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Articles de revues sur le sujet "Confined flow heated from below"
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Joo Sik Yoo, Moon-Uhn Kim et Do H. Choi. « Convective instability of a fluid layer confined in a vertical annulus heated from below ». International Journal of Heat and Mass Transfer 31, no 11 (novembre 1988) : 2285–90. http://dx.doi.org/10.1016/0017-9310(88)90160-3.
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Koizumi, Hiroyoshi. « Flow pattern formation and the transition to chaos in a confined container heated locally from below ». International Journal of Thermal Sciences 46, no 10 (octobre 2007) : 953–62. http://dx.doi.org/10.1016/j.ijthermalsci.2006.12.001.
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Yang, K. T. « Transitions and Bifurcations in Laminar Buoyant Flows in Confined Enclosures ». Journal of Heat Transfer 110, no 4b (1 novembre 1988) : 1191–204. http://dx.doi.org/10.1115/1.3250620.
Texte intégralRésumé :
Recent advances in experimental and numerical studies of flow instability, bifurcation, and transition to turbulence for buoyant flow in three-dimensional rectangular enclosures heated from below and from the sides are reviewed, with emphasis on the physical causes of various instabilities and bifurcations as well as the observed and calculated routes to chaotic motions. Also discussed are the current successes and shortcomings of numerical simulations of experimental data and observations. Finally, unresolved critical issues and needs for future research are also addressed.
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Poulikakos, D. « Natural Convection in a Confined Fluid-Filled Space Driven by a Single Vertical Wall With Warm and Cold Regions ». Journal of Heat Transfer 107, no 4 (1 novembre 1985) : 867–76. http://dx.doi.org/10.1115/1.3247515.
Texte intégralRésumé :
This paper reports heat and fluid flow results which describe the phenomenon of natural convection in an enclosure heated and cooled along a single vertical wall. In the first part of the paper, the case where the side-heating effect is positioned above the side-cooling effect is considered. Numerical simulations and scale analysis show that the temperature field in this configuration transforms from one of incomplete vertical penetration to one of incomplete horizontal penetration depending on the values of the Rayleigh number based on the enclosure height (Ra) and the height-to-length aspect ratio of the enclosure (H/L). The heat transfer scales differ substantially from one type of penetrative convection to the other in agreement with the numerical findings. The parametric domain of validity of the conclusions of this part of the study is outlined on the H/L-Ra plane. When the heated portion of the driving side wall is positioned below the cooled portion the flow spreads throughout the cavity. This configuration results in an enhancement of the overall heat transfer through the enclosure.
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Kessler, R. « Nonlinear transition in three-dimensional convection ». Journal of Fluid Mechanics 174 (janvier 1987) : 357–79. http://dx.doi.org/10.1017/s0022112087000168.
Texte intégralRésumé :
Steady and oscillatory convection in a rectangular box heated from below are studied by means of a numerical solution of the three-dimensional, time-dependent Boussinesq equations. The effect of the rigid sidewalls of the box on the spatial structure and the dynamical behaviour of the flow is analysed. Both conducting and adiabatic sidewalls are considered. Calculated streamlines illustrate the three-dimensional structure of the steady flow with Prandtl numbers 0.71 and 7. The onset and the frequency of the oscillatory instability are calculated and compared with available experimental and theoretical data. With increasing Rayleigh number a subharmonic bifurcation and the onset of a quasi-periodic flow can be observed. A comparison of the different time-dependent solutions shows some interesting relations between the spatial structure and the dynamical behaviour of the confined flow.
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Ferahta, Fatima Zohra, et Cherifa Abid. « Effect of Coupling Radiation Convection on Heat Transfer in the air gap of a Solar Collector ». MATEC Web of Conferences 330 (2020) : 01018. http://dx.doi.org/10.1051/matecconf/202033001018.
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In order to study the effect of convection-radiation coupling occurring in the air gap of a solar thermal collector, numerical simulations were conducted for various thicknesses of the air gap with and without radiation. The studied geometry is a closed cavity which represents the confined space between the absorber and the glass. The cavity is inclined at an angle equal to 45 ° and is uniformly heated from below. The flow is three-dimensional and in unsteady state. First, the simulations were conducted considering only convection in the air gap, in this case the radiation is neglected and in a second time, the coupling between convection and radiation was taken onto account. In the first case the results show that the increase of the air-gap thickness leads to an intensification of the natural convection which develops from laminar, chaotic to turbulent regime. When the radiation is taken into account, the results show that the flow regimes are substantially modified, the convection-radiation coupling reduces the temperature of the hot wall, which contributes to the reduction of the intensity of natural convection in the cavity. This observation is verified by the evolution of the temperature field at the absorber and the heat exchange coefficient. So in conclusion, this study allowed us to see the evolution of heat transfer in the air layer between the glass and the absorber, in the absence and in the presence of radiation. Taking into account the radiation in the cavity is essential for the modeling of flows in a cavity (which is often neglected).
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Kvarving, Arne Morten, Tormod Bjøntegaard et Einar M. Rønquist. « On Pattern Selection in Three-Dimensional Bénard-Marangoni Flows ». Communications in Computational Physics 11, no 3 (mars 2012) : 893–924. http://dx.doi.org/10.4208/cicp.280610.060411a.
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AbstractIn this paper we study Bénard-Marangoni convection in confined containers where a thin fluid layer is heated from below. We consider containers with circular, square and hexagonal cross-sections. For Marangoni numbers close to the critical Marangoni number, the flow patterns are dominated by the appearance of the well-known hexagonal convection cells. The main purpose of this computational study is to explore the possible patterns the system may end up in for a given set of parameters. In a series of numerical experiments, the coupled fluid-thermal system is started with a zero initial condition for the velocity and a random initial condition for the temperature. For a given set of parameters we demonstrate that the system can end up in more than one state. For example, the final state of the system may be dominated by a steady convection pattern with a fixed number of cells, however, the same system may occasionally end up in a steady pattern involving a slightly different number of cells, or it may end up in a state where most of the cells are stationary, while one or more cells end up in an oscillatory state. For larger aspect ratio containers, we are also able to reproduce dislocations in the convection pattern, which have also been observed experimentally. It has been conjectured that such imperfections (e.g., a localized star-like pattern) are due to small irregularities in the experimental setup (e.g., the geometry of the container). However, we show, through controlled numerical experiments, that such phenomena may appear under otherwise ideal conditions. By repeating the numerical experiments for the same non-dimensional numbers, using a different random initial condition for the temperature in each case, we are able to get an indication of how rare such events are. Next, we study the effect of symmetrizing the initial conditions. Finally, we study the effect of selected geometry deformations on the resulting convection patterns.
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Maughan, J. R., et F. P. Incropera. « Secondary flow in horizontal channels heated from below ». Experiments in Fluids 5, no 5 (1987) : 334–43. http://dx.doi.org/10.1007/bf00277712.
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Prasad, V., F. C. Lai et F. A. Kulacki. « Mixed Convection in Horizontal Porous Layers Heated From Below ». Journal of Heat Transfer 110, no 2 (1 mai 1988) : 395–402. http://dx.doi.org/10.1115/1.3250498.
Texte intégralRésumé :
Numerical studies are reported for steady, mixed convection in two-dimensional horizontal porous layers with localized heating from below. The interaction mechanism between the forced flow and the buoyant effects is examined for wide ranges of Rayleigh number Ra* and Peclet number Pe*. The external flow significantly perturbs the buoyancy-induced temperature and flow fields when Pe* is increased beyond unity. For a fixed Peclet number, an increase in Rayleigh number produces multicellular recirculating flows in a domain close to the heat source. This enhances heat transfer by free convection. However, for a fixed Ra*, an increase in forced flow or Peclet number does not necessarily increase the heat transfer rate. Hence, there exists a critical Peclet number as a function of Ra* for which the overall Nusselt number is minimum. The heat transfer is, generally, dominated by the buoyant flows for Pe* < 1 whereas the contribution of free convection is small for Pe* > 10 when Ra* ≤ 10.
