Journal articles: 'Natural Convection'

1

Schmidt, Frank W. "Natural convection." International Journal of Heat and Fluid Flow 7, no. 3 (September 1986): 240. http://dx.doi.org/10.1016/0142-727x(86)90030-5.

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W. Schmidt, Frank. "Natural convection." International Journal of Heat and Fluid Flow 7, no. 1 (March 1986): 27. http://dx.doi.org/10.1016/0142-727x(86)90039-1.

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Zhang, Tong, Shanshan Geng, Xin Mu, Jiamin Chen, Junyi Wang, and Zan Wu. "Thermal Characteristics of a Stratospheric Airship with Natural Convection and External Forced Convection." International Journal of Aerospace Engineering 2019 (September 8, 2019): 1–11. http://dx.doi.org/10.1155/2019/4368046.

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Though convective heat transfer is one of the main factors that dominate the thermal characteristics of stratospheric airships, there is no specific correlation equations for the calculation of convective heat transfer of airships. The equations based on flat plate and sphere models are all in use. To ameliorate the confusing situation of diverse convective heat transfer equations and to end the misuse of them in the thermal characteristic analysis of stratospheric airships, a multinode steady-state model for ellipsoid airships is built. The accuracy of the five widely accepted equations for natural convective heat transfer is compared and analysed on the proposed large-scale airship model by numerical simulation, so does that of the five equations for external forced convective heat transfer. The simulation method is verified by the available experimental data. Simulation results show that the difference of the five natural convection equations is negligible, while that of the five external forced convection equations must be considered in engineering. Forced convection equations with high precision and wide application should be further investigated.

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Abtahi, Arman, and J. M. Floryan. "Natural convection in a corrugated slot." Journal of Fluid Mechanics 815 (February 23, 2017): 537–69. http://dx.doi.org/10.1017/jfm.2017.73.

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Analysis of natural convection in a horizontal slot formed by two corrugated isothermal plates has been carried out. The analysis is limited to subcritical Rayleigh numbers$Ra$where no secondary motion takes place in the absence of corrugations. The corrugations have a sinusoidal form characterized by the wavenumber, the upper and lower amplitudes and the phase difference. The most intense convection occurs for corrugation wavelengths comparable to the slot height; it increases proportionally to$Ra$and proportionally to the corrugation height. Placement of corrugations on both plates may either significantly increase or decrease the convection depending on the phase difference between the upper and lower corrugations, with the strongest convection found for corrugations being in phase, i.e. a ‘wavy’ slot, and the weakest for corrugations being out of phase, i.e. a ‘converging–diverging’ slot. It is shown that the shear forces would always contribute to the corrugation build-up if erosion was allowed, while the role of pressure forces depends on the location of the corrugations as well as on the corrugation height and wavenumber, and the Rayleigh number. Placing corrugations on both plates results in the formation of a moment which attempts to change the relative position of the plates. There are two limiting positions, i.e. the ‘wavy’ slot and the ‘converging–diverging’ slot, with the latter being unstable. The system would end up in the ‘wavy’ slot configuration if relative movement of the two plates was allowed. The presence of corrugations affects the conductive heat flow and creates a convective heat flow. The conductive heat flow increases with the corrugation height as well as with the corrugation wavenumber; it is largest for short-wavelength corrugations. The convective heat flow is relevant only for wavenumbers of$O(1)$, it increases proportionally to$Ra^{3}$and proportionally to the second power of the corrugation height. Convection is qualitatively similar for all Prandtl numbers$Pr$, with its intensity increasing for smaller$Pr$and with the heat transfer augmentation increasing for larger$Pr$.

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Moctar, Moctar, Thierry Sikoudouin Maurice Ky, Amadou Konfé, Boureima Dianda, Salifou Ouédraogo, and Dieudonné Joseph Bathiébo. "Natural Convection Modeling in a Solar Tower." Indian Journal of Science and Technology 14, no. 48 (December 26, 2021): 3475–93. http://dx.doi.org/10.17485/ijst/v14i48.1357.

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Shen, Xiang Yang, Jing Ding, and Jian Feng Lu. "Turbulent Convective Heat Transfer in a Transversely Grooved Tube with Natural Convection Effect." Applied Mechanics and Materials 741 (March 2015): 458–61. http://dx.doi.org/10.4028/www.scientific.net/amm.741.458.

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The turbulent convective heat transfer in a transversely grooved tube of molten salt with natural convection effect has been numerically investigated. In general, the average Nusselt number with and without considering natural convection in transversely grooved tube was almost equal. According to the simulated results, the heat transfer coefficient of transversely grooved tube in upside region was lower than that of downside region. The effect of natural convection on unilateral heat transfer in transversely grooved tube was more obvious with lower Reynolds number and higher inlet temperature, and the effect of natural convection on unilateral heat transfer was lower with bigger groove deep.

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Kumar, Mahesh. "Experimental study on natural convection greenhouse drying of papad." Journal of Energy in Southern Africa 24, no. 4 (November 1, 2013): 37–43. http://dx.doi.org/10.17159/2413-3051/2013/v24i4a3144.

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In this paper, the convective heat transfer coefficients of papad for greenhouse drying under a natural convection mode are reported. Various experiments were conducted during the month of April 2010 at Guru Jambheshwar University of Science and Technology Hisar, India (29o5’5” N 75o45’55” E). Experimental data obtained for the natural convection greenhouse drying of papad was used to evaluate the constants in the Nusselt number expression by using simple linear regression analysis. These values of the constant were used further to determine the values of the convective heat transfer coefficient. The average value of a convective heat transfer coefficient was determined as 1.23 W/m2 oC. The experimental error in terms of percent uncertainty was also evaluated.

