Boru içindeki akışta pürüzlülük
The Effects of roughness on the flow in a tube
- Tez No: 46330
- Danışmanlar: PROF.DR. A. KEMAL DAĞSÖZ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1995
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 45
Özet
ÖZET Bu çalışmada, dikdörtgen kesitli yapay pürüzlülük için belli sınır şartları altında R pürüzlülük fonksiyonunu, pürüz geometrisini belirleyen parametreler cinsinden veren bağıntılar elde edilmiştir. Daha sonra elde edilen bu bağıntılardan bulunan sonuçlar, literatürdeki diğer araştırmacıların elde ettiği sonuçlarla karşılaştırılmış ve olumlu sonuçlar bulunmuştur. Pürüzlülük fonksiyonu, dairesel kesitli kanal içinde, dikdörtgen kesitli pürüzlü ortamdaki akış için kullanıldığında deneylerden elde edilen sonuçlara yaklaşmış olduğu görülmektedir. Elde edilen pürüzlülük fonksiyonu kullanılarak, kanal içindeki basınç düşümü hesapları ve buna bağlı olarak ısı geçişi hesapları yapılabilmektedir. VI
Özet (Çeviri)
SUMMARY THE EFFECTS OF ROUGHNESS ON THE FLOW IN A TUBE Gases are not good heat transfer media due to their very low density. However, they have been extensively used as coolant in reactors due to their low neutron absorption and low chemical activity. Much effort and ingenuity has been devoted to the increasement of the heat transfer in gas cooled reactors to increase to core power density and electrical power generating costs. It is known that the heat transfer in a fully developed turbulent flow depends on the material properties of the fluid. Fluids with a low Prandtl number have relatively good heat transfer properties. Fluids with a mean or high Prandtl number frequently require technical measures to improve the heat transfer so that the heat transferring equipment can be kept small. In some applications, such as gas turbine airfoil cooling design, The heat transfer enhancement is required on two opposite walls of the cooling passages in order to remove more from airfoil external surfaces which are -directly exposed to the hot gases flow. The internal passages can be approximately modeled as that in the flow in rectangular channels with two opposite rib-roughened walls. In the commercial gas cooled reactors this improvement has been achieved by means of extended heat-transfer surfaces or by the so-called artificial roughness on the surface of the fuel element. In that case the efficiency with respect to the heat-transfer properties of the roughness is a function of the shape and arrangement of the roughness elements. mIn the high temperature reactors even higher power densities are achieved by means of higher coolant pressures and much higher fuel surface temperatures made possible by the adoption of graphite as cladding and structural material. Artificial roughness is often used in nuclear reactors to improve the thermal performance of the fuel elements. Although these are made up of clusters of rods, the experiments to measure the heat-transfer and friction coefficients of roughnesses are performed with single rods contained in smooth tubes.The artificial roughness is made up of small ribs at regular intervals on the heat-transfer surface, which act as turbulence promoters breaking up the viscous sub layer in the fluid region nearest to the wall. Heat transfer and friction correlations are developed for turbulent flow in tubes having a repeated-rib roughens. Since the roughness of the wall, as a consequence of the higher turbulence produced in the flow, not only increase the heat transfer but also produces additional losses of pressure. Thermodynamic design of the reactor calls for knowledge generated by roughness. Therefore, a considerable number of experimental investigations have been performed and will be performed in the future in order to find the most appropriate forms of roughness. For investigations involving a broad variations of the geometrical roughness parameters it will be useful with a simple channel geometry which limits experimental costs. This resulted in the use of the concentric annular gap with rough inner tube in a large number of experiments. It should be notice that the increase in the heat transfer is accompanied by an increase in the pressure drop of the fluid flow but an appropriate figure of merit, St / 1|/ where the Stanton number St is a dimensionless number proportional to the heat transfer coefficient and y is the friction factor, proportional to the pressure drop, is generally greater for a rough surface than for a smooth one. The ratio is called the thermal performance of the roughness. Inspite of many studies conducted on a variety of rough surfaces, a lack of sufficient knowledgement on the flow mechanism over rough surfaces denies the prediction of friction factors as well as heat transfer rates by analytical methods. Thus the need for the evaluation of similarity low arises when the smooth channels are replaced by artificially roughened channels. VIIIAlthough quiet a few works had appeared previously, the first important work on roughness was published by Nikuradse in 1933. Later, Dipprey and Sabersky and Webb et al. developed friction and heat transfer similarity laws, which are complimentary to Nikuradse's friction similarity law. Their model is based on a heat-momentum transfer analogy, applied to a two-region flow model. Measured velocity distributions show that the effects of viscosity and surface roughness differ in the inner and outer region of the turbulent boundary layer. They assumed the existence of two regions, namely ; (1) inner region, where the law of the flow is applicable (2) outer region, where velocity defect law is effective. In the inner region, the velocity distribution is assumed to depend only on local conditions and represented by * u+= 4 =fiZJL-) (D u u The outer regions assumed to be intensive to both roughness of the surface and viscosity of fluid flowing inside the tube. The velocity distribution in this outer region can be represented by »*-?*. = f(y/h) (2, u In contrast the velocity near the wall is sensitive, both to viscosity and the type of roughness; This phenomenon is termed law of the wall similarity and is described by equation (1). Law of the wall similarity implies that wall region velocity distribution for geometrical similar roughness depends only h and is indepent of the pipe Reynolds number. The combination of the law of the wall and the velocity distribution equation for the turbulent dominant part near the wall region, which is given by u+= 2.5 \n(y/h)+R(h+) (3) IXThe physical meaning of h+ and R(h+) is quite clear, h is the Reynolds number based on the height of the roughness and friction velocity and R(h ) is the dimensionless flow velocity related to the friction velocity at the tip of the roughness. The friction similarity law for rough surfaces can be given by R(h+) = y[s/y +2.5 ln(2/z/£>)+3.75 (4) Schlichting showed that equation (3) are valid with good approximation for other macroscopic geometries besides the pipes, such as rectangular ducts or flat plates, at least not very far from the wall in this second case. A large number of results from measurements of the pressure drop can be seen from the literature. The selection of the roughness should depend on the intended material of the surface and on the cost and practicability of manufacturing methods for producing roughness of the eventually selected height on the material. For the walls of a duct, metallic materials are the type of materials most likely to be of concern in practical problems. Furthermore, for the best heat transfer benefit, the rib or other roughness element should also be metallic. If non-metallic roughness elements were used, their very low thermal conductivity would tend to reduce somewhat the heat benefit of the roughness. Present study served the purpose of finding relations, which would allow a calculation of the roughness parameter R to be made from the given roughness geometry. Aeration for the roughness parameter R of the velocity profile in a fully rough flow is established as a function of the geometrical parameters of roughness elements. The evaluation is based on a detailed analyses of experimental results described in the literature. The geometrical parameters are the ratio of distance to width of the roughness, the ratio of the height of the roughness to the length of the velocity profile from the rough wall to the position of zero shear stress. All calculations were carried out on a computer, using the least-squares method, R was considered to be the function of t -b/ h, h / b and h / y.For better accuracy of the relations indicated further systematic experimental studies will certainly have to be carried out over a broad range of h and in the transition range between a hydraulically smooth and a fully rough flow. Such studies could be valuable for the application of artificial roughness in heat transfer equipment. The experiments are therefore generally performed either with flow inside rough tubes or with a single rough rod, where heat is generated electrically by Joule effect, contained in a smooth tube thermally insulated from ambient. XI
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