Дисертації з теми "Horizontal convection"
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1
Chiu-Webster, Sunny. "Horizontal convection and glass furnaces." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611923.
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2
Kerr, O. S. "Horizontal effects in double-diffusive convection." Thesis, University of Bristol, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380229.
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3
Jansen, Adrian J. "Natural convection above a horizontal heat source." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from the National Technical Information Service, 1993. http://handle.dtic.mil/100.2/ADA267212.
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4
Hort, Matthew C. "Transient natural convection within horizontal cylindrical enclosures." Thesis, University of Surrey, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.313250.
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5
Grine, K. "Free convection problems from a semi-infinite horizontal plate." Thesis, University of Manchester, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.383889.
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6
Ali, Hafiz-Muhummad. "Free-convection condensation on single horizontal pin-fin tubes." Thesis, Queen Mary, University of London, 2011. http://qmro.qmul.ac.uk/xmlui/handle/123456789/2322.
Повний текст джерелаАнотація:
New experimental data are reported for free-convection condensation of ethylene glycol and R-113 on three-dimensional pin-fin tubes. Effects of pin geometry and tube thermal conductivity (for copper, brass and bronze giving a mean range of 400, 120 and 80 W/m K over the range of temperature of interest) were investigated. All tests were performed at near atmospheric pressure with downward flowing vapour at low velocity. Heat-transfer enhancement was found to be approximately twice the corresponding active surface area of the tubes, i.e. the surface area of the parts of the tube and pin surface not covered by condensate retained by surface tension. For ethylene glycol, the best performing pin-fin tube gave a heat-transfer enhancement of 5.8, about 24 % higher than the ‘equivalent’ two-dimensional integral-fin tube (i.e. with the same finroot diameter, longitudinal fin spacing and thickness and fin height). For R-113, the best enhancement was 5.9, about 10 % higher than the equivalent integral-fin tube. For both fluids tested, vapour-side, heat-transfer enhancement was found to increase with decreasing circumferential pin spacing and increasing pin height. Circumferential pin thickness had little effect on heat-transfer enhancement. Effects of tube thermal conductivity were found to be more significant for ethylene glycol than R-113. Retention angle measurements were made under static conditions (without condensation) and were found to be larger than for equivalent integral-fin tubes. An expression for condensate retention angle on pin-fin tubes was proposed and found to agree with the measured retention angles to ±15%. A semi-empirical model for condensation heat transfer on horizontal pin-fin tubes has been developed which accounts for the combined effect of gravity and surface tension. The model predicts the majority of available data to ±20 %.
7
Dyko, Mark P. "Three-dimensional buoyancy-driven convection in horizontal cylindrical annuli /." The Ohio State University, 2002. http://rave.ohiolink.edu/etdc/view?acc_num=osu1486402288261566.
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8
Straneo, Fiammetta. "Dynamics of rotating convection including a horizontal stratification and wind /." Thesis, Connect to this title online; UW restricted, 1999. http://hdl.handle.net/1773/10996.
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9
Lagana, Anthony. "Mixed convection heat transfer in vertical, horizontal, and inclined pipes." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/mq29607.pdf.
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10
Lagana, Anthony. "Mixed convection heat transfer in vertical, horizontal, and inclined pipes." Thesis, McGill University, 1996. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=27234.
Повний текст джерелаАнотація:
An experimental apparatus was designed and constructed for the study of laminar mixed convection heat transfer in vertical, horizontal and inclined tubes. The working fluid was distilled water, with bulk temperatures in the range of 8$ sp circ$C to 31$ sp circ$C.An innovative design allows, for the first time, flow visualization over the entire heated portion of the test section. The key element of this design is a thin, electrically conductive gold-film heater suitably attached to the outside surface of a plexiglas pipe: the gold film is approximately 80% transparent to electromagnetic radiation in the visible wavelength band. This test section was mounted inside a transparent vacuum chamber to insulate it from the environment. A dye injection technique was used to visualize the mixed-convection flow patterns. The apparatus was also designed and instrumented to allow the measurement of both circumferential and axial temperature variations over the heated tube.
The flow-visualization results revealed the following: (i) a steady recirculating flow pattern, followed by laminar flow instability in vertical tubes; (ii) steady spiralling flow patterns in inclined and horizontal tubes, that confirmed earlier numerical predictions. The temperature results agreed qualitatively with earlier published experimental and numerical data. Local and overall Nusselt numbers can be calculated using the data presented, but this is not within the scope of this thesis.
11
Blasick, Ann Marie. "Onset of flow instability in heated horizontal annuli." Thesis, Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/19538.
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12
MURRAY, BRUCE THOMAS. "EXPERIMENTAL AND NUMERICAL INVESTIGATION OF DOUBLE-DIFFUSIVE CONVECTION IN A HORIZONTAL LAYER OF POROUS MEDIUM." Diss., The University of Arizona, 1986. http://hdl.handle.net/10150/183899.
Повний текст джерелаАнотація:
The onset conditions and the behavior of the developed secondary flow were investigated for double-diffusive convection in a horizontal layer of porous medium. The work concentrated on the case in which the layer is heated from below and saturated with a fluid having a stabilizing concentration gradient. Because the component with the larger diffusivity (heat) is destabilizing and the component with the smaller diffusivity (solute) is stabilizing, the motion at onset is predicted to be oscillatory according to linear stability theory. Experiments were conducted in a rectangular tank 24 cm long x 12 cm wide x 4 cm deep filled with glass beads 3 mm in diameter. The saturating fluid was distilled water and NaCl was the solute. The basic state salinity profiles were slowly diffusing in time, because the salt concentration was not maintained fixed at the solid top and bottom boundaries. Sustained oscillations were not detected at onset in the experiments; instead, there was a dramatic increase in the heat flux at the critical temperature difference. After more than one thermal diffusion time, the heat flux reached a steady value, which increased monotonically if the temperature difference was increased further. When the temperature difference was reduced, the heat flux exhibited hysteresis. Flow visualization indicated that the convection pattern of the developed flow was three-dimensional. In order to better model the experiments, linear theory was extended to include the effects of temperature-dependent thermal expansion coefficient and viscosity for water and the actual solute boundary conditions in the experiment. These extensions of the linear theory required numerical solution procedures. In addition, nonlinear solutions were obtained using finite differences, assuming the problem is two-dimensional. In the nonlinear calculations, the oscillatory motion predicted by linear theory was found to be unstable at finite amplitude. The breakdown of the initial oscillatory motion is followed by a large increase in the heat transport, similar to what was observed in the experiments. Both steady and oscillatory nonlinear asymptotic solutions were found, depending on the governing parameter values. Hysteresis in the heat curve was also obtained.
13
Stone, Rebecca E. "Entrainment, detrainment, and large-scale horizontal gradients in oceanic deep convection." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1999. http://handle.dtic.mil/100.2/ADA367100.
Повний текст джерелаАнотація:
Dissertation (Ph.D. in Physical Oceanography) Naval Postgraduate School, June 1999."June 1999". Dissertation supervisor(s): Roland W. Garwood. Includes bibliographical references (p. 63-67). Also available online.
14
Barozzi, Giovanni Sebastiano. "Combined convection and other effects in heat transfer in horizontal flows." Thesis, City, University of London, 1993. http://openaccess.city.ac.uk/16971/.