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Xia, Chunmei, et Jayathi Y. Murthy. « Buoyancy-Driven Flow Transitions in Deep Cavities Heated From Below ». Journal of Heat Transfer 124, no 4 (16 juillet 2002) : 650–59. http://dx.doi.org/10.1115/1.1481356.
Texte intégralRésumé :
A numerical investigation has been conducted of flow transitions in deep three-dimensional cavities heated from below. The first critical Rayleigh number, RaI, below which the flow is at rest, and the second critical Rayleigh number, RaII, for transition from steady state to oscillatory flow, have been found for cavities of aspect ratios Ar in the range 1–5. Transition to chaos has also been examined for these cases. The results show that RaI=3583,2.545×104 and 5.5×105 and RaII=4.07×105,1.65×106 and 1.30×107 for aspect ratios of 1, 2, and 5 respectively. The route to chaos is PPeriodic→QP2(Quasi-periodic with two incommensurate frequencies)→QP3(Quasi-periodic with three incommensurate frequencies)→NChaotic for Ar=1 with the Rayleigh number varying from 4.07×105 to 4.89×105. The route is PPeriodic→P2(Periodic doubling)→I(Intermittent)→P(Periodic)→N(Chaotic) for Ar=2 over a Ra range of 1.65×106 to 1.83×106. The interval between periodic and chaotic flow is very short for Ar=5.
Thèses sur le sujet "Confined flow heated from below"
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Mehta, Sunil. « Pattern Formation in a Confined Porous Medium Heated From Below ». Thesis, City University London, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.529458.
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Silano, Gabriella. « Numerical simulations of thermal convection at high Prandtl numbers ». Doctoral thesis, Università degli studi di Trieste, 2009. http://hdl.handle.net/10077/3211.
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2007/2008In this thesis we present the results of an extensive campaign of direct numerical simulations of Rayleigh-B\'enard convection at high Prandtl numbers ($10^{-1}\leq Pr \leq 10^4$) and moderate Rayleigh numbers ($10^{5}\leq Pr \leq 10^9$). The computational domain is a cylindrical cell of aspect-ratio (diameter over cell height) $\Gamma=1/2$, with the no-slip condition imposed to the boundaries. By scaling the results, we find a $1/\sqrt{Pr}$ correction to apply to the free-fall velocity, obtaining a more appropriate representation of the large scale velocity at high $Pr$. We investigate the Nusselt and the Reynolds number dependence on $Ra$ and $Pr$, comparing the results to previous numerical and experimental work. At high $Pr$ the scaling behavior of the Nusselt number with respect to $Ra$ is generally consistent with the power-law exponent $0.309$. The Nusselt number is independent of $Pr$, even at the highest $Ra$ simulated. The Reynolds number scales as $Re\sim \sqrt{Ra}/Pr$, neglecting logarithmic corrections. We analyze the global and local features of viscous and thermal boundary layers and their scaling behavior with respect to Rayleigh and Prandtl numbers, and with respect to Reynolds and Peclet numbers. We find that the flow approaches a saturation regime when Reynolds number decreases below the critical value $Re_s\simeq 40$. The thermal boundary layer thickness turns out to increase slightly even when the Peclet number increases. We explain this behavior as a combined effect of the Peclet number and the viscous boundary layer influences. The range of $Ra$ and $Pr$ simulated contains steady, periodic and turbulent solutions. A rough estimate of the transition from steady to unsteady flow is obtained by monitoring the time-evolution of the system until it reaches stationary solutions ($Ra_U\simeq 7.5 \times 10^6$ at $Pr=10^3$). We find multiple solutions as long-term phenomena at $Ra=10^8$ and $Pr=10^3$ which, however, do not result in significantly different Nusselt number. One of these multiple solutions, even if stable for a long time interval, shows a break in the mid-plane symmetry of the temperature profile. The result is similar to that of some non-Boussinesq effects. We analyze the flow structures through the transitional phases by direct visualizations of the temperature and velocity fields. We also describe how the behavior of the flow structures changes for increasing $Pr$. A wide variety of large-scale circulations and plumes structures are found. The single-roll circulation is characteristic only of the steady and periodic solutions. For other solutions, at lower $Pr$, the mean flow generally consists of two opposite toroidal structures; at higher $Pr$, the flow is organized in multi-cell structures extending mostly in the vertical direction. At high $Pr$, plumes detach from sheet-like structures. The different large-scale-structure signatures are generally reflected in the data trends with respect to $Ra$, but not in those with respect to $Pr$. In particular, the Nusselt number is independent of $Pr$, even when the flow structures appear strongly different varying $Pr$. In order to assess the reliability of the data-set we perform a systematic analysis of the error affecting the data. Refinement grid analysis is extensively applied.
---------------------------------------------------------------------------------------- In questa tesi presentiamo i risultati di un'estensiva campagna di simulazioni numeriche dirette della convezione di Rayleigh-B\'enard ad alti numeri di Prandtl ($10^{-1}\leq Pr \leq 10^4$) e moderati numeri di Rayleigh ($10^{5}\leq Pr \leq 10^9$). Il dominio computazionale \`e una cella cilindrica di allungamento (diametro su altezza cella) $\Gamma=1/2$, con condizioni di non-slittamento ai contorni. Scalando i risultati, troviamo una correzione di $1/\sqrt{Pr}$ da applicare alla velocit\`a di caduta libera, ottenendo una rappresentazione pi\`u appropriata della velocit\`a di larga scala ad elevati $Pr$. Investighiamo la dipendenza del numero di Nusselt e del numero di Reynolds da $Ra$ e $Pr$, comparando i risultati con precedenti lavori numerici e sperimentali. Ad elevati $Pr$ il comportamento di scala del numero di Nusselt rispetto a $Ra$ \`e generalmente compatibile con l'esponente di legge di potenza $0.309$. Il numero di Nusselt \`e indipendente da $Pr$, anche per il pi\`u alto $Ra$ simulato. Il numero di Reynolds scala come $Re\sim \sqrt{Ra}/Pr$, a meno di correzioni logaritmiche. Analizziamo le caratteristiche locali e globali degli strati limite viscosi e termici, ed il loro comportamento di scala rispetto ai numeri Rayleigh e Prandtl, e rispetto ai numeri Reynolds e Peclet. Troviamo che il flusso approccia un regime di saturazione quando il numero di Reynolds scende sotto il valore critico $Re_s\simeq 40$. Lo spessore dello strato limite termico comincia a crescere leggermente anche quando in numero di Peclet aumenta. Spieghiamo questo comportamento come un effetto combinato delle influenze del numero di Peclet e dello strato limite viscoso. L'intervallo di $Ra$ e $Pr$ simulato contiene soluzioni stazionarie, periodiche e turbolente. Una stima approssimata della transizione da flusso stazionario a non stazionario \`e ottenuta monitorando l'evoluzione temporale del sistema fino al raggiungimento di soluzioni stazionarie o statisticamente stazionarie ($Ra_U\simeq 7.5 \times 10^6$ a $Pr=10^3$). Troviamo soluzioni multiple come fenomeni di lungo termine a $Ra=10^8$ e $Pr=10^3$ che, comunque, non comportano differenze significative nel numero di Nusselt. Una di queste soluzioni multiple, anche se stabile per un lungo intervallo di tempo, mostra una rottura della simmetria del profilo di temperatura rispetto al piano mediano. Il risultato \`e simile a quello di alcuni effetti di non-Boussinesq. Analizziamo le strutture del flusso nelle fasi di transizione tramite visualizzazioni dirette dei campi di velocit\`a e temperatura. Descriviamo inoltre come il comportamento delle strutture del flusso cambia al crescere di $Pr$. Un'ampia variet\`a di circolazioni di larga scala e strutture a pennacchio vengono trovate. La circolazione a singolo anello \`e caratteristica solo delle soluzioni stazionarie e periodiche. Per le altre soluzioni, a $Pr$ pi\`u bassi, il flusso medio \`e generalmente composto da due strutture toroidali opposte; a $Pr$ pi\`u alti, il flusso \`e organizzato in strutture multi-cellulari che si estendono maggiormente in direzione verticale. Ad alti $Pr$, pennacchi si staccano da strutture simili a fogli. Le impronte delle differenti strutture di larga scala si riflettono generalmente nell'andamento dei dati rispetto a $Ra$, ma non rispetto a $Pr$. In particolare, il numero di Nusselt \`e indipendente da $Pr$, anche quando le strutture del flusso appaiono molto differenti al variare di $Pr$. Per stabilire l'affidabilit\`a dell'insieme dei dati, effettuiamo un'analisi sistematica degli errori a cui i dati sono soggetti. L'analisi di raffinamento della griglia \`e largamente applicata.
