Caratterizzazione, modellazione numerica e valutazione delle prestazioni di una parete traspirante con isolante fibroso

Dynamic insulation is a form of breathing building envelope, characterised by the movement of air though the pores of insulation layers, so that the wall acts both as a filter and a heat exchanger. This technology was born in the 60’: since then, it has been investigated the performances of the system, related to the reduction of building energy requirements and to the high wall filter efficiency, and the mean lacks, related especially to the interior surface temperature reduction and to the deeper complexity of building design. This research deals with heat and mass transfer mechanisms inside porous materials and their theoretical approach, in order to treat the physical problem considering microscopic interactions between the solid and fluid phase, air flux effects and real geometry of the solid matrix. The volume average technique and the concept of representative elementary volume are applied to define two equations on which fluid dynamic and energetic problems are based. It has been defined four quantities that characterise the porous media and describe macroscopically the microscopic interactions in the medium: permeability and Ergun coefficient and thermal dispersion and tortuosity. Rockwool fibrous insulation, which is the object of this work, has been characterised in terms of physical properties of both phases and REV dimension. This last has been derived by the image analysis technique, on real samples’ sections’ photographs, processing the results, about porosity, granulometric distribution and autocorrelation function, through a statistical approach. Calculation mesh have been chosen and modelled, in order to perform 144 CFD parametric simulations, for six levels of air velocity and two thermal gradients: in each REV, velocity, pressure and temperature domains have been obtained. Numerical results have been used to calculate permeability and Ergun coefficient and to correlate thermal tortuosity and dispersion to macroscopic quantities (porosity and Péclet number). Based on these results, the effective performances of breathing wall have been studied, modelling the system in a finite differences’ algorithm developed in Matlab code. The analyses, carried out in winter, in contraflux regime, have illustrated that with dynamic insulation better performances, in terms of energy requirements, could be achieved only for low air velocity, especially for big and high ventilated buildings. Greater energy savings should be obtained in conjunction with conventional air heat recovery methods. At low velocity the reduction of interior surface temperature doesn’t imply serious problems of thermal comfort.