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KORTYAEVA, Darya O., and Maxim N. NIKITIN. "NUMERICAL STUDY OF NATURAL CONVECTION IN ENCLOSED VOLUME." Urban construction and architecture 6, no. 3 (September 15, 2016): 146–50. http://dx.doi.org/10.17673/vestnik.2016.03.24.

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A numerical study of natural convection was conducted with Code Saturne soft ware package. A numerical model based on combined k-ω SST turbulence model was developed. The results of simulation of natural convection in enclosed volume of air were obtained in two variants of boundary conditions specification: by heat flux and by heat transfer coefficient. The problem was solved in a non-stationary formulation using a pressure-velocity coupling algorithm PISO. This simulation model adequacy is evaluated. Experimental data on the temperature profile in the central section is used as a benchmark criteria. Assumptions about the destructive factors reducing the accuracy of the solution, are partly supported by the results of comparative analysis of the intensity of convective mixing. Assumptions partially confirmed by the results of comparative analysis of the intensity of convective mixing, performed on the basis of upward velocity profiles for the heated air.

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Oresta, Paolo, Laura Fabbiano, and Gaetano Vacca. "Wind Reversal in Bubbly Natural Convection." Applied Sciences 10, no. 22 (November 20, 2020): 8242. http://dx.doi.org/10.3390/app10228242.

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The multi-phase Rayleigh–Bènard convection has been weakly investigated, even though it plays a leading role in the theoretical and applied physics of the heat transfer enhancement. For the case of moderate turbulent convection, a rather unexpected result is an unusual kind of wind reversal, in the sense that the fluid is found to be strongly influenced by the bubbles, whereas the bubbles themselves appear to be little affected by the fluid, despite the relative smallness of the Stokes numbers. The wind reversal induced by the bubbles dispersed in the fluid is a new and remarkable phenomenon in multi-phase flows that provides further perspectives in understanding the complex physics leading the enhancement of thermal convection. For this reason, the fundamental research proposed in this paper aimed to identify a space of control parameters and the physical mechanisms responsible for the wind reversal induced by dispersed bubbles in a confined convective flow. The strength of the following description lies in an innovative numerical approach, based on the multi-scale physics induced by the coupling of the local thermal and mechanical mechanisms arising between each bubble and the surrounding fluid. The continuous phase has been solved numerically using the direct numerical simulation (DNS) technique and each bubble has been tracked by means of a particle Lagrangian model.

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Zakharov N.S., Pokusaev B.G., Vyazmin A.V., Nekrasov D.A., Sulyagina O.A., and Moshin A.A. "Research of heat transfer processes in hydrogels by holographic interferometry and gradient thermometry." Technical Physics Letters 48, no. 5 (2022): 7. http://dx.doi.org/10.21883/tpl.2022.05.53551.19058.

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The study of natural convection in structured optically transparent materials using pure and combined agarose-gelatin gels was carried out by optical holography. The article presents data on visualization of the occurrence and development of convective flows in such gels with non-stationary conductive heating from below. The similarities and differences of the conditions of heat transfer and the occurrence of convection in structured materials and droplet liquids are analyzed. For the first time experimentally obtained data on the effect of two interpenetrating and interacting structured media on the transition from conductive to convective heat transfer. Keywords: natural convection in gels, optical holography, hydrogels, three-dimensional bioprinting.

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11

Zhang, Lu, Kai Leong Chong, and Ke-Qing Xia. "Moisture transfer by turbulent natural convection." Journal of Fluid Mechanics 874 (July 15, 2019): 1041–56. http://dx.doi.org/10.1017/jfm.2019.463.

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We present an experimental and numerical study of natural convection with moist air as convecting fluid. By simplifying the system as two-component convection, an experimental method is proposed for indirectly measuring the moisture transfer rates in buoyancy-driven flows. We verify the results using direct numerical simulations. It is found that the non-dimensionalized transfer rates for both sensible heat ($Nu_{T}$) and water vapour ($Nu_{e}$) are essentially determined by a generalized Grashof number $Gr$ (the ratio of combined buoyancy generated by the imposed temperature and vapour pressure gradients to viscous force), and are only weakly dependent on the buoyancy ratio $\unicode[STIX]{x1D6EC}$ (the ratio of buoyancy induced by temperature variation to that due to vapour pressure variation). Moreover, we show that the full set of control parameters $\{Gr,\unicode[STIX]{x1D6EC},Pr,Sc\}$ is more suitable than other choices for characterizing the two-component system under investigation. As a special case, the Schmidt number dependence for passive scalar transport rates in buoyancy-driven flows is also deduced.

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RITCHIE, LINDSEY T., and DAVID PRITCHARD. "Natural convection and the evolution of a reactive porous medium." Journal of Fluid Mechanics 673 (February 17, 2011): 286–317. http://dx.doi.org/10.1017/s0022112010006269.