XXI Ciclo
1976
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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.
Texte intégralRésumé :
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.
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Liu, Li-kang, et 劉立崗. « Fluid Flow and Heat Transfer from Heated Extended Surfaces with Confined Slot Jet Impingement ». Thesis, 2004. http://ndltd.ncl.edu.tw/handle/26334066493416053938.
Texte intégralRésumé :
博士國立清華大學
動力機械工程學系
92
A series of experimental studies on the fluid flow and heat transfer characteristics from unconfined/confined heated smooth or extended surfaces by using different cooling methods have been performed. Two types of cooling methods such as purely natural convection and mixed convection due to slot jet impingement and buoyancy are employed in the present study. The relevant parameters influencing fluid flow and heat transfer performance in natural convection and slot jet impingement studies are listed, respectively. They are: (1) natural convection － the steady-state Grashof number (Grs), ratio of jet separation distance to nozzle width (H/W) and ratio of extended surface height to nozzle width (Hes/W). The ranges of these parameters studied are Grs=3.37x105~1.26x106, H/W=1-10 and Hes/W=0.74-3.40. (2) slot jet impingement － the steady-state Grashof number (Grs), ratio of jet separation distance to nozzle width (H/W), ratio of extended surface height to nozzle width (Hes/W) and jet Reynolds number (ReD). The ranges of these parameters studied are Grs=3.52x105~5.63x105, H/W=1-10, Hes/W=0.74-3.40 and ReD=57-1411. Their effects on fluid flow and heat transfer characteristics in natural convection and slot jet impingement have been systematically explored. For unconfined/confined smooth or extended surfaces with natural convection cooling, the transient-/steady-state local and average heat transfer characteristics are studied. The results manifest that the maximum transient/steady-state local Nusselt number exists in the region near the edge of the heated smooth or extended surface, and the transient/steady-state local Nusselt number decreases along the distance from the surface edge toward the surface center. The transient/steady-state local and average Nusselt number increases with increasing Grs, H/W or Hes/W. The effects of Grs, H/W and Hes/W on the Nuns/Nuns,o distribution are not significant; and the Nuns/Nuns,o distribution can be expressed as a generalized profile, which is independent of Grs, H/W and Hes/W. By the statistical sensitivity analysis of ANOVA F-test, Grs has the most significant effect on steady-state average Nusselt number. In addition, new correlations of steady-state average Nusselt number in terms of relevant influencing parameters for unconfined/confined smooth or extended surfaces are presented. In the hydrodynamic aspect for confined smooth or extended surfaces with slot jet impingement, the fluid flow characteristics including the local mean streamwise velocity distribution, local turbulence intensity distribution, mean streamwise velocity decay and turbulence intensities along jet centerline are investigated. From the results, the local mean streamwise velocity distributions at Z/W=1 are not significantly affected by the H/W for the cases of H/W 8 at low jet Reynolds numbers, say ReD< 910; while they will be close to a uniform profile when the H/W ratio decreases from H/W=8 to H/W=2, especially for the cases at ReD≥ 910. The local mean streamwise velocity distributions at are significantly influenced by Hes/W, and they will become more broadly as the Hes/W ratio increases. Besides, For all the cases without or with a target surface at ReD< 910, the local turbulence intensities at Z/W=1 are less than 5% in the region of 0 x/W< 0.5. As the jet Reynolds number progressively increases to a higher value, say ReD≥ 910, the local turbulence intensities will sharply increase at x/W> 0.3. In the heat transfer aspect for confined smooth or extended surfaces with slot jet impingement, the transient-/steady-state local and average heat transfer characteristics are successively explored. The results reveal that the highest heat transfer during the transient period occurs at the surface center of confined heated smooth or extended surface. The transient local Nusselt number decreases along the distance from the surface center toward the surface edge. As for exploring the effects of Grs, H/W, Hes/W and ReD, the transient/steady-state local and average Nusselt numbers are almost independent of Grs; and they are more significantly affected by ReD and Hes/W as compared with H/W, and increase with increasing ReD or Hes/W. Maximum local and average Nusselt numbers can be found between H/W=3 and H/W=5 for confined smooth surfaces. For confined extended surfaces, the transient/steady-state local and average Nusselt number decrease monotonically with increasing H/W. Furthermore, the effects of Grs, H/W and Hes/W on the Nums/Nums,o distribution are insignificant; and the Nums/Nums,o distribution can be expressed as a generalized profile, which is only dependent of ReD. The Nums/Nums,o distributions will become locally independent and the Nums/Nums,o ratio will keep at unity at ReD=85 and 162 for confined smooth and extended surfaces, respectively. By the statistical sensitivity analysis of ANOVA F-test, ReD has the most significant effect on steady-state average Nusselt number. Finally, two new composite correlations of steady-state average Nusselt number for mixed convection from confined smooth or extended surfaces due to slot jet impingement and buoyancy are presented, respectively.
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李岳勳. « Numerical simulation of convective air flow structures in a vertical cylinder heated from below and flow stabilization by axial cylinder rotation ». Thesis, 1997. http://ndltd.ncl.edu.tw/handle/55103668235026710359.
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Shu, Ding-Shi, et 許丁士. « Buoyancy Driven Vortex Flow Patterns in Mixed Convection of Air through a Blocked Horizontal Flat Duct Heated from below ». Thesis, 2001. http://ndltd.ncl.edu.tw/handle/37776667551793057755.
Texte intégralRésumé :
碩士國立交通大學
機械工程系
89
Buoyancy Driven Vortex Flow Patterns in Mixed Convection of Air through a Blocked Horizontal Flat Duct Heated from below Student: Ding-Shi Shu Advisor: Prof. Tsing-Fa Lin Institute of mechanical Engineering National Chiao Tung University ABSTRACT Experimental flow visualization combined with transient temperature measurement are carried out here to study the possible stabilization of the buoyancy driven vortex flow in mixed convection of air in a bottom heated horizontal flat duct by placing a rectangular solid block on the duct bottom. Two acrylic blocks having dimensions 40x20x5 mm3 (block A) and 40x20x10 mm3 (block B)are tested. The blocks are placed on the longitudinal centerline of the duct bottom at selected locations. How the location and orientation of the rectangular block affect the stability of the vortex flow is investigated in detail. An open loop mixed convective appratus established earlier by Yu et al.[17] was chosen in this investigation and the test section is a high aspect ratio (A=12) rectangular duct. Experiments are conducted for the Reynolds number varying from 3 to 30 and Rayleigh number from 3,000 to 6,000, covering a wide range of the buoyancy-to-inertia ratio. For longitudinal vortex flow, the presence of the blocks placed near the duct entry causes the onset points of the longitudinal rolls to move significantly upstream especially for the roll pair directly behind the block. Besides, the longitudinal vortex flow in the exit portion of the duct is destabilized by the block. The transverse vortex flow is found to be only slightly affected by the block when it is placed in the exit half of the duct. There is significant deformation of the transverse rolls as they pass over the block. However, they restore to their regular shape in a short distance. Significant decay in the flow oscillation is noted in the region right behind the block. Elsewhere the flow oscillates at nearly the same frequency and amplitude as that in the unblocked duct. When the block is placed near the duct entry the vortex flow is significantly changed. Stabilization of the vortex flow behind the block is more pronounced. This flow stabilization is more prominent for block B with its height being twice of block A. Placing the block with its longsides normal to the flow direction can also enhance the flow stabilization. More specifically, behind block B we have steady longitudinal rolls. For mixed vortex flow, placing the block near the duct inlet causes the transverse rolls to change to regular or deformed longitudinal rolls in the duct depending on the buoyancy-to-inertial ratio and orientation of the block. The flow stabilization by the block is substantial. Again the flow stabilization can be enhanced by increasing the block height and placing the block with its longsides normal to the forced flow direction.
