Dikdörtgen kesitli yapay pürüzlülük için pürüzlülük fonksiyonu korelasyonları
The Correlations of roughness parameters for two dimensional rectangular ribs in the channels
- Tez No: 21974
- Danışmanlar: PROF. DR. ALPİN KEMAL DAĞSÖZ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 95
Özet
ÖZET Boru ve kanallarda ısı taşınımını arttırmak için yüzeylerde akışa dik veya eğik olarak yapay pürüzlülük yapılır. Yüzey üzerindeki pürüzler akısı tedirgin ederek, yüzeyden taşımınla geçen ısı miktarını artırmaktadır. Bu çalışmada dikdörtgen kesitli pürüzler için, RCh* 5 pürüzlülük fonksiyonunu pürüzlülük geometrisi parametreleri cinsinden ampirik olarak veren bağıntılar elde edilmiştir. VII
Özet (Çeviri)
SUMMARY THE CORRELATIONS OF ROUGHNESS PARAMETERS FOR TWO DIMENSIONAL RECTANGULAR RIBS IN THE CHANNELS Some fluid coolants such as gases are not good heat transfer media due to their very low density. However, they have been extensively used in reactors due to their low neutron absorbtion and low chemical activity. Much effort and inguety has been devoted to increase the heat transfer. 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 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. Artificial roughness is often used in nuclear reactors to improve the thermal performance of fuel elements.This 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 sublayer in the fluid region nearest to the wall. It is known that roughness couldnt only increase the heat transfer. It should be noted 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 /X where the Stanton number St is a dimen- sionless number proportional to the heat transfer coeffi cient and X is the friction factor, proportional to the pressure drop is generally greater for a rough surface than for a smooth one. In spite of many studies conducted on a variety of rough surfaces, a lack of sufficient knowledge 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 quite a few works had appeared previously, the first important work on this topic was published byNikuradse in 1933. Later, Dippey and Sabersky and Webbet developed friction and heat transfer similarity laws, which are complimentary to Nikuradse's friction similarity law. Their model is based on a heat-momentum a- nalogy, applied to a two region flow model. They assumed the existance of two regions» namely inner region, where the law of the flow is applicable and outer region, where velocity defect law is effective. This thesis includes all kinds of knowledge on this theoretical base. Their model is based on a heat -momentum transfer analogy, applied to a two region flow model. They as sumed the existance of twoo regions, namely CD inner region, where the law of flow is applicable. C25 outer region, where velocity defect law is effective. In the inner region, the velocity distribution assumed to depend only the local conditions like y,t >y. and represented by, is u u * c* Cyu > = i : u 4>L the law of the wall and the velocity distribution eq'n part near the wall region, C3) Hence from the knowledge of velocity distribution inside rough tubes, the friction factor for the turbulent flow of fluid can be obtained by integration of the eq'n the entire cross -section of the tube. The similarity law for rough surfaces can does be C3) for friction given by 1/2 RCh >=C8/\).3 lnCh/R> + 3 C4) IXThe selection of the roughness should depend on the material of the surface and on the cost and practibility of manufacturing methods for producing roughness, of the eventually selected height, on that material. The selection of the roughness should depend on the intended material of the surface and on the cost and practibility of manufacturing methods for production. For the walls of a duct, metallic materials must be used because of their very thermal conductivy properties. Tine analysis of the different authors showed that the rectangular roughness parameter RCh ) is the function of the height of the ribs Ch), the pitch of the ribs Ct), the width of the ribs and the lenght of the velocity profile Cy~) from the rough wall to the position of zero zero shear stress, namely, R=RCt/h,h/b,h/y) or R=R C t-b/h, h/b > h/y 5 For studies on rough circular tubes the roughness parameter R according to velocity profile equation or pressure drop equation can be determined. For annular gapsand r ectangular channels the zero shear stress method developed by Maubach can be used which will be described in the following sections. This study served the purpose of finding relations which would allow a calculation of the roughness parame ter R to be made from the given roughness geometry and thickness of the flow layer. All calculations were carried out on a PC computer. Using the least squares method,^ R was considered to be the function of t-b/h, h/b, h/y as showed by, R=cCt-b,-'h> e* n l t-b )log - b The accuracy of these relations is related to the geometrical tolarences of the roughness elements, the quality of the data measured in the ° individual studies, the method of transformation and finally the assumptionthat the friction factor is independent of Reynolds number applies to a sufficiency high“roughness Reynolds number”dimensionless h. For better accuracy of the relations indicated system further systematic experimental studies will certainly have to be carried out over a broad range of h and in the transition range between a hydrautically smooth and a fully rough flow. Such studies could be valuable for the application of artificial roughnesses in heat transfer equipment. The fuel elements of a gas cooled reactor » both thermal or fast are formed by clusters of rods, which are in part or compeletely, provided with artificially rough surfaces. The walls of subassembly shroud which contains the fuel pins are of course always smooth. Heat transfer experiments with these fuel elements or fuel elements models, however, take a long time and are very expensive. Furthermore, the experimental data of these complicated geometries are diffucult to inter pr ete and to generalize. This analysis of all the results obtained from measurements on rectangular roughness known to authors has shown that it is possible to indicate+ a general relationship of the roughness parameter RCh ) as a fun- tion of geometry for a fully rough flow. The relationship found applies over a very broad range of the three geometry parameters.Evaluation of measured results requires agreement to be reached on the channel dimen sions. In this study the volumetric diameter was used throughout. The pressure drop in channels with rectangular roughnesses can be calculated on the basis of the rela tions found. Artificial roughness asa means of improving heat transfer gains more and more interest ? especially for application to gas cooled reactors. The thermodynamic designof the reactor core calls for knowledge of the heat transfer data and pressure loses generated by rough nesses. 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 varia tion of the geometrical roughness parameters it. will be useful to work with a simple channel geometry which XIlimits experimental costs. This resulted in the use of the concentric annular gap with rough inner tube in a large number of experiments. In the following sections, turbulent flow in the concentric annular gap will be described and a method will be indicated which allows the interperation of pressure loss measurements. In this way, general parameters can be derived from the experimental results which help to calculate the hyrdaulic behaviour of the flow. When we measure the friction factor, we measure the average value of the dimensionless velocity profile and by assuming that Nikuradse's friction similarity law is valid, we obtain roughness parameter. What we measure, however,u of the actual velocity profile and not the average value of a perfectly logarithmic velocity profile. Therefore we obtain a higher value of roughness parameter, and this increase is more pronounced when the region of discepancy from the logarithmic profile is larger relatively to the length of the velocity profile, i.e. for greater values of h/y^.The h/y**efect cannot, at least for this type of roughness ribs, be explained by the choice of the definition of the hydraulic diameter ( volumetric > based on the tip or the root of the ribs}. The explanation of the h/y* given above is in contradiction with the findings of the literature survay of Dalle Donne-Meerwald which showed that the h/y* effect is a function of the. rib shape. However, we have already seen that correlations of experimental data coming from a literature survey of many differents sources can be in considerable error when relatively moderate different effects, like h/y^'s, are investigated. To really have the h/y* effect for different types of ribs, it should be necessary to make the same sort of experiment which is made for rectangular ribs. Ch/y- > 0.235> In the range the height of the ribs is too large in comparison with the length of the velocity profile and it has no meaning any more to speak of artificial roughness or of logarithmic velocity profile, average of cross sections where there is the rib and cross sections where there is no rib, due to the considerable contraction of the flow vein over the ribs. XII
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