Articles de revista
http://hdl.handle.net/2117/3191
2015-08-05T13:03:12ZMulti-layered solid-PCM thermocline thermal storage for CSP. Numerical evaluation of its application in a 50 MWe plant
http://hdl.handle.net/2117/76396
Multi-layered solid-PCM thermocline thermal storage for CSP. Numerical evaluation of its application in a 50 MWe plant
Galione Klot, Pedro Andrés; Pérez Segarra, Carlos David; Rodríguez Pérez, Ivette María; Torras Ortiz, Santiago; Rigola Serrano, Joaquim
Thermocline storage concept is considered as a possible solution to reduce the cost of thermal storage in concentrated solar power (CSP) plants. Recently, a multi-layered solid-PCM (MLSPCM) concept—consisting of a thermocline-like tank combining layers of solid and phase change filler materials—has been proposed. This approach was observed to result in lower thermocline degradation throughout charge/discharge cycles, due to the thermal buffering effect of the PCM layers located at both ends of the tank. MLSPCM prototypes designed for a pilot scale plant were numerically tested and compared against other designs of single-tank thermocline systems, such as: solid-filled thermocline, tanks filled with a single encapsulated PCM and cascaded-PCM configurations. Results showed promising results of the MLSPCM configurations for their potential use in CSP plants.
In this work, the MLSPCM concept is used for designing a thermal energy storage (TES) system for a CSP plant with the dimensions and operating conditions of a parabolic trough plant of 50 MWe, similar to Andasol 1 (Granada, Spain). The performance evaluation of each of the proposed prototypes is virtually tested by means of a numerical methodology which considers the heat transfer and fluid dynamics phenomena present in these devices. Two sets of cases are considered, one with the objective of testing the TES systems individually, by defining specific operating conditions and taking the systems to a periodic steady state; and another, aiming to evaluate their performance after several days of operation in a CSP plant, in which the weather variability and the thermal behavior of the tank walls and foundation are simulated. Thermal performance parameters, such as total energy and exergy stored/released and the efficiency in the use of the storage capacity, are calculated and compared with those obtained by other thermocline-like configurations (single-solid and single-PCM), and with a reference 2-tank molten-salt system. Obtained results allow to continue considering the MLSPCM concept as an interesting alternative for thermal storage in CSP facilities.
2015-09-01T00:00:00ZMulti-layered solid-PCM thermocline thermal storage concept for CSP plants. Numerical analysis and perspectives
http://hdl.handle.net/2117/76330
Multi-layered solid-PCM thermocline thermal storage concept for CSP plants. Numerical analysis and perspectives
Galione Klot, Pedro Andrés; Pérez Segarra, Carlos David; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio; Rigola Serrano, Joaquim
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.
2015-03-15T00:00:00ZFrom extruded-2D to fully-3D geometries for DNS: a multigrid-based extension of the Poisson solver
http://hdl.handle.net/2117/21618
From extruded-2D to fully-3D geometries for DNS: a multigrid-based extension of the Poisson solver
Gorobets, Andrei; Trias Miquel, Francesc Xavier; Soria Guerrero, Manel; Pérez Segarra, Carlos David; Oliva Llena, Asensio
Direct numerical simulation (DNS) of incompressible flows is an essential tool for improving the understanding of the physics of turbulence and for the development of better turbulence models. The Poisson equation, the main bottleneck from a parallel point of view, usually also limits its applicability for complex geometries. In this context, efficient and scalable Poisson solvers on fully-3D geometries are of high interest.In our previous work, a scalable algorithm for Poisson equation was proposed. It performed well on both small clusters with poor network performance and supercomputers using efficiently up to a thousand of CPUs. This algorithm named Krylov-Schur-Fourier Decomposition (KSFD) can be used for problems in parallelepipedic 3D domains with structured meshes and obstacles can be placed inside the flow. However, since a FFT decomposition is applied in one direction, mesh is restricted to be uniform and obstacles to be 2D shapes extruded along this direction.The present work is devoted to extend the previous KSFD algorithm to eliminate these limitations. The extension is based on a two-level Multigrid (MG) method that uses KSFD as a solver for second level. The algorithm is applied for a DNS of a turbulent flow in a channel with wall-mounted cube. Illustrative results at Re τ = 590 (based on the cube height and the bulk velocity Re h = 7235) are shown.