Повний текст джерелаАнотація:
For many internal flow situations the effect of buoyancy is to cause significant modifications of the internal flow field and heat transfer rate. The present study focuses on combined forced and free convection under laminar flow conditions in horizontal cylindrical ducts. There, the superposition of secondary, buoyancy induced circulations to the basic forced flow, and the presence of peripheral and axial conduction in the duct wall, give rise to a three-dimensional conjugate heat transfer problem. Such a combined convection feature had not been investigated previously. A novel parallel predictive and experimental study of combined convection for laminar flow in cylindrical ducts is carried out here. The finite volume code FLOW3D from Harwell is used to treat the fully elliptic three-dimensional thermal-flow problem. The choice derives from a complete and detailed survey of the numerical techniques used in the context of combined convection. The predictive work relates specifically to a new experimental study, which has the object of obtaining fresh data for combined convection in horizontal duct flow. A 3 m long, 16 mm I.D. copper pipe is used, with uniform peripheral electrical heating. Wall temperature measurements are taken at twelve axial positions. The experiment covers the range of stable mixed convection and strictly laminar flow conditions, with the Reynolds number ranging from 500 to 1000, and the modified Rayleigh number, Raq, from 1x10⁵ to 5x10⁶. The scope of the experiment is to provide data for comparison with numerical predictions. These, in turn, are designed to model the experimental conditions very accurately, including, in particular, the effects of peripheral and axial wall conduction. The study is complemented with various analyses intended to understanding properly both pure forced and natural convection modes. These are investigated separately in the first part of the work. Overall, this work provides fresh experimental and predictive evidence on various features relevant to the onset and the development of buoyancy induced secondary flows in round ducts under heating conditions. More specifically, the effects, of conduction in the duct wall are highlighted, and demonstrated to have a definite influence on wall temperature, and Nusselt number distributions, even in the case of long, thin-walled ducts.
15
Borjini, Mohamed Naceur. "Etude numérique du couplage rayonnement-convection naturelle d'un espace annulaire d'axe horizontal." Perpignan, 1998. http://www.theses.fr/1998PERP0316.
Повний текст джерелаАнотація:
Le couplage de la convection naturelle et du rayonnement dans un espace annulaire d'axe horizontal est etudie numeriquement le milieu participant est gris ; il absorbe, emet et diffuse isotropiquement le rayonnement. Les equations de la convection naturelle, laminaire, bidimensionnelle et permanente seront ecrites avec les variables vorticite et fonction de courant. Le modele numerique utilise la methode du volume de controle et, pour resoudre l'equation du transfert radiatif, la methode des volumes finis. Les resultats issus de notre code de calcul sont compares a ceux donnes dans la litterature. Des resultats originaux relatifs au couplage rayonnement-convection naturelle en geometries annulaires ainsi que dans des secteurs bidimensionnels sont trouves. L'influence du rayonnement sur les champs des temperatures et des vitesses ainsi que sur le transfert de chaleur est presentee et discutee pour des nombres de rayleigh inferieurs ou egaux a 2. 10#5 et pour des valeurs du parametre de couplage rayonnement-conduction variant entre zero et l'infini.
16
Wang, Chong-Sheng. "Laminar natural convection heat transfer from isothermal horizontal cylinders with annular fins." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0011/NQ30657.pdf.
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17
Bentley, Jonathan Foy. "Evolution of natural convection in a horizontal cylinder with different end temperatures." Thesis, Georgia Institute of Technology, 1991. http://hdl.handle.net/1853/18184.
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18
Tsitsopoulos, Vasileios. "Modelling of buoyant flows associated with large area fires and indirect free convection." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/modelling-of-buoyant-flows-associated-with-large-area-fires-and-indirect-free-convection(7e849d5b-6b95-4534-b387-63387f5ff26f).html.
Повний текст джерелаАнотація:
Experimental observations indicate the presence of attached, gravity induced, horizontal buoyant currents above large area fires. Their driving mechanism is indirect and resembles the one observed above heated horizontal plates. Classic plume modelling is satisfactory for providing information for the flow far from the source. In dealing with large areas and directing attention to the flow close to the source, the classic plume theory should fail because the radial pressure gradient that is responsible for the driving of the flow is squeezed in the long and thin classic plume assumption. For this we propose a new plume structure for the description of the buoyant flow above a circular region of large radius L as “The flow field must be divided into three regions. A region where the flow is predominantly horizontal and attached to the surface, a transition region from horizontal to vertical where separation of the attached current takes place, and a region where vertical flow is established and classic plume theory can be applied”. A model for the description of the gross properties of the horizontal currents is developed under the term “horizontal plume”. The modified Richardson number for the horizontal plume a, being analogous to the radius of the large area, is studied asymptotically in the limit a → ∞ and second order uniformly valid semi-analytical solutions are obtained. The hot plate experiment was set up in order to test the model and facilitate its improvement. A chapter is dedicated to the data analysis coming from thermocouple readings and visualisation of the flow using particle image velocimetry.In the remainder of this thesis two classic problems of laminar natural convection are revisited. That of the first order laminar boundary layer above an isothermal circular plate of radius a and the first order laminar boundary layer above the semi- infinite plate inclined to horizontal. In both cases allowances to variable property effects were made through the introduction of a nondimensional parameter λT, with its value set to zero implying the assumption of the Boussinesq approximation. For the circular plate, fourth order series solutions were obtained valid at the edge of the plate where the effects of λT and Prandtl number Pr are studied. Furthermore a finite difference scheme for the numerical solution of the nonsimilar partial integro- differential equation was developed using the Keller Box method and compared with results obtained from the commercial finite element software COMSOL Multiphysics 3.5a. For the semi-infinite plate, fourth order series approximations valid at the edge of the plate were obtained, while an extensive analysis for the effect of λT, Pr and inclination parameter σ was performed on the flow. Positions of the separation points when the inclination is negative (σ < 0) as a function of Pr and λT were recovered.
19
Abid-David, Chérifa. "La convection mixte dans un conduit horizontal : instabilités thermiques dans la transition "laminaire-turbulent"." Aix-Marseille 1, 1993. http://www.theses.fr/1993AIX11007.
Повний текст джерелаАнотація:
On presente le phenomene de convection mixte dans un conduit horizontal, chauffe a la paroi, pour le regime laminaire et lors de la transition laminaire-turbulent. Dans le premier cas, les resultats experimentaux revelent la presence d'un gradient de temperature dans une section droite; on developpe alors un modele analytique permettant d'aboutir a l'expression de la temperature de paroi en coordonnees cylindriques. Les coefficients de transfert paroi-fluide sont egalement evalues. Lorsque la vitesse du fluide augmente, la temperature de paroi presente des fluctuations temporelles. Ces instabilites sont etudiees d'un point de vue phenomenologique, statistique et on montre egalement qu'elles resultent de la dynamique d'un systeme non lineaire conduisant a la notion du chaos
20
Steffen, Elizabeth Laird. "Observations of vertical and horizontal aspects of deep convection in the Labrador Sea by fully Lagrangian floats /." Thesis, Connect to this title online; UW restricted, 2003. http://hdl.handle.net/1773/11028.
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21
Barbosa, Saldana Juan Gabriel. "Numerical simualtion of mixed convection over a three-dimensional horizontal backward-facing step." Diss., Texas A&M University, 2005. http://hdl.handle.net/1969.1/2353.
Повний текст джерелаАнотація:
A FORTRAN code was developed to numerically simulate the mixed convective flow over a three-dimensional horizontal backward-facing step. The momentum and energy equations under the assumption of the Boussinesq approximation were
discretized by means of a finite volume technique. The SIMPLE algorithm scheme was applied to link the pressure and velocity fields inside the domain while an OpenMP
parallel implementation was proposed to improve the numerical performance and to accelerate the numerical solution. The heating process corresponds to a channel heated from below at constant temperature keeping insulated all the other channel walls. In addition, the back-step was considered as a thermally conducting block and its influence in the heating process was explored by holding different solid to fluid thermal conductivity ratios. The effects over the velocity and temperature distribution of buoyancy forces, acting perpendicular to the mainstream flow, are studied for three different Richardson numbers Ri=3, 2, and 1 and the results are compared against those of pure forced convection Ri=0. In these simulations the Reynolds number is fixed at 200 while the bottom wall temperature is adjusted to fulfill the conditions for the different Ri. Under this assumption, as Ri increases the buoyancy effects are the dominant effects in the mixed convective process.