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We describe a mathematical model of buoyancy-driven flow and solute transport in a saturated porous medium, the porosity and permeability of which evolve through precipitation and dissolution as a mineral is lost or gained from the pore fluid. Imposing a vertically varying equilibrium solubility creates a density gradient which can drive convective circulation. We characterise the onset of convection using linear stability analysis, and explore the further development of the coupled reaction–convection system numerically. At low Rayleigh numbers, the effect of the reaction–permeability feedback is shown to be destabilising through a novel reaction–diffusion mechanism; at higher Rayleigh numbers, the precipitation and dissolution have a stabilising effect. Over longer time scales, reaction–permeability feedback triggers secondary instabilities in quasi-steady convective circulation, leading to rapid reversals in the direction of circulation. Over very long time scales, characteristic patterns of porosity emerge, including horizontal layering as well as the development of vertical chimneys of enhanced porosity. We discuss the implications of these findings for more comprehensive models of reactive convection in porous media.

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13

Worster, M. Grae. "Natural convection in a mushy layer." Journal of Fluid Mechanics 224 (March 1991): 335–59. http://dx.doi.org/10.1017/s0022112091001787.

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Governing equations for a mushy layer are analysed in the asymptotic regime Rm [Gt ] 1, where Rm is an appropriately defined Rayleigh number. A model is proposed in which there is downward flow everywhere in the mushy layer except in and near localized chimneys, which are characterized by having zero solid fraction. Upward, convective flow within the chimneys is driven by compositional buoyancy. The radius of each chimney is determined locally by thermal balances within a boundary layer that surrounds it. Simple solutions are derived to determine the structure of the mushy layer away from the immediate vicinity of chimneys in order to demonstrate the gross effects of convection upon the solidification within the layer.

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14

Siebers, D. L., R. F. Moffatt, and R. G. Schwind. "Experimental, Variable Properties Natural Convection From a Large, Vertical, Flat Surface." Journal of Heat Transfer 107, no. 1 (February 1, 1985): 124–32. http://dx.doi.org/10.1115/1.3247367.

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Natural convection heat transfer from a vertical, 3.02 m high by 2.95 m long, electrically heat surface in air was studied. The air was at the ambient temperature and the atmospheric pressure, and the surface temperature was varied from 60 C to 520 C. These conditions resulted in Grashof numbers up to 2 × 1012 and surface-to-ambient temperature ratios up to 2.7. Convective heat transfer coefficients were measured at 105 locations on the surface by an energy balance. Boundary layer mean temperature profiles were measured with a thermocouple. Results show that: (1) the turbulent natural convection heat transfer data are correlated by the expression Nuy=0.098Gry1/3TwT∞−0.14 when all properties are evaluated at T∞; (2) variable properties do not have a significant effect on laminar natural convection heat transfer; (3) the transition Grashof number decreases with increasing temperature; and (4) the boundary layer mean temperaturue profiles for turbulent natural convection can be represented by a “universal” temperature profile.

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15

Ostrach, S. "Natural Convection in Enclosures." Journal of Heat Transfer 110, no. 4b (November 1, 1988): 1175–90. http://dx.doi.org/10.1115/1.3250619.

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There exists a great diversity of buoyancy flows in enclosures that are of interest in science and technology. These buoyancy flows pose new and challenging physical and mathematical problems. Emphasis is given to the complexities of the phenomena, viz., the coupling of the flow and transport and of the boundary-layer and core flows, the interaction between the flow and the driving force, which alters the regions in which the buoyancy acts, and the occurrence of flow sub-regions (cells and layers). The importance of scaling analysis and experiments to determine flow details are discussed and the essentials of scaling techniques are outlined. The implications of these for numerical methods are presented, and the inadequacies of purely numerical solutions are pointed out. Representative works covering a broad range of problems are discussed.

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16

Pelletret, Roger, and Hala Khodr. "Air transfer, natural convection." Batiment International, Building Research and Practice 18, no. 5 (September 1990): 284–91. http://dx.doi.org/10.1080/01823329008727059.

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17

Gao, Xiaoping, Jeonghee Lee, and Henry S. White. "Natural Convection at Microelectrodes." Analytical Chemistry 67, no. 9 (May 1995): 1541–45. http://dx.doi.org/10.1021/ac00105a011.

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18

Babus'Haq, Ramiz, and S. Douglas Probert. "Fundamentals of natural convection." Applied Energy 40, no. 1 (January 1991): 79–81. http://dx.doi.org/10.1016/0306-2619(91)90053-z.

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19

Buevich, Yu A., and V. N. Mankevich. "Natural thermal-concentration convection." Journal of Engineering Physics 57, no. 5 (November 1989): 1277–84. http://dx.doi.org/10.1007/bf00871260.

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20

Miklavčič, M., and C. Y. Wang. "Completely passive natural convection." ZAMM - Journal of Applied Mathematics and Mechanics / Zeitschrift für Angewandte Mathematik und Mechanik 91, no. 7 (February 22, 2011): 601–6. http://dx.doi.org/10.1002/zamm.201000030.

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Musielak, Grzegorz, Dominik Mierzwa, and Joanna Łechtańska. "Experimental Investigation of Enhancement of Natural Convective Heat Transfer in Air Using Ultrasound." Applied Sciences 13, no. 4 (February 15, 2023): 2516. http://dx.doi.org/10.3390/app13042516.

