Üzerinde hava akımı olan, yatay yakıt yatağındaki alevsiz yanmanın incelenmesi
Smoldering combustion in a horizontal fuel layer which is object to an air flow above fuel bed
- Tez No: 83098
- Danışmanlar: YRD. DOÇ. DR. ERHAN BÖKE
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1999
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 81
Özet
ÖZET Alevsiz yanma konusu daha çok yangına karşı alınacak tedbirlerin araştırıldığı çalışmalarda ele alınmıştır. Yangın emniyeti çalışmaları sırasında yanma karakterinin tayini önemli bir mesele haline gelmiştir. Daha önce yurdumuzda bu konuda yapılmış deneysel çalışma olmadığından günlük hayatta sıklıkla karşımıza çıkan alevsiz yanma konusunda bilgi birikimi mevcut değildir. Gerçekleştirilen bu çalışma ile öncelikle alevsiz yanma konusunda dünyada yapılmış deneysel bilgiler incelenmiştir. Bir çok farklı araştırmacının alevsiz yanma deney düzenekleri incelenerek çalışmanın amacına uygun bir deney düzeneği oluşturulmuştur, öncelikle yakıt olarak ağaç talaşı kullanılmıştır. Bu deney malzemesi ile deney düzeneğinin amaca uygunluğu tesbit edilmiştir. Günümüzde kullanım emniyetinin yükselen değeri nedeniyle yangın önleme çalışmalarına yapacağı katkı dolayısıyla bu çalışmanın bir başlangıç olduğu düşünülmektedir. Oksijen konsantrasyonuna bağlı olarak malzemelerin alevsiz yanma eğilimlerinin bilinmesi çok önemlidir. Zira malzeme iç yapısı, içindeki hava boşlukları, ısı ve malzeme yüzeyindeki hava akımının rol aldığı alevsiz yanmada asıl dikkat edilmesi gerekenin yakıcı gaz konsantrasyonu olduğu, deneylerden elde edilen en önemli neticedir. Yurdumuzda ısı izolasyonu imalatı konusunda uzmanlaşmış dünya çapında firmalar bulunmaktadır. Bu tez çalışması ile ortaya çıkan deney düzeneği neredeyse her tür malzemenin alevsiz yanma eğilimi konusunda deney yapılmasına olanak sağlamaktadır. Isı izolasyon malzemeleri duvar içlerinde kullanılmaktadır. Gözenekli yapıları nedeniyle yanabilir evsafta olanlar katlar arasında yangın geçişine neden olabilirler. Her ne kadar alevsiz yanma yavaş ilerleyen bir yanma türü olsa da yüksek ısılarda gerçekleşmektedir. Ortaya çıkan yüksek sıcaklık ise bu malzemeyi çevreleyen diğer malzemelerin tutuşmasına sebep olabilir. Toz patlamalarının gözlendiği depo ve silolarda yanma başlangıcında alevsiz yanma sıklıkla görülmektedir. Bu sebeple de alevsiz yanma mekanizmasının anlaşılması önem kazanmaktadır. Diğer taraftan alevsiz yanma esnasında ortaya çıkan yoğun CO dumanının zehirleyici etkisi nedeniyle reaksiyonun hangi şartlarda gerçekleştiğinin bilinmesi faydalar sağlayacaktır.