The numerical results indicate that the velocity field and the temperature distribution for pure forced convection are highly distorted if compared with the mixed
convective flow. If the Ri parameter is increased, then the primary re-circulation zone is reduced. Similarly, as the buoyancy forces become predominant in the flow, the
convective rolls, in the form of spiral-flow structures, become curlier and then higher velocity components are found inside the domain. The temperature field distribution showed that as the Ri is increased a thicker layer of high temperature flow is located at the channel??s top wall as a result of the higher rates of low-density flow moving to the top wall. The flow is ascending by the channel sidewalls, while descending by the channel span-wise central plane. The parallel numerical strategy is presented and some results for the performance of the OpenMP implementation are included. In this sense, linear speedup was obtained when using 16 possessors in parallel.
22
Kuo, Hai-perng. "Stability and finite amplitude natural convection in a shallow cavity with horizontal heating /." The Ohio State University, 1986. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487265143146491.
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23
Kukrer, Cenk Evren. "Direct Numerical Simulation Of Liquid Flow In A Horizontal Microchannel." Master's thesis, METU, 2005. http://etd.lib.metu.edu.tr/upload/12606495/index.pdf.
Повний текст джерелаАнотація:
Numerical simulations of liquid flow in a micro-channel between two horizontal plates are performed. The channel is infinite in streamwise and spanwise directions and its height is taken as m, which falls within the dimension ranges of microchannels. The Navier-Stokes equations with the addition of Brinkman number (Br) to the energy equation are used as the governing equations and spectral methods based approach is applied to obtain the required accuracy to handle liquid flow in the microchannel. It is known for microchannels that Br combines the effects of conduction and viscous dissipation in liquids and is also a way of comparing the importance of latter relative to former. The present study aims to simulate the unusual behavior of decreasing of Nu with increasing Re in the laminar regime of microchannels and to show that Br can be introduced to explain this unexpected behavior. Consequently, it is seen at the end of the results that secondary effect of the Br is observed for the single-phase convective heat transfer. Therefore, a laminar flow of a liquid in a microchannel shows different characteristics compared to a similar flow in a macrochannel. To observe the differences, three different cases are run over each of a range of Reynolds numbers: one with no axial conduction assumption that corresponds to a case similar to macrochannel flow, another case with axial conduction included in the energy equation to simulate one of the main differences and lastly a case with the inclusion of Br number in the governing equations.
A similar study is made for natural convection with the same numerical set-up for the same three cases. Formation of Rayleigh-Benard cells are observed for the critical numbers widely accepted in the literature. The results are compared with each other to see the effects of axial conduction and Br inclusion, in addition to Ra for natural convection.
24
Chaengbamrung, Apichart. "Turbulent plumes generated by a horizontal area source of buoyancy." Access electronically, 2005. http://www.library.uow.edu.au/adt-NWU/public/adt-NWU20060227.102144/index.html.
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25
Wangdhamkoom, Panitan. "Characteristics of multimode heat transfer in a differentially-heated horizontal rectangular duct." Thesis, Curtin University, 2007. http://hdl.handle.net/20.500.11937/1007.
Повний текст джерелаАнотація:
This study presents the numerical analysis of steady laminar flow heat transfer in a horizontal rectangular duct with differential heating on the vertical walls. Three heating configurations: one uniform wall temperature (CS1) and two linearly varying wall temperature cases (CS2 and CS3) are analysed. The study considers the combined effects of natural convection, forced convection and radiation heat transfer on the overall heat transfer characteristics. Air, which is assumed to be a non-participating medium, is chosen as the working fluid. A computational fluid dynamics solver is used to solve a set of governing equations for a range of parameters.For chosen duct aspect ratios, the numerical model simulates the flow and heat transfer for two main effects: buoyancy and radiation heat transfer. Buoyancy effect is represented by Grashof number, which is varied from 2,000 to 1,000,000. The effect of radiation heat transfer is examined by choosing different wall surface emissivity values. The weak and strong radiation effect is represented by the emissivity values of 0.05 and 0.85 respectively. Three duct aspect ratios are considered - 0.5, 1 and 2. The heat transfer characteristics of all the above heating configurations - CS1, CS2, and CS3 are analysed and compared. The numerical results show that, for all heating configurations and duct aspect ratios, the overall heat transfer rate is enhanced when the buoyancy effect increases. Since buoyancy effect induces natural circulation, this circulation is therefore the main mechanism that enhances heat transfer. Radiation heat transfer is found to significantly influence convection heat transfer in high Grashof numbers.
26
Hickey, Christopher N. "Natural convection from a horizontal heater in response to steady and pulsatile input powers." Thesis, Monterey, California. Naval Postgraduate School, 1992. http://hdl.handle.net/10945/23977.
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27
U-Cheul, Shin. "Etude théorique de la convection naturelle thermique à l'interieur d'une calotte cylindrique d'axe horizontal." Perpignan, 1993. http://www.theses.fr/1993PERP0144.
Повний текст джерелаАнотація:
L'auteur etudie, en regime laminaire et permanent, la convection naturelle thermique bidimensionnelle, qui existe dans une enceinte delimitee par une paroi cylindrique d'axe horizontal et une plaque plane inclinee, en introduisant la vorticite et les coordonnees bicylindriques dans deux cas. Dans le premier, il suppose que cette enceinte est remplie d'un fluide newtonien et dans le second, d'une matiere poreuse. L'influence des differents parametres du systeme, notamment de l'inclinaison par rapport au plan horizontal et du facteur de forme est etudiee. Les valeurs moyennes et locales des nombres de nusselt ainsi que les distributions des isothermes et des lignes de courant sont presentees et discutees. Les resultats mettent en evidence l'influence du facteur de forme sur la valeur du nombre de nusselt moyen et montrent que l'inclinaison du systeme affecte differemment le nombre de nusselt moyen selon les valeurs du facteur de forme
28
Vaux, Samuel. "Etude numérique des instabilités hydrodynamiques dans un cylindre soumis à un gradient horizontal de température." Ecully, Ecole centrale de Lyon, 2004. http://www.theses.fr/2004ECDL0029.
Повний текст джерелаАнотація:
Lors des processus de croissance cristalline, le contrôle des instabilités hydrodynamiques dans le bain fondu est primordial afin d'obtenir des cristaux de qualité. Dans ce travail de thèse, nous étudions par simulation numérique directe (D. . N. S. ) la convection naturelle dans un cylindre horizontal chauffé latéralement en envisageant différents métaux liquides à faible nombre de Prandtl et des cylindres de rapport de forme distinct. Nous étudions ainsi l'apparition de l'instabilité oscillatoire en déterminant son seuil, sa fréquence, les caractérisiques de sa structure ainsi que son origine par analyse énergétique. L'utilisation de la Proper Orthogonal Decomposition (P. O. D. ) nous permet par ailleurs d'extraire les modes principaux de l'écoulement oscillatoire et de déterminer en partie la dynamique du systèmeDuring crystal growth processes, the control of the hydrodynamic instabilities in the melt is primordial in order to obtain good quality crystals. In this thesis, we study by direct numerical simulation (D. N. S. ) natural convection in a laterally heated horizontal cylinder. We consider different low Prandtl number liquid metals and different aspect ratio cylinders. We then study the appearance of the oscillatory instability determining its threshold, nature, the characteristics of its structure and its origin by energetic analysis. The use of Proper Orthogonal Decompostion (P. O. D. ) allows the extraction of the principal modes of the flow and important features of the dynamic of the system
29
Varrall, Kevin. "Etude expérimentale de l'écoulement de convection mixte à travers un orifice horizontal reliant deux compartiments." Thesis, Aix-Marseille, 2016. http://www.theses.fr/2016AIXM4314/document.
