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Novev, Yavor Kirilov. "Natural convection in electrochemical systems." Thesis, University of Oxford, 2018. http://ora.ox.ac.uk/objects/uuid:b8badcfd-e376-4ff6-b2da-b8f821871777.
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This thesis is concerned with modelling natural convective flows and specifically with their role in electrochemistry. The studies described here demonstrate that many electroanalytical techniques are prone to non-negligible natural convective effects, thus making the standard assumption for purely diffusional mass transport inapplicable. The chosen approach focusses on investigating idealized systems and establishing orders of magnitude for the quantities of interest. The complexity of the observed natural convective flows and their strong dependence on factors such as container geometry serve as compelling arguments for rigorously excluding natural convection in experimental measurements. The text is structured as follows. Chapter 1 introduces the theoretical framework used in the rest of the text and gives an outline of the electrochemical techniques to which the results in later chapters apply. Chapter 2 surveys the literature on natural convection in electrochemistry and emphasizes recent developments. Chapter 3 studies the natural convection induced by the intrinsic heat of an electrochemical reaction, specifically its effect on mass transport in chronoamperometry and cyclic voltammetry. Chapters 4-6 deal exclusively with coupled heat and momentum transport. Chapter 4 considers the thermal convective flows that arise in an idealized cell for scanning electrochemical microscopy (SECM) and the surrounding air under conditions of imperfect thermostating. Chapter 5 is dedicated to thermal convection in an SECM cell that is being thermostated from below through a solid substrate. This chapter demonstrates the influence of the spatial distribution of substrate thermal conductivity on the observed flows and highlights this effect by using a simpler model of the SECM cell than Chapter 4. Chapter 6 investigates the thermal convection in a novel thermostated cell for electrochemical measurements. Chapter 7 contains the main conclusions from the studies described in the thesis. Appendices A, B and C provide additional data for Chapters 3, 5 and 6, respectively.
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Ihle, Bascuñán Christian. "Spatiotemporal Features of Natural Convection." Tesis, Universidad de Chile, 2011. http://repositorio.uchile.cl/handle/2250/102700.
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Esta tesis, consistente en una recopilación de artículos de investigación originales
autocontenidos, se ocupa del estudio de los mecanismos físicos que explican algunas
características de la dinámica de convección térmica con aplicación a convección
penetrativa, frecuentemente observada en lagos y reservorios chilenos. Este fenómeno
consiste en la aparición de un campo de flujo derivado del enfriamiento superficial de
una masa de de fluido donde potencialmente puede existir una estratificación de densidad
previa. Si bien este problema ha sido extensivamente estudiado empleando experimentos
de pequeña escala (desde 1 mm hasta unos pocos centímetros), no es el caso para
sistemas naturales de mayor tamaño, donde los flujos son comúnmente turbulentos
y la dinámica asociada está además acoplada con perturbaciones espaciotemporales,
incluyendo temperatura ambiente y vientos locales. El presente trabajo se ocupa de
algunas de estas interrogantes, incluyendo las condiciones requeridas para la aparición
de convección penetrativa bajo condiciones de borde térmicas que dependen del tiempo
y suponiendo ausencia de viento. Primero, se consideró el caso más simple de un
enfriamiento superficial repentino, modelado como una capa horizontal infinita, inicialmente
en reposo, de fluido de Boussinesq. La siguiente fase de este estudio consistió en la
elaboración de un modelo teórico simplificado, propuesto como una base para dar cuenta
de la estabilidad de sistemas de pequeña escala frente a patrones de forzamiento térmico
sinusoidales, buscando así un símil al efecto de enfriamiento vespertino o nocturno en
lagos en los casos donde además hay turbulencia media nula antes del comienzo del
flujo convectivo. Un segundo aspecto de este trabajo de tesis fue el estudio del efecto
de la presencia de fuentes y sumideros térmicos cercanos. Para condiciones débiles de
calentamiento y enfriamiento, se ha encontrado que el estudio de esta configuración es
equivalente al estudio de la interacción entre plumas térmicas y corrientes de densidad en
régimen laminar.
Se ha perseguido los objetivos mencionados empleando una combinación de métodos,
incluyendo simulaciones numéricas, técnicas analíticas de perturbación para el estudio
de la estabilidad de los sistemas referidos modelados a través de las ecuaciones de
Navier-Stokes y energía, además de la realización de experimentos. En este último
caso, se propone una técnica de medición simultánea de los campos vectoriales de
velocidad (usando PIV) y gradiente de densidad (usando schlieren sintético). La naturaleza
inherentemente delicada de los experimentos llevados a cabo hizo necesario el desarrollo
de sistemas de control ad-hoc. Como resultado de estas actividades, ha sido posible
vincular las propiedades del fluido con parámetros adimensionales (incluyendo los números
de Prandtl y Rayleigh), para dar cuenta de los tiempos de inicio de convección y frecuencia
de forzamiento térmico en la superficie (entre otros). Del estudio de inhomogeneidades
espaciotemporales, se encontró que las plumas térmicas bidimensionales laminares
pueden sobrevivir el impacto con una corriente de gravedad modificando, sin embargo, su
posición original.
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Khane, Vaibhav B. "Analogy based modeling of natural convection." Diss., Rolla, Mo. : Missouri University of Science and Technology, 2009. http://scholarsmine.mst.edu/thesis/pdf/Khane_09007dcc807046fe.pdf.
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Thesis (M.S.)--Missouri University of Science and Technology, 2009.Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed November 25, 2009) Includes bibliographical references (p. 23-24).
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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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Omranian, Seyed Ali. "The computation of turbulent natural convection flows." Thesis, University of Manchester, 2011. https://www.research.manchester.ac.uk/portal/en/theses/the-computation-of-turbulent-natural-convection-flows(f3b90728-2194-48cb-8162-a94f190b7792).html.
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Fournier, Martin. "Natural convection in two-dimensional irregular cavities." Thesis, University of British Columbia, 1986. http://hdl.handle.net/2429/26288.
