Modelling and simulation of a ventilated double window Jorge S. Carlos a, * , Helena Corvacho b , Pedro D. Silva a , J.P. Castro-Gomes a a C-MADE, Centre of Materials and Building Technologies, University of Beira Interior, Covilhã, Portugal b Faculty of Engineering of the University of Porto (FEUP), Civil Engineering Department, Building Division, Laboratory of Building Physics, Porto, Portugal article info Article history: Received 8 July 2009 Accepted 20 August 2010 Available online 18 September 2010 Keywords: Ventilated double window Heat transfer Airflow Numerical simulation abstract Double windows are a currently adopted construction system in Portuguese residential buildings to prevent air leakage and thermal discomfort. We have changed this construction system so that it could be able to pre-heat the ventilation air between the windows. Simple changes consist in introducing vents at the base of the outer window to allow a supply of fresh air. Heat that escapes from inside through the inner window and solar radiation heat up the air between the two windows. Due to wind pressure and stack effect, the air rises and enters the room through a vent at the inside top of the system warmer than the outdoor air. A simulation program was built and validated with the use of test facilities, where different configurations of the system were tested. This paper presents the mathematical model and some results of a simulation work based on it. Parametrical studies were carried out by varying the airflow rate, the outdoor air temperature and the solar irradiance to predict the temperature rise and the useful energy of the delivered air. Results indicate an encouraging thermal performance of the ventilated double window offering itself as an alternative to cold natural ventilation. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The renovation of the air inside a building requires the admit- tance of fresh air from outside. If the air enters the building directly from outside its temperature is certainly close to the outdoor air temperature. In winter season, this can be a problem which contributes to undesirable and uncomfortable cold ventilation associated to heating energy consumption needs. The ventilated double window is composed of two parallel windows forming a gap through which airflows. The gap that is formed between the windows is supplied with fresh air from outside through vents on the base of the outer window. The air that circulates through this gap is warmed by the heat extracted from the window pane surfaces which are warmed by the heat loss transmitted from indoors and by solar gains. Pre-heated rising airflow, by stack effect and wind pressure enters the room through a vent on the top of the roller shutter’s case, so the ventilated double window can provide the required ventilation air warmer than the air outside. The calculation of the heat transfer through a ventilated double window is a complex task. Natural convection is difficult to model due to the buoyancy driven ventilation and the external wind pressure on the building adds further complexity to the situation. The tempera- tures and airflows are the result of many simultaneous thermal and flow processes. Furthermore, the climactic conditions that interact are highly dynamic. Heat transfer to and from the ventilated air gap is probably the most complex part of the overall process. These also depend on geometric and physical properties of the various compo- nents of the ventilated double window. Since there are so many parameters and variables involved for such a configuration, computer modelling is needed to predict the performance of a ventilated double window. Complex fluid and thermal computer models can be applied simultaneously, working interactively with each other [1]. These can be less comfortable and not suitable for use by building designers, so a simpler approach is needed to capture salient aspects of the thermal behaviour of such a window system. This can be done using the simplified model presented in ISO 15099 [2]. In this standard guide- lines are given to analyse window’s performance and, in the case of the double ventilated window, the variation of the air temperature along the height of the air cavity of the window is considered making it a two-dimensional simplified approach. A detailed computational code was set up to characterize the performance of the ventilated double window. After validating the code by comparing its results with the results obtained through well-known and fully validated software tools and with experi- mental measurements its performance may be analyzed in a reli- able way for different combinations of characteristics of the system. The computational code consists of two main parts:  Airflow analysis;  Thermal analysis. * Corresponding author. E-mail address: jcarlos@ubi.pt (J.S. Carlos). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng 1359-4311/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2010.08.021 Applied Thermal Engineering 31 (2011) 93e102