Повний текст джерелаАнотація:
Afin de répondre à des problématiques bâtimentaires et des enjeux de sécurité incendie, cette thèse aborde l’écoulement de convection mixte à travers un orifice horizontal reliant deux compartiments. L’objectif est d’améliorer la connaissance et la modélisation de l’échange de gaz de masse volumique variable à travers l’orifice. Une étude expérimentale à échelle réduite couplée à une approche théorique est proposée. L'étude est d'abord focalisée sur l’influence du rapport géométrique L/D de l’orifice sur la variation de débit échangé pour un régime de convection naturelle. Les mesures non intrusives de ces débits, par suivit de l'interface entre deux liquides non miscibles lors d'une première approche densimétrique, ainsi que par Stéréo PIV en sortie d'orifice dans une approche thermique, permettent de décrire le processus d'échange bidirectionnel et de conforter les corrélations existantes.Des expériences en régime de convection mixte visent ensuite à caractériser l’influence d’une ventilation mécanique (en soufflage et en extraction) sur les débits échangés. La confrontation des corrélations existantes avec les points expérimentaux montre des écarts importants. Une modification de la corrélation de Cooper 89 est proposée et permet d'en accroître la précision. En parallèle, une approche théorique issue des équations de Navier Stokes simplifiées et sous l’approximation de Boussinesq permet de discuter la construction des corrélations existantes. L'ajustement de coefficients de pertes de charge à partir des points expérimentaux permet de proposer un modèle plus performant que ceux disponibles dans la littératureTo answer to building issues and fire safety challenges, this thesis deals with the mixed convection flow through a horizontal orifice linking two compartments. The aim is to improve the understanding and modeling of the exchange of variable density gas through the opening. A small scale experimental study and a theoretical approach are proposed.The study is first focussed on the impact of the geometrical ratio L/D of the opening on the exchanged flow rate variation for free convection regime. Non-intrusive measurements of these flow rates, via the tracking of the interface between two non miscible liquids in an isothermal approach, and thanks to the SPIV in a thermal approach, permit to describe the bidirectional exchange process and to consolidate existing correlations.Experiments in mixed convection regime aim to study the impact of mechanical ventilation (in blowing and extracting mode) on the exchanged flow rates. The comparison between existing correlations and experimental data shows large differences. A change making the coorelation of Cooper 89 more accurate is proposed. A theoretical approach from the simplified Navier Stokes equations and with the Boussinesq approximation permits to discuss the construction of existing correlations. From this theory, a model more accurate than those available in the literature is proposed thanks to an adjustment of discharge coefficients from experimental data
30
Yu, Youmin. "The Effects of Gravity Modulation on The Instability of Double-Diffusive Convection in a Horizontal Tank." Diss., The University of Arizona, 2006. http://hdl.handle.net/10150/195265.
Повний текст джерелаАнотація:
The effects of gravity modulation on the instability of double-diffusive convections in a horizontal tank with aspect ratio (width/height) of 11 have been investigated by experiments and numerical simulations. The stably stratified fluid layer is set up with ethanol-water solution of 0.0 and 2.0% (by weight). The tank is fixed on a platform that can oscillate in the vertical direction. A constant temperature difference is maintained across the tank at thermal Rayleigh number . The fluid layer becomes unstable as the initially stable solute gradient slowly decreases due to the non-diffusive boundary conditions. The experiments determine that the instability onset under steady gravity is at with onset vortices of wavelength and oscillatory frequency . When the tank is oscillated at modulation frequency and amplitude , the fluid layer is destabilized slightly with a critical and onset vortices of and . A two-dimensional numerical simulation has accurately reproduced the experimental results of steady gravity, and demonstrated that the slight destability effect of gravity modulation is contributed by the asymmetry of the actual gravity modulation.Further simulations have yielded following results: (1) Under steady gravity, the kinetic energy and mechanical work components oscillate synchronously with . Under modulated gravity, they only oscillate synchronously with when is low, whereas not only synchronously with locally but also synchronously with globally when is high; (2) The resonance phenomenon predicted by Chen (2001) also exists under the present lab conditions. Such instability is in the sub-harmonic mode and the destability effect increases as increases. (3) The double-diffusive fluid layer may experience density-mode instability before the double-diffusive instability onset at certain and . Such density-mode instability is generally in the sub-harmonic mode, although it may be in the synchronous mode when is low and is large. This instability accelerates the mixing of the density gradient across the fluid layer and thus affects the succeeding double-diffusive instability; (4) When the background gravity is absent, the purely modulated gravity destabilizes the fluid layer when is low. On the contrary, it stabilizes the fluid layer when is high and the instability onset is in the synchronous mode.
31
Stoddard, Ryan Manse. "Onset of flow instability and critical heat flux in horizontal, thin, uniformly-heated annuli." Thesis, Georgia Institute of Technology, 2000. http://hdl.handle.net/1853/17135.
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Fritzgerald, Claire Louise. "Forced-convection condensation heat-transfer on horizontal integral-fin tubes including effects of liquid retention." Thesis, Queen Mary, University of London, 2011. http://qmro.qmul.ac.uk/xmlui/handle/123456789/2353.
Повний текст джерелаАнотація:
Accurate and repeatable heat-transfer data are reported for forced-convection filmwise condensation of steam and ethylene glycol flowing vertically downward over two single, horizontal instrumented integral-fin tubes and one plain tube. Vapour-side, heat-transfer coefficients were obtained by direct measurement of the tube wall temperature using specially manufactured, instrumented tubes with thermocouples embedded in the tube walls. Both tubes had fin height of 1.6 mm and fin root diameter of 12.7 mm, with fin thickness and spacing of 0.3 mm and 0.6 mm, respectively for the first tube and 0.5 mm and 1.0 mm respectively for the second. Tests were performed at atmospheric pressure for steam with nominal vapour velocities from 2.4 m/s to 10.5 m/s and at three pressures below atmospheric with nominal vapour velocities from 8.4 m/s to 57 m/s for steam and 13 m/s to 82 m/s for ethylene glycol. The data show that both the finned tubes provide an increase in heat flux at the same vapour-side temperature difference with increasing vapour velocity. Visual observations were made and photographs obtained of the condensate retention angle at each combination of vapour velocity and pressure. It was observed that the curvature of the meniscus was distorted by the increase in vapour velocity and in many cases, the extent of condensate flooding changed compared to its value in the quiescent vapour case. In parallel, experiments involving simulated condensation on finned tubes were conducted using horizontal finned tubes in a vertical wind tunnel. Condensate was simulated by liquid (water, ethylene glycol and R-113) supplied to the tube via small holes between the fins along the top of the tube. Downward air velocities up to 24 m/s were used and retention angles were determined from still photograph. Eight tubes with a diameter at the fin root of 12.7 mm were tested. Five tubes of which had fin height of 0.8 mm and spacing between fins of 0.5 mm, 0.75 mm, 1.0 mm, 1.25 mm and 1.5 mm and three tubes had fin height 1.6 mm with fin spacings 0.6 mm, 1.0 mm and 1.5 mm. The results were repeatable on different days and suggested, for all tubes and fluids, that the retention angle asymptotically approached a value around 80o to 85o (from either lower or higher values at zero vapour velocity) with increase in air velocity. Good agreement was found with observations taken during the condensation experiments.
