures and it is minimum at the stoichiometric mixture. By rising the initial temperature from 306 K to 398 K the percentage difference increases from 3.3% to 17.5% at Ø=0.8, 1.9% to 10.5% at Ø=1.0, and from 5.7% to 19.9% at Ø=1.3. The simulation results are very close to the experimental results at low initial temperatures. It is also noticed that the two mechanism USC 2.0 and San Diego have similar results of laminar burning velocity under the same conditions. Furthermore, these two mechanisms have better prediction accuracy than GRI 3.0.

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