Özet (Çeviri)
SMOLDERING COMBUSTION IN A HORIZONTAL FUEL LAYER WHICH IS OBJECT TO AN AIR FLOW ABOVE FUEL BED. SUMMARY The studies made on smoldering combustion are generally focused on fire safety studies. In this conclusion, determining the combustion characteristic has become an important problem. Since there has not been any earlier study in our country on this subject, we have no data about smoldering combustion tendency of frequently used heat insulation materials in Turkey. The previously made experimental studies in world were inspected during this study. And a combination of all experimental tasks of different researchers is constructed. Firstly wood shaving is used as fuel. The reason for that relatively easy to burn material to be used is, it is easy to obtain and it makes possible to test the experimental system. Second fuel has been sliced tabocco which has similar characteristics to wood as a selulosic material. With that flexible system established, experiments made on two different materials and results are introduced on graphs. It will be possible to make experiments on real objective materials with the already found experimental system. This study is supposed to be a beginning with the rising value of usage safety in world of today. The knowledge of smoldering combustion tendency relative to Oxygen concentration is very important. The most remarkable knowledge derived from this study is concentration of smarting gas in a combustion process, which is involved on inner structure, air bubbles inside, heat and air flow above fuel surface. There are worldwide companies focused on heat insulation materials production in Turkey. The experiment system that is constructed for this thesis study can be used to determine the tendency of nearly any kind of material. Introduction The advanced countries are continuously developing their life style and safety precautions. Fire is a highly destructive disaster in any place. So there are many studies made on fire safety. It can be consented that the importance of smoldering combustion is assumed by year 1760. With rising industrial applications, people tend to find out how to prevent fires. Early researches approved that smoldering process in a possible fire beginning determines the character of fire. So it became very important to find out under which conditions smoldering combustion occurs. The experimental observations show that the major parameters of smoldering Hcombustion are the Oxygen concentration and burning temperature. It is very clear that the investigation of smoldering combustion in a simulator obtaining the starting conditions of a fire can give valuable knowledge for fire prevention studies. It has been shown by experimental studies and accidental occurences in industrial facilities that a layer of organic dust can, under certain conditions, udergo sustained smoldering combustion. The pupose of this investigation was to exemine the combustion, propagation and ignition properties of horizontal layers of wood dust and coal. Combustion wave velocities have been measured and are presented as a function of average combustion wave depth, ignition depth and type of material tested. Tests have been made with and without a forced horizontal airflow over the layer surface. The effect of the airflow has been shown to be an increase in the combustion wave velocity and under certain conditions, the possibility of transition from smoldering to flaming combustion. The majority of studies found in the literature concerned with smoldering combustion and/or pyrolysis of solid materials have focused on either wood or pure cellulose. One of the earliest studies in this area was made by Palmer Generally speaking, cellulosic materials such as wood, paper and related fibrous products do not burn directly, but under the influence of a sufficiently strong source of ignition they decompose as shown in relevant figure, by two alternative pathways. In the first pathway which operaties at higher temperatures, pyrolysis or thermal decomposition of the cellulosic materials provide a mixture of combustible gases. These gases mix with air and fuel the flaming combustion that could rapidly spread in the gas phase. In the second pathway, which operates at lower temperatures, pyrolysis gives mainly carbonaceous char and a mixture of containing water and corbon dioxide which is not very flammable. Oxidation of the resulting char then provides glowing combustion which is a localized and slower process, proceeding as a front in the solid state, rather than as a flame in the gas phase. Combustible Volatile Flaming Fragments Combustion Cellulose Water, Char and Glowing Corbon Dioxide ignition Thermal Decomposition of Cellulose During the flaming combustion, a relatively high rate of heat release and heat flux generated by the combustion of volatile products provide the energy required for gasification and rapid spread of fire. The process is usually initiated by a pilot flame or a similar high intensity source of ignition. XIIIn smoldering combustion, the lower intensity heat flux required for pyrolysis and charring of the substrate is provided by slowly proceeding oxidation of the char. The process is usually initiated by a low intensity and prolonged source of ignition, such as a burning cigarette which chars the substrate, and is observed with low intensity fibrous or porous materials. In these materials the char is slowly oxidized by the in-diffusing air and slow rate of heat release in the absence of substantial heat loss by radiation, convection or conduction provides the low heat flux reqired for further charring and propagation of the smoldering combustion. This process could terminate by insufficient heat flux which refers to self- extinguishment or exessive heat flux resulting in flaming combustion. As mentioned before, the main element of a smoldering combustion process is Oxygen concentration. In the experiment done, it is referred to as air flow above fuel bed. But the flow must be homogenous and laminar. Otherwise measured values would not