Çok ince toz zerrelerinde ısı geçişi

ÖZET Artan enerji harcamaları ve sık sık başgösteren enerji krizleri birçok sahada enerji tasarrufu tedbirlerini almayı zorunlu kılmıştır. Isı izolasyonunda da, klasik izolasyon metodlarına ve malzemelerine kıyasla günümüzde yeni gelişmeler elde edilmiştir. Köpük, toz, lif gibi değişik maddelerin yalıtım aracı olarak kullanımı bu sahada istenilen nitelikte izolasyon sistemi elde edilmesini sağlamıştır. Çok ince toz zerreleri ile ısı izolasyonu teknolojisi, son on yıldır kendisini büyük çapta hissettiren yeni bir yöntemdir. Bu sayede, çok düşük ısı iletim katsayıları elde edilmiştir. Ayrıca, hafif, kolay sekil alabilen, az yer kaplayan, sağlığa zararı olmayan yeni bir izolasyon sistemi gerçeklenmiştir. Bu çalışmada, çok ince mikronize tozlar ile yapılan izolasyonlardaki baskın ısı geçiş modlarının neler oldukları ve nasıl minimize edildikleri açıklanmıştır. Daha sonra, toplam ısı transfer katsayısının sürekli ve geçici rejimlerde deneysel olarak nasıl tesbit edildiği üzerinde durulmuştur. Sonuçta, elde edilen bulgulara göre, çok ince toz zerreleri ile yapılan izolasyonun üstünlüğü, birkez daha gözler önüne serilmiştir. -V- SUMMARY HEAT TRANSFER THROUGH ULTRA-FINE POWDERS Thermal insulation has long been a subject of fundemantal importance due to its wide application in engineering systems. Several modes of heat transfer in typical porous insulation systems, primarily solid - solid conduction between the particles or fibers, gaseous conduction and convection through the pores of materials, and radiative heat transfer through the materials, must be minimized in order to attain very good thermal performance. Heat transfer through a variety of insulation materials and insulation conditions has been studied extensively in the past. Several review articles on heat transfer in thermal insulation have been reported. The models derived by scientists were not correct enough. None old model predicts the high degree of.temperature depence observed. Their inadequacy is attributed to the assumption that the particles are opaque to radiation whereas both Al and SiO ar e partially transmissive. Deficiencies in the prediction of the conductive component by some model is attributed to their inability to characterize the effects of particle shape and size distribution. The models of Godbee - Ziegler, Kunii - Smith gave reasonable predictions for the conductive component of the silica beds at the lower temperatures for which radiant transfer is negligible. It has been written all old models below. It is notable that although the models differ markedly between themselves, they all give predictions within a factor of two of each other but none predicts the degree of temperature dependence found: 1 - The Wakao and Kat o model 2 - The Imura and Takegoshi model 3 - The Kunii and Smith model 4 - The Godbee and Ziegler model 5 - The Bauer and Schlunder and Zehner models 6 - The Laubitz and Russell models Each of tested models for the prediction of the effective thermal conductivity of systems at temperatures up to 950 °C gave a different degree of temperature dependence. If all above mentioned models are compared with the new models based on gas thermal conductivity it is clear that they are too primitive to reach at a correct result. One of the new concepts which has been advanced recently for thermal insulations involves in reducing the gaseous conduction contribution in powdered insulations by utilizing ultra-fine particles with diameters of the order. -VI- ?of approximately 70 °A. Indeed, a study of consolidated ultra-fine powder insulations in the form of rigid SiO aerogel tiles for double-pane window applications has been applied. In these insulations, gas conduction can be reduced greatly if the pore size between the solid particles is less than the mean free path of the gas, e.g. about 200 °A for air at 300 K. The resulting effective thermal conductivity through the void spaces will be lower than that of still air, and the high performance at vacuum-assisted insulations can be achieved without evacuating the system. Natural concection in such a system can be neglegted due to the small pore size. The solid - solid conduction is also negligible if the solid volume fraction fs of a material consisting of roughly spherical particles is less than approximately 0.10. The dominant heat transfer modes in ultra - fine powder insulation CUFPI) are gaseous conduction an thermal radiation. Here, a SiO ultra - fine powder called aerosil 380 with particle diameters close to 70 °A has been utilized. The goal is to determine the primary heat transfer mode in the system and to set up models for predicting the effective thermal conductivity of materials as a function of mean temperature and solid volume fraction. Experimantaly, effective thermal conductivity has been measured in terms of steady state and unsteady state. Two different apparatus under various conditions of temperature, solid volume fraction, density, have been installed to measure total thermal conductivity. 1- Steady State, Guarded Hot Plate Experiment: Various measurement specifications have evolved in the course of developing a standard method for testing insulations. This experiment is for steady state measurement of one - dimensional heat transfer. The Guarded Hot Plate Method measures the heat flow through insulations installed between hot and cold plates. Usually, the hot plate is electrically heated and the cold plate is water cooled. Heat transfer is determined via measurement of the electrical power required to maintain the surfaces at certain prescribed temperatures. In this stuady, the Guarded Hot Plate method, being the simplest and most widely accepted method, is adopted to measure the heat transfer in Ultra Fine Powder Insulations. 