9,982 research outputs found
A Lagrangian approach to modeling heat flux driven close-contact melting
Close-contact melting refers to the process of a heat source melting its way
into a phase-change material. Of special interest is the close-contact melting
velocity, or more specifically the relative velocity between the heat source
and the phase-change material. In this work, we present a novel numerical
approach to simulate quasi-steady, heat flux driven close-contact melting. It
extends existing approaches found in the literature, and, for the first time,
allows to study the impact of a spatially varying heat flux distribution. We
will start by deriving the governing equations in a Lagrangian reference frame
fixed to the heat source. Exploiting the narrowness of the melt film enables us
to reduce the momentum balance to the Reynolds equation, which is coupled to
the energy balance via the velocity field. We particularize our derivation for
two simple, yet technically relevant geometries, namely a 3d circular disc and
a 2d planar heat source. An iterative solution procedure for the coupled system
is described in detail and discussed on the basis of a convergence study.
Furthermore, we present an extension to allow for rotational melting modes.
Various test cases demonstrate the proficiency of our method. In particular, we
will utilize the method to assess the efficiency of the close-contact melting
process and to quantify the model error introduced if convective losses are
neglected. Finally, we will draw conclusions and present an outlook to future
work
Finite element formulation to study thermal stresses in nanoencapsulated phase change materials for energy storage
Nanoencapsulated phase change materials (nePCMs) â which are composed of a core with a phase change material and of a shell that envelopes the core â are currently under research for heat storage applications. Mechanically, one problem encountered in the synthesis of nePCMs is the failure of the shell due to thermal stresses during heating/cooling cycles. Thus, a compromise between shell and core volumes must be found to guarantee both mechanical reliability and heat storage capacity. At present, this compromise is commonly achieved by trial and error experiments or by using simple analytical solutions. On this ground, the current work presents a thermodynamically consistent and three-dimensional finite element (FE) formulation considering both solid and liquid phases to study thermal stresses in nePCMs. Despite the fact that there are several phase change FE formulations in the literature, the main novelty of the present work is its monolithic coupling â no staggered approaches are required â between thermal and mechanical fields. Then, the FE formulation is implemented in a computational code and it is validated against one-dimensional analytical solutions. Finally, the FE model is used to perform a thermal stress analysis for different nePCM geometries and materials to predict their mechanical failure by using Rankineâs criterion
A two-scale Stefan problem arising in a model for tree sap exudation
The study of tree sap exudation, in which a (leafless) tree generates
elevated stem pressure in response to repeated daily freeze-thaw cycles, gives
rise to an interesting multi-scale problem involving heat and multiphase
liquid/gas transport. The pressure generation mechanism is a cellular-level
process that is governed by differential equations for sap transport through
porous cell membranes, phase change, heat transport, and generation of osmotic
pressure. By assuming a periodic cellular structure based on an appropriate
reference cell, we derive an homogenized heat equation governing the global
temperature on the scale of the tree stem, with all the remaining physics
relegated to equations defined on the reference cell. We derive a corresponding
strong formulation of the limit problem and use it to design an efficient
numerical solution algorithm. Numerical simulations are then performed to
validate the results and draw conclusions regarding the phenomenon of sap
exudation, which is of great importance in trees such as sugar maple and a few
other related species. The particular form of our homogenized temperature
equation is obtained using periodic homogenization techniques with two-scale
convergence, which we investigate theoretically in the context of a simpler
two-phase Stefan-type problem corresponding to a periodic array of melting
cylindrical ice bars with a constant thermal diffusion coefficient. For this
reduced model, we prove results on existence, uniqueness and convergence of the
two-scale limit solution in the weak form, clearly identifying the missing
pieces required to extend the proofs to the fully nonlinear sap exudation
model. Numerical simulations of the reduced equations are then compared with
results from the complete sap exudation model.Comment: 35 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1411.303
Experimental validation of the exact analytical solution to the steady periodic heat transfer problem in a PCM layer
Phase change materials (PCM) are used in many industrial and residential applications for their advantageous characteristic of high capacity of latent thermal storage by means of an isothermal process. In this context, it is very useful to have predictive mathematical models for the analysis of the thermal performance and for the thermal design of these layers. In this work, an experimental validation of an analytical model that resolves the steady periodic heat transfer problem in a finite layer of PCM is presented. The experimental investigation was conducted employing a PCM with thermophysical and thermochemical behavior very close to those hypothesized in the formulation of the analytical model. For the evaluation of the thermophysical properties of the PCM sample used, an experimental procedure created by the authors was employed. In all tests realized in a sinusoidal and non-sinusoidal periodic regime, the comparison between the measured and calculated trends of the temperature at different sample heights and of the surface heat fluxes show an excellent agreement. Moreover, also having verified the analytical total stored energy, the analytical model constitutes a valid instrument for the evaluation of the latent and sensible contribution and the trend in time of the position of the bi-phase interface.The work was partially funded by the Spanish government (ENE2015-64117-C5-1-R (MINECO/FEDER), ENE2015-64117-C5-3-R (MINECO/FEDER), and ULLE10-4E-1305). GREA is certified agent TECNIO in the category of technology developers from the Government of Catalonia. The authors would like to thank the Catalan Government for the quality accreditation given to their research group (2014 SGR 123). This project has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under Grant agreement NÂș PIRSES-GA-2013-610692 (INNOSTORAGE) and from European Union's Horizon 2020 research and innovation programme under grant agreement NÂș 657466 (INPATH-TES). Alvaro de Gracia would like to thank Ministerio de Economia y Competitividad de España for Grant Juan de la Cierva, FJCI-2014-19940. JuliĂ Coma would like to thank the Departament d'Universitats, Recerca i Societat de la InformaciĂł de la Generalitat de Catalunya for his research fellowship (2016FI_B2 00147). Aran SolĂ© would like to thank Ministerio de EconomĂa y Competitividad de España for Grant Juan de la Cierva, FJCI-2015-25741
The pear-shaped fate of an ice melting front
A fluid-structure interaction problem with the melting of water around a
heated horizontal circular cylinder is analysed with numerical simulations.