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One of the methods to improve convective heat exchange is the application of ultrasound assistance. However, little is known about ultrasound application in the air. The main purpose of this study is to investigate the effect of ultrasound on natural convection cooling. The tests are based on the cooling of the metal samples (in four different shapes) preheated to a temperature of 60 °C. Cooling takes place in free convection without and with the use of ultrasound at different powers (50 W, 100 W, 150 W, and 200 W). The study uses a mathematical model based on a small Biot’s number assumption. The values of the convective heat exchange coefficients are determined by using an approximation of the experimental results. The coefficients obtained are an increasing exponential function of the applied ultrasound power. This study indicates the possibility of using ultrasound to improve heat transfer by free convection.

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22

Sano, Takao. "Transient natural convection between horizontal concentric cylinders Natural convection between concentric cylinders." Fluid Dynamics Research 1, no. 1 (August 1986): 33–47. http://dx.doi.org/10.1016/0169-5983(86)90005-5.

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Bahadure, Mr Saurabh D., and Mr G. D. Gosavi. "Enhancement of Natural Convection Heat Transfer from Perforated Fin." International Journal of Engineering Research 3, no. 9 (September 1, 2014): 531–35. http://dx.doi.org/10.17950/ijer/v3s9/903.

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Rivera-Salinas, Jorge-Enrique, Karla-Monzerratt Gregorio-Jáuregui, Heidi-Andrea Fonseca-Florido, Carlos-Alberto Ávila-Orta, Eduardo Ramírez-Vargas, José-Antonio Romero-Serrano, Alejandro Cruz-Ramírez, Víctor-Hugo Gutierréz-Pérez, Seydy-Lizbeth Olvera-Vazquez, and Lucero Rosales-Marines. "Numerical Study Using Microstructure Based Finite Element Modeling of the Onset of Convective Heat Transfer in Closed-Cell Polymeric Foam." Polymers 13, no. 11 (May 28, 2021): 1769. http://dx.doi.org/10.3390/polym13111769.

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The thermal performance of closed-cell foams as an insulation device depends on the thermal conductivity. In these systems, the heat transfer mode associated with the convective contribution is generally ignored, and studies are based on the thermo-physical properties that emerge from the conductive contribution, while others include a term for radiative transport. The criterion found in the literature for disregarding convective heat flux is the cell diameter; however, the cell size for which convection is effectively suppressed has not been clearly disclosed, and it is variously quoted in the range 3–10 mm. In practice, changes in thermal conductivity are also attributed to the convection heat transfer mode; hence, natural convection in porous materials is worthy of research. This work extends the field of study of conjugate heat transfer (convection and conduction) in cellular materials using microstructure-based finite element analysis. For air-based insulating materials, the criteria to consider natural convection (Ra=103) is met by cavities with sizes of 9.06 mm; however, convection is developed into several cavities despite their sizes being lower than 9.06 mm, hence, the average pore size that can effectively suppress the convective heat transfer is 6.0 mm. The amount of heat transported by convection is about 20% of the heat transported by conduction within the foam in a Ra=103, which, in turn, produces an increasing average of the conductivity of about 4.5%, with respect to a constant value.

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Jani, Jaronie Mohd, Sunan Huang, Martin Leary, and Aleksandar Subic. "Analysis of Convective Heat Transfer Coefficient on Shape Memory Alloy Actuatorunder Various Ambient Temperatures with Finite Difference Method." Applied Mechanics and Materials 736 (March 2015): 127–33. http://dx.doi.org/10.4028/www.scientific.net/amm.736.127.

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The demand for shape memory alloy (SMA) actuators for technical applications is steadily increasing; however SMA may have poor deactivation time due to relatively slow convective cooling. Convection heat transfer mechanism plays a critical role in the cooling process, where an increase of air circulation around the SMA actuator (i.e. forced convection) provides a significant improvement in deactivation time compared to the natural convection condition. The rate of convective heat transfer, either natural or forced, is measured by the convection heat transfer coefficient, which may be difficult to predict theoretically due to the numerous dependent variables. In this work, a study of free convective cooling of linear SMAactuators was conducted under various ambient temperatures to experimentally determine the convective heat transfer coefficient. A finite difference equation (FDE) was developed to simulate SMA response, and calibrated with the experimental data to obtain the unknown convectiveheat transfer coefficient, h. These coefficients are then compared with the available theoretical equations, and it was found that Eisakhaniet. almodel provides good agreement with the Experiment-FDE calibrated results. Therefore, FDE is reasonably useful to estimate the convective heat transfer coefficient of SMA actuator experiments under various conditions, with a few identified limitations (e.g. exclusion of other associative heat transfer factors).

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Rieksts, Karlis, Inge Hoff, Elena Scibilia, and Jean Côté. "Laboratory investigations into convective heat transfer in road construction materials." Canadian Geotechnical Journal 57, no. 7 (July 2020): 959–73. http://dx.doi.org/10.1139/cgj-2018-0530.

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This paper presents a laboratory investigation into natural air convection and the establishment of intrinsic permeability of road and railway construction materials. The laboratory investigations were performed using a heat transfer cell with an inner volume of 1 m3. The study shows the importance of natural air convection and a practical method for establishing the intrinsic permeability of coarse granular materials. Three different open-graded crushed rock materials and two lightweight aggregates were tested. All materials were tested for downward (conduction only) and upward (convection and conduction) heat flow conditions. The experimental results revealed that all three crushed rock materials are prone to developing natural air convection in thermal gradients of 4.5 to 11 °C/m, depending on the particle size distribution. Foam glass aggregates showed a convective heat transfer flow at the fairly low temperature gradient of 6.5 °C/m. No natural air convection was achieved in expanded clay aggregates within the temperature gradients imposed. Intrinsic permeability values were established based on the experimental results. The intrinsic permeability of crushed rock materials ranged from 1.1 to 2.2 × 10−6 m2 while that of foam glass materials was 0.9 × 10−6 m2.

