Düşey perdeli duvarlarda doğal taşınım ile ısı geçişinin sayısal çözümü
On Natural convection heat transfer from veiled walls
- Tez No: 126695
- Danışmanlar: YRD. DOÇ. DR. İ. YALÇIN URALCAN
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Doğal taşınım, Isı geçişi, Sayısal çözüm, Natural convection, Heat transfer, Numerical solution
- Yıl: 2002
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Enerji Bilim Dalı
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Sabit sıcaklıkta ısıtılan düşey duvardan ortama doğal taşımınla ısı geçişi karakteristiklerinin, duvara paralel olarak yerleştirilen ve ısıtılmayan bir levha tarafından nasıl etkilendiğinin sayısal çözümleme ile tespiti için sunulan bu çalışma dört ana bölümden oluşmaktadır. Birinci bölümde doğal taşınım olayı ve bazı uygulamaları açıklanmış, ve akabinde, bu çalışmanın amacı ve önemi vurgulanmıştır. Düşey tek levha ve paralel levha halleri için, değişik sınır şartlarında daha önce yapılmış çalışmalar, ikinci bölümde özetlenmiştir. Bir sonraki bölümde ise,“Fluent Flow Modeling V 4.51”akış modelleme programı kullanılarak yapılan sayısal çözüm, bu modelleme programında kullanılan yöntem, parametre ve değerlendirmede izlenen yol açıklanmış ve dördüncü bölümde de sonuçlar ve analizleri verilmiştir. Yapılan çözüm, gözlem ve daha önce yapılan çalışmalardan, hava için, taşınımla ısı geçişinin en aza indiği, optimum kanal geometrisinin, kenar oranının, H«62.5 değerlerinde elde edildiği anlaşılmıştır. H>62.5 aralığında, hakim olan ısı geçiş rejiminin iletim olduğu ve küçülen kenar oram ile, taşınımla ısı geçişinin, tek duvar haline oranla azaldığı görülmüştür. Kenar oranının daha da küçülmesi yani, kanal genişliğinin artması ile, akışkan hareketlerinin hızlanması sonucu, taşınım, hakim rejim haline gelmiş ve hatta kanal içindeki baca çekiş etkisinin artması ile ısı taşınımının, tek duvar haline oranla daha fazla olduğu tesbit edilmiştir. Kanal genişliği daha da artınca, baca çekişinin etkisi ortadan kalkmış ve ısı taşınımı yeniden azalmıştır. Belirli bir levha açıklığından sonra, ısıtılmayan levhanın kanal içine bakan yüzeyinde, duvardan aldığı ışınım ısısını taşınımla ortama vermesi sonucu, ayrı bir sınır tabakanın geliştiği görülmüştür. İki sınır tabakanın kesişmesi sonucu, kanal içinde tüm yükseldik boyunca, sıcaklık gradyeninin sıfir olduğu noktalarda, kanal genişliğince minimum akışkan sıcaklığının serbest akışkan sıcaklığından daha yüksek olduğu ve bu noktaların geometrik yerinin, izolasyon etkisi gösteren bir adyabatik akışkan tabakası oluşturduğu gözlemlenmiştir. Bu nedenle ısı taşınım miktarı, tek duvar haline oranla azalmış ve kenar oranının daha da küçülmesi ile sınır tabakalar arasındaki etkileşimin ortadan kalkması sonucu tek duvar halindeki taşınım karakteristiklerinin yeniden oluştuğu görülmüştür.
