ANALYSIS OF HEAT AND MASS FLUXES DURING MICROWAVE DRYING

Abstract

The kinetics of combined convective and micro-wave drying presents universally more than the two habitual drying phases. Superimposed to a general heating of the product, a brutal and short acceleration of the drying rate raises all flux densities at a very important level. This is particularly evident with the class of product we used in this study : non porous polymeric gels.

Moreover, an unusual correlation between the kinetics and the surface temperature around the micro-wave specific phase might appear : unexpectedly, the evaporation flux density decreases when the surface temperature increases noticeably.

In order to elucidate this apparent complexity and understand the underlying physical phenomena, we analysed term by term all fluxes according to their origin. The splitting was achieved through mathematical modelling and partially validated by the recording of temperatures and mass fluxes.

Usually with convective drying, all rate changes are governed by the state o f the product : hygroscopicity and porosity development. Here in the case of combined micro-wave drying, all accidents on the drying curves are determined by the heat generation and the heat transfer situation. The constant rate period is also governed by the balance of heat fluxes. The accelerated period corresponds to a heat wave arriving to the surface. Most of the time, the product gives out heat to the surrounding air : this heat is lost for the drying but this cooling effect may he used for the protection of the sample.     

Modelisation of Ni-P electroless deposition in ammoniacal solutions

A reaction path for the electroless deposition of Ni-P layers is developed. The steady-state polarisation and impedance responses are calculated. The comparison between the experimental results and the calculation shows the adequacy of the model. The mechanism accounts for most of the experimental features observed for the steady-state and the impedance behaviours and their dependencies with pH, hypophosphite and nickel sulphate concentrations.     

Physicochemical characterization of the layer-by-layer self-assembly of polyphenol oxidase and chitosan on glassy carbon electrode

This paper demonstrates the feasibility to build up layer-by-layer (L-b-L) self-assemblies of polyphenol oxidase (PPO) and quaternized chitosan (CHI⁺) on a glassy carbon (GC) rotating disk electrode. This work highlights the promising properties of this modified polysaccharide to immobilize PPO in self-assembled structures under conditions that preserve its catalytic activity. The film consists of a diffusional underlayer coupled with an outermost active layer of (PPO/CHI⁺)_n. UV-vis spectroscopy and quartz crystal microbalance (QCM) were used to follow the multilayer film formation. Referring to the low amount of enzyme immobilized in the multilayer structure, the amperometric response of the biosensor reached an excellent sensitivity of about 2000 A mol⁻¹ cm, proving that an important part of the entrapped enzyme is accessible to the substrate molecule while keeping a good level of activity. The experimental validation of the theoretical model that was carried out on the basis of the catechol and phenol-polyphenol oxidase model system shows that the amperometric responses are in excellent agreement with the model. From a comparison of the experimental results with theory, we were able to characterize the diffusion through the film which includes two different areas and extract its enzymatic activity related to catechol and phenol substrates.     

