Holografik interferometre yardımıyla ısı ve kütle geçişi
Başlık çevirisi mevcut değil.
- Tez No: 100896
- Danışmanlar: PROF.DR. OSMAN F. GENCELİ
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2000
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 74
Özet
ÖZET Bu çalışmada, yatay silindirlerde doğal taşınım, sabit ısı akısı sınır şartında ısı transferi ve bileşik ısı ve kütle transferi incelenerek, literatürde verilen ampirik formüllerle karşılaştırılması amacı ile yapılan bu teorik ve deneysel çalışmada, konu yedi ana bölümde incelenmiştir. Birinci bölümde, doğal taşınım olayı ve bazı uygulamaları ile bu çalışmanın amacı ve önemi vurgulanmıştır. İkinci bölümde, yatay silindirlerde doğal taşınım ile ısı geçişine ve bileşik ısı ve kütle geçişine ait yapılan çalışmalar özetlenmiş ve verilen ampirik formüller karşılaştuılmışur. Üçüncü bölümde, yatay silindir etrafındaki akış için değişik sınır şartlarında ölçek analizi (scale analysis) kullanılarak teorik bir inceleme yapılmıştır. Dördüncü bölümde, deneylerde uygulanan yöntemler hakkında bilgi verilmiştir. Beşinci bölümde, deney tesisatı ve deneylerin yapılışı hakkında ayrıntılı bilgi verilmiştir. Altıncı bölümde, elde edilen deney sonuçları verilmiş ve literatürdeki sonuçlarla karşılaştınlmıştır. Ve son olarak yedinci bölümde ise, elde edilen sonuçlar değerlendirilerek ileride yapılacak çalışmalar hakkında öneriler verilmiştir. Deney sonuçlan ayrıntılı olarak ekler bölümünde bulunmaktadır. Sonuçlar, literatürdeki çalışmalar ile karşılaştırılarak uyumlu olanlar saptanmıştır.
Özet (Çeviri)
SUMMARY Natural convection heat transfer occurs whenever a body is placed in a fluid at a higher or a lower temperature than that of the body. As a result of the temperature difference, heat flows between the fluid and the body and causes a change in the density of the fluid in the vicinity of the surface. The difference in density leads to downward flow of the lighter one. If the motion of the fluid is caused solely by differences in density resulting from temperature gradients, without the aid of a pump or a fan, the associated heat transfer mechanism is called natural convection. Natural convection currents transfer internal energy stored in the fluid in essentially the same manner as forced convection currents. However, the intensity of the mixing motion is generally less in natural convection, and consequently the heat transfer coefficients are lower than in forced convection. Although natural convection heat transfer coefficients are relatively small, many devices depend largely on this mode of heat transfer for cooling. In the electrical engineering field, transmission lines, transformers, rectifiers, and electrically heated wires such as the heating elements of an electric furnace are cooled in party by natural convection. As a result of the heat generated internally, the temperature of these bodies rises above than that of the surroundings. As the temperature difference increases, the rate of heat flow also increases until a state of equilibrium is reached where the rate of heat generation is equal to the rate of heat dissipation. Natural convection is the dominant heat flow mechanism from radiators, walls of a building, or the stationary human body in a quiescent atmosphere. The determination of the heat load on heating and air-conditioning equipment and computers requires, therefore, knowledge of the natural convection heat transfer coefficients. Natural convection is also responsible for heat losses from pipes carrying steam or other heated fluids. XINatural convection is also responsible for heat losses from pipes carrying steam or other heated fluids. Natural convection has been proposed in nuclear power applications to cool the surfaces of bodies in which heat is generated by fission. In all of the after mentioned examples the body force responsible for the convection currents is the gravitational attraction. Gravity, however, is not the only body force that can produce natural convection. In certain aircraft applications there are components such as the blades of gas turbines and helicopter ramjets, which rotate at high speeds. Associated with these rotative speeds are large centrifugal forces whose magnitudes, like the gravitational force, are also proportional to the fluid density and hence can generate strong natural-convection currents. Cooling of rotating components by natural convection is therefore feasible even at high heat fluxes. Exact evaluation of the heat transfer coefficient for natural convection from the boundary layer is very difficult. The problem has been solved only for simple geometries, such as a vertical flat plate and a horizontal cylinder. The phenomenon of natural convection observed by the Greeks over 2000 years ago and phrased by Archimedes somewhat as follows: A body immersed in a fluid experiences a buoyant or lifting force equal to the mass of the displaced fluid. Hence, a submerged body rises when its density is less than that of the surrounding fluid and sink when its density is greater. The buoyant effect is the driving force in natural convection. Mass transfer can result from several different phenomena. There is a mass transfer associated with convection in that mass is transported from one place to another in the flow system. This type of mass transfer occurs on a macroscopic level and is usually treated in the subject of fluid mechanics. Mass diffusion may also result from a temperature gradient in a system; this is called thermal diffusion. Similarly, a concentration gradient can give rise to a temperature gradient and a consequent heat transfer. These two effects are termed coupled phenomena and may be treated by the methods of irreversible thercoupled phenomena and may be treated by the methods of irreversible thermodynamics. When a mixture of gases or liquids is contained such that, there exists a concentration gradient of one or more of the constituents across the system, there xiiwill be a mass transfer on a microscopic level as the result of diffusion from regions of high concentration to regions of low concentration. The local coefficient of heat transfer for a gas on a horizontal cylinder is a function of position, however, for a horizontal cylinder the variation occurs around the periphery of the cylinder rather than along the axis. The highest value of heat transfer coefficient is on the top of the cylinder, where the condensate film is thinnest, and the lowest value of the heat transfer coefficient is on the underside. Over the years it has been found that average free convection heat transfer coefficients can be represented in the following functional form for a variety of circumstances Nu = C(GrfPrf)m where the subscript f indicates that the properties in the dimensionless groups are evaluated at the film temperature., _ 00+ ^w This master degree thesis, presents the results of an experimental investigation of heat transfer by natural convection from horizontal cylinders to the air at uniform heat flux boundary condition. The experimental range of the Rayleigh number has been considered from 105 to 106 where the Prandtl number of air is taken as a constant value (Pr=0.7). Because of the practical importance of the natural convection phenomena on the horizontal cylinders, it has attracted the interest of many investigators. Table 2-2 constitute, a list of previously published empirical and semi-empirical equations for heat transfer by natural convection from an infinitely long horizontal isothermal cylinder immersed in a body of fluid that is finite in extent. A curcory examination of these nine equations reveals that they do not agree closely with one another. For example, for cylinders in air and Ra = 1, the value of Nusselt calculated from equation (8) exceeds that calculated from equation (6) by approximately 30 % of the value calculated by equation (8). Further more, as might be expected, these equations xuido not represent experimental data obtained from various sources equally well, particularly at low Rayleigh numbers (Ra
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