2010-01-01T00:00:00ZDNS of turbulent natural convection flows on the MareNostrum supercomputer
http://hdl.handle.net/2117/21617
DNS of turbulent natural convection flows on the MareNostrum supercomputer
Trias Miquel, Francesc Xavier; Gorobets, Andrei; Soria Guerrero, Manel; Oliva Llena, Asensio
A code for the direct numerical simulation (DNS) of incompressible turbulent flows that provides a fairly good scalability for a wide range of computer architectures has been developed. The spatial discretization of the incompressible Navier-Stokes equations is carried out using a fourth-order symmetry-preserving discretization. Since the code is fully explicit, from a parallel point of view, the main bottleneck is the Poisson equation. In the previous version of the code, that was conceived for low cost PC clusters with poor network performance, a Direct Schur-Fourier Decomposition (DSFD) algorithm was used to solve the Poisson equation. Such method, that was very efficient for PC clusters, can not be efficiently used with an arbitrarily large number of processors, mainly due to the RAM requirements (that grows with the number of processors). To do so, a new version of the solver, named Krylov-Schur-Fourier Decomposition (KSFD), is presented here. Basically, it is based on the Direct Schur Decomposition (DSD) algorithm that is used as a preconditioner for a Krylov method (CG) after Fourier decomposition. Benchmark results illustrating the robustness and scalability of the method on the MareNostrum supercomputer are presented and discussed. Finally, illustrative DNS simulations of wall-bounded turbulent flows are also presented.
2009-01-01T00:00:00ZLow-frequency unsteadiness in the vortex formation region of a circular cylinder
http://hdl.handle.net/2117/21548
Low-frequency unsteadiness in the vortex formation region of a circular cylinder
Lehmkuhl Barba, Oriol; Rodríguez Pérez, Ivette María; Borrell Pol, Ricard; Oliva Llena, Asensio
The presence of low-frequency fluctuations in the wake of bluff bodies have been observed in several investigations. Even though the flow past a circular cylinder at Re = 3900 (Re = U ref D/ν) has been the object of several experimental and numerical investigations, there is a large scattering in the average statistics in the near wake. In the present work, the flow dynamics of the near wake region behind a circular cylinder has been investigated by means of direct numerical simulations and statistics have been computed for more than 858 shedding cycles. The analysis of instantaneous velocity signals of several probes located in the vortex formation region, point out the existence of a low-frequency fluctuation at the non-dimensional frequency of f m = 0.0064. This large-scale almost periodic motion seems to be related with the modulation of the recirculation bubble which causes its shrinking and enlargement over the time. Two different configurations have been identified: (i) a high-energy mode with larger fluctuations in the shear-layer and in the vortex formation region (Mode H) and (ii) a low-energy mode with weaker fluctuations in the shear layer (Mode L). The influence of such a low-frequency in the wake topology has been studied not only by means of the phase-average flow field for each mode, but also by the analysis of the time-average first- and second-order statistics of each wake mode. The results are compared with the long-term averaged solution and with results in the existing literature.
Electronic version of an article published as "Physics of fluids", vol. 25, no 8, 2013. DOI: http://dx.doi.org/10.1063/1.4818641.
2013-08-23T00:00:00ZDirect numerical simulation of a NACA0012 in full stall
http://hdl.handle.net/2117/21438
Direct numerical simulation of a NACA0012 in full stall
Rodríguez Pérez, Ivette María; Lehmkuhl Barba, Oriol; Borrell Pol, Ricard; Oliva Llena, Asensio
This work aims at investigating the mechanisms of separation and the transition to turbulence in the separated shear-layer of aerodynamic profiles, while at the same time to gain insight into coherent structures
formed in the separated zone at low-to-moderate Reynolds numbers. To do this, direct numerical simulations of the flow past a NACA0012 airfoil at Reynolds numbers Re = 50,000 (based on the free-stream velocity and the airfoil chord) and angles of attack AOA = 9.25 and AOA = 12 have been carried out. At low-to-moderate Reynolds numbers, NACA0012 exhibits a combination of leading-edge/trailing-edge stall which causes the massive separation of the flow on the suction side of the airfoil. The
initially laminar shear layer undergoes transition to turbulence and vortices formed are shed forming a von Kármán like vortex street in the airfoil wake. The main characteristics of this flow together with its
main features, including power spectra of a set of selected monitoring probes at different positions on the suction side and in the wake of the airfoil are provided and discussed in detail.