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wang, Yi-Lung, et 王壹龍. « Vortex Flow and Thermal Characteristics Resulting from a Confined Round Jet of Air Impinging onto a Large Heated Horizontal Disk ». Thesis, 2008. http://ndltd.ncl.edu.tw/handle/09323225080355931040.
Texte intégralRésumé :
碩士國立交通大學
機械工程系所
96
An experiment is carried out in the present study to investigate the vortex flow patterns resulting from a round air jet impinging over a large confined heated horizontal circular disk of 16 inch in diameter. In the present experiment the jet flow rate is varied from 0.4 to 10.0 (standard liter per minute) for the jet Reynolds number ranging from 27 to 676 with the injection pipe diameter Dj = 20.0 mm and jet-disk separation distance H = 12.5mm and 25.0 mm. The temperature difference between the disk and the air injected at the inlet of the jet is varied from 0 to 14.0℃ for the Rayleigh number Ra ranging from 0 to 7,340. The results from the flow visualization for H=25.0 mm indicate that the vortex flow is characterized by the primary and/or secondary inertia-driven circular rolls along with several buoyancy-driven rolls. The number and strength of the buoyancy induced rolls increase with the buoyancy-to-inertia ratio, which can vary from 1 to 5. Besides, at a low Gr/Rej2 slightly deformed circular buoyancy rolls prevail in the test section. For slightly higher Gr/Rej2 the flow is dominated by the highly deformed curved rolls. The roll pattern becomes somewhat irregular for a further increase in Gr/Rej2 . For all cases examined here the vortex flow is unsteady at long time after the initial transients have died out and is irregular to a certain degree since the buoyancy effect is rather strong for the large impinging plate tested here. In addition to the roll deformation, the mutual pushing, merging and splitting of the buoyancy rolls occur nonperiodically in time. This unsteady and irregular vortex flow is also reflected in the data for the air temperature variations with time. Some quantitative buoyancy-driven vortex flow characteristics such as the size and location of the buoyancy rolls are summarized and correlated empirically. Moreover, a flow regime map is provided to delineate various induced vortex flow patterns and the boundaries separating various vortex flow patterns are also correlated empirically. When the jet-disk separation distance is reduced to 12.5 mm for ΔT varied from 8.0℃ to 14.0℃ ( corresponding to the Rayleigh number ranging from 1,470 to 2,670 ), circular waves traveling in the radial direction and radial vortex rolls appear in the confined impinging jet flow. These waves are somewhat deformed and are not axisymmetric. The radial rolls originate in the stagnation region of the impinging jet and grow in size in the radial direction. The size of the radial rolls is also nonuniform in the circumferential direction. Besides, the moving waves and radial rolls can coexist in the flow. Moreover, the ranges of the jet Reynolds number and Rayleigh numbers leading to the new vortex flow patterns are determined. Flow regime maps delineating various vortex flow patterns including all inertia- and buoyancy-driven roll and wave patterns found here are given. And the boundaries among various vortex flow patterns are empirically correlated.
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Wu, Jia-Hong, et 吳佳鴻. « Characteristics of Unstable Vortex Flow Resulting from a Round Jet of Air Impinging onto a Heated Horizontal Disk Confined in a Vertical Cylindrical Chamber ». Thesis, 2004. http://ndltd.ncl.edu.tw/handle/2x3f86.
Texte intégralRésumé :
碩士國立交通大學
機械工程系所
92
An experiment combining flow visualization and temperature measurement is carried out in the present study to investigate the possible presence of new inertia-driven vortex rolls and some unique characteristics of the time-dependent mixed convective vortex flow resulting from a high speed round air jet impinging onto a heated horizontal circular disk in a vertical cylindrical chamber. The flow photos taken from the side and top views of the vortex flow in the chamber aim to unravel these new vortex flow characteristics. In the present experiment the jet-to-disk separation distance is varied from 10.0 to 30.0 mm and the jet flow rate is varied from 0 to 12.0 slpm (standard liter per minute) for the jet Reynolds number Rej ranging from 0 to 1,623. The temperature difference between the disk and the air injected into the chamber is varied from 0 to 25.0℃ for the Rayleigh number Ra ranging from 0 to 63,420. The results from the flow visualization clearly show that at sufficiently high Rej the inertia-driven tertiary and quaternary rolls can be induced. At even slightly higher Rej the vortex flow becomes unstable due to the inertia-driven flow instability. Only for H=20.0 mm the flow is subjected to the buoyancy-driven instability. Because of the simultaneous presence of the inertia- and buoyancy-driven instabilities, a reverse flow transition can take place in the chamber with H=20.0 mm. At the large H of 30.0 mm the flow unsteadiness results from the mutual pushing and squeezing of the inertia- and buoyancy-driven rolls since they are relatively large and contact with each other. It is also noted that the critical jet Reynolds number for the onset of tertiary and quaternary rolls increase with for H=10.0 & 20.0 mm. But for H=30.0 mm the opposite is true, indicating that raising can destabilize the vortex flow.
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Lo, Wen-Hsien, et 羅文賢. « Suppression of Vortex Flow Resulting from a Round Jet of Air Impinging onto a Heated Horizontal Disk Confined in a Vertical Cylindrical Chamber by Disk Rotation ». Thesis, 2005. http://ndltd.ncl.edu.tw/handle/49780753298375342957.
Texte intégralRésumé :
碩士國立交通大學
機械工程系所
93
An experiment combining flow visualization and temperature measurement is carried out in the present study to explore the possible suppression of the buoyancy-driven stable and unstable vortex flow resulting from a round jet of air impinging onto a heated horizontal disk in a vertical cylindrical chamber by the disk rotation. In this study the experiment is mainly conducted for the jet flow rate varied from 0 to 12.0 slpm (standard liter per minute) with two different injection pipes (diameter 10.0 and 22.1 mm) and the temperature difference between the disk and the air injected into the chamber is varied from 0 to 25.0℃ at a fixed jet-to-disk separation distance of 20.0 mm. The disk rotation speed is varied from 0 to 50 rpm. Thus the jet Reynolds number, Rayleigh number and rotational Reynolds number range respectively from 0 to 1,623, from 0 to 18,790, and from 0 to 3,892. The results from the flow visualization clearly show that typically the steady mixed convective air jet impinging onto the rotating disk consists of three circular vortex rolls. The inner vortex roll is generated by the deflection of the impinging jet at the disk surface and hence termed as the inertia-driven vortex roll. The middle vortex roll is mainly formed by the centrifugal pumping action produced by the disk rotation and hence termed as the rotation-induced roll. The buoyancy-induced vortex roll resulting from the temperature difference between the heated disk and the inlet air prevails in the outer zone of the processing chamber. At a high disk rotation rate, the buoyancy roll can be significantly suppressed and even wiped out by the disk rotation. Besides, the primary inertia-driven roll is stretched out to become slender and weaker. Moreover, the inertia-driven secondary, tertiary, and quaternary rolls dominated at high Rej can be entirely wiped out. We further note that the disk rotation can effectively suppress the inertia-driven and/or buoyancy-driven time-dependent and nonperiodic vortex flows. The unstable vortex flows can be completely stabilized by the disk rotated at a high speed and the flows become steady. The disk rotation can also reduce the radial temperature variation in the flow and significantly delay the onset of the buoyancy-driven roll. Based on the present data, a flow regime map is provided to delineate the the axisymmetric and nonaxisymmetric vortex flows with various disk rotation rates for H=20.0 mm.
