1,721,026 research outputs found
Forced convection with slug flow and viscous dissipation in a rectangular duct
No full textStationary forced convection in a rectangular duct is investigated in the case of slug flow by taking into account the effect of viscous dissipation. Axially-varying heat fluxes are prescribed on the four duct walls. Under the assumption that the axial heat conduction in the fluid is negligible, an analytical solution for the thermal entrance region is obtained by employing a superposition method. More precisely, the superposition method allows one to reduce the three-dimensional boundary value problem to a two-dimensional problem which is solved by the Laplace transform technique. The dimensionless temperature and the axially local Nussek number are determined. Special attention is devoted to the eight fundamental boundary conditions of axially uniform wall heat fluxes and to the case of a peripherally uniform wall heat flux which undergoes an exponential axial variation
Hyperbolic thermal waves in a solid cylinder with a non-stationary boundary heat flux
No full textThe non-stationary heat conduction in an infinitely long solid cylinder with a time-dependent boundary heat flux is studied for a material with a non-vanishing thermal relaxation time. An analytical solution of the hyperbolic energy equation together with its boundary and initial conditions is obtained by the Laplace transform method. The temperature distribution and the heat flux density distribution are studied both for a constant boundary heat flux and for an exponentially decaying boundary heat flux. The compatibility of these distributions with the local equilibrium hypothesis is analysed. © 1997 Elsevier Science Ltd
Urban green technologies for energy saving: Numerical simulation of heat transfer between green façades and green roofs and the local outdoor environment
No full textThe last decades, a robust urbanization and climate change effects generated an increase in environmental problems in water management, energy consumption, air pollution, biodiversity, warming at urban level. According with the UN Sustainable Development Goal approach, our cities and human settlements have to become "more inclusive, safe, resilient and sustainable". In other words our cities have to become "greener", considering the help in that sense that can derive by a widespread application of urban green technologies. These technologies, as green roofs or green walls, can represent a very important tool in terms of thermal insulation, heat island effect reduction and in general energy saving at building and district scale. In recent years the mechanisms that govern exchanges between green walls (roofs and façades) and outdoor environmental air in terms of heat and mass transfer have been studied more and more in the literature. The approaches mostly used for these studies are experimental local measurements in order to characterize heat and mass transfer between the building and the environment, while theoretical studies, especially numerical studies, are still lacking. In this work, after a general overview related with environmental benefit deriving from Green Technologies at urban level, a short study results are proposed related with heat and mass transfer between green façades and green roofs and the local environment around buildings. As a case study, the transpiration cooling of vegetation as a measure to mitigate outdoor air temperatures is investigated in the case of an experimental green roof that is present over a building, in the laboratories area, inside the Campus of the Bologna University Engineering School
Cfd characterization of street canyon heating by solar radiation on building walls
No full textHeat exchange processes between building walls and external air within street canyons is an important topic in air quality modelling considering that the thermal fields largely affect local flow dynamics and pollutant concentration distribution. Despite the abundance of numerical studies, many questions still remain unanswered, ultimately limiting the inclusion of heat effects in mesocale atmospheric models. Of interest in this study is the assessment of radiative processes at building walls in terms of emissivity (i.e. is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature), shape factors (i.e. the proportion of the radiation which leaves the surface of a building and strikes toward the surface of another building) and their relation to the Richardson number. The study is approached via the computational fluid dynamics code OpenFOAM, with large eddy simulations (LES) extension to model turbulence within the canyon and Boussinesq approach to model heat transfer. Consequences on pollutant concentrations are also analysed. A model for setting the thermal boundary conditions on the building surfaces is developed, based on a database that gives the correlations between the emissivity of a surface and the heat flux emitted by this surface, under certain weather conditions, as a function of the latitude and longitude of the surface, the orientation, the time of the day. As a novel result an exponential law between the transfer coefficient and Ri (as well as with the emissivity) is proposed as a function of the canyon aspect ratio
Thermal instability of the buoyant flow in a vertical cylindrical porous layer with a uniform internal heat source
Full text linkThe buoyancy-induced parallel flow in a vertical cylindrical porous layer is
analysed. A radial thermal gradient caused by a uniformly distributed heat
source is assumed to induce the buoyant flow. The layer boundaries are modelled
as isothermal and permeable to an external fluid reservoir. The onset of the
convective instability is analysed by linearising the governing equations for
the perturbations. The governing parameters driving the instability are the
heat-source Rayleigh number and the ratio between the internal radius and the
external radius. Neutral stability curves and the critical values of the
Rayleigh number, the perturbation wave number and the angular frequency are
computed numerically. It is shown that axisymmetric modes form the most
dangerous mode of instability.Comment: 12 pages, 4 figure
Large-Eddy Simulations of Pollutant Removal Enhancement from Urban Canyons