33
Hung, Ming-Cheng. "Transitions in convection of a low Prandtl number fluid driven by a horizontal temperature gradient /." The Ohio State University, 1989. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487668215807295.
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34
Wangdhamkoom, Panitan. "Characteristics of multimode heat transfer in a differentially-heated horizontal rectangular duct." Curtin University of Technology, Department of Mechanical Engineering, 2007. http://espace.library.curtin.edu.au:80/R/?func=dbin-jump-full&object_id=17353.
Повний текст джерелаАнотація:
This study presents the numerical analysis of steady laminar flow heat transfer in a horizontal rectangular duct with differential heating on the vertical walls. Three heating configurations: one uniform wall temperature (CS1) and two linearly varying wall temperature cases (CS2 and CS3) are analysed. The study considers the combined effects of natural convection, forced convection and radiation heat transfer on the overall heat transfer characteristics. Air, which is assumed to be a non-participating medium, is chosen as the working fluid. A computational fluid dynamics solver is used to solve a set of governing equations for a range of parameters.For chosen duct aspect ratios, the numerical model simulates the flow and heat transfer for two main effects: buoyancy and radiation heat transfer. Buoyancy effect is represented by Grashof number, which is varied from 2,000 to 1,000,000. The effect of radiation heat transfer is examined by choosing different wall surface emissivity values. The weak and strong radiation effect is represented by the emissivity values of 0.05 and 0.85 respectively. Three duct aspect ratios are considered - 0.5, 1 and 2. The heat transfer characteristics of all the above heating configurations - CS1, CS2, and CS3 are analysed and compared. The numerical results show that, for all heating configurations and duct aspect ratios, the overall heat transfer rate is enhanced when the buoyancy effect increases. Since buoyancy effect induces natural circulation, this circulation is therefore the main mechanism that enhances heat transfer. Radiation heat transfer is found to significantly influence convection heat transfer in high Grashof numbers.
35
Mouhtadi, Driss. "Etude numérique de la convection naturelle thermique engendrée par des blocs générant de la chaleur dans un canal horizontal et dans une géométrie cubique." Thesis, Cergy-Pontoise, 2012. http://www.theses.fr/2012CERG0564/document.
Повний текст джерелаАнотація:
L'objectif de ce travail est d'étudier les détails des écoulements et des transferts thermiques induits par convection naturelle au sein d'un canal (simulation bidimensionnelle) et au sein d'une cavité cubique (simulation tridimensionnelle) munis de blocs chauffants. La chaleur dégagée par les blocs résulte d'une génération volumique uniforme de la chaleur, d'une température chaude constante ou d'un flux surfacique uniforme. On utilise l'air (Pr=0.72) comme fluide. Les paramètres de l'étude sont le rapport des conductivités thermiques du bloc solide et du fluide (0.1≤k*≤200), le nombre de Rayleigh (〖10〗^4≤Ra≤〖10〗^7) et la hauteur relative des blocs (1/8≤B≤1/2). La détermination des conditions de validité du modèle à blocs isothermes et du modèle à blocs libérant un flux surfacique uniforme, en fonction du rapport des conductivités thermiques et des autres paramètres du problème, compte parmi les principaux objectifs de ce travail. Les résultats obtenus montrent que l'écoulement et le transfert thermique ainsi que les conditions de validité des modèles mentionnés sont fortement influencés par les paramètres de contrôle et par la multiplicité de solutions trouvée en régime convectifThe object of this work is to study the details of the flow and heat transfer induced by natural convection in a channel (2D simulation) and in a cubic cavity (3D simulation) containing heating blocks. The heat released by the blocks results from a uniform volumetric heat generation, a constant hot temperature or a uniform surface flux. Air (Pr=0.72) is used as working fluid. The parameters of the study are the thermal conductivities ratio of solid blocks and fluid (0.1≤k*≤200), the Rayleigh number (〖10〗^4≤Ra≤〖10〗^7) and the relative height of the blocks (1/8≤B≤1/2). Among the main objects of this work is the determination of the conditions of validity of the model with isothermal blocks and the model with blocks releasing a uniform surface flux, as functions of the thermal conductivities ratio and the other parameters of the problem. The results obtained show that the flow and heat transfer and the conditions of validity of the models mentioned are strongly affected by the control parameters and the multiplicity of solutions found in the convective regime
36
Begum, Latifa. "Natural and mixed convection in a horizontal cylindrical annulus with and without fins on inner cylinder." Thesis, McGill University, 2007. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=112556.
Повний текст джерелаАнотація:
Determination of the heat transfer coefficients for natural and mixed convection in horizontal annuli is important for designing double pipe heat exchangers and for energy storage systems. In part one and two of this study, the 2D numerical solution of the laminar natural convection of water in six internally finned horizontal annuli has been obtained. The fins are attached to the external surface of the inner cylinder. Only the symmetrical half of the horizontal annulus with three equally spaced longitudinal divergent solid and porous fins are considered. The parameters of the problem are Rayleigh number, fin height, permeability and porosity of the porous fin, etc. The above parameters are suitably varied to ascertain their effects on fluid flow and heat transfer. The results show that traditional solid fins provide much higher heat transfer rates compared to the porous fins. Part three of this work deals with mixed convective heat transfer (laminar natural and forced convections) of water in a vented annulus. The forced flow conditions are imposed by providing an inlet at the top and an outlet at the bottom. For various parameters of the problem, the average and local Nusselt numbers along the inner cylinder are calculated for water for both aiding and opposing flows. The fourth part of this study deals with numerical modeling of natural convection of nanofluids in a horizontal cylindrical annulus. Simulations are carried out for Cu-water nanofluids. The results, in general, show that nanoparticles systematically decrease the natural convective heat transfer coefficient on the inner cylinder. Practical and useful correlations are provided for calculating average heat transfer rates from the inner cylinder in the form of average equivalent thermal conductivity and average Nusselt number for all of the four cases discussed above. These correlations are new and will be helpful in designing heat exchangers.
37
Ozdemir, Mehmet Ozan. "Optimum Design Of Parallel, Horizontal And Laminar Forced Convection Air-cooled Rectangular Channels With Insulated Lateral Surfaces." Master's thesis, METU, 2009. http://etd.lib.metu.edu.tr/upload/3/12610710/index.pdf.
Повний текст джерелаАнотація:
The objective of this thesis is to predict numerically the optimal spacing between parallel heat generating boards. The isothermal boards are stacked in a fixed volume of electronic package enclosed by insulated lateral walls, and they are cooled by laminar forced convection of air with prescribed pressure drop. Fixed pressure drop assumption is an acceptable model for installations in which several parallel boards in electronic equipment receive the coolant from the same source such as a fan.
In the numerical algorithm, the equations that govern the process of forced convection for constant property incompressible flow through one rectangular channel are solved. Numerical results of the flow and temperature field in each rectangular channel yield the optimal board-to-board spacing by which maximum heat dissipation rate from the package to the air is achieved. After the results of the optimization procedure are given, the correlations for the determination of the maximum heat transfer rate from the package and optimal spacing between boards are, respectively, derived in terms of prescribed pressure difference, board length, and density and kinematic viscosity of air.
In conclusion, the obtained correlations are compared and assessed with the available two-dimensional studies in literature for infinite parallel plates. Furthermore, existing two-dimensional results are extended to a more generalized three-dimensional case at the end of the thesis.
38
Doder, Dejan. "Experimental analysis of the pressure characteristic curve of a forced convection boiling flow in single horizontal channel." Thesis, Norges Teknisk-Naturvitenskaplige Universitet, 2013. http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-20917.
