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Isıl toplayıcı-depolayıcı duvarlı pasif sistemlerde sınır tabaka akışının sayısal incelenmesi

Başlık çevirisi mevcut değil.

  1. Tez No: 55524
  2. Yazar: RÜYA CARAN
  3. Danışmanlar: PROF.DR. A. NİLÜFER EĞRİCAN
  4. Tez Türü: Yüksek Lisans
  5. Konular: Makine Mühendisliği, Mechanical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1996
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 52

Özet

ÖZET Bu çalışmada, önemli bir pasif sistem çeşidi olan ısıl toplayıcı-depolayıcı duvarın (Trombe duvarı) hava kanalında, masif duvar yüzeyi üzerinde oluşan sınır tabaka akışının, iki boyutlu, laminer akış kabulleri yapılarak zamana bağlı incelemesi yapılmıştır. Bölüm l'de, yenilenebilir enerji kaynaklarının temiz ve bol bulunur olma özelliklerine değinilerek, bu enerji kaynaklarından güneş enerjisinin önemi anlatılmıştır. Güneş enerjisinin hacim ısıtma amacıyla kullanılmasında yararlanılan pasif sistemlerin çeşitleri ve çalışma prensipleri hakkında bilgi verilmiştir. Bölüm 2'de pasif sistem tiplerinden Trombe duvarı tanıtılmıştır. Trombe duvarının hava kanalındaki akışa ait sayısal denklemlerin verilmesi ve sınır tabaka akışının çözülmesinde kullanılan SIMPLE yönteminin tanıtılması Bölüm 3 'de yapılmışlar. Hava kanalı yüzeylerindeki sıcaklıkların yüzey boyunca sabit olduğu ve kanala girişte hava hızının kanal genişliği boyunca değişmediği kabulleri yapılmıştır. Yüzey sıcaklıklanyla, girişteki hızın elde edilmesi için TRNSYS isimli güneş enerjisi sistemlerinin simulasyonu için geliştirilen paket programdan yararlanıldı. Bu bölümde ayrıca, hazırlanan bilgisayar programına ait akış diyagramı verilerek programın çalışma şekli şematik olarak gösterilmiştir. Bölüm 4'de, programın çeşitli koşullarda çalıştırılmasıyla elde edilen sonuçlar açıklanmıştır. Kanal girişindeki Re sayılarına göre, değişik sıcaklık aralıklarında elde edilen eş hız ve sıcaklık grafikleri verilmiştir. Ayrıca İstanbul'a ait 1994 yılı meteorolojik datalarının kullanılmasıyla, TRNSYS'den belirli aylarda Trombe duvarı hava kanalında sınır şartları elde edilmiştir. Masif duvar yüzeyinin hava kanalı tarafındaki yüzeyinde sayısal çözüm yapılarak sözkonusu aylar için boyutsuz ısı akıları, Nu sayısı, elde edilmiştir. Bu konuda, daha sonra yapılacak çalışmalarda yardımcı olacağı umularak yapılan öneriler bu bölümün sonunda yeralmıştır. XI

Özet (Çeviri)