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Natural convection in two-dimensional irregular cavities was simulated by numerically solving the steady-state conservation equations written in terms of stream function, vorticity and temperature dependent variables and for a general orthogonal coordinate system. It was assumed that the Boussinesq approximations were valid, that the fluid was Newtonian and that the properties other than density were constant. The use of orthogonal coordinates and the above set of dependent variables was found to have several advantages over the use of Cartesian or non-orthogonal systems and the set of primitive dependent variables (velocities, pressure and temperature). The body-fitted orthogonal coordinate system was numerically generated by means of the weak constraint method of Ryskin and Leal [26], Special forms of the Wood and second-order vorticity boundary conditions were derived for a general two-dimensional body-fitted orthogonal coordinate system. Finite difference techniques were used to solve the resulting set of differential equations.
The effects of the mapping characteristics, the vorticity boundary conditions and the finite difference grid size on the accuracy of the natural convection solution were investigated first. For the cavity geometries studied,, it was observed that, except for grid boundary conditions which led to undesirable grids, most combinations of grid and vorticity boundary conditions gave results of acceptable accuracy (relative error less than one percent) as long as a sufficiently fine grid size (28x28 or finer) was employed.
The effects of the cavity geometry and the Rayleigh number on natural convection were investigated in Part II. It was found that increasing the Rayleigh number always acted to enhance both the natural convection circulation and the heat transfer rate, a result which was easily explained by examining the source term of the momentum equation. The effect of the cavity geometry was more complex but these results could also be interpreted by examining the influence of the cavity shape in impeding or enhancing fluid circulation and the opposing effects of the distance between isothermal walls on conductive and convective heat transfer.
The possibility of using a similar numerical procedure to simulate a melting or a freezing process was investigated in Part III. Numerical predictions of the circulating flow in the liquid phase of an ice forming process were obtained by digitizing the photographic image of a real ice interface and using the true non-linear relationship between density and temperature for water at low temperature. The numerical results were in reasonable agreement with the flow visualization experiments carried out by Eckert [42].Applied Science, Faculty of
Chemical and Biological Engineering, Department of
Graduate
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Safier, Paul Alan. "Electrically-Driven Natural Convection in Colloidal Suspensions." Diss., Tucson, Arizona : University of Arizona, 2005. http://etd.library.arizona.edu/etd/GetFileServlet?file=file:///data1/pdf/etd/azu%5Fetd%5F1122%5F1%5Fm.pdf&type=application/pdf.
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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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King, Kevin John. "Turbulent natural convection in rectangular air cavities." Thesis, Queen Mary, University of London, 1989. http://qmro.qmul.ac.uk/xmlui/handle/123456789/25786.
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The velocity and temperature fields of several air cavities have been surveyed. The cavities operated in the transitional boundary layer regime with vertical, opposing, isothermal heated and cooled walls. The cavity height, width, temperature difference and wall insulation were all changed during the study, with the aspect ratio varying from 4 to 10, and RaH varying from 2,263x10 to 4.486x101e. The local velocity and temperature were measured simultaneously using a laser Doppler anemometer and a 25jim chromel-alumel thermocouple. This allowed the turbulence quantity tT to be measured directly, as well as the mean and root mean square of the fluctuations of velocity and temperature. Several other quantities, which have not previously been available, were derived from the measured data, these were the wall shear stress, the mean lateral velocity, u'v', and v'T'. The effect of a decrease of the level of insulation on the vertical walls was to decrease the non-dimensional temperature of the fluid at the vertical centre-line. Different thermal boundary conditions on the horizontal walls resulted in significant differences between the heated and cooled wall, thermal and velocity, boundary layers. A decrease in the cavity width was seen to alter the characteristics of the mean velocity and temperature profiles when the width was less than twice the lateral extent of either boundary layer in a cavity with a larger width. Near wall distributions of u'v' have shown that the viscous sub-layer was approximately 4mm thick. Calculations of power spectral density, together with inspection of time histories, have confirmed that a laminar flow was present at the bottom of the heated wall. P.S.D. calculations showed that the dominant frequencies of transition were multiples of a base frequency and dependent on the local temperature drop between the wall and the "environment". The power relationship between frequency and power spectral density has been shown to depend on the local vertical temperature gradient. Three sub-ranges were identified in the velocity spectra, whereas four were identified in the temperature spectra. The equivalent ranges in the velocity and temperature spectra exhibited different powers on the frequency, with those of the temperature field being larger.
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Owinoh, Antony Zachariah. "Natural convection driven by heating from below." Thesis, University of Cambridge, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.625011.
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Day, Jerod. "Laminar Natural Convection From Isothermal Vertical Cylinders." Thesis, University of North Texas, 2012. https://digital.library.unt.edu/ark:/67531/metadc177190/.
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Laminar natural convection heat transfer from the vertical surface of a cylinder is a classical subject, which has been studied extensively. Furthermore, this subject has generated some recent interest in the literature. In the present investigation, numerical experiments were performed to determine average Nusselt numbers for isothermal vertical cylinders (103 < RaL < 109, 0.5 < L/D <10, and Pr = 0.7) with and without an adiabatic top in a quiescent ambient environment which will allow for plume growth. Results were compared with commonly used correlations and new average Nusselt number correlations are presented. Furthermore, the limit for which the heat transfer results for a vertical flat plate may be used as an approximation for the heat transfer from a vertical cylinder was investigated.
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Qiu, Hongtao. "Natural convection in a non-homogeneous enclosure." Ann Arbor, Mich. : ProQuest, 2008. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3303953.
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Thesis (Ph.D. in Mechanical Engineering)--S.M.U.Title from PDF title page (viewed Mar. 16, 2009). Source: Dissertation Abstracts International, Volume: 69-03, Section: B, page: 1917. Adviser: Jose L. Lage. Includes bibliographical references.
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Hama, Katsuhiko. "Film boiling heat transfer in various liquids under natural convection and forced convection conditions." Kyoto University, 2006. http://hdl.handle.net/2433/143793.
