1,721,131 research outputs found
A Semi-Empirical Model for Predicting the Effective Dynamic Viscosity of Nanoparticle Suspensions
No full textA semi-empirical model for predicting the effective dynamic viscosity of nanofluids, based on a single set of experimental data available in the literature for silica nanoparticles suspended into ethanol, is presented and discussed. The equation, which includes the overall effects of the friction at the solid-liquid interface, the nanoparticle colloidal interactions, and the formation of aggregates, expresses the ratio between the effective dynamic viscosity of the suspension and the dynamic viscosity of the pure base liquid as a function of the nanoparticle size and concentration. The predicted results are in rather good agreement with a wide variety of data relative to nanofluids consisting of several combinations of solid and liquid phases, extracted from different sources. The accuracy and ease of application of the proposed equation make it interesting from the engineering point of view, for both numerical simulation purposes and thermal design tasks
Heat transfer correlations for free convection from upward-facing horizontal rectangular surfaces
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Interactive free convection from a pair of vertical tube-arrays at moderate Rayleigh numbers
Full text linkSteady laminar free convection from a pair of vertical arrays of equally-spaced, horizontal isothermal cylinders set in free air, is studied numerically. A specifically developed computer-code based on the SIMPLE-C algorithm is used for the solution of the mass, momentum and energy transfer governing equations. Simulations are performed for pairs of tube-arrays consisting of 1-4 circular cylinders, for center-to-center horizontal and vertical spacings from 1.4 to 24 cylinder-diameters, and from 2 to 12 cylinder-diameters, respectively, and for values of the Rayleigh number based on the cylinder-diameter in the range between 10(2) and 10(4). It is found that any cylinder may exhibit either enhanced or reduced Nusselt numbers with respect to the case of single tube-array, depending on its location in the array, on the geometry of the array, as well as on the Rayleigh number. Heat transfer dimensionless correlating equations are also proposed. (c) 2006 Published by Elsevier Ltd
A wet surface three-fluid type heat exchanger for energy recovery in air-conditioning plants
No full textThe cooling performance of a recovery equipment for energy saving in air-conditioning processes is analyzed. The thermal behaviour of the system proposed, essentially consisting of a counterflow, three-fluid type, wet surface heat exchanger, where outside air is cooled simultaneously by humidified exhaust air and dehumidified supply air to be heated before its introduction into the ambient, is studied through a specifically developed finite-difference numerical model. Simulations are executed for different outdoor air and supply air psychrometric states relevant to typical summer climatic conditions, as well as for several values of the air-passage width, mean air velocity and heat exchanger length, whose influence on the cooling performance of the system discussed is pointed out. The numerical results obtained are expressed in terms of correlations among dimensionless parameters. Comparisons with the recovery efficiency of a three-fluid type, dry surface heat exchanger, as well as more traditional air-to-air recuperators for energy transfer between outdoor and exhaust airstreams, with both wet and dry heat transfer surfaces, are also reported
Algoritmi della famiglia SIMPLE per la risoluzione numerica di campi termofluidodinamici
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Buoyancy-induced heat and momentum transfer in a sidewall-heated square cavity partially divided by an active vertical partition
No full textA theoretical analysis of steady laminar natural convection in air-filled, square enclosures with differently heated sidewalls and adiabatic endwalls, partially divided by a single isothermal vertical plate located in the interior of the cavity, is presented. A numerical model based on the SIMPLE algorithm has been used for the solution of the mass, momentum and energy transfer governing equations. Simulations have been performed for several values of the Rayleigh number of the enclosure, as well as for different temperatures, sizes and locations of the divider, whose influence upon the flow structure type, the local temperature distribution and the heat transfer rates are analyzed and discussed. In particular, it is found that the presence of the thermally active divider may result in either an increase or a decrease in the heat transfer rate at each sidewall with respect to the more traditional case of non-partitioned enclosure. A dimensionless correlation equation for the heat transfer rate at the sidewalls of the enclosure is also proposed
Correlating equations for free convection heat transfer from horizontal isothermal cylinders set in a vertical array
Full text linkSteady laminar free convection from flat vertical arrays of equally-spaced, horizontal isothermal cylinders set in free air, is studied numerically. A specifically developed computer-code based on the SIMPLE-C algorithm is used for the solution of the mass, momentum and energy transfer governing equations. Simulations are performed for arrays of 2–6 circular cylinders, for center-to-center separation distances from 2 up to more than 50 cylinder-diameters, and for values of the Rayleigh number based on the cylinder-diameter in the range between 500 and 500.000. It is found that the heat transfer rate at the bottom cylinder remains the same as a single cylinder. In contrast, the downstream cylinders may exhibit either enhanced or reduced Nusselt numbers depending on their location in the array and on the geometry of the array. Heat transfer dimensionless correlating equations are proposed both for any individual cylinder in the array and for the whole tube-array. New correlation-equations for the calculation of the heat transfer rate from a single cylinder to the surrounding air are also proposed and compared to those available in the open literature
Effects of the thermal boundary conditions at the sidewalls upon natural convection in rectangular enclosures heated from below and cooled from above
No full textSteady laminar natural convection in air-filled, 2-D rectangular enclosures heated from below and cooled from above is studied numerically for a wide variety of thermal boundary conditions at the sidewalls. A specifically developed numerical model based on the SIMPLER algorithm is used for the solution of the mass, momentum and energy transfer governing equations. Simulations are performed for several values of both the width-to-height aspect ratio of the enclosure in the range between 0.66 and 8, and the Rayleigh number based on the cavity height in the range between 103 and 106, whose influence upon the flow patterns, the temperature distribution and the heat transfer rates are analyzed and discussed. Comparisons among the different thermal configurations considered are reported. In particular, with reference to the typical configuration wherein the sidewalls are adiabatic, it is found that the heat transfer effectiveness of the bottom wall increases (or slightly decreases) as each adiabatic sidewall is replaced by a cooled (or a heated) sidewall. An opposite behaviour is observed for the top wall. The heat transfer rate results obtained are expressed through dimensionless correlation-equations. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved
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