Numerical investigation of forced convection heat transfer from a sphere at low Prandtl numbers

Direct numerical simulations of the flow and forced convective heat transfer around a sphere at Reynolds numbers between = 500 and = 1000 are performed. We investigate the effects of the Prandtl number ( ) on the forced convective heat transfer from a sphere for various fluids having = 0.01, 0.1, 0.7. At the larger Prandtl number, the convective transport due to the vortex shedding process dominates over the diffusive transport. As the Prandtl number decreases, diffusive effects become important. Moreover, the thermal boundary layer increases with decrements of the Prandtl number, which results in a reduction in the local and mean non-dimensional heat transfer coefficient. It is seen that at = 500 and = 750, the vortex shedding process is asymmetric, which results not only in a non-zero lift coefficient, but in an asymmetric temperature field in the wake of the sphere at ≥ 0.1. The dual asymmetry in the flow and the convective heat transfer is smooth out when the Prandtl number reaches = 0.01 as the heat diffusion dominates and asymmetries in the vortex formation zone are no longer relevant in the heat transport. The descend in Prandtl number also produces an attenuation of the temperature fluctuations and thereby, in the turbulent heat transfer. As a direct consequence, two factors emerge at = 0.01: (1) a lower decay ratio of the temperature in the wake centreline, and (2) a larger wake spread compared to higher Prandtl numbers.This work has been partially financially supported by the Ministerio de Ciencia e Innovación, Spain (Ref. PID2020-116937RB-C22). We also acknowledge Red Española de Surpercomputación (RES) for awarding us access to the MareNostrum IV machine based in Barcelona, Spain (Ref. IM-2020-2-0013).Peer ReviewedPostprint (published version

Numerical simulation of the long-term cooling process applied to a storage tank with an internal gas flue

The transient cooling of a fluid initially at rest, inside a storage tank with an internal gas flue submitted to heat losses to the ambient is studied. In order to identify the relevant non-dimensional groups that define the transient natural convection phenomenom that occurs, a nondimensional analysis is carried out. A parametric study by means of several numerical simulations led to correlate the Nusselt number to feed simplified models based on global balances.Peer ReviewedPostprint (published version

On the validity of the Boussinesq approximation in a tall differentially heated cavity with water

In the present work, the fluid flow and heat transfer inside an integrated solar collector installed on an advanced fac¸ade are investigated. According to Gray and Giorgini [1], the use of the Boussinesq approximation can be considered valid for variations of thermosphysical properties up to 10 % with respect to the mean value. In the configuration under study, there is a variation of about 20 % in the dynamic viscosity and 15 % in the thermal expansion coefficient. Thus, the main objective of this work is to analyse the validity of the Boussinesq approximation for the turbulent natural convection flow of water in a rectangular parallelepiped tank. The significance of the Boussinesq effects is studied comparatively by means of detailed DNS simulations.Peer ReviewedPostprint (author’s final draft

Development of a multi-functional ventilated façade with an integrated collector-storage: numerical model and experimental facility

The paper reports on physical modeling and experimental evaluation of glazed systems. A multi-functional ventilated façade with an integrated collector-storage is developed and mounted on a test cell facility under real weather conditions. The component uses solar radiation to produce solar heated water flow, which in turn can provide space heating or fulfill domestic hot water demands. Different operational modes of the ventilation channel are analyzed in order to improve the thermal performance of the buildings with glazed façades. The existing numerical platform for the prediction of the thermal performance of buildings and solar systems is used to implement a numerical model to address the multi-functional ventilated façade with integrated collector-storage element. The model uses the measured outdoor data as boundary conditions to obtain predictions by means of a general energy balance in the test room and the façade component. The model permits different levels of simulation depending on the desired precision in each element, applying a modular methodology. In this study, the convection heat transfer coefficient within the parallelepiped storage tank is obtained from a direct numerical simulation (DNS) of turbulent natural convection flow of water, while the remaining empirical information is obtained from the literature for similar geometries.Peer ReviewedPostprint (author’s final draft

Thermo-hydraulic analysis and numerical simulation of a parabolic trough solar collector for direct steam generation

Direct Steam Generation (DSG) is one of the most promising alternatives for parabolic trough solar plants to replace the synthetic oil and reduce the electricity cost. The focus of this work is to develop a comprehensive optical and thermo-hydraulic model for the performance prediction of DSG process under real operating conditions. Pressure drop and heat transfer characteristics are determined considering the effect of the non-uniform heat flux distribution due to the concentration of the sunlight. A numerical-geometrical method based on ray trace and finite volume method techniques is used to determine the solar flux distribution around the absorber tube with high accuracy. A heat transfer model based on energy balance is applied to predict the thermal performances of the different flow regimes in the DSG loop. The thermo-hydraulic behavior of the different DSG sections i.e. preheating, evaporation and superheating is investigated under different operating conditions. The validity of the model has been tested by being compared with experimental data from DISS test facility and other available models in the literature. The study also presents a comparative study of the effect of different parameters on the thermal gradient around the absorber tube. The analysis shows that the highest thermal gradient is occurring in the superheating section with a high risk of thermal bending and a potential damage risk. The model is also capable to evaluate the efficiency of a DSG loop for different conditions and help to take the appropriate control strategies to avoid flow instabilities in the DSG rows.Peer ReviewedPostprint (author's final draft

Three dimensionality in the wake of the flow around a circular cylinder at Reynolds number 5000