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Bokhari, I. H. "Turbulent natural convection in a tall rectangular cavity." Thesis, University of Manchester, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488249.
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Corson, Lindsey Thomson. "Geochemical effects on natural convection in porous media." Thesis, University of Strathclyde, 2012. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=18197.
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We describe a model of buoyancy-driven flow in a saturated reactive porous medium, the porosity and permeability of which evolve through precipitation and dissolution as a mineral is lost or gained from the pore fluid. We consider two scenarios: convection driven solely by solutally induced buoyancy effects with a vertically varying equilibrium solubility, and convection driven by a combination of thermally and solutally induced buoyancy effects where the solubility of the dissolved component depends on the temperature. In both cases we characterise the onset of convection using linear stability analysis, and explore the further development of the coupled reaction-convection system numerically. For solutal convection, at low Rayleigh numbers the effect of the reaction-permeability feedback is shown to be destabilising, while at higher Rayleigh numbers the porosity evolution has a stabilising effect. Over longer timescales, reaction-permeability feedback triggers secondary instabilities in quasi-steady convective circulation, leading to rapid reversals in the direction of circulation. Over very long timescales, characteristic patterns of porosity emerge, including horizontal layering as well as the development of vertical chimneys of enhanced porosity. For thermosolutal convection we find that, when the system is solutally unstable, the behaviour of the system is qualitatively the same as for solutal convection, regardless of whether the system is thermally stable or unstable. However, new, interesting behaviour is seen when the system is solutally stable. The long-term evolution of the porous layer depends on whether the underlying thermal or solutal gradient dominates. When the solutal gradient dominates, the reaction-permeability feedback triggers a secondary instability, resulting in the lateral migration of the concentration and temperature elds and rapid reversals in the direction of circulation. However, when the thermal gradient dominates, the thermal gradient dominates, the reation-permeability feedback tends to suppress the circulation, although it re-emerges after a long quiescent.
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Ziai, Said. "Turbulent natural convection in a large rectangular cavity." Thesis, University of London, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.307545.
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The thesis is part of a wider study of turbulent natural convection in cavities undertaken by the U.K. Atomic Energy Authority, to provide experimental data for the verification of computer codes needed in the design of Fast Breeder Reactors. The objectives of the present work were to design and construct a large rectangular air cavity in order to measure distributions of temperature, velocity and local heat transfer rates within the cavity. The cavity was designed to operate with eitherisothermal hot and cold walls, or a uniform heat flux hot wall and an isothermal cold wall. These walls were 3m high and liii wide and the distance between them (cavity thickness) was adjustable between .i.m and .5m. The horizontal and vertical side walls were as nearly adiabatic as possible. Temperature and velocity profiles were measured, at different heights in the vertical mid-plane, with 25j.un diameter themocouples and standard DISA hot-wire anemometer probes respectively. These measurements were for isothermal walls and cavity thickness of .5m. The low velocities and large temperature variations of the flow required a special low speed jet to be designed and constructed for calibrating the hot wires. The calibrations were for velocities up to l.Om/s at temperatures between ambient and 70°C with the flow direction set vertically upwards for the probe used near the hot wall and downwards for the probe used near the cold wall. The experimental data were recorded in digital form on magnetic tapes, using a PDP/ll based data logging system. Statistical analysis of these measurements, involving the local heat transfer rates, mean and standard deviation of temperatures and velocities, frequency spectra, etc. were later carried out on the QMC ICL 2980 computer. The data are compared with results from other experiments including some with a variety of different working fluids, and with the predictions of available computer codes.
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Zhao, Yongling. "Instability and Transition of Natural Convection Boundary Layers." Thesis, The University of Sydney, 2014. http://hdl.handle.net/2123/13126.
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The present work is concerned with the instability and transition of vertical natural convection boundary layers, which are investigated by numerical simulations and particle image velocimetry (PIV) experiments. Two-dimensional direct stability analyses are firstly implemented to study the instability characteristics of the boundary layers, and three-dimensional direct numerical simulations are then conducted to investigate the transitions of the boundary layers. Two broad categories of transitions, that is, the natural transition and the controlled transition, are investigated in this thesis. For the investigation of the controlled transitions, the K-type and H-type transitions are examined respectively. Furthermore, a PIV measurement of the flow characteristics of the boundary layers under natural transition and a preliminary PIV experiment of the K-type transition are conducted to provide qualitative and quantitative evidences for validating the numerical results.
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Ke, Junhao. "Direct numerical simulation of an unsteady natural convection boundary layer." Thesis, University of Sydney, 2021. https://hdl.handle.net/2123/24382.
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The present thesis considers the flow mechanics of a natural convection boundary layer (NCBL) along an isothermally heated vertical wall. Large scale direct numerical simulations are carried out to investigate the laminar stability and the turbulent mechanics of the flow. In this study, a computationally efficient temporal framework, where periodic boundary conditions are imposed in the homogeneous directions, has been used to develop a temporally evolving (instead of a spatially evolving) flow.
The stability properties of the laminar temporally developing NCBL, with Prandtl number 0.71, are numerically investigated in the configuration of a temporally evolving parallel flow. By assuming the timescales of the laminar base flow and the perturbations are separate, the instantaneous linear stability of the flow is investigated by an eigenvalue approach with a quasi-steady assumption, whereby the unsteady base flow is frozen in time. Temporal responses of the discrete perturbation modes are numerically obtained by solving the two-dimensional linearised disturbance equations using a `frozen' base flow as an initial-value problem at various 〖Gr〗_δ. The resultant amplification rates of the discrete modes are compared with the quasi-steady eigenvalue analysis, and both two-dimensional and three-dimensional direct numerical simulations (DNS) of the temporally evolving flow. The amplification rate predicted by the linear theory compares well with the direct numerical simulation solutions up to a transition point. The extent of the linear regime where the perturbations linearly interact with the base flow is thus identified. The value of the transition 〖Gr〗_δ, according to the three-dimensional DNS results, is dependent on the initial perturbation amplitude. Beyond the transition point, the DNS results diverge from the linear stability predictions as nonlinear mechanisms become important.
