Experimental investigation of the steady state behaviour of Breathing Walls by means of a novel laboratory apparatus

Abstract

Breathing Walls are envelope components, based on porous materials, crossed by a natural or forced airflow. Since they behave both as recovery heat exchangers and active insulation, reducing the conductive heat flux, they represent a promising envelope technology, allowing to reduce energy consumption in buildings.From the modeling point of view, an analytical model can be found in literature, describing heat and mass transfer across Breathing Walls in steady state conditions. However, to the best of the authors' knowledge, the model lacks an exhaustive experimental validation. Therefore, in this paper, the novel laboratory apparatus named Dual Air Vented Thermal Box developed at Politecnico of Milano is presented. The apparatus is used to experimentally investigate the steady state behavior of a 1 m 2 Air Permeable Concrete sample, crossed by an airflow at different velocities up to 12 mm/s.The temperature profile inside the sample, measured in different positions, is compared with the model predictions. While in the central portion of the wall a very good agreement is found, the experimental results at the top and at the bottom of the wall suggest a non-uniform velocity field entering the sample. A qualitative confirmation of this hypothesis is provided by CFD simulations on the apparatus, clearly showing a mixed convection regime on both sides of the wall. The results lead to state the validity of the one-dimensional analytical model in literature, although a careful application should take into account adjusted boundary conditions, consisting in an airflow velocity possibly variable with height.

Experimental validation of a steady periodic analytical model for Breathing Walls

“…Overall, it was demonstrated how this technical solution is able to recover part of the conductive heat losses by pre-heating the fresh air coming from outside, so that the Breathing Wall can be seen as a recovery heat exchanger whose efficiency depends on the air flow velocity [11]. Moreover, in winter contra-flux steady state condition it was demonstrated both analytically [8] and experimentally [12,13] that the heat exchange at pore scale leads to an exponential trend of the temperature distribution and to the cooling of the inner surface [14,15]. Since the air crosses inward the solid matrix at very low speed, the permeable layer acts also as high efficiency filter for PM2-10 [16].…”

“…The steady-state solution was already presented in past works [8,26] and experimentally validated in [12,13]. Starting from Eq.…”

“…Since the steady-state behaviour of Breathing Walls has already been thoroughly discussed in the past [8,12,13], no further investigation will be presented in this paper.…”

“…As far as modeling of BW is concerned, a onedimensional analytical model for heat transfer in steady state was proposed in the 90's under first type [8] and third type [22] boundary conditions. Such model was recently experimentally validated in laboratory, although the opportunity to consider a velocity distribution of the inlet air was observed [12]. Modeling BW in transient state has mainly been approached numerically through finite difference methods [20,23], even includ-ing the effect of the air gap channel between the porous layer and the external cladding [18].…”

“…The experimental validation of the steady periodic analytical model reported in this paper is performed on a BW sample tested in the Dual Air Vented Thermal Box. The laboratory rig, firstly presented in [12,13], has been specially improved in order to reproduce dynamic boundary conditions. The performance of the apparatus and the results of the comparison between the measurements and the model calculations are presented and discussed in the paper.…”