Full text linkTechniques for improving the removal of pollution from urban canyons are crucial for air quality control in cities. The removal mainly occurs at the building roof level, where it is supported by turbulent mixing and hampered by roof shear, which tends to isolate the internal canyon region from the atmospheric flow. Here, a modification of roof infrastructures is proposed with the aim of increasing the former and reducing the latter, overall enhancing the removal mechanisms. The topic is investigated by numerical experiment, using large-eddy simulation to study the paradigmatic case of a periodic square urban canyon at Re= 2 × 10 4. Two geometries are analyzed: one with a smooth building roof, the other having a series of solid obstacles atop the upwind building roof. The pollutant is released at the street level. The simulations are successfully validated against laboratory and numerical datasets, and the primary vortex displacement detected in some laboratory experiments is discussed. The turbulence triggered by the obstacles destroys the sharp shear layer that separates the canyon and the surrounding flow, increasing the mixing. Greater vertical turbulent mass fluxes and more frequent ejection events near the upwind building (where pollution accumulates) are detected. Overall, the obstacles lead to a reduction in the pollution concentration within the canyon of about 34 %
Numerical modelling of droplet formation in a micro cross-junction
No full textIn this paper, liquid-liquid droplet formation in a cross-junction between stadium-shaped cross-section micro-channels has been studied by means of a CFD OpenFOAM approach. The particularity of the micro-junction considered in this paper is that within the junction the micro-channels have a restriction in the cross-section, while the shape remains stadium-shaped. The junction has the function to focus the dispersed flow and the droplets formed in certain regimes have dimensions that can be related to the dimensions of the junction. The object of this study is to characterise the dimensions of the droplets formed in this micro-junction. The evolution of the droplets are obtained for several ratios of the volume flow rates of the two fluids. A correlation between the dimensionless length of the droplet and the ratio of the volume flow rates are obtained and compared with other correlations available in the literature for different junctions
Flow and convection in metal foams: A survey and new CFD results
Full text linkMetal foams are widely studied as possible tools for the enhancement of heat transfer from hot bodies. The basic idea is that a metal foam tends to significantly increase the heat exchange area between the hot solid body and the external cooling fluid. For this reason, this class of porous materials is considered as a good candidate for an alternative to finned surfaces, with different pros and cons. Among the pros, we mention the generally wider area of contact per unit volume between solid and fluid. Among the cons is the difficulty to produce different specimens with the same inner structure, with the consequence that their performance may be significantly variable. This paper will offer a survey of the literature with a focus on the main heat transfer characteristics of the metal foams and the energy balance model based on Local Thermal Non-Equilibrium (LTNE). Then, a numerical simulation of the heat transfer at the pore-scale level for an artificial foam with a spatially periodic structure will be discussed. Finally, these numerical results will be employed to assess the macroscopic modeling of the flow and heat transfer in a metal foam. More precisely, the Darcy–Forchheimer model and the LTNE model adopted to describe the momentum and energy transfer in metal foams have been validated for metallic periodic structures
Nanofluid suspensions as heat carrier fluids in single U-tube borehole heat exchangers
Full text linkThe borehole heat exchanger (BHE) is a critical component to improve energy efficiency and decreasing environmental impact of ground-source heat pump systems. The lower thermal resistance of the BHE results in the better thermal performance and/or in the lower required borehole length. In the present study, effects of employing a nanofluid suspension as a heat carrier fluid on the borehole thermal resistance are examined. A 3D transient finite element code is adopted to evaluate thermal comportment of nanofluids with various concentrations in single U-tube borehole heat exchangers and to compare their performance with the conventional circuit fluid. The results show, in presence of nanoparticles, the borehole thermal resistance is reduced to some extent and the BHE renders a better thermal performance. It is also revealed that employing nanoparticle fractions between 0.5% and 2 % are advantageous in order to have an optimal decrement percentage of the thermal resistance
Life Cycle Assessment of a Lithium-Ion Battery Pack Unit Made of Cylindrical Cells
Full text linkSaving energy is a fundamental topic considering the growing energy requirements with respect to energy availability. Many studies have been devoted to this question, and life cycle assessment (LCA) is increasingly acquiring importance in several fields as an effective way to evaluate the energy demand and the emissions associated with products’ life cycles. In this work, an LCA analysis of an existent lithium-ion battery pack (BP) unit is presented with the aim to increase awareness about its consumption and offering alternative production solutions that are less energy intensive. Exploiting the literature data about cradle-to-grave and cradle-to-gate investigations, and after establishing reasonable approximations, the main BP sub-elements were considered for this study, such as the plastic cells support, the Li-ion cells brick, the PCBs for a battery management system (BMS), the liquid-based battery thermal management system (BTMS) and the BP container. For each of these components, the impacts of the extraction, processing, assembly, and transportation of raw materials are estimated and the partial and total values of the energy demand (ED) and global warming potential (GWP) are determined. The final interpretation of the results allows one to understand the important role played by LCA evaluations and presents other possible ways of reducing the energy consumption and (Formula presented.) emissions
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