Chapitres de livres sur le sujet "Confined flow heated from below"
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Gholijani, Alireza, Sebastian Fischer, Tatiana Gambaryan-Roisman et Peter Stephan. « High Resolution Measurements of Heat Transfer During Drop Impingement onto a Heated Wall ». Dans Fluid Mechanics and Its Applications, 291–310. Cham : Springer International Publishing, 2022. http://dx.doi.org/10.1007/978-3-031-09008-0_15.
Texte intégralRésumé :
AbstractDrop impact on a hot surface heated above the saturation temperature of the fluid plays an important role in spray cooling. The heat transferred from the wall to the fluid is closely interrelated with drop hydrodynamics. If the surface temperature is below the Leidenfrost temperature, the heat transport strongly depends on the transport phenomena in the vicinity of the three-phase contact line. Due to extremely high local heat flux, a significant fraction of the total heat flow is transported through this region. The local transport processes near the three-phase contact line, and, therefore, the total heat transport, are determined by the wall superheat, contact line velocity, system pressure, fluid composition, surface structure and physical properties on the wall. The effect of the aforementioned influencing parameters on fluid dynamics and heat transport during evaporation of a single meniscus in a capillary slot are studied in a generic experimental setup. The hydrodynamics and evolution of wall heat flux distribution during the impact of a single drop onto a hot wall are also studied experimentally by varying the impact parameters, wall superheat, system pressure, and wall topography. In addition, the fluid dynamics and heat transport behavior during vertical and horizontal coalescence of multiple drops on a heated surface are studied experimentally.
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Warrick, Arthur W. « Saturated Flow ». Dans Soil Water Dynamics. Oxford University Press, 2003. http://dx.doi.org/10.1093/oso/9780195126051.003.0008.
Texte intégralRésumé :
Saturated conditions generally exist below a water table, either as part of the permanent groundwater system (aquifer) or in the vadose zone as perched water. For isotropic and steady-state conditions, such systems can be modeled by Laplace’s equation. Because it is linear, Laplace’s equation is much easier to solve than the variably saturated forms of Richards’ equation and, hence, provides a convenient place to begin. Analyses of water flow for drainage and groundwater systems borrow heavily from the classical (and old!) work in heat conduction, hydrodynamics, and electrostatics. This section presents analytical solutions for subsurface drainage and well discharge in fully penetrating confined aquifers (the solutions are the same). Included are the definition of stream functions and demonstrations of the Cauchy–Riemann relations. A comparable numerical solution is presented, and also for the ponded drainage and well discharge, and the results compared with the analytical solutions. A more complex example is then presented concerning drainage below a curved water table. These results are followed by travel-time calculations relevant to solute movement from the soil surface to a drainage system. A short section covering analytical techniques with three-dimensional images is then given, followed by a section covering additional topics, which includes a complex image example (two dimensional) and some relationships for Fourier series. Consider a point source in a two-dimensional x—y plane, as in figure 3-1. The origin corresponds to a source that is assumed to be an infinite line perpendicular to the x—y plane. If the steady flow rate is Q, then the conservation of mass results in . . . Q = Jr(2πr) (3-1) . . . where Jr is the Darcian flow in the r direction and evaluated at a polar radius r. The dimensions of Q are [L2T-1] corresponding to a volume of flow per unit time from a unit length of the line perpendicular to the x—y plane. Values of Q are taken to be positive for water entering the system.
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Whiteman, C. David. « Diurnal Mountain Winds ». Dans Mountain Meteorology. Oxford University Press, 2000. http://dx.doi.org/10.1093/oso/9780195132717.003.0019.
Texte intégralRésumé :
Diurnal mountain winds develop over complex topography of all scales, from small hills to large mountain massifs and are characterized by a reversal of wind direction twice per day. As a rule, winds flow upslope, up-valley, and from the plain to the mountain massif during daytime. During nighttime, they flow downslope, down-valley, and from the mountain massif to the plain. Diurnal mountain winds are strongest when skies are clear and winds aloft are weak. Diurnal mountain winds are produced by horizontal temperature differences that develop daily in complex terrain. The resulting horizontal pressure differences cause winds near the surface of the earth to blow from areas with lower temperatures and higher pressures toward areas with higher temperatures and lower pressures. The circulations are closed by return, or compensatory, flows higher in the atmosphere. Four wind systems comprise the mountain wind system, which carries air into a mountain massif at low levels during daytime and out of a mountain massif during nighttime. The slope wind system (upslope winds and downslope winds) is driven by horizontal temperature contrasts between the air over the valley sidewalls and the air over the center of the valley. The along-valley wind system (up-valley winds and down-valley winds) is driven by horizontal temperature contrasts along a valley’s axis or between the air in a valley and the air over the adjacent plain. The cross-valley wind system results from horizontal temperature differences between the air over one valley sidewall and the air over the opposing sidewall, producing winds that blow perpendicular to the valley axis and toward the more strongly heated sidewall. The mountain-plain wind system results from horizontal temperature differences between the air over a mountain massif and the air over the surrounding plains, producing large-scale winds that blow up or down the outer slopes of a mountain massif. The mountain-plain circulation and its upper level return flow are not confined by the topography but are carried over deep layers of the atmosphere above the mountain slopes. Because diurnal mountain winds are driven by horizontal temperature differences, the regular evolution of the winds in a given valley is closely tied to the thermal structure of the atmospheric boundary layer within the valley, which is characterized by a diurnal cycle of buildup and breakdown of a temperature inversion.
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Erman, Burak, et James E. Mark. « Critical Phenomena and Phase Transitions in Gels ». Dans Structures and Properties of Rubberlike Networks. Oxford University Press, 1997. http://dx.doi.org/10.1093/oso/9780195082371.003.0009.
Texte intégralRésumé :
The term “gel” has been used in a wide variety of contexts, and there have been difficulties in reaching an all-inclusive, workable definition for it. Perhaps the simplest way to proceed is to list some of its most important characteristics: It is a solidlike material that when deformed responds in the manner of a typical elastic body, but generally with a very small modulus. If it does show plastic flow, then this occurs above a threshold value of the stress, with full recoverability below this limit. It typically consists of two or more components: one a liquid in substantial quantity, and the other generally a polymeric network. One of the most direct ways of obtaining a gel is to place a network into a solvent known to be capable of dissolving the network chains in the absence of cross-links. In fact, a unique property of a highly extensible elastomer (resulting from a low degree of cross-linking) is its ability to swell greatly when exposed to a good solvent. A gel with less than 10-6 mol cm-3 of cross-links, for example, may increase its volume more than thousandfold when immersed in a suitable solvent. The extent to which such a network will swell depends specifically not only on the degree of cross-linking, but also on the interactions between the chains and the solvent. While the degree of cross-linking is established during the preparation of a network, the extent of the interaction of chains and solvent may be modified as desired, and therefore the degree of swelling may be controlled. A gel can be made to swell or shrink continuously by changing the quality of the solvent with which it is in contact. Alternatively, it may go through critical conditions and, in fact, can exhibit phase transitions, depending on the type of the polymer-solvent interaction and the extent of cross-linking. The discrete shrinkage of the gel, by changing the polymer-solvent interaction parameter, is a volume phase transition similar to the gas-liquid transition of a condensing gas. The possibility of such phase transitions was, notably, first discussed by Dusek and collaborators many years ago. Their treatment was confined to nonionic networks.