2013-10-15T00:00:00ZLimits of the Oberbeck–Boussinesq approximation in a tall differentially heated cavity filled with water
http://hdl.handle.net/2117/21437
Limits of the Oberbeck–Boussinesq approximation in a tall differentially heated cavity filled with water
Kizildag, Deniz; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio; Lehmkuhl Barba, Oriol
The present work assesses the limits of the Oberbeck–Boussinesq (OB) approximation for the resolution of turbulent fluid flow and heat transfer inside a tall differentially heated cavity of aspect ratio G = 6.67 filled with water (Pr = 3.27, Ra = 2.12e11). The cavity models the integrated solar collector-storage element installed on an advanced façade. The implications of the Oberbeck–Boussinesq approximation is submitted to investigation by means of direct numerical simulations (DNS) carried out for a wide range of temperature differences. Non-Oberbeck–Boussinesq (NOB) effects are found to be relevant, especially beyond the temperature difference of 30 °C, in the estimation of heat transfer, stratification, and flow configuration.
2014-01-01T00:00:00ZHeat transfer analysis and numerical simulation of a parabolic trough solar collector
http://hdl.handle.net/2117/21435
Heat transfer analysis and numerical simulation of a parabolic trough solar collector
Amine Hachicha, Ahmed; Rodríguez Pérez, Ivette María; Capdevila Paramio, Roser; Oliva Llena, Asensio
Parabolic trough solar collector is the most proven industry-scale solar generation technology today available. The thermal performance of such devices is of major interest for optimising the solar field output and increase the efficiency of power plants. In this paper, a detailed numerical heat transfer model based on the finite volume method for these equipment is presented. In the model, the different elements of the receiver are discretised into several segments in both axial and azimuthal directions and energy balances are applied for each control volume. An optical model is also developed for calculating the non-uniform solar flux distribution around the receiver. This model is based on finite volume method and ray trace techniques and takes into account the finite size of the Sun. The solar heat flux is determined as a pre-processing task and coupled to the energy balance model as a boundary condition for the outer surface of the receiver. The set of algebraic equations are solved simultaneously using direct solvers. The model is thoroughly validated with results from the literature. First, the optical model is compared with known analytical solutions. After that, the performance of the overall model is tested against experimental measurements from Sandia National Laboratories and other un-irradiated receivers experiments. In all cases, results obtained shown a good agreement with experimental and analytical results.
2013-11-01T00:00:00ZSymmetry-preserving discretization of Navier-Stokes equations on collocated unstructured meshes
http://hdl.handle.net/2117/21111
Symmetry-preserving discretization of Navier-Stokes equations on collocated unstructured meshes
Trias Miquel, Francesc Xavier; Lehmkuhl Barba, Oriol; Oliva Llena, Asensio; Pérez Segarra, Carlos David; Verstappen, R.W.C.P.
A fully-conservative discretization is presented in this paper. The same principles followed by Verstappen and Veldman (2003) [3] are generalized for unstructured meshes. Here, a collocated-mesh scheme is preferred over a staggered one due to its simpler form for such meshes. The basic idea behind this approach remains the same: mimicking the crucial symmetry properties of the underlying differential operators, i.e., the convective operator is approximated by a skew-symmetric matrix and the diffusive operator by a symmetric, positive-definite matrix. A novel approach to eliminate the checkerboard spurious modes without introducing any non-physical dissipation is proposed. To do so, a fully-conservative regularization of the convective term is used. The supraconvergence of the method is numerically showed and the treatment of boundary conditions is discussed. Finally, the new discretization method is successfully tested for a buoyancy-driven turbulent flow in a differentially heated cavity.
2014-02-01T00:00:00ZImproved semi-analytical method for air curtains prediction
http://hdl.handle.net/2117/21065
Improved semi-analytical method for air curtains prediction
Giráldez García, Héctor; Pérez Segarra, Carlos David; Rodríguez Pérez, Ivette María; Oliva Llena, Asensio
The present study is devoted to the analysis and prediction of the efficiency of air curtains. The attention is focused on improving existing semi-analytical methods with the information from CFD simulations and experimental measurements. The goal is to obtain an accurate simplified model which describes the three-dimensional behaviour of the air jet without requiring large time consuming calculations. The interaction of the air curtain with other agents, e.g. pedestrians and flying insects, is also revised. The model is validated against both experimental data and advanced LES calculations carried out by the authors. Furthermore, a parametric study shows the air curtain behaviour for different configurations.
2013-11-01T00:00:00Z