For the turbulent NCBL flows, three-dimensional direct numerical simulations (DNS) with different initial conditions were carried out to investigate the turbulent mechanics up to 〖Gr〗_δ=1.2×10^8. The turbulent NCBL is examined in two distinct regions separately: a near-wall boundary-layer-like region and an outer bulk plume-like region. In the near-wall region, a constant heat flux layer (see also in George & Capp, 1979; Ho ̈lling & Herwig, 2005) and a constant forcing layer are identified for the turbulent NCBL. In the close vicinity of the wall (y^+<5) a laminar-like sublayer has developed, and the temperature profile follows the linear relation, consistent with the studies of spatially developing flows (Tsuji & Nagano. 1988a); whereas such a linear relation cannot be observed for the velocity profile due to the extra buoyancy. Similar to earlier studies (Ng et al., 2017), this buoyancy effect is shown to asymptotically approach zero with increasing 〖Gr〗_δ. Further away from the wall (y^+>50), there is a log-law region for the mean temperature profile as reported by Tsuji & Nagano (1988a). In this region, the turbulent length scale which characterises mixing scales linearly with distance from the wall once 〖Gr〗_δ is sufficiently large. By taking the varying buoyancy into consideration with the robust mixing length model, a modified log-law for the mean velocity profile for y^+>50 is proposed. The effect of the initialization is shown to persist until relatively high 〖Gr〗_δ as a result of slow adjustment of the buoyancy (temperature) profile. Once these differences are accounted for, our two DNS cases and the spatially developing data of Tsuji & Nagano (1988a) show excellent agreement with the modified log-law.
Beyond a wall-normal distance δ_i, the NCBL can be characterised as an outer bulk plume-like region. This region is found to be well described by an self-similar integral model with profile coefficients (cf. van Reeuwijk & Craske, 2015) which are 〖Gr〗_δ-independent after 〖Gr〗_δ=10^7. The entrainment coefficient for the plume-like region is investigated by decomposing contributions from shear production, buoyancy, viscosity and boundary conditions. For the turbulent NCBL, the entrainment coefficient is found to be mainly affected by the buoyancy in the flow and appears constant beyond 〖Gr〗_δ=10^7. Solution to the self-similar integral model are analytically obtained by solving ordinary differential equations (ODE) with profile coefficients empirically obtained from the DNS results. The DNS results also suggest that the wall heat transfer of the NCBL is directly related to the top-hat scales which characterise the plume-like region. The Nusselt number of the NCBL is found to follow 〖Nu〗_δ∝〖Gr〗_δ^0.373, similar to the observations by Ng et al. (2017) for a vertical NCBL in differentially heated slot and He et al. (2012) for a Rayleigh--Be ̀nard convection. This power-law correlation is higher than the empirical 1/3-power-law correlation reported for spatially developing NCBLs at lower 〖Gr〗_δ, but appears consistent with the ultimate heat transfer with a logarithmic correction suggested by Grossmann & Lohse (2011). Using empirical correlations for the wall shear stress, it is shown that the buoyancy effect in the near-wall region would become negligible, and the near-wall mechanics of the NCBL would become similar to that of a neutrally buoyant turbulent boundary layer above 〖Gr〗_δ>2×10^9 for the present study.
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Campbell, Alasdair Neil. "The effects of natural convection on low temperature combustion." Thesis, University of Cambridge, 2007. https://www.repository.cam.ac.uk/handle/1810/255080.
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When a gas undergoes an exothermic reaction in a closed vessel, spatial temperature gradients can develop. If these gradients become sufficiently large, the resulting buoyancy forces will move the gas, i.e. there is natural convection. The nature of the resulting flow is determined by the Rayleigh number, Ra = (β g ΔT L^3) / (κ ν). The evolution of such a system will depend on the interactions of natural convection, diffusion of both heat and chemical species, and chemical reaction. This study is concerned with a gas-phase system undergoing Sal'nikov's reaction: P → A → B, in the presence of natural convection. This kinetic scheme is used as a simplified representation of a cool flame, which is a feature of the low temperature combustion of a hydrocarbon vapour. Sal'nikov's reaction is one of the simplest to display thermokinetic oscillations, such as those seen in cool flames. The behaviour of Sal'nikov's reaction in the presence of natural convection was investigated using a combination of analytical and numerical techniques. First, a numerical model was developed to compute the temperature, velocity and concentrations when a simple exothermic reaction occurs in a spherical batch reactor, the results of which could be compared with previous experimental measurements. Subsequently, a scaling analysis of Sal'nikov's reaction proceeding in a spherical reactor was performed. This yielded significant insight into the general behaviour of this and similar systems. The forms of the analytical scales were confirmed through comparison with the results from numerical simulations. These scales were used to predict how the system responds to changes in certain key process variables, such as the pressure and the size of the reactor. It was shown that the behaviour of this system is governed by the ratios of the characteristic timescales for diffusion, reaction and natural convection. These ratios were used to define a regime diagram describing the system. The behaviour in different parts of this regime diagram was characterised and regions in which oscillations occur were identified.
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Dafa'Alla, Adil Ahmed. "Turbulent natural convection in a tall closed air cavity." Thesis, University of Manchester, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.236699.
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Worthington, D. R. E. "The cooling of electronic power supplies by natural convection." Thesis, University of Exeter, 1987. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.380691.
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Liu, Ting-Yueh. "Effects of natural and forced convection on thermal explosions." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610835.
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Ghodeswar, Kaustubh. "Natural Convection in a Porous Medium Saturated by Nanofluid." Cleveland State University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=csu1293169230.
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Hashemi, Mohammad. "Lattice Boltzmann Simulation of Natural Convection During Dendritic Growth." University of Akron / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=akron1459444594.
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Ettefagh, Javad. "Natural convection and thermal instabilities in open-ended structures /." The Ohio State University, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487676847115494.
