توسعه و اعتبار تجربی یک مدل شبیه سازی برای باز نمای تهویه مشترک

The investigation of the thermal and fluid dynamical behaviour of open joint ventilated façades is a challenging task due to the complex airflows generated inside of the naturally ventilated cavity by the existence of open joints. For this reason, the use of advanced fluid measurement and simulation techniques is highly recommended. This paper focuses in the development and experimental validation of a simulation model for these façade systems. More specifically, different turbulence and radiation models available in the commercial computational fluid dynamic codes have been tested on a three-dimensional model and the results have been compared to particle image velocimetry measurements. The correlation between experimental and numerical data has been used in order to select the simulation procedure for this type of façades. Best fittings have been found when using the RNG k-epsilon turbulence model and the Discrete Ordinate radiation model. Using the selected scheme, parametrical simulations have been performed to investigate the effect of increasing the cavity height, and correspondingly, the number of slabs. Results show that ventilation air flow inside the cavity is enhanced by incident radiation as well as by the height of the façade.

Generalising about the fluid and thermal performance of open joint ventilated façades (OJVF) is somehow difficult because of the big range of constructive solutions existing in the market. Slabs can be metallic, ceramic or made from stone. Additionally, the dimensions and proportions of the slabs, the shape and size of the open joints, as well as the metallic structure frame supporting the exterior coating differ from manufacturer to manufacturer. Apart from the difficulties derived from the constructive solutions, the existence of open joints distributed along the exterior coating has a great influence in the fluid and thermal behaviour of this façade system in comparison to other continuous ventilated façades, such as double glazed ventilated façades (DGVF), whose behaviour is rather well known, as detailed in the studies made by Manz [1], Safer et al. [2], Baldinelli [3], Fuliotto et al. [4] and Coussirat et al. [5] among other authors. The existence of open joints enables the outdoor air to freely enter and leave the ventilated cavity all along the wall, producing discontinuities and instabilities in the flow inside the ventilated cavity, which is highly dependent not only in the façade geometry by also on the solar incident radiation, outdoor temperature and wind conditions. All these factors, summed to the general lack of data related to these construction systems and the absence of validated models, evidence that there is still a lot of work to do before having a global criterion to determine the energy behaviour of OJVF.

The work presented in this article aim to shed some more light on the topic OJVF. A tree-dimensional CFD model has been developed to simulate OJVF under radiation conditions and it has been validated with PIV experimental data. The suitability of the different turbulence and radiation models available in the commercial CFD codes has been investigated. Simulations have been performed on a 3D model with the same geometry, materials and boundary conditions as the experimental unit used by Sanjuan et al. [18]. The comparison between experimental and numerical data indicates that the k-epsilon two equation turbulence models and the DO radiation models show best fittings in temperature and velocity profiles. Still, all the turbulence models fail to predict the size of the recirculation vortexes formed at the entrance of the slabs. Using the selected turbulence and radiation models, a parametrical simulation on a 2D model of a real façade has been performed. The effect of varying the cavity height (increasing the number of slabs), and different levels of incident radiation has been investigated. The results show that the ventilation produced by buoyancy (chimney effect) is enhanced by cavity height and incident radiation. Additionally, it has been found that, regardless of the number of joints, the flow enters the cavity distributed along the slabs below the central height (the percentage of entering flow diminishes with the height), and leaves the cavity also distributed along the upper joints (the percentage of the flow increases with the height). The presented 2D results (mass flow though the joints, ventilation mass flow and pressure profiles) can be used as benchmarking to validate other models, such as those based on energy balances (analytical, non-dimensional, etc.), lumped models, or those based on nodal airflow networks.