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Zhao, Yongling, Chengwang Lei, and John C. Patterson. "Natural transition in natural convection boundary layers." International Communications in Heat and Mass Transfer 76 (August 2016): 366–75. http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.06.004.

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Engström, Maria, and Bo Nordell. "Seasonal groundwater turnover." Hydrology Research 37, no. 1 (February 1, 2006): 31–39. http://dx.doi.org/10.2166/nh.2006.0003.

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Seasonal air temperature variations and corresponding changes in groundwater temperature cause convective movements in groundwater similar to the seasonal turnover in lakes. Numerical simulations were performed to investigate the natural conditions for thermally driven groundwater convection to take place. Thermally driven convection could be triggered by a horizontal groundwater flow. Convection then starts at a considerably lower Rayleigh number (Ra) than the general critical Rayleigh number (Rac) assuming that 10°C groundwater is cooled to 4°C, i.e. heated from below convection in porous media. This study supports the hypothesis that seasonal temperature variations, under certain conditions, initiate and drive thermal convection.

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Prakash, M. "Numerical Studies on Natural Convection Heat Losses from Open Cubical Cavities." Journal of Engineering 2013 (2013): 1–9. http://dx.doi.org/10.1155/2013/320647.

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The natural convection heat losses occurring from cubical open cavities are analysed in this paper. Open cubical cavities of sides 0.1 m, 0.2 m, 0.25 m, 0.5 m, and 1 m with constant temperature back wall boundary conditions and opening ratio of 1 are studied. The Fluent CFD software is used to analyse the three-dimensional (3D) cavity models. The studies are carried out for cavities with back wall temperatures between 35°C and 100°C. The effect of cavity inclination on the convective loss is analysed for angles of 0° (cavity facing sideways), 30°, 45°, 60°, and 90° (cavity facing vertically downwards). The Rayleigh numbers involved in this study range between 4.5 × 105and 1.5 × 109. The natural convection loss is found to increase with an increase in back wall temperature. The natural convection loss is observed to decrease with an increase in cavity inclination; the highest convective loss being at 0° and the lowest at 90° inclination. This is observed for all cavities analysed here. Nusselt number correlations involving the effect of Rayleigh number and the cavity inclination angle have been developed from the current studies. These correlations can be used for engineering applications such as electronic cooling, low- and medium-temperature solar thermal systems, passive architecture, and also refrigeration systems.

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WITHAM, FRED, and JEREMY C. PHILLIPS. "The dynamics and mixing of turbulent plumes in a turbulently convecting environment." Journal of Fluid Mechanics 602 (April 25, 2008): 39–61. http://dx.doi.org/10.1017/s0022112008000682.

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The turbulent motion of buoyant plumes released into turbulently convecting environments is studied. By assuming that the turbulent environment removes fluid from the plume at a rate proportional to a characteristic environmental velocity scale, we derive a model describing the fluid behaviour. For the example of pure buoyancy plumes, entrainment dominates near the source and the plume radius increases with distance, while further from the source removal, or extrainment, of plume material dominates, and the plume radius decreases to zero. Theoretical predictions are consistent with laboratory experiments, a major feature of which is the natural variability of the convection. We extend the study to include the evolution of a finite confined environment, the end-member regimes of which are a well-mixed environment at all times (high convective velocities), and a ‘filling-box’ model similar to that of Baines & Turner (1969) (low convective velocities). These regimes, and the motion of the interface in a ‘filling-box’ experiment, match experimental observations. We find that the convecting filling box is not stable indefinitely, but that the density stratification will eventually be overcome by thermal convection. The model presented here has important applications in volcanology, ventilation studies and environmental science.

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Zhang, Jian, Liang Wang, Yu Jie Xu, Yi Fei Wang, Zheng Yang, and Hai Sheng Chen. "Natural Convective Heat Transfer Characteristics of the Bundle Heat Exchanger in the Latent Heat Microcapsulated Phase Change Material Slurry." Materials Science Forum 852 (April 2016): 969–76. http://dx.doi.org/10.4028/www.scientific.net/msf.852.969.

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As a novel latent functionally thermal fluid, microcapsulated phase change material slurry (MPCMS) has many potential applications in the fields of energy storage, air-conditioning, refrigeration and heat exchanger, etc. In order to investigate the heat storage and heat transfer performance of MPCMS, natural convection in a rectangular enclosure heated by bundle heat exchanger has been studied numerically in this paper. The effects of mass concentration (Cm) of MPCMS, the vertical spaces of bundle heat exchanger on the natural convective heat transfer are investigated. The results indicate that, MPCMS with Cm=30% shows the best natural convectionperformance, and a lower position of bundle heat exchanger can strengthen the natural convection.

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Jiang, Hechuan, Xiaojue Zhu, Varghese Mathai, Xianjun Yang, Roberto Verzicco, Detlef Lohse, and Chao Sun. "Convective heat transfer along ratchet surfaces in vertical natural convection." Journal of Fluid Mechanics 873 (June 28, 2019): 1055–71. http://dx.doi.org/10.1017/jfm.2019.446.