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« * ** Fig. 39 Cyclone-type homogenizer mixing chamber. (From Ref. 41.) chamber. The symmetry axes of these entry ports are perpendicular to the symmetry axis of the interaction chamber. This design is presented in Fig. 40, with only four entry ports. This machine is called Novamix® (a registered name for Micro Vesicular Sys-tems). It was originally designed to process and produce nonphospholipid lamellar mi-crostructures or lipid vesicles. The lipid vesicles are composed of two immiscible aqueous and lipid phases. The lipid phase consists, generally, of solid polyoxyethylene-derived amphiphiles that form micelles in aqueous media. Under the proper mixing conditions, i.e., a combination of shear, heat, and turbulence, followed by appropriate cooling, the micelles of these types of lipids fuse to form lipid vesicles. The two phases are metered carefully and heated in separate reservoirs and finally pumped to the interaction chamber for pro-cessing. The interaction chamber and pump heads are confined in an insulated com-partment that is maintained at the required temperature for the production of the lipid vesicles. The outlet is attached to a chilling device that cools the product at the required rate [43]. The flow pattern is similar to that of a cyclone, i.e., the flow of liquid is in a vertically positioned rotating cylinder along its vertical axis. The streamlines are con-centric circles with their radii decreasing toward the center of the cylinder. The de-crease is a function of cylinder radius, flow rate of fluid (speed of rotation), and other parameters like viscosity, density, and surface tension of the formulation. In curved type of flow with changing radii, there exists a pressure gradient, i.e. dPIdr = V /r (8) where P = pressure ; r = vessel (interaction chamber) radius ; V = tangential linear velocity ; and p= the liquid density. Since the change in pressure is positive for a positive radius change, the pressure at successive points increases from the concave to the convex side of the streamline [39]. The exact change in pressure depends on the variation in tangential linear velocity, which is proportional to the speed of the rotation and the ra-dius. The flow pattern in the interaction chamber is neither a free vortex, due to the presence of an initial momentum from the pumps, nor a forced vortex, for the stream- ». Dans Pharmaceutical Dosage Forms, 368–69. CRC Press, 1998. http://dx.doi.org/10.1201/9781420000955-55.
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« measurements. This paper is confined to the different forms of sampling odourous gases for olfactometric measurements and the problems involved. It refers to existing guidelines for olfactometric measurements in the countries of the EEC, as well. 2. TYPES OF SAMPLING Samples of odourous gas may be collected in unconcentrated or concentrated form. Concentrated sampling is usually neces sary when gas chromatography or other chemical analytical meth ods are to be used. Unconcentrated sampling is provided if o-dour threshold concentrations are required (2). Depending on the type of olfactometer used dynamic sam pling or static sampling are provided. The principle of dynam ic sampling is shown in Figure 1. It requires a part-flow of the odourous gas to be continoulsy extracted from the source and subsequently directed to the olfactometer. This sampling method implies that the measurements are carried out close to the source. An advantage of the method is that there is the possibility of controlling a process, directly, and in case of the break-down of the process this can be noticed right away. A disadvantage of the dynamic method is that odour sources that are not readily accessible require a relatively great ef fort in order to install the olfactometer and suitable sam pling pipes which often should be insulated or heated to avoid adsorption or condensation (3). When static sampling is used a partial stream of the o-dourous air is collected in a sampling vessel. Samples are taken from this vessel or bag to dilute the odourous air for the olfactometer using syringes or on-line tubings. When using this method odour measurement with the panel can be carried out at any arbitrary location, if the vessel is a transport able one. An example for static sampling is given in Figure 2. 3. PROBLEMS OF SAMPLING the main problems encountered when sampling odourous air derive from surface effects of the sampling tubes and vessels, namely by - adsorption, - desorption, and - condensation. This depends mainly on the material of the tube, the vessel or the bag (adsorption) or on the nature of the gas, whether it is hot and/or containes a high amount of humidity (condensa tion). On the other hand the sample can be altered by trace components bleeding from the material of the walls of the ves sel or the tube (desorption). The following factors are to be observed for valid static sampli ng. aTTTToTce of_m£teri aj_ For tWe sampling of odourous gases glas vessels, stain less steel tanks (4) and flexible plastic bags (5) were tested. The initial concentrations of the test gases decrease consider ably with storage time in glass and steel vessels. In recent years bags made of Polyethylene(6), Teflon (3) and Tedlar (7), (8) were usually used. Figure 3 shows a graph from SCHUETZLE ». Dans Odour Prevention and Control of Organic Sludge and Livestock Farming, 59. CRC Press, 1986. http://dx.doi.org/10.1201/9781482286311-18.
Texte intégralActes de conférences sur le sujet "Confined flow heated from below"
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Ridouane, El Hassan, et Antonio Campo. « Time-Depending Pitchfork Bifurcation in the Thermal Convection Flow Confined to an Isosceles Triangular Cavity Heated from Below ». Dans 9th AIAA/ASME Joint Thermophysics and Heat Transfer Conference. Reston, Virigina : American Institute of Aeronautics and Astronautics, 2006. http://dx.doi.org/10.2514/6.2006-3776.
Texte intégral
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Bagchi, Aniruddha, et Francis A. Kulacki. « Natural Convection in Horizontal Fluid-Superposed Porous Layers Heated Locally From Below ». Dans ASME 2010 International Mechanical Engineering Congress and Exposition. ASMEDC, 2010. http://dx.doi.org/10.1115/imece2010-39604.
Texte intégralRésumé :
This paper reports numerical studies of steady two-dimensional natural convection in fluid-superposed porous layers heated locally from below. The numerical simulation is based on the Darcy-Brinkman-Forchheimer model for the porous layer and focuses on the parametric domain in which the flow is well established, i.e., the overall Rayleigh number is several orders of magnitude larger than the critical value. An emphasis is placed on revealing the effects of two dimensionless parameters on the overall Nusselt number: the porous layer-to-cavity height ratio (η = Hm/H) and the heater-to-cavity base length ratio (δ = LH/L). Calculations cover η = 0.25, 0.5, 0.75, δ = 0.25, 0.5, 1, and overall Rayleigh numbers from 103 to 106. For a fixed height ratio, overall Nusselt numbers increase with a decrease in the heater length. For a given heater length ratio, overall Nusselt number increases with an increase in the height of the overlying fluid layer. Recirculating flow is confined primarily to the overlying fluid layer with some penetration into the upper part of the porous layer. The present results represent an extension of the well studied problem of buoyant convection in superposed layers with a fully heated lower surface.
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Li, Yanjun, Ya-Ting T. Liao et Paul Ferkul. « Concurrent-Flow Flame Spread Over a Thin Solid in a Narrow Confined Space in Microgravity ». Dans ASME 2019 International Mechanical Engineering Congress and Exposition. American Society of Mechanical Engineers, 2019. http://dx.doi.org/10.1115/imece2019-11908.
Texte intégralRésumé :
Abstract A numerical study is pursued to investigate the aerodynamics and thermal interactions between a spreading flame and the surrounding walls as well as their effects on fire behaviors. This is done in support of upcoming microgravity experiments aboard the International Space Station. For the numerical study, a three-dimensional transient Computational Fluid Dynamics combustion model is used to simulate concurrent-flow flame spread over a thin solid sample in a narrow flow duct. The height of the flow duct is the main parameter. The numerical results predict a quenching height for the flow duct below which the flame fails to spread. For duct heights sufficiently larger than the quenching height, the flame reaches a steady spreading state before the sample is fully consumed. The flame spread rate and the pyrolysis length at steady state first increase and then decrease when the flow duct height decreases. The detailed gas and solid profiles show that flow confinement has competing effects on the flame spread process. On one hand, it accelerates flow during thermal expansion from combustion, intensifying the flame. On the other hand, increasing flow confinement reduces the oxygen supply to the flame and increases conductive heat loss to the walls, both of which weaken the flame. These competing effects result in the aforementioned non-monotonic trend of flame spread rate as duct height varies. This work relates to upcoming microgravity experiments, in which flat thin samples will be burned in a low-speed concurrent flow using a small flow duct aboard the International Space Station. Two baffles will be installed parallel to the fuel sample (one on each side of the sample) to create an effective reduction in the height of the flow duct. The concept and setup of the experiments are presented in this work.
