Definition of parameters useful to describe dynamic thermal behavior of hollow bricks

Dynamic thermal behavior of hollow bricks is attracting much interest nowadays as there is much concern on energy performance of building envelope. In fact, high thermal inertia of outer walls provides mitigation of the daily heat wave, which reduces the cooling peak load and the related energy demand. Different approaches have been used to study dynamic thermal behavior within the papers available on unsteady heat transfer through hollow bricks. Actually, the usually employed methods for calculation of unsteady heat transfer through walls are based on the hypothesis that such walls are composed by homogeneous layers, so they are not suitable for many common building components. In this framework, a study on the dynamic thermal performance of hollow bricks is brought forth in the present paper. A critical review of available data from the literature is provided. Literature data are used to propose a parameter useful to predict dynamic thermal behavior. A finite-volume method is used to solve two-dimensional unsteady thermal fields in some standard bricks with different imposed temperature solicitations, with a numerical code developed by the authors. New results are used to check the effectiveness of the proposed parameters

Cloaking via mapping for the heat equation

This paper explores the concept of near-cloaking in the context of time-dependent heat propagation. We show that after the lapse of a certain threshold time instance, the boundary measurements for the homogeneous heat equation are close to the cloaked heat problem in a certain Sobolev space norm irrespective of the density-conductivity pair in the cloaked region. A regularised transformation media theory is employed to arrive at our results. Our proof relies on the study of the long time behaviour of solutions to the parabolic problems with high contrast in density and conductivity coefficients. It further relies on the study of boundary measurement estimates in the presence of small defects in the context of steady conduction problem. We then present some numerical examples to illustrate our theoretical results.Comment: 31 pages, 7 figures, 1 tabl

Thermal Instability in Gravitationally-Stratified Plasmas: Implications for Multi-Phase Structure in Clusters and Galaxy Halos

We study the interplay among cooling, heating, conduction, and magnetic fields in gravitationally stratified plasmas using simplified, plane-parallel numerical simulations. Since the physical heating mechanism remains uncertain in massive halos such as groups or clusters, we adopt a simple, observationally-motivated prescription which enforces global thermal equilibrium when averaged over large scales. The plasma remains susceptible to local thermal instability, however, and cooling drives an inward flow of material. In contrast to previous results, we argue that the thermal stability of the plasma is independent of its convective stability. We find that the ratio of the cooling timescale to the dynamical timescale t_cool/t_ff controls the saturation of the thermal instability: when t_cool/t_ff < 1, the plasma develops extended multi-phase structure, whereas when t_cool / t_ff > 1 it does not. (In a companion paper, we show that the criterion for thermal instability in a spherical potential is somewhat less stringent, t_cool / t_ff < 10.) When thermal conduction is anisotropic with respect to the magnetic field, the criterion for multi-phase structure is essentially independent of the thermal conductivity of the plasma. Our criterion for local thermal instability to produce multi-phase structure is an extension of the cold vs. hot accretion modes in galaxy formation that applies at all radii in hot halos, not just to the virial shock. We show that this criterion is consistent with data on multi-phase gas in the ACCEPT sample of clusters; in addition, when t_cool / t_ff > 1, the net cooling rate to low temperatures and the mass flux to small radii are suppressed enough relative to models without heating to be qualitatively consistent with star formation rates and x-ray line emission in groups and clusters.Comment: This is an electronic version of an article published in Monthly Notices of the Royal Astronomical Society, Volume 419, Issue 4, pp. 3319-333

Effectiveness of CFD simulation for the performance prediction of phase change building boards in the thermal environment control of indoor spaces

This is the post-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2013 ElsevierThis paper reports on a validation study of CFD models used to predict the effect of PCM clay boards on the control of indoor environments, in ventilated and non-ventilated situations. Unlike multi-zonal models, CFD is important in situations where localised properties are essential such as in buildings with complex and large geometries. The employed phase change model considers temperature/enthalpy hysteresis and varying enthalpy-temperature characteristics to more accurately simulate the phase change behaviour of the PCM boards compared to the standard default modelling approach in the commercial CFD codes. Successful validation was obtained with a mean error of 1.0 K relative to experimental data, and the results show that in addition to providing satisfactory quantitative results, CFD also provides qualitative results which are useful in the effective design of indoor thermal environment control systems utilising PCM. These results include: i) temperature and air flow distribution within the space resulting from the use of PCM boards and different night ventilation rates; ii) the fraction of PCM experiencing phase change and is effective in the control of the indoor thermal environment, enabling optimisation of the location of the boards; and iii) the energy impact of PCM boards and adequate ventilation configurations for effective night charging.This work was funded through sponsorship from the UK Engineering and Physical Sciences Research Council (EPSRC), Grant No: EP/H004181/1

Implementation of a new urban energy budget scheme in the MetUM. Part I: Description and idealized simulations

This paper describes the formulation of a new urban scheme, MORUSES (Met Office–Reading Urban Surface Exchange Scheme) for use in the Met Office Unified Model. The implementation of the new scheme ensures that (1) the new scheme offers more flexibility in the parametrization of the building properties, and hence provides a more realistic representation of the fluxes; (2) the bulk outputs are in satisfactory agreement with previous observational studies; and (3) the impact of the new scheme on the energy balance fluxes is similar to the impact of the current urban scheme when set up to mimic it. As well as having a better physical basis, MORUSES also gains in flexibility in applications and adaptations to different urban materials as well as urban planning. The new scheme represents the urban area as a composition of two tiles, a canyon and a roof, using a simple 2D geometry. Sensitivity analysis to canyon geometry and thickness of the roof canopy emphasizes the gain in flexibility captured by the new scheme. Copyright © 2010 Royal Meteorological Society and Crown Copyrigh

Model Reduction for Multiscale Lithium-Ion Battery Simulation

In this contribution we are concerned with efficient model reduction for multiscale problems arising in lithium-ion battery modeling with spatially resolved porous electrodes. We present new results on the application of the reduced basis method to the resulting instationary 3D battery model that involves strong non-linearities due to Buttler-Volmer kinetics. Empirical operator interpolation is used to efficiently deal with this issue. Furthermore, we present the localized reduced basis multiscale method for parabolic problems applied to a thermal model of batteries with resolved porous electrodes. Numerical experiments are given that demonstrate the reduction capabilities of the presented approaches for these real world applications

Transient thermal modelling of substation connectors by means of dimensionality reduction

This paper proposes a simple, fast and accurate simulation approach based on one-dimensional reduction and the application of the finite difference method (FDM) to determine the temperatures rise in substation connectors. The method discretizes the studied three-dimensional geometry in a finite number of one-dimensional elements or regions in which the energy rate balance is calculated. Although a one-dimensional reduction is applied, to ensure the accuracy of the proposed transient method, it takes into account the three-dimensional geometry of the analyzed system to determine for all analyzed elements and at each time step different parameters such as the incremental resistance of each element or the convective coefficient. The proposed approach allows fulfilling both accuracy and low computational burden criteria, providing similar accuracy than the three-dimensional finite element method but with much lower computational requirements. Experimental results conducted in a high-current laboratory validate the accuracy and effectiveness of the proposed method and its usefulness to design substation connectors and other power devices and components with an optimal thermal behavior.Postprint (published version