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Urban, Pavel. "Helium Cryostat for Experimental Study of Natural Turbulent Convection." Doctoral thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2010. http://www.nusl.cz/ntk/nusl-233938.
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V disertační práci je popsán heliový kryostat s experimentální válcovou konvekční celou pro studium proudění při přirozené konvekci za velmi vysokých Rayleigho čísel Ra (až do řádu 10^15) a Nusseltových čísel Nu (až do řádu 10^4). Pracovní látkou je studené 4He, které umožňuje díky výhodným fyzikálním vlastnostem dosažení velmi vysokých hodnot čísel Ra. Návrh kryostatu je založen na koncepci lázňových NMR kryostatů s nízkým odparem kryokapalin. Ve středu kryostatu je umístěna konvekční cela o průměru 300 mm a výšce 300 mm. Celu tvoří horní a spodní dno a výměnná střední část. Tyto díly jsou spojeny rozebíratelnými přírubami těsněnými indiovým drátem. Výměnná část umožňuje snadnou modifikaci geometrie cely. Hlavní přednost kryostatu spočívá v minimálním vlivu konstrukce cely a použitých materiálů na studovanou konvekci. Cela kryostatu je navržena pro pracovní tlaky do 250 kPa.
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Sánchez, Martín Xavier. "Processos convectius en un sistema aquàtic natural: l'estany de Banyoles." Doctoral thesis, Universitat de Girona, 2008. http://hdl.handle.net/10803/7810.
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En aquesta tesi s'aborda l'estudi de la turbulència convectiva, a partir d'un treball experimental realitzat a l'estany de Banyoles. Més concretament, els objectius s'emmarquen en dues categories diferents. En la primera, es pretén sistematitzar el marc teòric-conceptual de la turbulència i més concretament de la turbulència convectiva, el qual es troba molt dispers a la bibliografia i sovint enfocat des de perspectives prou diverses. Dins aquest primer objectiu també es pretén sistematitzar tot un conjunt d'eines experimentals de tractament de dades que ens permetran obtenir els valors de les velocitats de dissipació de l'energia cinètica turbulenta i de la variància de temperatura de forma automàtica y obtenir els fluxos turbulents. També volem abordar la caracterització del les escales turbulentes, pel que treballarem amb l'escala de Thorpe i utilitzarem l'anàlisi d'ondetes del senyal de microstructura tèrmica. Dins la segona categoria d'objectius, aquests aplicats directament a l'estany de Banyoles, es pretén aplicar els conceptes desenvolupats al primer objectiu en dos sistemes convectius, un de tipus plomall i l'altre del tipus multidifusiu. Dins aquest context, en els dos primers capítols es presenta teòricament com s'obtenen els fluxos turbulents i s'introdueixen els fenòmens convectius, i en el tercer i quart capítols es descriuen tot un conjunt d'eines d'anàlisi experimental que ens serviran per a l'estudi del cas concret de la convecció al fons de l'estany de Banyoles.
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Swartz, Matthew M. "Experimental Study of Turbulent Natural Convective Condensation In the Presence of Non-Condensable Gas on Vertical and Inclined Surfaces." Research Showcase @ CMU, 2017. http://repository.cmu.edu/dissertations/918.
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Pressurized water reactor nuclear plants, currently under construction, have been designed with passive containment cooling systems. Turbulent, natural-convective condensation, with high non-condensable mass fraction, on the walls of the containment vessel is a primary heat transfer mechanism in these new plant designs. A number of studies have been completed over the past two decades to justify use of the heat and mass transfer analogy for this scenario. A majority of these studies are founded upon natural-convective heat transfer correlations and apply a diffusion layer model to couple heat and mass transfer. Reasonable success in predicting experimental trends for vertical surfaces has been achieved when correction factors are applied. The corrections are attributed to mass transfer suction, film waviness or mist formation, even though little experimental evidence exists to justify these claims. This work examines the influence of film waves and mass transfer suction on the turbulent, natural-convective condensing flow with non-condensable gas present. Testing was conducted using 0.457 m x 2.13 m and a 0.914 m x 2.13 m condensing surfaces suspended in a large pressure vessel. The test surfaces could be rotated from vertical to horizontal to examine the inclination angle effect. The test facility implements relatively high accuracy calorimetric and condensate mass flow measurements to validate the measured heat and mass transfer rates. Test results show that application of the Bayley (1955) and Al-Arabi and Sakr (1988) heat transfer correlations using the heat and mass transfer analogy is appropriate for conditions in which the liquid film remains laminar. For transitional and wavy film flows, a clear augmentation in heat transfer was observed due to disruption of the gas layer by film waves. This result has implications for the scalability of existing correlations. A new correlation is proposed and results compared to several other datasets.
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Kalapurakal, Dipin. "Numerical Simulation of Magnetohydrodynamic (MHD) Effect on Forced, Natural and Mixed Convection Flows." University of Akron / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=akron1342115168.
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Mehrtash, Mehdi. "Numerical Investigation Of Natural Convection From Plate Finned Heat Sinks." Master's thesis, METU, 2011. http://etd.lib.metu.edu.tr/upload/12613530/index.pdf.
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Finned heat sink use for electronics cooling via natural convection is numerically investigated. An experimental study from the literature that is for vertical surfaces is taken as the base case and the experimental setup is numerically modeled using commercial CFD software. The flow and temperature fields are resolved. A scale analysis is applied to produce an order-of-magnitude estimate for maximum convection heat transfer corresponding to the optimum fin spacing. By showing a good agreement of the results with the experimental data, the model is verified. Then the model is used for heat transfer from inclined surfaces. After a large number of simulations for various forward and backward angles between 0-90 degrees, the dependence of heat transfer to the angle and Rayleigh number is investigated. It is observed that the contributions of radiation and natural convection changes with the angle considerably. Results are also verified by comparing them with experimental results available in literature.
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Mulolani, Isaac. "Similarity analysis of natural convection flow due to chemical diffusion." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp01/MQ31625.pdf.
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Sotocinal, Samson A. "Design and testing of a natural convection solar fish dryer." Thesis, McGill University, 1992. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=60638.