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Kruger, Sunita, et Leon Pretorius. « Numerical Investigation of Natural Convection in a Mono-Span Greenhouse ». Dans ASME 2012 11th Biennial Conference on Engineering Systems Design and Analysis. American Society of Mechanical Engineers, 2012. http://dx.doi.org/10.1115/esda2012-82365.
Texte intégralRésumé :
In this paper, the use of computational fluid dynamics is evaluated as a design tool to investigate the indoor climate of a confined greenhouse. The finite volume method using polyhedral cells is used to solve the governing mass, momentum and energy equations. Natural convection in a cavity corresponding to a mono-span venlo-type greenhouse is numerically investigated using Computational Fluid Dynamics. The CFD model is designed so as to simulate the climate above a plant canopy in an actual multi-span greenhouse heated by solar radiation. The aim of this paper is to investigate the influence of various design parameters such as pitch angle and roof asymmetry and on the velocity and temperature patterns inside a confined single span greenhouse heated from below. In the study reported in this paper a two-dimensional CFD model was generated for the mono-span venlo-type greenhouse, and a mesh sensitivity analysis was conducted to determine the mesh independence of the solution. Similar two-dimensional flow patterns were observed in the obtained CFD results as the experimental results reported by Lamrani et al [2]. The CFD model was then modified and used to explore the effect of roof pitch angle and roof asymmetry at floor level on the development of the flow and temperature patterns inside the cavity for various Rayleigh numbers. Results are presented in the form of vector and contour plots. It was found that considerable temperature and velocity gradients were observed in the centre of the greenhouse for each case in the first 40mm above the ground, as well as in the last 24mm close to the roof. Results also indicated that the Rayleigh number did not have a significant impact on the flow and temperature patterns inside the greenhouse, although roof angle and asymmetry did. The current results demonstrate the importance of CFD as a design tool in the case of greenhouse design.
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Choi, Chang K., et MinChan Kim. « BUOYANCY EFFECTS IN PLANE COUETTE FLOW HEATED UNIFORMLY FROM BELOW ». Dans International Heat Transfer Conference 10. Connecticut : Begellhouse, 1994. http://dx.doi.org/10.1615/ihtc10.3230.
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Rhee, Hyop S., Christopher J. Freitas et Robert L. Street. « Experimental and Numerical Investigation of a Natural Convection Flow Heated from Below ». Dans International Heat Transfer Conference 8. Connecticut : Begellhouse, 1986. http://dx.doi.org/10.1615/ihtc8.2810.
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Remillieux, Marcel C. « Experimental Study on the Stabilization of the No-Motion State in the Rayleigh-Benard Convection Problem ». Dans ASME 2006 International Mechanical Engineering Congress and Exposition. ASMEDC, 2006. http://dx.doi.org/10.1115/imece2006-16350.
Texte intégralRésumé :
We demonstrate experimentally that through the use of proportional-differential control, it is possible to stabilize the no-motion state of a fluid layer heated from below, cooled from above, and confined in an upright, circular cylinder (the Rayleigh-Be´nard problem). An array of 24 independently controlled heaters (thermal actuators), microfabricated on a silicon wafer, constitutes the bottom boundary of the test cell. A cooling system maintains the top boundary at a constant temperature. Silicon diodes located at the mid-height of the cell, above the actuators, measure the fluid's temperature. The multi-input, multi-output controller adjusts the heaters' power in proportion to the deviation of the fluid's temperatures, as recorded by the diodes, from preset values associated with the no-motion, conductive state. First, a set of experiments was conducted in the absence of a controller to determine the uncontrolled, reference state. Advantage is taken of the linear dependence of the mid-height temperature on the power input in the no-motion state. The preset temperatures are determined by extrapolating the mid-height temperatures to the desired input power values. A proportional controller is then engaged. We show that as the controller's gain increases so does the critical Rayleigh number for the onset of convection. The proportional controller allows us to increase the critical Rayleigh number by as much as a factor of 1.4. When the controller's gain is larger than a critical value, the system becomes time-wise oscillatory (Hopf bifurcation) and the controller's performance deteriorates. The oscillatory convection can be significantly damped out by engaging a proportional-differential (PD) controller. The PD controller allows us to further increase the critical Rayleigh number for the onset of convection to as much as a factor or 1.7 compared to the uncontrolled case. Further increases in the critical Rayleigh number were not possible due to the actuators' saturation. We also compared the supercritical flow patterns at the mid-height of the test cell in the presence of the controller with the flow patterns in the absence of a controller. The proportional controller modified the flow pattern from a single convective cell with ascending fluid in one half of the cell and descending in the other half, to fluid ascending at the center of the cell and descending at near the lateral wall. Our work represents an improvement over previous experimental investigations on the stabilization of Rayleigh-Be´nard convection in which the critical Rayleigh number was increased by only a factor of 1.2. Almost uniform temperature distribution at the mid-height is obtained through the combined action of proportional and derivative controllers. The Rayleigh-Be´nard convection is suppressed under conditions when, in the absence of a controller, flow would persist.
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Choi, C. K., С B. Shin et S. T. Hwang. « THERMAL INSTABILITY IN THERMAL ENTRANCE REGION OF PLANE COUETTE FLOW HEATED UNIFORMLY FROM BELOW ». Dans International Heat Transfer Conference 8. Connecticut : Begellhouse, 1986. http://dx.doi.org/10.1615/ihtc8.2870.
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Golob, Matthew, Clayton Nguyen, Sheldon Jeter et Said Abdel-Khalik. « Solar Simulator Efficiency Testing of Lab-Scale Particle Heating Receiver at Elevated Operating Temperatures ». Dans ASME 2016 10th International Conference on Energy Sustainability collocated with the ASME 2016 Power Conference and the ASME 2016 14th International Conference on Fuel Cell Science, Engineering and Technology. American Society of Mechanical Engineers, 2016. http://dx.doi.org/10.1115/es2016-59655.
Texte intégralRésumé :
Particle Heating Receivers (PHR) offer a range of advantages for concentrator solar power (CSP). PHRs can facilitate higher operating temperatures (>700°C), they can allow for inexpensive direct storage, and they can be integrated into cavity receiver designs for high collection efficiency. In operation, PHRs use solid particles that are irradiated and heated directly as they fall through a region exposed to concentrated sunlight. The heated particles can subsequently be stored in insulated bins, with the stored thermal energy reclaimed via heat exchanger to secondary working fluid for the power cycle in CSP. In this field Georgia Tech has over five years’ experience developing PHR technology through the support of the DOE SunShot program and similar research efforts. Georgia Tech has dealt with the crucial challenges in particle receiver technology: particulate flow behavior, particulate handling, and particulate heat transfer. In particular, Georgia Tech has specialized in innovative advances in the utilization and design of discrete structures in PHRs (DS-PHR) to prolong particulate residence time in the irradiated zone. This paper describes the development and results of lab-scale testing for DS-PHRs especially in the Georgia Tech high flux solar simulator (GTHFSS). The GTHFSS is a bank of high intensity xenon lamps with elliptical reflectors designed to replicate a concentrated solar source. Two series of tests have been undertaken: batch and continuous operation. Initially the DS-PHR has been tested in a batch apparatus in which a substantial but still limited quantity of preheated particulate flows through from an elevated bin through the irradiated PHR into a weighing box collecting bin. The use of a weighing box is advantageous since the flow rate of particulate is otherwise especially hard to measure. Temperature rise measurements and mass flow rate measurements allow calculation of energy collection rates. Calorimetry measurements, also described in the paper, are used to verify the incident concentrated radiation allowing the calculation of the collection efficiency. This preliminary series of experiments have been completed using the batch apparatus, with the efficiencies of the lab-scale DS-PHR being determined for a range of temperatures. Efficiencies above 90% have been measured at low temperatures roughly corresponding to the so-called optical efficiency, which is the rate of energy collection at low temperature and minimal heat loss. Batch experiment data indicates a collection efficiency of approximately 81–85% at an average particle operating temperature of 500°C. Lab-scale batch results at 700°C in proved to be unstable, and as such a rework employing a continuous recirculation loop is underway. While the batch apparatus is convenient for preliminary work, it is challenging to reach steady state operation in the mixing and measurement section below the DS-PHR, which limits this apparatus in higher temperature experiments. Consequently, the experiment is being reconfigured for continuous flow, in which the particulate will be heated and recirculated by a high temperature air conveyor. The advantage of the high temperature conveyor has already been proved by its successful integration as a heater and mixer in the hot bin of the batch apparatus. Such a compact device was also quite advantageous in the limited confines of a typical laboratory simulator such as the GTHFSS. While continuous flow prevents the exceedingly desirable use of an uninterrupted mass measurement device, highly accurate mass flow data is still expected based on the use of a perforated plate flow control station. This device relies on the Berverloo effect to maintain a constant flow of particulate through an array of orifices, for which the flow is largely independent of upstream conditions. A weighing box will be used to calibrate and verify the mass flow. This paper will report on efficiency measurements with the batch flow experiments and present the preliminary steps taken to conduct the recirculation experiment. The bulk the research reported in the paper is sponsored by and done in support of the DoE Sun Shot initiative.