Повний текст джерелаАнотація:
Forced convection boiling flow, often referred to as two-phase flow, is a common phenomenon which occurs in many industrial processes. The relationship between the total pressure drop in a channel containing two-phase flow and the mass flux of the flow is known as the pressure characteristic curve. If any part of pressure characteristic curve contains a negative slope, flow instabilities might occur in the channel. Flow instabilities are unwanted because they can reduce the efficiency of the industrial process. In this study experimental work has been done to analyze the behavior of the pressure characteristic curve in two-phase flow by applying a sensitivity analysis. The parameters which were analyzed in the sensitivity analysis were inlet pressure, subcooling temperature, total heating power and heating power distribution. Also, experimental results from this study were compared with the results from a numerical study of the characteristic pressure curve of two-phase flow [1].The experimental study showed that all the analyzed parameters influence the behavior of the pressure characteristics curve. The comparison with the numerical study showed the same trends for behavior for the pressure characteristic curve for all analyzed parameters except for the total heating power.From the analyzed parameters it was shown that the inlet pressure has the largest relative influence on the behavior of the pressure characteristic curve compared to the other parameters. Lowering the inlet pressure leads to the largest change in the mass flux range where flow instabilities can occur. Lowering the inlet pressure also leads to the largest change in the negative slope the pressure characteristic curve.
39
Memory, Stephen B., Vance Hiro Adams, and P. J. Marto. "Theoretical study of laminar film condensation on horizontal elliptical tubes under conditions of free and forced convection." Thesis, Monterey, California: Naval Postgraduate School, 1993. http://hdl.handle.net/10945/24236.
Повний текст джерела
40
Petrone, Giuseppe. "Etude numérique des écoulements de convection naturelle et mixte dans un espace annulaire horizontal : approche par analyse de stabilité linéaire." Marne-la-Vallée, 2004. http://www.theses.fr/2004MARN0225.
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41
"Experimental studies of Rayleigh-Bénard convection and horizontal convection." 2013. http://library.cuhk.edu.hk/record=b6115868.
Повний текст джерелаАнотація:
本論文由兩部分組成。第一部分介紹了關於瑞利－伯纳德對流的兩項工作。第一項工作在五個側向寬高比不同的矩形對流槽中研究了空間約束效應對流體動力學和傳熱效率的影響。實驗發現，整體流場隨著約束強度的增加而明顯減弱，而且大尺度環流的流向反轉變得更為頻繁。令人驚訝的是，儘管流動變慢，總體傳熱效率卻顯著地提高。仔細分析表明，空間約束改變了邊界層中羽流的形態和動力學特性，表現為形成的羽流結構更為有序和充滿活力，從而導致邊界層變得更薄更均勻，傳熱效率由此得以提高。第二項工作研究了不同溫度邊界條件對湍流熱對流的影響。實驗在兩個對流槽中進行：其中一個在上邊界固定溫度而在下邊界固定熱通量（HC 對流槽），另一個則在兩個邊界都保持恆定溫度（CC對流槽）。研究發現，在恆定溫度的邊界條件下，溫度邊界層的厚度對瑞利數的依賴關係滿足于1/3的標度律，區別于在恆定熱通量下的行為（標度律近似為2/7）。此外，CC 對流槽中的流場強度平均比HC對流槽中的要強∼9% ，其流向反轉頻率也要快1.5倍。我們把這些現象歸咎於不同溫度邊界條件下羽流的發射強度不同。論文的第二部分對另外一種重要的熱對流系統，水平對流，進行了實驗研究。在加熱和冷卻位于系統上邊界同一高度的條件下，我們觀測到了一個貫穿整個對流槽的大尺度環流。通過染料示蹤表明，流場中的迴流并不一定需要損耗能量來跨越溫度邊界層，因此可以更有效地維持其流動強度。對於熱傳輸效率以及溫度邊界層厚度隨瑞利數的變化關係，實驗觀測到了一個0.3的標度律，不同于經常被報道的理論值（0.2），這可能是因為在高瑞利數下流動狀態發生了轉變。本研究的结果表明，表面温度梯度不仅可以驱动大尺度环流，而且其強度也比人们普遍认为的要强。
This thesis consists of two self-contained parts, both related to the topic of thermal convection.
In the first part, two sets of studies on turbulent Rayleigh-Bénard convection are presented.
The first study addresses the confinement effects on the heat transport and flow dynamics in quasi-2D geometry. The experiments were conducted in five rectangular cells with the same height and length, while the width being varied to produce a lateral aspect ratio Γ ranging from 0.6 to 0.1. As expected, with decreasing Γ, i.e. increasing the level of confinement, the overall flow slows down and more plumes travel through the bulk region, thus resulting in a large-scale circulation with more frequent reversals. Surprisingly, despite a slower flow, the global heat transport efficiency enhances significantly. Detailed examinations from experiment and simulation show that this enhancement is brought about by the changes in the dynamics and morphology of the thermal plumes in the boundary layers: the confined geometry produces more coherent and energetic plume clusters that in turn result in more uniform and thinner thermal boundary layers. This study demonstrates how changes in turbulent bulk flow can influence the boundary layer dynamics and shows that the quasi-2D geometry is very different from the true 2D and also the 3D systems.
The second study investigates the influence of thermal boundary conditions. Two experimentally achievable configurations are examined; one is fixed-flux at the bottom boundary and fixed-temperature at the top (HC cell), while the other is fixed-temperature at both boundaries (CC cell). It is found that, rather surprisingly, the Rayleigh number dependence of the thermal boundary layer thickness follows a 1/3 power law approximately under the condition of constant temperature, in contrast to the behavior in the case of constant flux (close to a 2/7 scaling). It is further found that the flow strength is on average ∼ 9% larger in the CC cell, and its flow reversal frequency is ∼ 1.5 faster than that for the HC case, which may be understood as changes in the plume emission dynamics.
The second part deals with horizontal convection. The experiment was conducted with a long apparatus (∼ 2.4 m) with the heating and cooling imposed over the top boundary. Flow visualization studies revealed a full penetrating circulation and suggest that the returning flow does not necessarily overcome the stratification to penetrate the entire thermal BL, in such a way that it can “save its kinematic energy thus supports the motion more efficiently. Both the heat transport efficiency and thermal BL thicknesses are found to follow a 0.3 power law, in contrast to the often-reported value of 0.2. This is likely caused by a flow-regime transition in the high Ra number range. This study demonstrates experimentally that a surface temperature gradient is capable of driving a large-scale circulation and its strength is much stronger than what is generally believed by the oceanography community.
Detailed summary in vernacular field only.
Detailed summary in vernacular field only.
Huang, Shidi = 瑞利-伯納德對流及水平對流的實驗研究 / 黃仕迪.
Thesis (Ph.D.) Chinese University of Hong Kong, 2013.
Includes bibliographical references (leaves 97-110).
Abstracts also in Chinese.
Huang, Shidi = Ruili-Bonade dui liu ji shui ping dui liu de shi yan yan jiu / Huang Shidi.
42
Narambhatla, Sambamurthy *. "Conjugate Natural Convection In Horizontal Annuli." Thesis, 2004. http://etd.iisc.ernet.in/handle/2005/1236.
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43
SHI, CHONG-GUANG, and 施重光. "Mixed convection and thermophoresis in a horizontal circular tube." Thesis, 1992. http://ndltd.ncl.edu.tw/handle/24958376569282124710.