2- Unsteady State, Rapid Measurement Experiment: To measure rapidly effective thermal conductivity, another commen method is to use Rapid Measurement Technic. There is such an apparatus at the Mechanical Engineering Faculty in I.T.U.. By means of prop consisting of thermocouple and heater, the voltage difference between aerosil 380 and reference container is measured. In this study, heat changes as timewise. Convector is used to feed the circuit electrically. Heater is made of CCr-Ni Alloy) Cromel, and also thermocouple is made of CNi-Al Alloy) Alumil. Voltage is noted for each minute to find k value. -VII-There is a constant value in order to calculate temperature so voltage values are divided into 40.7. After drawing the graph, slope is read. At the and of experiments, very low thermal conductivity values are attained such as, 0.025, 0.036, 0.020 W / m K. Aerosil 380, being main material of this study consists of spherical particles which have an average diameter of 70 °A. If a chain of particles from one gram of aerosil 380 could be strung together, this would result in a length equivalent to 20 times the distance from the earth to the moon. As particle sizes decrease, the specific surface increases. Conversely, when particle sizes increase, the specific surface decreases. Aerosil particles in general, have a smooth surface free of pores. The process of high temperature flame hydrolysis was introduced by Degussa in 1942. It was shortly thereafter developed for large scale industrial use,and the product manufactured by this method was introduced worldwide under the AEROSIL trade mark. The manufacture of aerosil takes place through the hydrolysis of a volatile.- si lane compound in an oxygen - hydrogen gas flame. The result is an extremely fine - particle size silicon dioxide, which is characteristic for this process. It is possible to obtain variations of the product's properties by appropriate controlled modifications of the reaction conditions. Aerosil as a problem solver has very large field from cosmetics, lubrications to fire extinguishing powders, toner s7 also table salt and tomato powder and so on. It has been determined about 200 aeras to add aerosil sof ar. Some of them have been summarized as follows; 1- Aerosil thickens and structures liquid systems. 2- Aerosil improves suspention behavior and redispersibility of solids in liquid systems. 3- Aerosil improves mechanical properties of elastomers. 4- Aerosil enhances the free flow and storage stability of powders. 5- Aerosil hydrophobizes. This is the water repellant properties of aerosil. 6- Aerosil supports catalysts. It is acquiring a growing prominence in the field of catalysis. 7- Aerosil improves electrical properties. It also reduces the number of charge - carriers in high - tension cables and thereby improves the insulation properties, particularly in the higher temperature range. -VIII-8- Aerosil insulates in high and low temperature ranges : Aerosil has proven to be very successful in thermal insulation - especially in the high temperature range up to 1000 C. Amorphous silicon dioxide is a very poor heat conductor. The particle size of aerosil enables it to form agglomerates, the interstices of which lie in the range of the average free path length of nitrogen and oxygen molecules and thus reduce the gas flow to a minimum. The high purity of aerosil hinders a premature sintering and ensures the stability of the specific surface area, even at high temperatures. Aerosil is also effective in very low temperature insulations for which the same properties are of importance. Aerosil containing powder mixtures or compressed preformed shapes are used to insulate industrial components, for example heat storage units, jet engines, tanks for liquified gases and pipelines on ships and power stations. Optimum performance is obtained with aerosil 380,as usual. 9- Aerosil raw material for the manufacture of high purity silicates. 10- Aerosil transforms liquid systems into free flowing powders in pastes, plant extracts, balsams and so on. 11- Aerosil improves the effectiveness of defoamers. As explained above clearly there are two different kinds of aerosil in terms of behaviour towards water. The first one is hydrophilic Another one is hydrophobic. Both are used in many ways.._ _. In this study, the heat transfer characteristics of ultra - fine powder insulations have been investigated in order to obtain a more effective insulation once more. An examination of the mechanisms of heat transfer for low solid fractions has revealed that gas conduction and thermal radiation are the dominant modes of heat transfer as it has been mentioned above. Ultra - fine spheres with a diameter of about 7 nm, make it possible to effectively reduce gas conduction as their inter particle clearance is smaller than the mean free path of air molecules at atmospheric conditions. Therefore, the material is a very good insulation at quite cold systems and also high temperatures, the models have been developed to predict the gas conduction in the insulation. Some of the new models have been analysed. All have been compared with each other in terms of the solid fraction, mean temperature and ambient condition. 1- Floating Sphere Model. 2- Si ngl i -Chained- Sphere Model. 3- Multi-Chained-Sphere Model. -IX-these new models gave the more reasonable results than the previous ones. Today, there are many current literatures on this subject. This technology is developing more and more, year by year. -X- 67