Dynamic meshing was used for evolving the flow domain in time as the melting
front extended radially outward from the cylinder; a node shuffle algorithm was
used to retain mesh quality across the significant mesh deformation. We
simulated one case above the density inversion point of water and one case
below, yielding pear-shaped melting fronts due to thermal plumes either rising
or falling from the cylinder, respectively. Results were compared with previous
experimental studies and the melting front profiles matched reasonably well and
melting rates were in agreement. We confirm that natural convection plays a
significant role in the transport of energy as the melt zone increases, and
needs to be considered for accurately modelling phase change under these
conditions.Comment: Accepted for the 12th International Conference on CFD in Oil & Gas,
Metallurgical and Process Industries. SINTEF, Trondheim, Norway. May 30th -
June 1st, 201
An adjusted analytical solution for thermal design in artificial ground freezing
Artificial ground freezing is a widely used, reliable method for excavation in water-bearing ground. The questions posed in the thermal design of ground freezing projects require solving moving boundary (Stefan) problems. Approximate analytical solutions, such as the ones by Stš
ander1 and Sanger and Sayles,2 have been
developed for thermal engineering design and are used by practitioners across the industry. For instance, Sanger
& Saylesâ solution is widely used for the single-freeze-pipe problem, but it has proven to be of limited accuracy.3
In the present paper, an adjustment to this formula is proposed based on the re-evaluation of their empirical
assumption that the ratio between the temperature penetration depth and the phase-change radius equals a
constant value of 3 regardless the conditions. A sensitivity study is performed using a verified numerical model as
a benchmark to study several problems with different initial and boundary conditions (initial, phase change and
freeze pipe temperatures) and thermal properties of the ground (water content, thermal conductivity and heat
capacity). This is done for the freezing times of 10 and 365 days, in order to consider the potential change of the
ratio with the freezing time. In this way, a calibrated formula is proposed to find appropriate values of this ratio
and a suitable adjustment to Sanger & Saylesâ solution is determined. Adjusting Sanger & Saylesâ solution in this
manner, a significantly higher and more consistent accuracy is achieved for different boundary and initial
conditions. This accuracy improvement was checked for real conditions from an engineering project, which
shows that the adjustment can be useful for thermal problems in engineering design of ground freezing
Radiation hydrodynamics integrated in the code PLUTO
The transport of energy through radiation is very important in many
astrophysical phenomena. In dynamical problems the time-dependent equations of
radiation hydrodynamics have to be solved. We present a newly developed
radiation-hydrodynamics module specifically designed for the versatile MHD code
PLUTO. The solver is based on the flux-limited diffusion approximation in the
two-temperature approach. All equations are solved in the co-moving frame in
the frequency independent (grey) approximation. The hydrodynamics is solved by
the different Godunov schemes implemented in PLUTO, and for the radiation
transport we use a fully implicit scheme. The resulting system of linear
equations is solved either using the successive over-relaxation (SOR) method
(for testing purposes), or matrix solvers that are available in the PETSc
library. We state in detail the methodology and describe several test cases in
order to verify the correctness of our implementation. The solver works in
standard coordinate systems, such as Cartesian, cylindrical and spherical, and
also for non-equidistant grids. We have presented a new radiation-hydrodynamics
solver coupled to the MHD-code \PLUTO that is a modern, versatile and efficient
new module for treating complex radiation hydrodynamical problems in
astrophysics. As test cases, either purely radiative situations, or full
radiation-hydrodynamical setups (including radiative shocks and convection in
accretion discs) have been studied successfully. The new module scales very
well on parallel computers using MPI. For problems in star or planet formation,
we have added the possibility of irradiation by a central source.Comment: 13 pages, 11 figures, accepted by Astronomy & Astrophysic
A mathematical model for the dissolution of particles in multi-component alloys
AbstractDissolution of stoichiometric multi-component particles is an important process occurring during the heat treatment of as-cast aluminum alloys prior to hot extrusion. A mathematical model is proposed to describe such a process. In this model equations are given to determine the position of the particle interface in time, using a number of diffusion equations which are coupled by nonlinear boundary conditions at the interface. This problem is known as a vector valued Stefan problem. A necessary condition for existence of a solution of the moving boundary problem is proposed and investigated using the maximum principle for the parabolic partial differential equation. Furthermore, for an unbounded domain and planar co-ordinates an asymptotic approximation based on self-similarity is derived. The asymptotic approximation is used to gain insight into the influence of all components on the dissolution. Subsequently, a numerical treatment of the vector valued Stefan problem is described. The numerical solution is compared with solutions obtained by the analytical methods. Finally, an example is shown
Scale/Analytical Analyses of Freezing and Convective Melting With Internal Heat Generation
Using a scale/analytical analysis approach, we model phase change (melting) for pure materials which generate constant internal heat generation for small Stefan numbers (approximately one). The analysis considers conduction in the solid phase and natural convection, driven by internal heat generation, in the liquid regime. The model is applied for a constant surface temperature boundary condition where the melting temperature is greater than the surface temperature in a cylindrical geometry. The analysis also consider constant heat flux (in a cylindrical geometry).We show the time scales in which conduction and convection heat transfer dominate
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