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Sa´nchez, F., F. Solorio et Ruben A´vila. « Conjugate Natural Convection in a Square Cavity Heated From Below ». Dans ASME 2004 International Mechanical Engineering Congress and Exposition. ASMEDC, 2004. http://dx.doi.org/10.1115/imece2004-59463.
Texte intégralRésumé :
This paper presents numerical results for two-dimensional steady-state natural convection in a square cavity. The upper and lower walls are kept at different constant temperatures, whereas the lateral walls have certain thickness and thermal conductivity and are externally insulated. Under these conditions we deal with a conjugate natural convection problem in which the heat conduction in the lateral walls is coupled with the internal convection. The continuity, momentum and energy equations were solved by using the finite volume method. The results here presented include: (i) the temperature distribution in the lateral walls and in the fluid, (ii) the velocity field, and (iii) the average Nusselt number at the upper and lower walls. It was found that the steady state fluid flow is strongly dependent on the initial temperature condition, when the fluid is initially at rest. The PIV technique allowed us to get some experimental data by measuring the velocity field in a two-dimensional square cavity. A good agreement between numerical and experimental results was found.
Rapports d'organisations sur le sujet "Confined flow heated from below"
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Lahav, Ori, Albert Heber et David Broday. Elimination of emissions of ammonia and hydrogen sulfide from confined animal and feeding operations (CAFO) using an adsorption/liquid-redox process with biological regeneration. United States Department of Agriculture, mars 2008. http://dx.doi.org/10.32747/2008.7695589.bard.
Texte intégralRésumé :
The project was originally aimed at investigating and developing new efficient methods for cost effective removal of ammonia (NH₃) and hydrogen sulfide (H₂S) from Concentrated Animal Feeding Operations (CAFO), in particular broiler and laying houses (NH₃) and hog houses (H₂S). In both cases, the principal idea was to design and operate a dedicated air collection system that would be used for the treatment of the gases, and that would work independently from the general ventilation system. The advantages envisaged: (1) if collected at a point close to the source of generation, pollutants would arrive at the treatment system at higher concentrations; (2) the air in the vicinity of the animals would be cleaner, a fact that would promote animal growth rates; and (3) collection efficiency would be improved and adverse environmental impact reduced. For practical reasons, the project was divided in two: one effort concentrated on NH₃₍g₎ removal from chicken houses and another on H₂S₍g₎ removal from hog houses. NH₃₍g₎ removal: a novel approach was developed to reduce ammonia emissions from CAFOs in general, and poultry houses in particular. Air sucked by the dedicated air capturing system from close to the litter was shown to have NH₃₍g₎ concentrations an order of magnitude higher than at the vents of the ventilation system. The NH₃₍g₎ rich waste air was conveyed to an acidic (0<pH<~5) bubble column reactor where NH₃ was converted to NH₄⁺. The reactor operated in batch mode, starting at pH 0 and was switched to a new acidic absorption solution just before NH₃₍g₎ breakthrough occurred, at pH ~5. Experiments with a wide range of NH₃₍g₎ concentrations showed that the absorption efficiency was practically 100% throughout the process as long as the face velocity was below 4 cm/s. The potential advantages of the method include high absorption efficiency, lower NH₃₍g₎ concentrations in the vicinity of the birds, generation of a valuable product and the separation between the ventilation and ammonia treatment systems. A small scale pilot operation conducted for 5 weeks in a broiler house showed the approach to be technically feasible. H₂S₍g₎ removal: The main goal of this part was to develop a specific treatment process for minimizing H₂S₍g₎ emissions from hog houses. The proposed process consists of three units: In the 1ˢᵗ H₂S₍g₎ is absorbed into an acidic (pH<2) ferric iron solution and oxidized by Fe(III) to S⁰ in a bubble column reactor. In parallel, Fe(III) is reduced to Fe(II). In the 2ⁿᵈ unit Fe(II) is bio-oxidized back to Fe(III) by Acidithiobacillus ferrooxidans (AF).In the 3ʳᵈ unit S⁰ is separated from solution in a gravity settler. The work focused on three sub-processes: the kinetics of H₂S absorption into a ferric solution at low pH, the kinetics of Fe²⁺ oxidation by AF and the factors that affect ferric iron precipitation (a main obstacle for a continuous operation of the process) under the operational conditions. H₂S removal efficiency was found higher at a higher Fe(III) concentration and also higher for higher H₂S₍g₎ concentrations and lower flow rates of the treated air. The rate limiting step of the H₂S reactive absorption was found to be the chemical reaction rather than the transition from gas to liquid phase. H₂S₍g₎ removal efficiency of >95% was recorded with Fe(III) concentration of 9 g/L using typical AFO air compositions. The 2ⁿᵈ part of the work focused on kinetics of Fe(II) oxidation by AF. A new lab technique was developed for determining the kinetic equation and kinetic parameters (KS, Kₚ and mₘₐₓ) for the bacteria. The 3ʳᵈ part focused on iron oxide precipitation under the operational conditions. It was found that at lower pH (1.5) jarosite accumulation is slower and that the performance of the AF at this pH was sufficient for successive operation of the proposed process at the H₂S fluxes predicted from AFOs. A laboratory-scale test was carried out at Purdue University on the use of the integrated system for simultaneous hydrogen sulfide removal from a H₂S bubble column filled with ferric sulfate solution and biological regeneration of ferric ions in a packed column immobilized with enriched AFbacteria. Results demonstrated the technical feasibility of the integrated system for H₂S removal and simultaneous biological regeneration of Fe(III) for potential continuous treatment of H₂S released from CAFO. NH₃ and H₂S gradient measurements at egg layer and swine barns were conducted in winter and summer at Purdue. Results showed high potential to concentrate NH₃ and H₂S in hog buildings, and NH₃ in layer houses. H₂S emissions from layer houses were too low for a significant gradient. An NH₃ capturing system was designed and tested in a 100-chicken broiler room. Five bell-type collecting devices were installed over the litter to collect NH₃ emissions. While the air extraction system moved only 10% of the total room ventilation airflow rate, the fraction of total ammonia removed was 18%, because of the higher concentration air taken from near the litter. The system demonstrated the potential to reduce emissions from broiler facilities and to concentrate the NH₃ effluent for use in an emission control system. In summary, the project laid a solid foundation for the implementation of both processes, and also resulted in a significant scientific contribution related to AF kinetic studies and ferrous analytical measurements.