1,721,020 research outputs found
Hybrid LES/DNS of turbulent forced and aided mixed convection to a liquid metal flowing in a vertical concentric annulus
No full textIn the present study fully-developed turbulent forced and mixed convection heat transfer to a liquid metal flowing upwards in a concentric annulus is numerically investigated by means of Large Eddy Simulation (LES). The inner-to-outer radius ratio is 0.5. The Reynolds number based on bulk velocity and hydraulic diameter is 8900, while the Prandtl number is set to a value of 0.026. A uniform and equal heat flux is applied on both walls. Three different buoyancy strengths are considered, corresponding to onset of turbulence reduction, maximum impairment and recovery condition on the inner wall while recovery and enhancement develop on the outer wall. Due to the difference between thermal and hydrodynamic turbulent scales in liquid metals it is shown that with the same grid resolution a LES is performed for the flow field and at the same time a "thermal" Direct Numerical Simulation (DNS) for the temperature field. From a detailed analysis of the two-point correlation functions of velocity and temperature fluctuations it emerges that a streamwise extent of 25d and 40d (being d the half gap width) is necessary for forced and mixed convection, respectively, while a quarter of circumference is enough in azimuthal direction for this radius ratio, Reynolds and Prandtl number. Comparison of the forced convection flow field with available DNS simulations shows very good agreement. Nusselt numbers evaluated from the few available literature correlations for liquid metals flowing in an annulus give unsatisfactory results, mainly on the inner wall. The mixed convection results are thoroughly analyzed and discussed in terms of friction factor, Nusselt number, first and second order statistics, budgets of turbulent kinetic energy and budgets of temperature variance. The obtained data are also useful for validating Reynolds-Averaged turbulence models. Moreover, simulations with two coarser grids at the condition of maximum turbulence reduction are also compared with the reference results of the fine LES. It results that when turbulence is impaired the grid resolution in circumferential direction can be strongly coarsened
A novel CFD model for design and performance prediction of recuperators for Indirect Evaporative Cooling
Full text linkIndirect evaporative cooling (IEC) appears to be a highly promising technology for incorporating and/or substituting traditional air conditioning systems, as it can guarantee good cooling performance with a reduced environmental impact. In this study, a Computational Fluid Dynamics (CFD) model for design and performance prediction of recuperators for IEC systems with dry primary and secondary channels was developed. The model was validated against experimental data for a cross-flow recuperator, obtaining a maximum difference between numerical and experimental results of 4.9% for the secondary air outlet temperature, 5.3% for the primary air outlet temperature, and 8.1% for the dry-bulb effectiveness. After validation, the model was used to find a new plate geometry which guarantees a 12.5-15.9% improvement in the dry-bulb effectiveness, without an excessive increase in the pressure losses along the channels
Parametric study of filler size and properties for a liquid-metal thermal energy storage
Full text linkLiquid metals are promising heat transfer fluids since they remain liquid in a wide temperature range and can transfer heat efficiently due to their high thermal conductivity. A first-of-its-kind lab-scale thermal energy storage system with filler material and with lead-bismuth as heat transfer fluid is currently tested at the Karlsruhe Institute of Technology, while a 100-kWh storage system is under construction. This numerical study aims to analyse the influence of the filler parameters on the system's efficiency when the fluid used is a liquid metal. The filler should store part of the thermal energy, be efficiently discharged during the cyclic process and buffer the degradation of the thermocline during standby. For each of these purposes different particle diameters and values of some thermophysical properties of the filler, such as thermal conductivity, specific heat capacity and density, may be advantageous. Their influence on the thermocline extension is numerically investigated using a one-dimensional concentric dispersion model. The results of the parameter study show that for an efficient discharge process in a liquid metal dual-media storage, a small filler particle size is beneficial (d< 10 mm for the reference case chosen in this work). In contrast, the standby phase is favoured by larger diameters, here an order of 10–20 mm. Furthermore, a high thermal conductivity of the filler material improves the discharge performance, due to the enhanced heat transfer, but leads to an accelerated growth of the thermocline during standby. For this case, the optimum value is 5–10W/mK. Moreover, using a filler material with a high volumetric heat capacity leads to the best overall performance. A full factorial analysis shows that the filler diameter has the strongest effect on the discharge behaviour, while, during standby, the volumetric heat capacity has the largest influence
Hydrodynamic characterization of Gyroid, Diamond and Split-P Triply Periodic Minimal Surfaces as porous medium