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We report on a combined experimental and numerical study of convective heat transfer along ratchet surfaces in vertical natural convection (VC). Due to the asymmetry of the convection system caused by the asymmetric ratchet-like wall roughness, two distinct states exist, with markedly different orientations of the large-scale circulation roll (LSCR) and different heat transport efficiencies. Statistical analysis shows that the heat transport efficiency depends on the strength of the LSCR. When a large-scale wind flows along the ratchets in the direction of their smaller slopes, the convection roll is stronger and the heat transport is larger than the case in which the large-scale wind is directed towards the steeper slope side of the ratchets. Further analysis of the time-averaged temperature profiles indicates that the stronger LSCR in the former case triggers the formation of a secondary vortex inside the roughness cavity, which promotes fluid mixing and results in a higher heat transport efficiency. Remarkably, this result differs from classical Rayleigh–Bénard convection (RBC) with asymmetric ratchets (Jiang et al., Phys. Rev. Lett., vol. 120, 2018, 044501), wherein the heat transfer is stronger when the large-scale wind faces the steeper side of the ratchets. We reveal that the reason for the reversed trend for VC as compared to RBC is that the flow is less turbulent in VC at the same $Ra$. Thus, in VC the heat transport is driven primarily by the coherent LSCR, while in RBC the ejected thermal plumes aided by gravity are the essential carrier of heat. The present work provides opportunities for control of heat transport in engineering and geophysical flows.

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Ren, Xiaofei, Feifei Liu, and Zheng Xin. "A Novel Thermal Lattice Boltzmann Method for Numerical Simulation of Natural Convection of Non-Newtonian Fluids." Processes 11, no. 8 (August 2, 2023): 2326. http://dx.doi.org/10.3390/pr11082326.

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A modified thermal Bhatnagar–Gross–Krook Lattice Boltzmann (BGK-LB) model was developed to study the convection phenomenon of non-Newtonian fluids (NNFs). This model integrates the local shear rate into the equilibrium distribution function (EDF) of the flow field and keeps the relaxation time from varying with fluid viscosity by introducing an additional parameter. In addition, a modified temperature EDF was constructed for the evolution equation of the temperature field to ensure the precise recovery of the convection–diffusion equation. To validate the accuracy and effectiveness of the proposed model, numerical simulations of benchmark problems were performed. Subsequently, we investigated the natural convection of power–law (PL) fluids and examined the impact of the PL index (n = 0.7–1.3) and Rayleigh number (Ra = 103–5 × 105) on the flow and temperature fields while holding the Prandtl number (Pr = 7) constant. The obtained results indicate that, for a given value of n, the convective intensity exhibits a positive correlation with Ra, which is illustrated by the rising trend in the average Nusselt number (Nu¯) with increasing Ra. Additionally, shear-thinning fluid (n < 1) exhibited increased Nu¯ values compared to the Newtonian case, indicating an enhanced convection effect. Conversely, shear-thickening fluid (n > 1) exhibits reduced Nu¯ values, indicating weakened convective behavior.

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Yapici, Kerim, and Salih Obut. "Benchmark results for natural and mixed convection heat transfer in a cavity." International Journal of Numerical Methods for Heat & Fluid Flow 25, no. 5 (June 1, 2015): 998–1029. http://dx.doi.org/10.1108/hff-02-2014-0036.

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Purpose – The purpose of this paper is to numerically investigate steady, laminar natural and mixed convection heat transfer in a two-dimensional cavity by using a finite volume method with a fourth-order approximation of convective terms, with and without the presence of nanoparticles. Highly accurate benchmark results are also provided. Design/methodology/approach – A finite volume method on a non-uniform staggered grid is used for the solution of two-dimensional momentum and energy conservation equations. Diffusion terms, in the momentum and energy equations, are approximated using second-order central differences, whereas a non-uniform four-point fourth-order interpolation (FPFOI) scheme is developed for the convective terms. Coupled mass and momentum conservation equations are solved iteratively using a semi-implicit method for pressure-linked equation method. Findings – For the case of natural convection problem at high-Rayleigh numbers, grid density must be sufficiently high in order to obtain grid-independent results and capture reality of the physics. Heat transfer enhancement for natural convection is observed up to a certain value of the nanoparticle volume fraction. After that value, heat transfer deterioration is found with increasing nanoparticle volume fraction. Originality/value – Developed a non-uniform FPFOI scheme. Highly accurate benchmark results for the heat transfer of Al2O3-water nanofluid in a cavity are provided.

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Desrayaud, Gilles, and Alberto Fichera. "Numerical Analysis of General Trends in Single-Phase Natural Circulation in a 2D-Annular Loop." Science and Technology of Nuclear Installations 2008 (2008): 1–10. http://dx.doi.org/10.1155/2008/895695.

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The aim of this paper is to address fluid flow behavior of natural circulation in a 2D-annular loop filled with water. A two-dimensional, numerical analysis of natural convection in a 2D-annular closed-loop thermosyphon has been performed for various radius ratios from 1.2 to 2.0, the loop being heated at a constant flux over the bottom half and cooled at a constant temperature over the top half. It has been numerically shown that natural convection in a 2D-annular closed-loop thermosyphon is capable of showing pseudoconductive regime at pitchfork bifurcation, stationary convective regimes without and with recirculating regions occurring at the entrance of the exchangers, oscillatory convection at Hopf bifurcation and Lorenz-like chaotic flow. The complexity of the dynamic properties experimentally encountered in toroidal or rectangular loops is thus also found here.

