Analytical modelling of PCM supercooling including recalescence for complete and partial heating/cooling cycles

ACLPhase change material (PCM) experiencing supercooling and phase change hysteresis are widely reported in the literature. However, only few studies model analytically such PCM, and they mostly focus on either supercooling or phase change hysteresis, but rarely on both phenomena. Moreover, partial phase change cycles, with an incomplete melting or solidification, are rarely considered even if these processes occur frequently in latent heat thermal energy storage (LHTES) systems. The objective of this study is to model analytically the thermal behaviour of a PCM experiencing supercooling and phase change hysteresis, for complete and partial phase change cycles. The developed method, based on a heat source term, enables to model the recalescence process during the solidification. Currently rarely considered, the influence of the cooling rate on the supercooling degree and the recalescence process is evaluated with a phenomenological approach. For partial cycles, the different behavior between heating and cooling is modelled with the hysteresis model “curve scale” already validated in literature. The experimental validation is carried out on a PCM brick sample, monitoring both the heat flux and the PCM temperature, which enables to characterize a greater mass of PCM compared to direct scanning calorimeter (DSC) analysis. The selected PCM undergoing supercooling during solidification is PEG6000, a polymer suitable for domestic hot water (DHW) storage. Results show a good agreement between experimental and numerical results for complete heating and cooling cycles. The behavior laws used to model the solidification with the supercooling and recalescence processes are validated for the cooling rate range tested. The modelling is also satisfactory concerning the experiments on partial solidification for different temperature plateaus. However, the model fails to correctly represent the thermal behavior for a cooling process after a partial melting of the PCM. To conclude, the developed model enables to represent accurately the thermal behaviour of a PCM experiencing supercooling and phase change hysteresis for most of the phase change processes studied. Investigations on the thermal conditions influencing the crystalline structure and the effect on the phase change dynamic are suggested to improve supercooling modelling. The developed model needs to be validated for PCM having a higher supercooling degree, such as sugar alcohols or salt hydrates

Innovative use of fluorescein for the air path study within light-weight wall assemblies

The impact of air infiltration on the hygro-thermal performance of a wall is closely linked to the air dispersion inside it, but there is a lack of experimental studies and methods for the air path investigation within light-weight wall assemblies. A new technique has therefore been developed, consisting in an innovative use of fluorescein micro-particles as tracer inside the insulation material. It is a destructive method but it has the great advantage of not being intrusive unlike the use of any type of sensor. The experimental protocol is detailed in this paper. This technique was tested on a number of preliminary tests which showed consistent results and a good repeatability of the measurement. A fluorescein transport model was developed to facilitate the comparison between the experimental fluorescein concentration mappings and the numerical velocity fields. This method was then applied to a specific configuration: an air channel in contact with porous media. A simple analysis of the resulting fluorescein concentration mappings enabled to draw conclusions on the impact of parameters such as the flow velocity or the insulation material on the air infiltration. It has also given evidences of phenomena such as the appearance of thin air gaps between the components of the wall assembly. The results were compared to a numerical study with the fluorescein transport model coupled to a CFD model

Towards a better analytical modelling of the thermodynamic behaviour of phase change materials

ACLModelling of latent heat thermal storage systems is mainly achieved with an analytical formulation describing the thermodynamic state of the phase change material (PCM). In literature, different models exist to describe the evolution of the liquid fraction, the effective heat capacity and the enthalpy as a function of temperature. The main issue is to correctly establish these behaviour laws and to identify the thermophysical properties of each PCM. Analytical models proposed in literature often have obvious limitations, such as the presence of discontinuities or the impossibility to represent asymmetric phase change dynamics. Such approaches a priori do not allow to model accurately the thermodynamic behaviour of the PCM. Given the limitations identified on existing models, a new analytical model is proposed in this paper. Moreover, it is not uncommon to observe the presence of two heat flux peaks during the phase change process of a PCM, as with the studied PCM (RT58). Therefore, we propose a method to model the two heat flux peaks, which are rarely considered in literature. PCM properties are determined thanks to a four-step identification process, using experimental data and the different analytical models studied. Based on the numerical modelling of the rectangular container used for the experiments, and on the identified thermodynamic properties of the enclosed PCM, a comparative study between the new analytical model and existing models is carried out. It appears that modelling only a single heat flux peak is not satisfactory for the studied PCM, and taking into account a second heat flux peak decreases the error on the exchanged energy by a factor of 2.5 to 6.5 depending on models. The new model leads to an accuracy gain of a factor of 1.5 to 3.3 compared with existing models. We do not recommend the model based on a linear evolution of the liquid fraction as the resulting error is between 1.6 and 3.3 times greater than for the other models. The study carried out is based on both melting and solidification processes, and concerns a PCM which does not undergo supercooling