Full text linkTriply Periodic Minimal Surfaces are gaining significant attention as engineered porous media for applications in fluid transport and thermal management systems due to their unique geometric properties. However, accurate prediction of pressure drop across TPMS structures remains a challenge, particularly in transitioning flow regimes. This study addresses this gap by investigating the hydrodynamic behavior of Gyroid, Diamond, and Split-P geometries using computational fluid dynamics simulations across a range of Reynolds numbers, from viscous to weakly inertial regimes. Two modeling frameworks were utilized: the Ergun equation, commonly used for packed beds, and the Darcy-Forchheimer equation, enhanced with newly developed correlations for permeability and inertial drag factor. An adapted Kozeny-Carman equation was also applied for permeability prediction. The developed correlations, expressed as power-law functions of porosity and tortuosity, demonstrated high accuracy, with relative errors below 10 % for most configurations and a maximum error of 21 % for the more complex Split-P1 geometry. Validation in larger-scale geometries, such as pipes filled with TPMS, confirmed the scalability and robustness of the proposed models, even when accounting for variations in the hydraulic diameter due to wall effects. The results demonstrate the superior suitability of the Darcy-Forchheimer equation with the developed permeability and inertial drag factor models, particularly for complex geometries like Split-P. In contrast, the Ergun equation fails to accurately predict pressure drop across the investigated TPMS, underscoring its limitations for these geometries. Furthermore, while the inclusion of tortuosity in the correlations provides additional detail, it does not offer significant advantages over the simpler permeability-porosity relation for any of the investigated TPMS, making the latter a more practical choice for design and optimization applications in systems such as heat sinks and porous flow device
Heat transfer of liquid metal flow in a tube heated on the half of the circumference - Concept of a test loop
No full textThis proposal deals with forced convection of liquid metal fluid flow inside a tube, which is heated inhomogeneously. Liquid metals have very low Prandtl number, resulting in a thermal behavior considerably different from that of ordinary fluids (e.g. air or water). A special test section is developed, allowing investigating different thermal boundary conditions, including a non-homogeneous heat flux profile, which is of particular interest in receivers of concentrated solar power plants. This contribution gives information about the specifications and restrictions of the test loop and the test section. Also accompanying CFD simulations are described and first results are presented
Turbulent heat transfer in a liquid metal tube flow with azimuthally inhomogeneous heat flux
Full text linkAn experimental study of the convective heat transfer in a turbulent liquid metal tube flow with azimuthally inhomogeneous heat flux is presented. Prior to the liquid metal experiments, the validation of the test section was realized using water. These results showed a very good agreement with literature data. For the liquid metal experiments, an eutectic alloy of gallium, indium and tin (GaInSn) was used. The Péclet number was varied between 1400 and 3600, thus in a regime of forced convection. Experiments with homogeneous heating over the full circumference of the tube and inhomogeneous heating over half of the circumference with the other half being insulated, were performed. The azimuthally averaged Nusselt number and the temperature distribution in the tube wall were investigated. The results suggest that the azimuthally averaged Nusselt number for water and liquid metal tube flows with inhomogeneous heating over the circumference can be calculated sufficiently well with literature correlations for uniform heat flux. For an inhomogeneous heat flux the azimuthal temperature gradient in the tube wall increases for higher Reynolds number and is more pronounced for GaInSn than for water. Furthermore, impurities like oxide particles significantly decrease the liquid metal convective heat transfer coefficient
LES simulations and Nusselt number decomposition of turbulent mixed convection of liquid metals flowing in a vertical pipe
No full textFully developed turbulent aided mixed convection to liquid metals with Prandtl number of 0.025 and 0.005 flowing vertically in a homogeneously heated pipe is numerically investigated with Large Eddy Simulations. These are performed at Reynolds numbers, based on the radius, in the range from 2650 to 7500 and for several Richardson numbers, encompassing the heat transfer reduction and its subsequent recovery. The results are analyzed based on a novel Nusselt number decomposition for mixed convection that allows to quantitatively identify the different contributions to the Nusselt number and thus to the heat transfer mechanism. The main contributions in liquid metals mixed convection are the laminar one and those due to the laminar and turbulent buoyancy terms. The maximum heat transfer impairment, i.e. the laminarization state, corresponds to vanishing values of the Reynolds stress contribution only at low Reynolds numbers below Re=7500. For sufficiently high Reynolds numbers, the heat transfer recovery beyond laminarization occurs before the Reynolds stress becomes negative in the core of the pipe. The comparison with RANS simulations shows that these are able to reproduce qualitatively well the Nusselt number decrease and recovery at different Richardson numbers, i.e. for different buoyancy contributions
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Non-homogeneous thermal boundary conditions in low Prandtl number pipe flows
No full textThe effect of non-homogeneous thermal boundary conditions on temperature statistics in low Prandtl number turbulent pipe flows is studied numerically via direct numerical simulations. Two wall heat flux distributions, varying in azimuthal direction and motivated by concentrated solar power systems, are prescribed and their influence on the thermal field is presented. As a reference, also homogeneous thermal boundary conditions are simulated and compared the the non-homogeneous ones. The influence of the azimuthal variation of prescribed wall heat flux is assessed in terms of instantaneous velocity and temperature fields, local and global Nusselt numbers, averaged temperature distributions and the turbulent thermal diffusivity. The global Nusselt number appears to be unaffected by the thermal boundary conditions, whereas the local Nusselt number deviates appreciably
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