Özet (Çeviri)
ON NATURAL CONVECTION HEAT TRANSFER FROM VEILED WALLS SUMMARY In this master degree thesis, a numerical analysis of natural convection in an open- ended vertical channel has been performed with air in the channel and in the ambient surrounds the channel. Heating conditions are such that, one of the principal walls of the channel-the heated wall- is maintained at a uniform temperature, Tw (elevated with respect to ambient), while the other principal wall is unheated. At first, numerical solutions are carried out for the single vertical isothermal wall case with air and results are compared with relevant previous works existing in literature. In the second step of the work, one thin plate as the same geometry as the one mentioned above, is placed parallel to the heated plate with different spacing. The effect of unheated plates on natural convection heat transfer from heated wall to ambient is solved and results are compared with the previous works. Rayleigh number based on the height of the channel or the plate, is kept in a range of 3xl07<RaL<8xl08, the aspect ratio of the channel in a range of 14.41<H<125. The channel or plate height L is maintained at 0.245 m. and the inter-wall spacing s is kept in a range 2 mm to 17 mm. (2, 4, 6, 8, 12 and 17 mm.) The study is composed of four main parts. In the first part of the study, physical explanation of natural convection phenomena and some of its applications are presented, and then the aim and the importance of searching the natural convection phenomena, for mentioned boundary conditions, are summarized. Previous works existing in literature dealing with both single vertical wall case and open-ended vertical channel case, are presented in the second chapter. VIIn the third chapter, numerical solution process by using Fluent Flow Modeling V 4.51 for the analysis of air, parameters, important modeling factors and evaluation procedures are described in detail. Finally in the fourth chapter, observations on the behavior of the problem are interpreted and results are given. The main reason of attempting to analyse the natural convection problem in a one side heated, open-ended vertical channel, is to determine the effect of a non-heated plate, placed parallel to a heated wall, on natural convection characteristics heat transfer from the heated wall to ambient. Analysis are focused on the determination of optimum channel geometry, represented by means of the aspect ratio H, which results in minimum amount of convection heat transfer from the heated wall to ambient and heat transfer regimes in successive ranges of H. Results obtained can be summarized in four steps: 1. Heat transfer measurements on single vertical isothermal wall in air, shows good agreement with the correlation of Churchill and Chu [3], Nu*= 0.825 +/ a387'Ra" w (2.1) (l-f (0.492 + Prf'f and also Ostrach [7]'s similarity solution of f -£ and T -£. 2. Observations on natural convection heat transfer from the heated principal wall to ambient showed that, heat transfer characteristics are effected in different manners in successive ranges of the aspect ratio, H. Conduction heat transfer seems to be the governing regime in channels of high aspect ratio. Convection heat transfer coefficient decreases with decreasing aspect ratio in this regime, until an optimum value of H, which results in minimum amount of convection heat transfer from the heated wall, is reached. If the aspect ratio is decreased more, hydrodynamic effects, suppressed in the narrow channel, spread through the fluid between the walls. This causes fluid particles start moving faster in the channel and from then on, convection becomes the governing heat transfer regime. In this case amount of heat transfer increases with decreasing aspect ratio. vnAs the aspect ratio decreases, it is seen that, the heat transfer coefficient exceeds the value attained in the single wall case. This behavior arises from the chimney draft effect maintained in the channel. The third regime is observed by means of temperature and velocity profiles and contours. In this regime, the heat transfer coefficient again decreases with decreasing aspect ratio and after a certain value it is seen that amount of natural convection heat transfer becomes less than that observed in the single wall case. Two factors cause this drop in the heat transfer amount; first is the disappearance of chimney draft effect, which decreases as the channel gets wider; and second factor is existence of a separate boundary layer on the non-heated wall. It is seen on the temperature profiles obtained, that, the non-heated wall no longer receives heat from the heated wall, by means of convection. The non-heated wall receives heat only by radiation from the heated wall, and gives this heat back to the fluid in and out of the channel, by convection. If the aspect ratio is decreased more, the heat transfer coefficient increased slightly and finally reaches the value attained for the single wall. As the non-heated plate is drawn away from the heated wall, the view factor between the surfaces decreases and so does the amount of radiation heat transfer that is received by the non-heated wall. The contraction between the two boundary layers vanishes and the boundary layer; on the heated wall, regains the attitude that is maintained in the single wall case. Decreasing the aspect ratio makes no difference in heat transfer characters, from then on. 3. The optimum aspect ratio that results in minimum amount of convection heat transfer for air is determined. All data obtained for the value of channel height and in a wide range of Rayleigh numbers show that, optimum aspect ratio is 4. Measured channel Nusselt and Rayleigh numbers, Nug and Ras, are compared with results of other investigators. The difference between numerical and analytical solutions of relevant works and the experimental results presented in this work, arises from the fact that; 4.1 in the analytical solution presented by Quintiere and Muller [8], both walls are thought to be heated asymmetrically. But it is seen that if y values mentioned in Quintiere and Muller [8] work is calculated and comparisons are made regarding to those y values there will be no difference at all. vm4.2 in the numerical analysis of Sparrow, Chrysler and Azevedo [9], the non-heated wall is assumed to be adiabatic. Both boundary conditions of these relevant works cause the amount of heat transfer, from the heated wall to ambient, decrease. 4.3 in the experiments presented by Uralcan [4] 4.3.1 the view factors between the plates are not taken into account, 4.3.2 in the numerical analysis presented in this work, both plates are considered as infinitive, 4.3.3 required insulation in the experiment's apparatus could not be achieved well. Heat transfer mechanism, in succesive ranges of H, are interpreted by means of a parameter, rj, defined as the ratio of convection heat transfer coefficient, determined for parallel plates case, to the one that is obtained for the single plate, at the same RaL number. This parameter may be called as the efficiency of the non-heated plate, which is thought to veil heat transfer from the heated plate. t\>1 means, amount of convection heat transfer from the heated plate to ambient in the parallel plates case, is more than the amount observed in the single plate case. Otherwise, it means that, the non-heated plate has made isolation effect and the amount of heat transfer by convection in the parallel plates case is less then that in the single plate case. EX
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