Повний текст джерелаАнотація:
碩士國立成功大學
航空太空工程研究所
80
MCVD製程是目前製造光纖的方法中最廣為採用的方法，其製造成本與反應氣體反應後 所產生之粒子的堆積效率有關，如何提高粒子堆積效率是本文探討的課題所在。 本文以雷射光束加熱－旋轉圓管，改變雷射強度及電射光束寬度，以及電射的中心位 置，並考慮流場高溫時所不能忽略的熱輻射效應。在沒有電射加熱時，探討圓管轉速 、入口速度、入口溫度對流場、溫度場和粒子濃度場之影響，尋求一較佳的粒子堆積 效的狀況。 對圓管旋轉時，在θ方向引入強制對流，使得浮力效應對二次流的影響較小，當轉速 為60rpm 和120rpm時溫度場變化不大。當轉速提高為300rpm時，溫度場趨向對稱，使 得管壁附近的溫度梯度減小，故粒子堆積效率較低。 以雷射光束加熱流場時，雷射強度越大，流場溫度越高，有助於提高粒子堆積效率， 並且存在一最佳的雷射光束分佈，而獲得最高的粒子堆積效率。當雷射光束分佈不是 十分集中時，雷射中心在管中央時有較高的粒子堆積效率；當雷射光束分佈十分集中 時，雷射中心位於圓管底端時粒子堆積效率較高。考慮熱輻射時，流場的溫度分佈較 為平緩，粒子堆積效率也會降低。
44
Chen, Li-Sun, and 陳立山. "Experimental Investigation of Natural Convection Over a Horizontal Plate." Thesis, 1993. http://ndltd.ncl.edu.tw/handle/53151605671856037485.
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45
Lee, Wei Yi, and 李唯溢. "Parametric Study on Natural Convection from Horizontal Finned Tubes." Thesis, 2016. http://ndltd.ncl.edu.tw/handle/72974438074859140720.
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46
林志傑. "Three-Dimensional Mixed Convection betwen Two Horizontal Eccentric Cylinders." Thesis, 1999. http://ndltd.ncl.edu.tw/handle/84203473063407501113.
Повний текст джерелаАнотація:
碩士國立成功大學
航空太空工程學系
87
Placing the power cable underground is an engineering tendency in the present days, especially in city areas and industrial zones. The constant heat flux is generated from the electrical resistance of the power cable which lies on the bottom of the conduit, while the concrete wall is adiabatic. This configuration does not permit of steady-state solution of a two-dimensional case. Many theoretical and experimental studies on natural convection in horizontal, eccentric annuli have been carried out. In most of these studies, a two-dimensional model was used in which the annuli were assumed to be coupled with thermal boundary conditions on the cylinder surfaces specified as either with two constant wall temperatures or one with constant wall temperature while the other with constant wall heat flux (including adiabatic surface). A comprehensive literature survey revealed that published work is largely nonexistent on the three-dimensional eccentric annuli between two horizontal cylinders, where their geometric configurations possess an open end. The existed three-dimensional studies on natural convection were limited to the cavity flow problem. The boundary conditions for this problem are as follows. The adiabatic condition is given on the outer cylinder (concrete conduit) surface, while a constant heat flux which is specified with the heat dissipated from the power cable is given on the inner cylinder (power cable). Due to the symmetric natural convection of the flow field with respect to the two free ends and to a vertical plane crossing the center of the cylinders, zero-gradient conditions are given there. Thus, the free end consists of inflow (fresh air) and outflow (heated air) at the same plane. This flow configuration leads to that there is no thermal fully developed field at the open end. Instead, the computational domain has to be extended into the outside environment so that the proper boundary condition can be specified. The "zonal grid" method is used to tread numerically this two-zone approach. In addition to natural convection, mixed convection is studied in this work. In mixed convection, the fluid enters the annuli at one free open end and the other one becomes the outlet plane. For a long enough axial distance in the flow field, the "fully developed" boundary conditions can be reasonably specified at the outlet plane. It is economic to use the "fully developed" boundary conditions at outlet plane as compared to the two-zone approach. The present work investigates that under what conditions (in terms of Gr/Re2 which represents the ratio of buoyant force to inertia force), the fully-developed outlet boundary conditions cab be properly used in the modeling. Effects of the eccentricity on the surface temperature distribution of the inner cylinder are also investigated in this work.
47
Wu, Sheng Ju, and 吳聖儒. "Wave instability and thermal convection between horizontal parallel plates." Thesis, 1997. http://ndltd.ncl.edu.tw/handle/06748381884902768668.
Повний текст джерелаАнотація:
博士國立清華大學
動力機械工程學系
85
The physical mechanism of Tollmien-Schlichting wave and heat transfercharacteristic in the fully developed region between horizontal parallelplates are analysed theoretically by a direct numerical simulation. In thisinvestigation, a direct numerical scheme is developed to study the temporalamplification of a two-dimensional disturbance in plane Poiseuille flow.Transient non-linear equations are applied in a region of a wavelengthmoving with the wave propagation speed. The complex amplitude involved inthe perturbation functions is considered as the initial input of thenon-linear stability equations. The growth and decay of disturbance withtime are presented, and the neutral stability curves is in good agreementwith the existing solutions. In the subcritical regime usingfinite-amplitude approach, the neutral stability curves shift toward lowerReynolds number, and the critical conditions as a function of the initialmagnitude A0 of the disturbance is presented. The friction factor increaseswith the increase in the Reynolds number in the supercritical regime. This thesis also investigates numerically the effect of asymmetricheating on wave instability and thermal convection in a horizontal parallelplate channel. The lower plate and upper plate are at two differenttemperature levels. By the linear stability theory, the critical Reynoldsnumber decreases due to the heating of the lower plate and increases due tothe heating of the upper plate. The result of linear theory shows thevariation of critical Rec with the varying Grashof number for Pr = 0.7 and7. By using a direct numerical simulation on the amplification of a 2-Ddisturbance, the thesis also studies the flow and heat transfercharacteristics in the post-critical regime with the effect of asymmetricheating. The numerical results show the non-linear and time-periodicbehavior of the streamlines, vorticities, velocity vectors, and isotherms. Subsequently, one attempt is to explore the effect of nonlinear basictemperature distribution due to the constant wall heat flux on the wavestability. The modified Grashof number Gr* characterizes the effect ofconstant wall heat flux. Gr* > 0 indicates the heating and Gr* < 0 standsfor the cooling of the channel. The governing parameters are Gr*, Pr and Re.By the linear stability theory, the critical Reynolds number alwaysincreases with the increase in the magnitude of Gr* for both heating andcooling. The boundary condition of the wall heat flux tends to stabilize theflow. For a fixed Gr*, the effect of axial temperature gradient becomessignificant for a small Pr. The T-S wave moves upward due to expandedreverse flow near the lower plate as Gr* > 0 and moves downward due toexpanded reverse flow close to the upper plate as Gr* < 0. This means thatthe axial temperature gradient can enlarge the effect of adverse temperaturegradient in the gravitational direction. The effect of axial temperaturegradient decreases with the increase in the Reynolds number because thebuoyancy force is suppressed by the inertia force. The friction factors andthe Nusselt numbers at the upper plate and the lower plate in thepost-critical regime are exhibited.
48
Lir, Jihn-Tsair, and 黎進財. "Vortex Structures of Air Flow in Near Critical Natural Convection in a Horizontal Shallow Cavity and Mixed Convecton in a Horizontal Flat Duct." Thesis, 2000. http://ndltd.ncl.edu.tw/handle/62394036230457662062.