SUMMARY NUMERICAL ANALYSIS OF BOUNDARY LAYER FLOW ON A THERMAL COLLECTOR- STORAGE WALL IN A PASSIVE SYSTEM In Turkey, the 36% of total energy consumption is used in buildings and 27% of this energy use is for space heating. This portion can be up to 40-50% in developed countries. Primarily, non-renewable energy sources are consumed to overcome this necessity. Now, it is a known fact that fossil fuels are not unlimited and the ecological system of the world cannot withstand the pollution caused by the fossil fuels until their life is over. So, long before fossil fuels are exhausted, environmental pollution will lead societies to find new energy sources. There are two kinds of environmental damage that fossil fuels cause: -Environmental pollution; including air pollution and acidification of environment, -Global warming; including ozone layer depletion and green house effect. The first and obvious prevention to be taken is to product and use energy more efficiently improving efficiency of the energy systems and using insulation materials. The second and most important step is to find alternative, ecology friendly energy sources. Solar energy is an important kind of renewable energy sources being clear, abundant and natural. The only way to the future is to find a way of sustainable development. Sustainable development means that the existence of future people is not endangered by current developments. Unfortunately, today's life is far from being sustainable. Environmental conscious energy consumption will lead us a final steady state world. In this future world, non-renewable energy sources will not be used because of depletion or their environmental impact and the world population will be at a stabilized level making a sustainable way of living easy. Solar energy is the only input that the world receives. The world absorbs solar radiation at the temperature of the sun and it emits the same amount of energy at the temperature of the outer atmosphere. The quality difference between the absorbed short wave radiation and the emitted long wave radiation is the energy received from sun. This energy, having a great work potential also fuels all the biological processes causing fossil fuels. Solar energy is used via two types of systems: -Active systems -Passive systemsPassive systems have gained considerable interest in the last few decades. The word“passive”means that solar energy is used to heat a building in winter and/or to cool it in summer without using moving equipments. Passive systems have four basic parts: -Absorber -Storage -Control -Glazing Passive solar energy use can be summarized dividing solar architecture components as like in the following figure: and Passive solar energy Choice of building site Orientation of building -open to south -closed towards north Compact shape of building Overhanging roofs for shadowing Choice and location of plants Choice and integration of components Components Windows (esp. towards south) Sun spaces Trombewall Glazing Control devices Thermal storage elements Adopted heating systems Storage effect of building structure Passive systems can be classified in five types: 1. Direct Gain System: The sun radiation comes directly into the inhabited spaces through glazing element. There should be a thermal storage material inside the building like a concrete floor or a massive wall insulated outside to store solar energy during the day and release it back during the night. 2.Collecting-Storage Wall (Trombe wall) System: It consists of a glazing placed in front of a massive wall with an air gap between. Massive wall is a conductive and dense wall and its sun facing surface should be painted dark. Radiation coming through the glazing is absorbed by the massive wall and transported indoor spaces by conduction and convection.3. Sun Space System: Sun spaces are a combination of direct gain and collecting-storage wall principles. They collect heat in a larger area, store this energy using a massive object and/or the wall separating room and the sunspace and release it into the room with a time lag. Sun spaces improve the comfort conditions of indoor spaces preventing glare and reducing the temperature fluctuation and can also be used as an additional living space. 4.Roof Pond System: In the roof pond system, the thermal storage material is the ceiling of the house. A movable insulation is required in this system. In winter season, heating function is achieved; in summer season, by use of movable insulation, a good cooling system can be obtained. 5.The Barra System: In this type of passive system, the southern wall is insulated and is detailed as a thermosyphonic air heating solar collector. The air is heated by radiation, goes upward and flows through the horizontal channels embedded inside the concrete ceiling. Ceiling also acts as a storage material. Air exits from the channels at the northern part of the building and heats the parts of the building far from the collector, then flows back to the inlets of the collector wall. The temperature distribution in the building is more even in this system when compared to other passive systems. Passive systems with a Trombe wall has been accepted as an efficient way of using solar energy. It provides three different functions simultaneously: 1. Solar energy coming through the glazing is collected on the massive wall. 2. Solar energy is absorbed by the wall and increases the temperature of the wall and the air in the gap. 3. Heat is transported via two parallel paths. One path is conduction through the wall; the other path is convective heat transfer by the air flowing through the ventilation holes into the room. Circulation of air is maintained by natural means or by a fan. Major advantages of Trombe wall system can be summarized like the following.: -The indoor temperatures are more stable than other passive systems. -The mass wall is a simple solar collector and glare problem caused by direct exposure to sunlight is avoided. -The storage function of the massive wall creating a time lag between collecting and releasing of solar energy supplies heat during cloudy hours and night. -The glazing plate acts as a weather protect tool for the wall. Disadvantages are: xiv-ifan effective shading is not provided, overheating problem can be faced in summer season. -Due to the limited depth of natural convection and thermal radiation effects inside the building, the effective heating is achieved only to a depth of 1.5 times the wall's height by the Trombe wall. -The glass plate requires cleaning. -Trombe wall system should be supplied with an auxiliary heating system in severe winter days and a cooling system in hot summer days. But the massive wall complicates the control of auxiliary heating systems. Heat transfer in Trombe wall system has been studied both numerically and experimentally by researchers all around the world. The knowledge about the system performance which would be obtained experimentally, can be reached cheaper and faster by numerical methods. The effect of system parameters and meteorological conditions on the efficiency can be evaluated by performing simulation under different conditions. There is a satisfying agreement about conductive heat transfer mechanism in Trombe wall systems. But the same can not be said about the second heat transfer path, i.e. convective heat transfer. In this study, the convective heat transfer problem is investigated by studying velocity and temperature distributions across the boundary layer on the outer surface massive wall. In the first chapter, importance of renewable energies, particularly solar energy, is talked about and the types of passive systems are introduced. In the second chapter, passive systems with a Trombe wall is described and in the third chapter air movement in Trombe wall channel is explained, the solution method of the partial differential equations is introduced. Assuming a two dimensional Newtonian fluid, constant property, laminar flow and the Boussinesq approximation, boundary layer governing equations are obtained like that: dp dü dv â+Pâx- + Pây- = ° dv f dv dv) dP H dt *\ ax dyj dy ^ dT ÖT 5T d2l - +U-T-+V- = a - =- at ox ay ex2 lax2. + pgP(T-T00) The following boundary conditions are adopted for the solution of above equations: u = v = 0; T = TW atx=0;0oo; 0p =ûfcE+avH)W + aNS+b Where, aE = DeA(|Pee|) + ||-Fe,0| aw = DwA(|Pew|) + |Fw,0|| aN = DnA(|Pen|) + |-Fn)0| as = DsA(|Pes|) + |Fs,0| 0 ppAxAy af - At b = ScAxAy + w = w + W (l-QIPew)5 (l-O.OSPew) puAx Pe = I r«t> J w O^Pew^lO Pe^lOTRNSYS, one dimensional simulation program of solar energy systems, was used to obtain the boundary conditions. Meteorological data of Istanbul city, containing the solar radiation, outside temperature and wind velocity was given to TRNSYS to calculate the wall and glazing surface temperatures and air velocity at the inlet of the channel at every hour of the typical day of a month. Using the SIMPLE method, described above, two dimensional, laminar boundary layer flow near the massive wall surface of the channel is solved and velocity and temperature distributions are obtained at one hour intervals. The heat transfer on the outer surface of the massive wall of a Trombe wall system in winter months is demonstrated via the dimensionless heat transfer quantities.

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