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A natural convection solar fish dryer consisting of a flat-plate solar collector, drying chamber, and an auxiliary heater was designed, constructed and tested in the Philippines. The dryer is capable of drying 5 kg of fish in 10 hours.Water was first heated in a flat-plate solar collector then through thermosyphon effect, heat and mass was moved to the heat exchanger where heat was transferred to the air. Heated air was allowed to flow through the drying chamber where trays of prepared samples of fish were laid. Pre-drying treatment of fish similar to those used in commercial practice, were used for individual drying experiments in order to permit a general evaluation of the system.
Seven drying experiments using different fish samples were conducted and the data generated was used to determine the efficiency of the system in terms of solar energy utilization. Results indicate that the system function efficiently at a minimal water temperature increase of 10$ sp circ$C, and the dryer operates at a system efficiency of 9 per cent which compares well with the findings of Yu Wai Man (1986) which found that natural convection solar dryers operate in the efficiency range between 7 to 14 per cent.
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Tian, Yongsheng. "Low turbulence natural convection in an air filled square cavity." Thesis, London South Bank University, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245135.
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LAGE, JOSÉ LUIS. "NUMERICAL ANALYSIS OF NATURAL CONVECTION IN A CHANNEL PARTIALLY OBSTRUCTED." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1986. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=19127@1.
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CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICONeste trabalho foi utilizado o método dos volumes de controle para solução das equações de balanço de massa, momentum e energia, relativas ao escoamento de ar através de um canal. Este é formado por duas paredes verticais, planas, paralelas e adiabáticas. A dissipação de calor, através de um elemento retangular isotérmico adjacente a uma das paredes, provoca a convecção natural. O pioneirismo está na utilização da condição de contorno de pressão para a solução de uma equação, que corrige s velocidades neste mesmo contorno, permitindo utilizar as equações de momentum em forma elíptica. É investigado o comportamento do número de Nusselt médio do canal para Rayleigh variando do Ra igual 153 a Ra igual 1223, com o elemento posicionado no início, no meio e no fim do canal, variando-se as aberturas do mesmo. Verifica-se também o comportamento do Nusselt local ao redor do elemento, assim como o campo de velocidade. Foi verificada a existência de uma distancia ótima entre as paredes do canal, para cada valor do número de Rayleigh. Conclui-se, portanto, que a geometria do elemento tem um efeito importante no processo de transferência de calor, não devendo ser desprezada.
The control volume method was used to solve numerically the equations related to mass, momentum and energy balance. Air flows trough a channel formed by two vertical, flat, parallel and adiabatic walls. The natural convection was caused by a retangular Isothermal element attached to one of the walls. The innovation of this work lies in the utilization of the momentum equations in its elliptical form. It was achieved using the pressure boundary condition to solve an equation that corrects the boundary velocity. The variation of the medium Nusselt number was insvestigated from Ra equal 153 to Ra equal 1223, attaching the element in three diferent positions: at the beginning, in the middle, at the end of the channel and varyin the distance between the walls. The local Nusselt number and the velocity Field around the element were also studied. An optimum distance between the walls for each Rayleign number was that the geometry, of the element has a decisive effect on the heat transfer process, and so it must not be neglected.
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FERREIRA, CLAUDIO MARCELO DOS SANTOS. "NATURAL CONVECTION AND RADIATION HEAT TRANSFER FROM SHROUDED VERTICAL FINS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1992. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=25010@1.
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FUNDAÇÃO CENTRO DE ANÁLISE, PESQUISA E INOVAÇÃO TECNOLÓGICAExperimentos foram realizados para se determinar as características de transferência de calor de modo combinado, por convecção natural e radiação, de um conjunto de aletas retangulares e verticais, com antepara, em ar. Os seguintes parâmetros foram avariados durante o curso dos experimentos: comprimento longitudinal das aletas, diferença de temperatura aleta-ambiente, altura das aletas e a folga entre aletas/antepara. Anteparas possuindo características térmicas distintas foram empregadas (altamente condutora e isolante). Para cada configuração variou-se o Número de Rayleigh entre 4,0 x 10(1) a 7,0 x 10(2). Encontrou-se que o posicionamento de uma antepara próxima às alentas reduz a taxa de troca de calor de modo combinado quando comparado ao caso sem antepara. Encontrou-se, também, que as anteparas metálicas forneceram taxas de transferência de calor maiores que as isoladas. Cálculos mostraram que, quando a folga aleta/antepara era nula, a parcela convectiva era dominante. Para outros valores da folga, a contribuição da componente radiativa era da mesma ordem de grandeza da convectiva. A contribuição da parcela radiativa era maior para baixos valores do Número de Rayleigh e para valores da folga aleta/antepara diferentes de zero.
Experiments were performed to determine the combined-mode natural convection/radiation heat transfer characteristics of a shrouded array of rectangular, vertical fins. The investigated parameters included the height and the longitudinal lenght of the fins, the clearance gap between the shroud and the fin tips and the fin to ambient temperature difference. Shroud walls having different thermal characteristics were used (highly conducting and insulated). For each configuration, the Rayleigh Number ranged from 4,0 x 10 (1) to 7,0 x 10 (2). It was found that the positioning of a shrouding surface close to the fins decreased the rate of combined mode heat transfer when compared to the unshrouded case. Greater heat transfer rates were found for the conducting wall shroud in comparison with the insulated one. Calculations performed showed that, when the clearance gap between the shroud and the fin tips was zero, the convective component was dominant. For other clearance gaps, the contribution of radiation was of the same order of magnitude of the convective one. The contribution of the radiative component was greater for small values of the Raileigh Number and for clearance gaps different from zero.
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Chen, Yin-Heng. "Study of solidification, shrinkage and natural convection in casting processes /." The Ohio State University, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487676847114631.
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Myo, Thant Maung Maung. "Experimental investigation of natural convection heat transfer in bundle pipeline." Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/9498.