Повний текст джерелаАнотація:
博士國立交通大學
機械工程系
89
The present study includes two parts. In the first part flow visualization is conducted to study the vortex flow patterns associated with the Rayleigh-Bénard convection in a horizontal shallow cavity of air. The cavity is a rectangular enclosure characterized by the aspect ratios Ax = 16 and Az = 20. Attention is focused on the convection rolls driven at slightly supercritical and subcritical buoyancies. The results show that at slightly subcritical Ra the induced vortex flow is in the form of rectangular rolls along the cavity sides and short straight parallel rolls in the cavity core. At slightly higher Ra near Rac ( = 1,708) more rectangular rolls appear and the short straight rolls in the cavity core merge together to form a serpentine roll. At slightly supercritical buoyancy with 2000 £ Ra £ 3,000 the entire cavity is filled with the straight rolls all parallel to the short sides of the cavity. At an even higher Ra of 4,000 the vortex rolls become irregular and time dependent. Moreover, the processes through which various vortex flow structures evolve during the transient stage are shown to be rather complicate and the vortex flow patterns during the flow formation are significantly affected by the heating rate in raising the buoyancy force. Furthermore, the wavenumber reduction at higher Ra for the parallel vortex roll pattern was noted to mainly result from the splitting of some rectangular rolls into cells and the subsequent merging of the cells into bigger rolls at the intermediate stage of the flow formation. In the second part of the study, combined experimental flow visualization and temperature measurement are carried out to investigate the spatial and temporal structure of the buoyancy driven vortex flow in a mixed convective air flow through a bottom heated horizontal flat duct. Particular attention is paid to the flow at a very low Reynolds number for 1.0 £ Re £ 5.0 and low Rayleigh number for 1,200 £ Ra £ 4,000. Here the possible presence of any vortex flow at subcritical Rayleigh number is also studied. The experiment is conducted in an open loop mixed convection apparatus by choosing a high aspect ratio rectangular duct ( A = 16 ) as the test section. The results from the flow visualization have revealed four new vortex flow patterns in addition to the oftenly seen patterns — longitudinal rolls (L rolls), moving transverse rolls (T rolls), and mixed longitudinal and transverse rolls (M rolls). The newly observed vortex flow patterns include stable longitudinal rolls along with nonperiodic traversing transverse waves, mixed longitudinal and transverse rolls as well as irregular cells, stable stationary transverse rolls in the duct entry and stable longitudinal rolls in the downstream, and U-rolls. The later two vortex patterns are only noted at Re = 1.0. Moreover, steady longitudinal rolls, nonperiodic traversing transverse waves and stationary inlet transverse rolls are induced even at subcritical Rayleigh number. The temporal and spatial characteristics of the new vortex flows are inspected in detail. The buoyancy driven flow formation processes resulting in various vortex flow structures are also investigated here. Many complicate processes during the vortex flow formation are noted, such as the generation of the L and T rolls, merging of L and T rolls to form U-rolls, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the moving of T rolls, roll bending and degeneration of T rolls. Moreover, the flow formation processes leading to the two new vortex flow structures are also examined carefully. Furthermore, a correlation is given to estimate the local onset locations of the longitudinal rolls. Meanwhile, the oscillation frequency and convection speeds of the transverse rolls are correlated from the present data. Finally, a flow regime map is provided to delineate various vortex flow structures observed in this study at the slightly supercritical and subcritical buoyancies.
49
Shaija, A. "Laminar Conjugate Natural Convection And Surface Radiation In Horizontal Annuli." Thesis, 2008. http://hdl.handle.net/2005/796.
Повний текст джерелаАнотація:
Numerical studies of two-dimensional laminar conjugate natural convection flow and heat transfer in horizontal annuli formed between inner heat generating solid cylinders and outer isothermal circular boundary are performed with and without the effect of surface radiation. The two configurations of the concentrically placed inner cylinder are a circular cylinder (CC configuration) and a square cylinder (SOS, i.e., Square-On-Side, configuration). The mathematical formulation consists of the continuity equation, momentum equations with Boussinesq approximation and the solid and fluid energy equations. Numerical solutions are obtained by discretising the governing equations on a collocated mesh (non-staggered mesh) and the pressure-velocity coupling is taken into account via the SIMPLE (Semi-Implicit Method for Pressure-Linked Equations) algorithm. A cylindrical polar coordinate system is employed for CC configuration and a Cartesian coordinate system is used for the SOS configuration. The convective terms are discretised with donor-cell differencing scheme and the diffusion terms, with central differencing scheme. The algebraic equations resulting from the discretisation of the governing equations are solved using the line by line TDMA (Tri-Diagonal Matrix Algorithm). A global iteration scheme over each time step is used for better coupling of temperature and the flow variables and steady-state solutions are obtained by time-marching.
Steady-state results of conjugate pure natural convection are obtained for the volumetric heat generation and outer radius based Grashof number ranging from 104 to 1010, for solid-to-fluid thermal conductivity ratios of 1, 5, 10, 50 and 100, and for the aspect ratios of 0.2 and 0.4, with air as the working medium (Pr=0.708) for the CC and SOS configurations. The flow and temperature distributions are presented in terms of isotherms and streamline maps. Results are presented for several quantities of interest such as local and average Nusselt numbers on the inner and outer boundaries, dimensionless local temperatures on the inner boundary and dimensionless maximum and average solid cylinder temperatures. The results show that the flow in the annulus is characterized by double or quadruple vortex patterns. Of the dimensionless maximum solid temperature, average solid temperature and average inner boundary temperature, the first two are much sensitive to solid-to-fluid thermal conductivity ratio.
Surface radiation effects are studied numerically in conjugation with natural convection. The coupling with surface radiation arises through the solid-fluid interface thermal condition. To account for the radiation effects, configuration factors among the subsurfaces of the inner and outer boundaries formed by the computational mesh are determined. Results are obtained for CC and SOS configurations for emissivities ranging from 0.2-0.8, with the other parameters as in pure natural convection case. It is found that even at low surface emissivity, radiation plays a significant role in bringing down the convective component and enhancing the total Nusselt numbers across the annulus. The presence of radiation is found to reduce the dimensionless temperatures inside the solid and homogenise the temperature distribution in the fluid. The radiative Nusselt number is about 50-70 % of the total Nusselt number depending on the radiative parameters chosen. This factor emphasizes the need for taking into account the coupling of radiation and natural convection for the accurate prediction of the flow and heat transfer characteristics in the annulus. The solution of the conjugate problem facilitates the determination of the solid temperature distribution, which is important in connection with the safety aspects of various thermal energy systems. Correlations as functions of Grashof number and thermal conductivity ratio are constructed for the estimation of various quantities of interest for the two configurations and aspect ratios for pure natural convection and for combined natural convection and radiation cases. The results are expected to be useful in the design of thermal systems such as spent nuclear fuel casks during transportation and storage, underground transmission cables and cooling of electrical and electronic components.
50
Jeng, Ying-Chou, and 鄭英周. "Mixed Convection in Horizontal Rectangular Channel Heated from the Sid." Thesis, 1998. http://ndltd.ncl.edu.tw/handle/37465435084606477793.
Повний текст джерелаАнотація:
碩士國立成功大學
航空太空工程學系
86
Experimental studies of mixed convection flow and heat transfer in horizontal rectangular channel heated from a side have been carried out. The channel is made of two vertical parallel plates with one of the plate heated uniformly and the opposite plate well insulated. The gap between the parallel plates is small and the height to the gap ratio of the channel cross section is 6.67. Both flow visualization and the heat transfer along the heated wall are measured. The Reynolds number ranges from 316 to 1500, the buoyancy parameter, from 0 to 20000 and Pr of the airflow is 0.7. Flow structure inside the channel is visualized by injecting smoke at the inletflowing along the heated side wall. The heated buoyant flow accumulates in the upper region of the channel, which grows in size as the buoyancy parameter increases. The accumulated flow is thermally stable and has a slower motion, which can reduce the heat transfer enhancement by the buoyancy force. The effect of the Reynolds number and the buoyancy par ameter on the heat transfer is presented and discussed. The normalized Nusselt numbers are in proportion to the Buoyancy parameter. This suggests that correlations of both the local and the averaged Nusselt number in terms of the buoyancy parameter can be successful, and thus obtained.