Impact of different construction details on air permeability of timber frame wall assemblies: Some experimental evidences from a three-scale laboratory study

International audiencePoor airtightness in buildings can lead to an over-consumption of energy and to many issues such as moisture damage and poor indoor climate. The timber frame constructions are particularly subject to air leakages, and further knowledge in this field is needed to meet the regulation requirements tightened by the development of low-energy and passive houses. This article focuses on a three-scale experimental study carried out in laboratories to quantify the impact of a number of construction details on timber frame wall airtightness. For this purpose, we built two original experimental setups and to complement an existing large-scale facility. Each setup enables to carry out pressurization tests at a different scale. The results put all together give quantitative information for more accurate simulations of building performance. Some specific construction details were investigated. It has been found in particular that the density of the insulation material is significant since a soft glass wool can have an air permeability three times higher than a rigid one with the same thermal performances. Moreover, it has been pointed out that the bond between the gypsum board and the insulation has a significant impact on the resulting pressure–flow law, and to ensure that there is no air gap the whole interface should be glued. The air flow directions also influence the flow values for high-pressure differences. Finally, at wall scale we have found that the sealing of the gypsum boards and the vapor barrier against the bottom wall plate is not very significant as long as the exterior side is sealed correctly. On the other hand, a proper sealing on both sides of a window is required because of the air gaps along it

Hygric characterization of wood fiber insulation under uncertainty with dynamic measurements and Markov Chain Monte-Carlo algorithm

International audienceThe present work is the hygric characterization of wood fibre insulation boards, using dynamic measurements of relative humidity and sample weight, analyzed in the frame of Bayesian inference for parameter identification under uncertainty. It is an attempt at identifying detailed profiles of moisture-dependent properties , and thus a relatively high number of parameters. Because of this ambition, some caution should be exercised once the outcome of the inversion algorithm is available: in addition to confidence intervals of parameters provided by the Bayesian framework, a simplified form of identifiability analysis is performed by analysing a posteriori parameter correlations and likelihood-based confidence intervals. The characterization methodology does not require for the model structure to have a differentiable analytical formulation, or for material samples to reach mass equilibrium between each RH step of the experimental process. Two separate experimental designs were used for material characterization and for validation, respectively. Results show a clear relation between available information (experimental data) and inference (confidence intervals of parameters). A single relative humidity step is not informative enough for a precise inference of moisture-dependent properties such as vapour permeability and moisture capacity. A two-step experiment however holds enough information to significantly reduce parameter uncertainty

A two-phase flow description of the initiation of underwater granular avalanches

International audienceA theoretical model based on a depth-averaged version of two-phase flow equations is developed to describe the initiation of underwater granular avalanches. The rheology of the granular phase is based on a shear-rate-dependent critical state theory, which combines a critical state theory proposed by Roux & Radjai (1998), and a rheological model recently proposed for immersed granular flows. Using those phenomenological constitutive equations, the model is able to describe both the dilatancy effects experienced by the granular skeleton during the initial deformations and the rheology of wet granular media when the flow is fully developed. Numerical solutions of the two-phase flow model are computed in the case of a uniform layer of granular material fully immersed in a liquid and suddenly inclined from horizontal. The predictions are quantitatively compared with experiments by Pailha, Nicolas & Pouliquen (2008), who have studied the role of the initial volume fraction on the dynamics of underwater granular avalanches. Once the rheology is calibrated using steady-state regimes, the model correctly predicts the complex transient dynamics observed in the experiments and the crucial role of the initial volume fraction. Quantitative predictions are obtained for the triggering time of the avalanche, for the acceleration of the layer and for the pore pressure

An efficient numerical model for liquid water uptake in porous material and its parameter estimation

41 pages, 19 figures, 3 tables, 35 references. Other author's papers can be downloaded at http://www.denys-dutykh.com/International audienceThe goal of this study is to propose an efficient numerical model for the predictions of capillary adsorption phenomena in a porous material. The Scharfetter-Gummel numerical scheme is proposed to solve an advection-diffusion equation with gravity flux. Its advantages such as accuracy, relaxed stability condition, and reduced computational cost are discussed along with the study of linear and nonlinear cases. The reliability of the numerical model is evaluated by comparing the numerical predictions with experimental observations of liquid uptake in bricks. A parameter estimation problem is solved to adjust the uncertain coefficients of moisture diffusivity and hydraulic conductivity