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This thesis describes work relating to the thermal management of hydrocarbon fluids issuing from (typically sub-sea) wells and passing through flow lines to a processing facility (typically an offshore platform). The hydrocarbon fluids must be kept at a temperature above that at which solids (typically waxes or hydrates) are formed. One way of achieving this “Thermal Management” is to place the flow lines inside a carrier pipe through which is also passed (in a separate pipe) a heating fluid (typically hot water). The design of such “bundle” systems is a significant challenge because of the complex combined natural convection and radiation heat transfer processes involved. In related work, Computational Fluid Dynamics (CFD) methods are being used to predict these complex processes and the principal objective of the work described in this thesis was to carry out new experiments to validate these predictions. Experiments were carried out on an industrial scale 5-Pipe Bundle covering the full range of bundle orientations from horizontal to vertical so as to be able to simulate horizontal and inclined flow lines and risers. The bundle chosen was 3 m long with a 30 inch (762 mm) internal diameter carrier pipe with 4 pipes mounted inside it. The four internal pipes were respectively a 14 inch (355.6 mm) outside diameter pipe, a 8 inch (219.1 mm) outside diameter pipe and two 4 inch (114.3 mm) outside diameter pipes. These 4 pipes represented the hydrocarbon product pipe, the test pipe and the heating flow and return pipes in a typical industrial bundle. A support mechanism on which the bundle can be rotated from 0 to 360° was designed and constructed. In the experiments, the surface temperatures of each pipe surface were kept constant but differences were imposed between the respective surfaces. Heat flow rates from or to each pipe surface in the bundle were measured using a calorimetric method and the results were compared with those predicted using the ANSYS CFD code. Good agreement was obtained between the heat flows measured and those calculated from the code. This thesis also describes an analytical and numerical work on natural convection heat transfer inside a vertical pipe taking into account the effect of solids formation by freezing of the fluid at the wall.
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Silva, Tony Barbosa da. "Parametric stellar convection models." Dissertação, 2015. https://repositorio-aberto.up.pt/handle/10216/82260.
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Shih, Chun-Hao, and 施俊豪. "Natural convection in electrical transformers." Thesis, 2001. http://ndltd.ncl.edu.tw/handle/26442324175423825180.
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碩士國立中央大學
機械工程研究所
89
Knowledge of the temperature and position of the hot spot is very important for the design and operation of power transformers. The rate of deterioration of the winding insulation increases with the conductor temperature. Thus it is necessary to know the hottest conductor temperature in order to ensure a reasonable life of the insulation. Pumping working fluid through a set of ducts generally cools the windings of large modern transformers. However, compared with forced convection cooling, natural convection cooling offers minimal safety requirements to avoid burnout. In the present study, the natural convection heat transfer in a disc-type pole-top transformer is investigated. The study depicts flow pattern of cooling fluid inside the windings of a transformer and provides an estimate of the position of the hot spot. The geometrical model consists of two arrays of rectangular heat-dissipating blocks arranged in line in a transformer tank. A two-dimensional, steady and turbulent flow is simulated. The set of transport equations is solved numerically using the finite volume technique. Solutions are presented for the temperature distribution in the disc coils and cooling horizontal ducts of a transformer. The attention is focused on the parameters, such as the Prandtl number, aspect ratio of the tank, heat dissipation rate, thermophysical properties, configuration, number and block ratio of the heated coils. The results show that the flow behavior has a primary clockwise circulating cell in the top region caused by buoyancy effects originating from the heat source. The bottom zone is almost stagnant and thermally stratified. The parametric study indicates that the block ratio, BR, and the aspect ratio, AR, have a significant influence. Increasing BR from 2 to 5, the maximum temperature drops by 20 percent. When aspect ratio increases from 0.257 to 0.4, the maximum temperature is lowered by 33 percent. The effects of other parameters are studied to arrive at qualitative suggestions that may improve the cooling design of the power transformers
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Lin, Pao-Hsiu, and 林保秀. "Natural Convection in Vertical Cylinder." Thesis, 2000. http://ndltd.ncl.edu.tw/handle/10911718705904583381.
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碩士國立中央大學
機械工程研究所
88
Unsteady numerical computation of natural convection are carry out in this paper. Our modeling configuration is three-dimensional vertical cylinder heated from below and adiabatic at sidewall. We use H2O(Pr=6.7) as melt and various three different types of aspect ratio(A=2,1,0.5, diameter D/height H) on the Rayleigh-Benard convection. As for A=2, steady flow structure would change to m1(non-axial symmetry) type form m0(axial symmetry) type once Ra(Rayleigh number) exceed 15000. However, we found that for A=1, 20000
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Khatamifar, Mehdi. "Conjugate natural convection boundary layers." Thesis, 2018. https://researchonline.jcu.edu.au/54302/1/54302-khatamifar-2018-thesis.pdf.
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In this study, the unsteady behavior of conjugate natural convection ow and heat transfer in a differentially-heated square cavity divided by a partition with finite thickness and thermal conductivity is studied using direct numerical simulation. A series of numerical simulation is carried out for three values of dimensionless partition thickness (i.e., Tₚ = 0:05; 0:1; and 0:2), two values of dimensionless partition position (i.e., Xₚ = 0:25 and 0:5), five values of the thermal conductivity ratio (i.e., kᵣ = 0:1; 1; 100; 500 and 1000) and six values of the Rayleigh value (i.e., Ra = 10³; 10⁴; 10⁵; 10⁶; 10⁷ and 10⁸). For all these cases, the aspect ratio of the cavity A = H=L = 1, and the Prandtl number Pr = 0:71 were used.
A computer code written in Visual C# programming language is developed in this study for all numerical simulations. The code operates by solving the conservation equations for heat, mass and momentum with the finite volume method. The main variables used throughout the code are velocities and pressure, and the SIMPLE algorithm is employed to solve the velocity and pressure fields. Each equation can be solved by TDMA (as a default algorithm) or other methods. The developed code can solve steady/unsteady, compressible/incompressible, and turbulent/laminar flows in a Cartesian coordinate system. The nomenclature of the TEACH code (originated at the Imperial Collage) is mainly used in this code to increase readability. To further improve the readability of this code, the code structure has been designed to have separate and independent sections. Another code has been written in Visual C# to do the post-processing of data using already produced binary files by the main code. The code is verified and validated against the published results of partitioned (data from two studies) and non-partitioned (data from 14 studies) cavities.