designate real conditions. For that reason, a test canal of 5 meters long is constructed. The cross-section of canal was 38 x 38 cm fuel bed is placed where the effect of little eddies in flow can be discarded. Which means 5,2 meters away from air blower. The maximum flow rate of air blower is 4600 m3/h which means an air speed of 9 meters per second. However, this much high speed can cause the small particles to travel above the unburned surface. A burning particle falling to unbumed area causes a new ignition. This effect is observed partially in the experiment with a 4 m/s over bed air flow. A metallic fuel container with the dimensions of 50 x 20 x 15 cm is used for this study. In order to correctly measure the temperature values, fuel container is heat insulated with insulation material resistant to 1200 °C. 1 cm diameter openings served to enter thermo couple wiring, gas analyzer probe and ignitor electric resistant wiring. Fuel container is designed for easy installation. Complete container can be rotated through air flow. The bolt housing design is made to allow that kind of a rotation. Thus this easy to mount design, it has been possible to observe forward - backward smoldering combustion mechanisms. Attached is the production drawings of container. As it can be seen, container is fixed to air canal by bolts. The surfaces facing between container and canal is insulated in order to prevent air leakage. All construction of container is done by an oven constructor. 27 thermo couples are arranged to form a three dimensional matrix in fuel bed. A ceramic tube is used sheath a ceramic cartridge which carries 3 thermo couples. Ceramic tube is formed of six holes. Each two holes are used for one couple of thermo couples. Since there are angular difference between three couples, all three are shaped to face the same direction. Although tubing caused a non uniformity in fuel bed, this effect is discarded. Due to equipment annoyance, there were no better choice to form a 3d matrix. Initial weight is recorded before each experiment. According to weight decrement and time consumed, combustion speed is determined. The average quantity in each experiment is around 3 kilograms. Container is installed below the air canal. Ignition is held with an electric resistance. It is found that longer ignition periods can cause a flaming combustion. The ignition duration has a major role on determining reaction characteristics. X1UAs it can be seen on the corrsponding figures, the temperature history for each experiment has three main divisions. First division has a constant temperature rising through 100 °C. This part is assumed to be an evaporation period of volatiles and water that fuel holds. The second division has a high increasing of temperature, where combustion takes place. Followed by that rapid increase, a rapid decreasing of temperature is observed. This area is assumed to be an extinguishing period. The slope then increases which is probably not related with combustion reaction directly. Area is assumed to be a cooling period of thermo couple carrier ceramic rods. Temperature distribution is observed for various air velocities. The shape and propagation velocity of smoldering wave is determined by using three dimensional matrix shaped thermocouples. Change on combustion properties by the change of over bed air stream velocity is inspected. It is found that the upper layers of fuel has higher reaction temperatures and rates. This effect is result of higher oxygen diffusion rates. Parallel to this observation, it is determined that there will always remain an unburned layer in every case. The unburned layer is observed in all 10 experiments performed. By the way, mathematical approach to such a problem is observed using numerical methods. A suitable mathematical function is derived for each experiment. These functions also yield to extremum which is refering to unburned layer. The combustion rate is determined with remaining fuel weight. On the other hand, the burning rate is also calculated by recorded temperature history. Taking a temperature value and inspecting the same value on other following thermo couples can allow to calculate that kind of a rate. The time passed for a downstream thermo couple to measure the taken temperature value is found. Since the geometric formation of thermo couple array is exactly known, the propagatin speed of burning front is calculated. It is shown that horizontal propagating speed is always higher then the vertical propagating speed. This is also due to decreasing oxygen diffusion rate by depth. Graphics are drawn for that two different propagating directions. Since the propagating kinetics is not only horizontal nor vertical a resultant propagating rate is also defined. It is found that higher over bed air flow speeds cause higher resultant propagating speeds. The change of speed due to over bed air speed is near linear on tobacco used experiments. The same change for wood shaving has more like a concave shape. All recorded data is observed by computer and put to graphics or table format for applicational purposes. The literature observation show that some similar studies are made in world but not in our country. The results of experimnts made for this study are compared with the ones made in different countries. The results are yielding a tight accomodation with that experiments. Experiments made in different countries are specifically for inspecting 2D propagation. From this point of view, this study is first of 3D inspections. Derived data from this experiments will also constitute a database for future studies in İ.T.Ü. or some other institutions. XXVConclusion Data derived is put to table format. So some future studies for constructing mathematical model will be easy. Below is the list of drawn tables. 1. Temperature change with respect to time. 2. Temperature change with respect to position 3. Propagation speed change with air speed. 4. Form of propagation (derived from temperature change data) XV
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