Yüksek hızlı sürekli yanma sistemlerinde alev kararlılığının incelenmesi
A Study on flame stabilization in high speed continuous combustion system
- Tez No: 39351
- Danışmanlar: PROF.DR. OĞUZ BORAT
- Tez Türü: Doktora
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 89
Özet
ÖZET Yüksek hızlı sürekli yanma sistemlerinde, akış içersine konulan bir katı cisim yardımıyla dolanım bölgesi meydana getirilerek kararlı alev sağlanması yaygın olarak uygulanmaktadır. Bu metodda alev kararlılığı, katı cisim cidarında akış hızının azalması, alev ile çevrelenen dolanım bölgesinin ısı rezervuarı gibi davranarak alev bölgesinden aldığı ısıyı kimyasal aktif Öğelerle beraber alevin başlangıç bölgesine taşıması ile oluşmaktadır. Alev kararlılığını konu alan teorik çalışmalarda daha çok dolanım bölgesinin ısıl dengesi ve kısmen de bu bölgeyi esas alan kinetik modeller incelenmiştir. Bu çalışmada ise dolanım bölgesi, ana akış bölgesi ve alev tarafından çevrelenen kama şeklindeki hazırlık bölgesi gözönüne alınmıştır. Hazırlık bölgesi için kinetik bir model oluşturulup bölgedeki oksijen dönüşüm oranını sağlayacak kinetik kalma zamanı bulunmuş ve ana akış hızına bağlı olarak kararlılığı kontrol eden bir parametre olarak değerlendirilmiştir. Yine aynı bölge için giren ve doğan ısılar ile çıkan ısının farkı hesaplanarak artan ısı değerinin pozitif olma şartı kararlılığı kontrol eden ikinci parametre olarak değerlendirilmiştir. Gaz halindeki LPG nin yakıt olarak kullanıldığı bir deney tesisatı kurulmuştur. Akış içersine yerleştirilen konik bir stabilizör ile kararlı alevin elde edildiği sistemde, soğuk akış halinde akış hızları ölçülerek dolanım bölgesinde hız profilleri çıkarılmıştır. Yanmalı halde ise ayni bölgede sıcaklık ölçmeleri yapılmış, ayrıca belirli HFK (hava fazlalık katsayısı) değerleri için kararlı yanmanın oluştuğu maksimum giriş hızları aranmıştır. Matematik modelin çözümü için hazırlanan bilgisayar programından kararlı yanma sınırlarına ait giriş hızı, sıcaklık, ısı ve ortalama kalma zamanı eğrileri HFK'nın fonksiyonu olarak bulunmuştur. Elde edilen hesap sonuçları deneysel sonuçlar ve literatürdeki çalışmalar ile karşılaştırılmıştır.
Özet (Çeviri)
A STUDY ON FLAME STABIUZATION İN HIGH SPEED CONTINUOUS COMBUST1ON SYSTEMS S U M M A R Y Recent developments in gas türbine technology required the design of more powerful combustion chambers, which burn the hydrocarbon fuel within a minimum space and maintain a stable combustion över a wide range of air - fuel ratio. There are several methods which can be applied to high speed flow velocity systems to stabilize the flame in the combustion chamber. For instance some methods utilize the recircuiation zone creates by a bluff body inserted in the flow ör by a sudden expansion behind a sharp edge. Some ather methods are swirling the flow and deflecting a secondary flow toward the main flow.ln this study, the stabilization of flame by the use of a conicai stabilizer is investigated. The mechanism of stabilization of flame with the aid of a recircuiation zone behind a solid body can be explained as follows: -When a solid body is placed in a flow, a boundary layer is developed on the surface of the body. The velocity of the flow is reduced in the boundary layer, so that the flow velocity and the flame speed approach each other. This is öne of the requirements for a stable flame. -The recirculating flow, which is set up behind the body, carries the heat and the gaseous chemical products from the combustion zone around the wake, back to combustion initiation region. Because of both increase in temperature and the presence of chemically active elements,Jn the precombustion zone, the rate of reaction betvveen fuel and air accelerates. When ali these conditions are simultaneously present, a stable combustion is obtained. Earlier works on flame stability are based on the heat balance between the recircuiation zone and the surrounding flow. The effects on flame stability of the geometrical characteristics of the wake and the characteristics of the main flow, such as mass flow and flow velocity have been investigated in previous works. There are also some kinetic models which assume that the recircuiation zone is a“well-stirred”reactor. VIIThe precombustion zone betvveen the wake and the main flow is considered in this work. in this zone, the fuel-air mixture is prepared for combustion. Apart from the heat balance, the rate of the chemical reaction is also taken into consideration in the model. The stability is controlled by this two criteria, the existance of positive excess heat and depending on the main flow velocity vvhether sufficient time exists for the conversion of fuel ör oxidant. in the theoretical part of this study, a survey on the stochiometric relations of hydrocarbon combustion is completed and the consumption rate of the oxygen molecules is defined. A computer program is developed for computing the results by using the adiabatic flame temperature as a function of air - fuel ratio. Further, the basic relations of reaction kinetics, reaction rates and rate constants are reviewed. The scavenging time is defined as the ratio of the precombustion zone lenght to the main flow velocity. The kinetic time required to reach a certain conversion ratio is also defined as mean residence time. Reynolds analogy is used for the calculation of heat and mass transfer rate between the wake and the surrounding flow. The temperature of the recirculation zone is computed as a function of the three temperatures, namely the temperature of main flow, the temperatures of the precombustion zone and the recirculating zone. The thermodynamic data are computed with the temperature obtained above. The thickness of the wedge shaped precombustion zone, which lies betvveen fresh air-fuel mixture and the wake, is calculated by using classical boundary layer equations. Since the conditions in the recirculation region on a stable combustion are very similar to the well-stirred reactor, the well - stirred reactor model is used in the study. Although the conditions of the precombustion zone do not match with the well - stirred reactor, in order to simplfy the computation, well - stirred reactor model is prefered. Also in this region the size and the conversion ratio are very small. The well-stirred reactor model is applied under the follovving steps: 1 - The temperature achieved at the end of combustion is assumed to be a function of the equivalence ratio and the conversion ratio. If the conversion ratio is equal to unity the temperature will be the adiabatic flame temperature which eorresponds to the equivalence ratio. 