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Rahman, M., and I. Mulolani. "Convective--Diffusive Transport with Chemical Reaction in Natural Convection Flows." Theoretical and Computational Fluid Dynamics 13, no. 5 (February 1, 2000): 291–304. http://dx.doi.org/10.1007/s001620050001.

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Dimassi, N., and L. Dehmani. "Natural Convection of a Solar Wall in a Test Room under Tunisian Climate." ISRN Renewable Energy 2012 (June 20, 2012): 1–9. http://dx.doi.org/10.5402/2012/573176.

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Tunisia has a great potential of solar energy, which is the most important source of renewable energy; however, sufficient benefit can be obtained from this clean energy source especially for building heating. This paper reports on an experimental investigation on the natural convection of a Trombe wall in a test room located in Borj Cedria under the Mediterranean climatic conditions of Tunisia. Temperatures were recorded throughout the test room, as were the air velocity and the climatic parameters affecting the Trombe wall operation. This work deals with analyzing the natural convection heat transfer process with the evaluation of Rayleigh number, Nusselt number, the wall convection coefficient, and the convective flux. The results show that many parameters affect the wall function and therefore the thermocirculation process. The flow along the wall was found to be composite and turbulent most of the time.

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Wong, Sun, and João Teixeira. "Extreme Convection and Tropical Climate Variability: Scaling of Cold Brightness Temperatures to Sea Surface Temperature." Journal of Climate 29, no. 10 (May 13, 2016): 3893–905. http://dx.doi.org/10.1175/jcli-d-15-0214.1.

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Abstract Changes in tropical convective events provide a test bed for understanding changes of extreme convection in a warming climate. Because convective cloud top in deep convection is associated with cold brightness temperatures (BTs) in infrared window channels, variability in global convective events can be studied by spaceborne measurements of BTs. The sensitivity of BTs, directly measured by an Atmospheric Infrared Sounder (AIRS) window channel, to natural changes (the seasonal cycle and El Niño–Southern Oscillation) in tropical sea surface temperature (SST) is examined. It is found that tropical average BTs (over the ocean) at the low percentiles of their probability distributions scale with tropical average SSTs (higher SST leading to colder BTs), with the lower percentiles being significantly more sensitive to changes in SST. The sensitivity is reduced for high percentiles of BT and is insignificant for the median BT, and has similar magnitudes for the two natural changes used in the study. The regions where the lower-percentile BTs are most sensitive to SST are near the edges of the convection active areas (intertropical convergence zone and South Pacific convergence zone), including areas with active tropical cyclone activity. Since cold BTs of lower percentiles represent stronger convective events, this study provides, for the first time, global observational evidence of higher sensitivity of changes in stronger convective activity to a changing SST. This result has important potential implications in answering the key climate question of how severe tropical convection will change in a warming world.

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PADET, JACQUES, RENATO M. COTTA, EMILIA C. MLADIN, and COLETTE PADET. "Mixed thermal convection: fundamental issues and analysis of the planar case." Anais da Academia Brasileira de Ciências 87, no. 3 (August 25, 2015): 1865–85. http://dx.doi.org/10.1590/0001-3765201520140254.

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This paper aims to renew interest on mixed thermal convection research and to emphasize three issues that arise from the present analysis: (i) a clear definition of the reference temperature in the Boussinesq approximation; (ii) a practical delimitation of the three convective modes, which are the forced convection (FC), mixed convection (MC) and natural (or free) convection (NC); (iii) and, finally, a uniform description of the set FC/MC/NC in the similarity framework. The planar case, for which analytical solutions are available, allows a detailed illustration of the answers here advanced to the above issues.

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Hireche, Zouhira, Lyes Nasseri, and Djamel Eddine Ameziani. "Heat transfer analysis of a ventilated room with a porous partition: LB-MRT simulations." European Physical Journal Applied Physics 91, no. 2 (August 2020): 20904. http://dx.doi.org/10.1051/epjap/2020200146.

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This article presents the hydrodynamic and thermal characteristics of transfers by forced, mixed and natural convection in a room ventilated by air displacement. The main objective is to study the effect of a porous partition on the heat transfer and therefore the thermal comfort in the room. The fluid flow future in the cavity and the heat transfer rate on the active wall have been analyzed for different permeabilities: 10−6 ≤ Da ≤ 10. The other control parameters are obviously, the Rayleigh number and the Reynolds number varied in the rows: 10 ≤ Ra ≤ 106 and 50 ≤ Re ≤ 500 respectively. The transfer equations write were solved by the Lattice Boltzmann Multiple Relaxation Time method. For flow in porous media an additional term is added in the standard LB equations, to consider the effect of the porous media, based on the generalized model, the Brinkman-Forchheimer-extended Darcy model. The most important conclusion is that the Darcian regime start for small Darcy number Da < 10−4. Spatial competition between natural convection cell and forced convection movement is observed as Ra and Re rise. The effect of Darcy number values and the height of the porous layer is barely visible with a maximum deviation less than 7% over the ranges considered. Note that the natural convection regime is never reached for low Reynolds numbers. For this Re values the cooperating natural convection only improves transfers by around 10% while, for the other Reynolds numbers the improvement in transfers due to natural and forced convections cooperation is more significant.

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Ousmane, M., B. Dianda, S. Kam, A. Konfe, T. Ky, and DJ Bathiebo. "Experimental study in natural convection." Global Journal of Pure and Applied Sciences 21, no. 2 (December 8, 2015): 155. http://dx.doi.org/10.4314/gjpas.v21i2.8.