The turbulent flow around a circular cylinder has been investigated at Re=5000Re=5000 using direct numerical simulations. Low frequency behavior, vortex undulation, vortex splitting, vortex dislocations and three dimensional flow within the wake were found to happen at this flow regime. In order to successfully capture the wake three dimensionality, different span-wise lengths were considered. It was found that a length LZ=2pDLZ=2pD was enough to capture this behavior, correctly predicting different aspects of the flow such as drag coefficient, Strouhal number and pressure and velocity distributions when compared to experimental values. Two instability mechanisms were found to coexist in the present case study: a global type instability originating in the shear layer, which shows a characteristic frequency, and a convective type instability that seems to be constantly present in the near wake. Characteristics of both types of instabilities are identified and discussed in detail. As suggested by Norberg, a resonance-type effect takes place in the vortex formation region, as the coexistence of both instability mechanisms result in distorted vortex tubes. However, vortex coherence is never lost within the wake.Peer ReviewedPostprint (author's final draft

Multi-layered solid-PCM thermocline thermal storage concept for CSP plants. Numerical analysis and perspectives

Thermocline storage concept has been considered for more than a decade as a possible solution to reduce the huge cost of the storage system in concentrated solar power (CSP) plants. However, one of the drawbacks of this concept is the decrease in its performance throughout the time. The objective of this paper is to present a new thermocline-like storage concept, which aims at circumventing this issue. The proposed concept consists of a storage tank filled with a combination of solid material and encapsulated PCMs, forming a multi-layered packed bed, with molten salt as the heat transfer fluid. The performance evaluation of each of the prototypes proposed is virtually tested by means of a detailed numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. The virtual tests carried out are designed so as to take into account several charging and discharging cycles until periodic state is achieved, i.e. when the same amount of energy is stored/released in consecutive charging/discharging cycles. As a result, the dependence of the storage capacity on the PCMs temperatures, the total energy and exergy stored/released, as well as the efficiencies of the storing process are compared for the different thermocline, single PCM, cascaded PCM and the proposed multi-layered solid-PCM (MLSPCM) configurations. The analysis shows that the multi-layered solid-PCM concept is a promising alternative for thermal storage in CSP plants.Peer ReviewedPostprint (author’s final draft

Techno-economic performance evaluation of solar tower plants with integrated multilayered PCM thermocline thermal energy storage: a comparative study to conventional two-tank storage systems

Copyright 2016 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing.Solar Tower Power Plants with thermal energy storage are a promising technology for dispatchable renewable energy in the near future. Storage integration makes possible to shift the electricity production to more profitable peak hours. Usually two tanks are used to store cold and hot fluids, but this means both higher investment costs and difficulties during the operation of the variable volume tanks. Instead, another solution can be a single tank thermocline storage in a multi-layered configuration. In such tank both latent and sensible fillers are employed to decrease the related cost up to 30% and maintain high efficiencies. This paper analyses a multi-layered solid PCM storage tank concept for solar tower applications, and describes a comprehensive methodology to determine under which market structures such devices can outperform the more conventional two tank storage systems. A detail model of the tank has been developed and introduced in an existing techno-economic tool developed by the authors (DYESOPT). The results show that under current cost estimates and technical limitations the multi-layered solid PCM storage concept is a better solution when peaking operating strategies are desired, as it is the case for the two-tier South African tariff scheme.Peer ReviewedPostprint (published version

Numerical study of the flow past a three-element high-lift airfoil at different angles of attack

A parametric study of the 30p30n three-element high-lift airfoil is conducted by changing the angle of attack at α = 5, 9 and 23◦ at a constant Reynolds number of Rec = 750, 000. Computational predictions are performed employing large eddy simulations.This work has been partially financially supported by the Ministerio de Economía y Competitividad, Secretaría de Estado de Investigación, Desarrollo e Innovación, Spain (ref. PID2020-116937RB-C21 and PID2020-116937RB-C22). We also acknowledge Red Española de Surpercomputación (RES) for awarding us access to the MareNostrum IV machine based in Barcelona, Spain (Ref IM-2022-3-0005); as well as the HPC RIVR consortium (www.hpc-rivr.si) and EuroHPC JU (eurohpc-ju.europa.eu under the project ID EHPC-REG-2022R01-030) for funding this research by providing computing resources of the HPC system Vega at the Institute of Information Science (www.izum.si). R. Montalà acknowledges support of Departament de Recerca i Universitats de la Generalitat de Catalunya through a FI-SDUR graduate fellowship program.Peer ReviewedPostprint (published version

Large-eddy simulations of fluid flow and heat transfer around a parabolic trough solar collector

This study reports on numerical simulations of a parabolic trough solar collector to predict the aerodynamic behaviour and the convection heat transfer from the heat collector element. In the study, the variation of fluid flow with different angles of attack has been taken into account. Calculations are performed using Large Eddy Simulations with a Variational Multiscale (VMS)approach for modelling the sub-grid scale stress tensor. The governing equations are discretised on a collocated unstructured grid arrangement by means of second-order spectro-consistent schemes. The numerical model is validated first with a cross flow around a horizontal cylinder. After that,aerodynamic coefficients at different angles of attack or pitch angles are calculated and compared to wind-tunnel experiments. It has been shown that, the orientation of the solar collector plays an important role in evaluating the aerodynamic performance and structural design criteria of the collector.Peer ReviewedPostprint (published version