2-An expression for the consumption rate of oxygen is written for the bimoleculer model of the oxygen-fuel system. 3-This expression is simplified after comparing with the equation given by ODGERS [18]. An expression for the mean residence time (TPSR) in reactor is derived using this equation of consumption rate of oxygen.4- The mean residence times (TPSR) are calculated both in recirculation zone and in precombustion zone, and they are compared with the scavenging time for controlling the existing of stability. The amount of heat transfered from and into the precombustion zone are also calculated. The precombustion zone, between the main flow and the recirculation zone, can be defined as the free turbulent shear layer on the recirculation zone. The heat transfer coefficent given by ECKERT [19] for the free turbulent shear layer is used for the heat transfer calcuiation. The heat transfer coefficent is calculated by the expression (h=0.105 p Cp Uo). Using this coefficent, the heat which is trasferred to precombustion region,QR, and is transfered from this region, Qo, are determined. The effect of the radiation on the heat transfer rate is also taken into account in this study. The heat release rate in precombustion zone is expressed in the form of Qz = f ( Hu, iZ). Finally the algebraic summation of the amounts of heat, QRZO t which is defined as excess heat is computed. For stability the excess heat QRZO should be positive. in the experimental part of the this study, stability is observed on a test stand. LPG (30% propane + %70 butane) is used as fuel in the experiments. The following equipments are used on the test stand. -An air blovver for combustion system -A throttle control valve for air flow adjustment -A laminar flow converter -A nozzle for accelareting the mixture -A conical stabilizer -An inclined manometer for air flow measurements -A LPG tube -A chamber for air-fuel mixture -A Hartmann-Braun orifice and a pressure transducer for fuel flow measurement. in the experiments a conical stabilizer with 15 mm diameter and 90° cone angle is used. The cone base is lying coaxially on the cross sectional plane of the air nozzle. Thus air - fuel mixture that is leaving the nozzle, has the form of an annular jet. This free jet expands as a result of air from the surrounding area. in the experiments which are performed without combustion the velocity profiles are obtained at various cross sections of the recirculation zone from the measured data. in the experiment with combustion, the temperatures are measured at the IXpoints, which lies both in the flame zone and in the recirculation zone. The maximum nozzle exit velocity at which the stable combustion still exists is also measured. The experimental results are as follows: - In the case of stable combustion, the first visible flame take place at the point, which lies in the distance of 1-5 mm from the conical stabilizer basement. If the flow velocity is increased at this stable point, the flame displaces downstream and blows off. - If the flow velocity is less then (3 m/s), the flame displaces upstream and flash back. - Maximum flow exit velocity of (28.5 m/s) is obtained for the equivalence ratio equal to unity. - Because of the air entering from the surroundings, the flame lenght increases. The measured temperature is not as high as in the closed system. Maximum temperature measured for open system is Tmax = 1 550 K. - Combustion is monitored by fotographs. The characteristic curves Uo = f (EAC) are obtained from the computer program, which are developed for analitical and numerical solutions of the mathematical model. These curves give the change of main stream velocity as a fuction of excess air coefficient. The characteristic curves are given in the tree graphical forms where E, activation energy, Xi, the distance of the flame to the edge of the conical stabilizer and Zi, the conversion ratio of oxygen in the precombustion region are parameters. After the interpretation of about curves the folloving results are concluded. - Although the form of the curves, Uo = f (EAC) agree with the classical experimantal results, these curves expanded into wider range due to varries effect of turbulance which can change the heat transfer coefficient. After a certain value, the length of Xi is not effective to the limits of stable combustion. When the conversion ratio, Zi, increases the temperatures and as a result of this, blow off velocity also increases. - When the activation energy is increased or decreased % 4, it is observed that stable combustion range changes in an important amount and inversly proportional to the activation energy. As the second gruop of curves, temparatures of flame and recirculation zone are found as a function of EAC. The highest temperatures for both zones were obtained for the value of EAC = 0.9. Any increase of the value of Xi, decrease the recirculation zone temperature. Third group curves represent the summation of the released heat and heat input from recirculation zone, and heat output to the main stream as a function ofexcess air coefficient. In a stable conditions the summation of the release heat and the input heat is less then the output heat. As a limit case excess heat approaches to zero, and stability is determined by the heat balance. The last group curves represent, mean residence time and scavenging time to the function of EAC for the precombustion zone. After examination of the curves it can be observed that the stability is determined by mean residence time when the heat balance is not effective. İn this study, the blow off limits which is determined both experimentally and numerically are agreed. Also the temperature distributions are agreed with the experimental results which determined by KUNDU [13]. The originality of this study is based on both the investigations of the heat balance in the precombustion zone instead of recirculation region and a new kinetic model. In this model the stability control by either kinetic mean residence time or the sign of excess heat. XI
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