Post‐construction liability and insurance

A new method for stochastic embodied greenhouse gas emissions modelling of construction materials is proposed. The method progressively integrates process analysis data into a prior probability distribution of embodied greenhouse gas emissions for a given material drawn from a disaggregated expansion of the UK Environmental Accounts. Current analyses of embodied CO_2eq emissions, whether performed at the economic sector or individual building system boundary level, are deterministic. Such deterministic models do not allow for the easy analysis of potential CO_2eq emissions reductions obtainable through preferential selection of low embodied CO_2eq material suppliers within material supply chains. Such analyses are important for assessing the extent to which emissions reductions can be achieved through economic mechanisms which drive preferential selection of low embodied carbon materials back through the supply chain - such as carbon taxes. It is argued that the need for estimating both the uncertainty, and the variability, in embodied CO_2eq data is best addressed through the development of a fully stochastic modelling framework, and that such frameworks are methodologically more appropriate for Kyoto Protocol compliance policy formulation. Cet article traite d'une nouvelle méthode de modélisation stochastique des émissions de gaz à effet de serre intrinsèque applicable aux matériaux de construction. Elle intègre progressivement des données résultant d'analyses de procédés dans une distribution de probabilité préalable d'émissions de gaz à effet de serre intrinsèque pour un matériau donné tiré d'une expansion désaggrégée des Environmental Accounts du Royaume Uni. Les analyses actuelles des émissions de CO_2eq intrinsèque, réalisées soit au niveau du secteur économique soit à celui de la limite des systèmes de construction individuels, sont déterministes. De tels modèles déterministes ne permettent pas d'analyser facilement les réductions des émissions de CO_2eq potentielles que l'on pourrait obtenir via une sélection préférentielle de fournisseurs de matériaux à faible CO_2eq intrinsèque au sein de chaônes d'approvisionnement de matériaux. De telles analyses sont importantes pour évaluer dans quelle mesure ces réductions d'émissions peuvent être réalisées au moyen de mécanismes économiques qui renvoient la sélection préférentielle de matériaux à faible teneur en carbone intrinsèque vers la chaône d'approvisionnement, comme la taxe sur les hydrocarbures. On prétend que le besoin d'estimer à la fois l'incertitude et la variabilité dans les données relatives au CO^2eq intrinsèque est mieux appréhendé par le développement d'un cadre de modélisation entièrement stochastique et que de tels cadres sont méthodologiquement plus appropriés pour la formulation de principes conformes au Protocole de Kyoto.     

MODELISATION PRINCIPLES FOR DRYING OF GELS

ABSTRACT

When one wants to understand and model the drying of high water content gels, one needs to gather all thermal physical parameters and to derive a physical model based on independently measured parameters. We chose to work experimentally and theoretically on water diffusion and drying of spheres of polyacrylamide gel.

We have demonstrated the strong influence of vitreous transition on the relationship of water diffusivity with moisture content. Values of the diffusion coefficient and temperature shift factor for diffusion have been proposed.

Convective and micro-wave drying kinetics show the presence of a drying phase purely specific to radiation absorption. Finally, we have proposed a mathematical model containing independent physical parameters, which' not only predicts mass fluxes but also analyses critical physical phenomena.     

D2SNet: Dynamics of diffusion and dynamic human behaviour in social networks

In this paper, we present an original and formal framework, the D2SNet model designed to combine both the social network evolution and the diffusion dynamics among individuals. We have conducted experiments on three social networks that show identical characteristics as real social networks. A formal definition of the model is provided and we describe its implementation in a simulation tool. We represent human behaviors and information dissemination strategies by standard and synthetic scheme. In a first step, we study the impact of network growing strategies only and we highlight important parameters such as the evolution speed and mainly the kind of strategies that favour or not the spread. Then we study a more complete evolution strategy that involves link creation and deletion. We provide a deep analysis on the impact of each parameter such as evolution speed, creation and deletion probabilities and dynamic human behaviors on the diffusion amplitude and coverage. Our study gives a novel and useful insight in the diffusion process in a dynamic context.     

Mathematical modeling of scroll compressors — part II: overall scroll compressor modeling

This paper presents the development of a comprehensive simulation model of a horizontal scroll compressor, which combines a detailed compression process model (Chen Y., Halm N., Groll E., Braun J. Mathematical modelling of scroll compressors — part I: compression process modeling, International Journal of Refrigeration 2002;25(6):731–750) and an overall compressor model. In the overall model, compressor components are analyzed in terms of nine different elements. Steady state energy balance equations are established applying the lumped capacitance method. In combination with the detailed compression process model, these equations were implemented into computer code and solved recursively. In this way, the temperature and pressure of the refrigerant in different compressor chambers, the temperature distributions in the scroll wraps, and the temperatures of the other compressor elements can be obtained. Thereafter, power consumption and efficiency of the compressor can be calculated. Tests were used to verify the overall model on a macroscopic basis. Using the simulation program based on the overall compressor model, a parametric study of the scroll compressor was performed, and the effects of internal leakage and heat transfer losses were investigated and some preliminary results were obtained. These results indicate that the comprehensive scroll compressor model is capable of predicting real compressor behavior and useful to the design and optimization of scroll compressors.