Empirical correlations are developed for the average Nusselt number by the iterative non-linear curve fitting (i.e., the Levenberg-Marquardt algorithm) which include the effects of Ra, Tₚ and Xₚ . It is found that the effect of Xₚ is negligible. For high kᵣ cases, the number of isotherms in the partition is very low or is not present. The low temperature difference in the partition leads to a negligible heat flux through the partition. In this situation, the partition can be considered as an isothermal wall and the heat transfer characteristics are similar to those of non-partitioned cavity cases, and consequently, the scaling relations for isothermals of a non-partitioned cavity can be used. However, higher temperature gradients are present in the partition in the low kᵣ cases, compared to that in the high kᵣ cases. Therefore, the temperature difference between the left and right sides of the partition is dependent on both kᵣ and Tₚ. In this situation, the thermal behavior of the partition shifts from an isothermal wall like to isoflux wall type. This type of partition has uniform heat flux, and the thermal resistance parameter role becomes important and the scaling relations extracted for isoflux wall in a non-partitioned cavity are more appropriate.
The overall behaviour of Nu(Ave) as kᵣ varies is identified to have three distinctive regions; thermal resistance, thermal transient and isothermal regions. The effect of Tₚ on Nu(Ave) is trivial for low Ra values. Tₚ and kᵣ have opposite effects on the thermal resistance parameter of the partition. Therefore, there is a point where kᵣ nullifies the effect of Tₚ and the increased thermal conductivity of the partition overcomes the thermal damping effect of the partition thickness. This situation happened around kᵣ = 100 for both the centrally positioned and off-centre partitioned cavities.
The transient Nu(Ave) at the hot sidewall (or cold sidewall) of a centrally partitioned cavity characterized by four regimes; conduction, quasi-steady, decaying and steady-state regimes. For cavities with off-centre partitions (in this study partition is close to the hot sidewall) this classification is different. The cold sidewall was characterized by the same four regimes presented for a centrally partitioned cavity. However, for the hot sidewall, a regime identified after decaying region and is called filling regime. Consequently, the five distinct regimes of Nu(Ave) at the hot wall of an off-centre partitioned cavity are conduction, quasi-steady, decaying, filling and steady-state.
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Silva, Tony Barbosa da. "Parametric stellar convection models." Master's thesis, 2015. https://repositorio-aberto.up.pt/handle/10216/82260.
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LI, ZONG-XIN, and 李宗興. "Transient natural convection between concentric shperes." Thesis, 1992. http://ndltd.ncl.edu.tw/handle/56701265639979338750.
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Narambhatla, Sambamurthy *. "Conjugate Natural Convection In Horizontal Annuli." Thesis, 2004. https://etd.iisc.ac.in/handle/2005/1236.
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Narambhatla, Sambamurthy *. "Conjugate Natural Convection In Horizontal Annuli." Thesis, 2004. http://etd.iisc.ernet.in/handle/2005/1236.
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Tasaka, Yuji. "Natural Convection induced by Internal Heat Generation." Doctoral thesis, 2005. http://hdl.handle.net/2115/28093.
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Han, Hong Shi, and 洪士翰. "Numerical Simulation of Natural Convection in Rectangular." Thesis, 1995. http://ndltd.ncl.edu.tw/handle/40499229883113902830.
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碩士國立中興大學
機械工程研究所
83
The heat transfer and flow field of natural convection in a two dimension rectangular enclosure located at varisus divided broad is calculated numerical by SIMPLE-R algorithm with Power-law Scheme. The governed equations governing the mass momentum ,and energy are casted into the suitable finite difference equations.The flow patten,temperature distribution and heat transfer characteristic is a function of the aspect ratio of computation domain,the broad are calculated in great detail in the present paper.In addition,the effect of Nusselt number and the inclined angle on the enclosure is discussed detailly.The computed results show that the heat transfer effect is improved when the Rayleigh number is increased.The flow patten and Nusselt number are strongly affected by the height and the number of heated broad and the inclined angle.For the same Rayleigh number,the aperature ratio affects the flow field and heat transfer characteristic is affected by aperature ratio in microgravity.
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Mo, Wu Ching, and 吳慶謨. "Combined Heat And Mass Transfer Natural Convection." Thesis, 1995. http://ndltd.ncl.edu.tw/handle/85065528046423709754.
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博士國立中央大學
化學工程研究所
83
In this thesis, the natural convection systems of the (1) uniform wall temperature with uniform wall concentration and of (2)the constant heat flux with uniform wall concentration on the vertical or horizontal flate plate were considered. The proper parameters,including the thermal buoyancy and the solutal buoyancy effects, was submitted for trasformation process. The governing equations of each system were trans- formed to the more easily solvable dimensionless equations. Both air and water were taken as the fluids of analysis medium. The profiles of velocity, temperature, concentration and the transfer coefficients of momemtun, heat, mass are quantitative expressed in the physical meaningful figures. By the analysis of the relative coefficients of heat and mass, we found that both the thermal buoyancy and the solutal buoyancy effects are important in the air system. But,in the water system,the thermal buoyancy effect become more import- ant than solutal buoyancy. For the convenience of engineering application, the very accurate correlations of heat transfer and mass transfer rates are developed for the cases of single and combined buoyancy forces.
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賴昱銘. "Natural convection in two trapezoidal porous cavities." Thesis, 2014. http://ndltd.ncl.edu.tw/handle/80565312779359396580.
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Wang, Chih Yao, and 王志耀. "Buoyancy Driven Natural Convection in Rectangular Enclosures." Thesis, 1993. http://ndltd.ncl.edu.tw/handle/37928480245032572476.
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