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42

Yao, L. S. "Natural convection along a wedge." Journal of Thermophysics and Heat Transfer 2, no. 1 (January 1988): 48–54. http://dx.doi.org/10.2514/3.61.

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Perić, Milovan. "Natural Convection in Trapezoidal Cavities." Numerical Heat Transfer, Part A: Applications 24, no. 2 (September 1993): 213–19. http://dx.doi.org/10.1080/10407789308902614.

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Hegseth, J. J., N. Rashidnia, and A. Chai. "Natural convection in droplet evaporation." Physical Review E 54, no. 2 (August 1, 1996): 1640–44. http://dx.doi.org/10.1103/physreve.54.1640.

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Osipov, Aleksei I., and A. V. Uvarov. "Natural convection in laser systems." Quantum Electronics 35, no. 2 (February 28, 2005): 123–27. http://dx.doi.org/10.1070/qe2005v035n02abeh002889.

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Prasad, V., F. A. Kulacki, and M. Keyhani. "Natural convection in porous media." Journal of Fluid Mechanics 150 (January 1985): 89–119. http://dx.doi.org/10.1017/s0022112085000040.

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Experimental results on free convection in a vertical annulus filled with a saturated porous medium are reported for height-to-gap ratios of 1.46, 1 and 0.545, and radius ratio of 5.338. In these experiments, the inner and outer walls are maintained at constant temperatures. The use of several fluid–solid combinations indicates a divergence in the Nusselt-number–Rayleigh-number relation, as also reported by previous investigators for horizontal layers and vertical cavities. The reason for this divergence is the use of the stagnant thermal conductivity of the fluid-filled solid matrix. A simple model is presented to obtain an effective thermal conductivity as a function of the convective state, and thereby eliminate the aforementioned divergence. A reasonable agreement between experimentally and theoretically determined Nusselt numbers is then achieved for the present and previous experimental results. It is thus concluded that a unique relationship exists between the Nusselt and Rayleigh numbers unless Darcy's law is inapplicable. The factors that influence the breakdown of Darcian behaviour are characterized and their effects on heat-transfer rates are explained. It is observed that, once the relation between the Nusselt and Rayleigh numbers branches out from that obtained via the mathematical formulation based on Darcy's law, its slope approaches that for a fluid-filled enclosure of the same geometry when the Rayleigh number is large enough. An iterative scheme is also presented for estimation of effective thermal conductivity of a saturated porous medium by using the existing results for overall heat transfer.

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Abtahi, Arman, and J. M. Floryan. "Natural convection and thermal drift." Journal of Fluid Mechanics 826 (August 8, 2017): 553–82. http://dx.doi.org/10.1017/jfm.2017.426.

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An analysis of natural convection in a horizontal, geometrically non-uniform slot exposed to spatially non-uniform heating has been carried out. The upper plate is smooth and isothermal, and the lower plate has sinusoidal corrugations with a sinusoidal temperature distribution. The distributions of the non-uniformities are characterized in terms of the wavenumber$\unicode[STIX]{x1D6FC}$and their relative position is expressed in terms of the phase difference$\unicode[STIX]{x1D6FA}_{TL}$. The analysis is limited to heating conditions which do not give rise to secondary motions in the absence of the non-uniformities. The heating creates horizontal temperature gradients which lead to the formation of vertical and horizontal pressure gradients which drive the motion regardless of the intensity of the heating. When the hot spots (points of maximum temperature) overlap either with the corrugation tips or with the corrugation bottoms, convection assumes the form of pairs of counter-rotating rolls whose size is dictated by the heating/corrugation wavelengths. The formation of a net horizontal flow, referred to as thermal drift, is observed for all other relative positions of the hot spots and corrugation tips. Both periodic heating as well as periodic corrugations are required for the formation of this drift, which can be directed in the positive as well as in the negative horizontal directions depending on the phase difference between the heating and corrugation patterns. The most intense convection and the largest drift occur for wavelengths comparable to the slot height, and their intensities increase proportionally to the heating intensity as well as proportionally to the corrugation amplitude, with the drift being a very strong function of the phase difference. Convection creates forces at the plates which would cause horizontal displacement of the corrugated plate and deform the corrugations if such effects were allowed. Tangential forces generated by the uniform heating always contribute to the corrugation buildup while similar forces generated by the periodic heating contribute to the buildup only when the hot spots overlap with the upper part of the corrugation. The processes described above are qualitatively similar for all Prandtl numbers$Pr$, with the intensity of convection and the magnitude of the drift increasing with a reduction in$Pr$.

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Marušić, Sanja. "Natural convection in shallow water." Nonlinear Analysis: Real World Applications 8, no. 5 (December 2007): 1379–89. http://dx.doi.org/10.1016/j.nonrwa.2005.10.009.

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Kagei, Y., M. Růžička, and G. Thäter. "Natural Convection with Dissipative Heating." Communications in Mathematical Physics 214, no. 2 (November 2000): 287–313. http://dx.doi.org/10.1007/s002200000275.

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Garg, Vijay K. "Natural convection between concentric spheres." International Journal of Heat and Mass Transfer 35, no. 8 (August 1992): 1935–45. http://dx.doi.org/10.1016/0017-9310(92)90196-y.

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Journal articles on the topic 'Natural Convection'

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