International Journal of Thermal Sciences 152 (2020) 106288 Available online 21 February 2020 1290-0729/© 2020 Elsevier Masson SAS. All rights reserved. Experimental and numerical natural convection in an asymmetrically heated double vertical facade Yassine Cherif a, ** , Emilio Sassine b, * , Stephane Lassue a , Laurent Zalewski a a Univ. Artois, ULR 4515, Laboratoire de G� enie Civil et g� eo-Environnement (LGCgE), B� ethune, F-62400, France b Laboratory of Applied Physics (LPA), Lebanese University, Faculty of Sciences, Fanar Campus, Lebanon A R T I C L E INFO Keywords: Natural convection Double vertical channel Radiative heat fux Convective heat fux ABSTRACT The present work addresses the numerical and experimental study of natural convection inside an asymmetri- cally heated open double vertical facade. Two heating cases were considered independently, the constant heat fux (Neumann condition) and the constant temperature (Dirichlet condition). The double facade has been modeled using a vertical two-dimensional channel with one wall being maintained at the heating condition and the other one insulated. The boundary conditions at the inlet and outlet were controlled through the addition of adiabatic walls upstream and downstream of the studied area. The airfow is assumed to be laminar and per- manent. This study is conducted for several modifed Rayleigh numbers ranging from 10 2 � Ra m �10 7 and different aspect ratios (A ¼ 25, 12.5, 8.34, 6.25, and 5). Various parameters have been evaluated and high- lighted, namely velocity and temperature profles. In the frst part of this study, the radiative heat transfer is not considered, comparison results give excellent agreement with the experimental work of Webb and Hill [18]. Similarly, the streamline results show a return fow through the outlet of the channel starting from a modifed Raleigh value of Ra m ¼ 10 4 . In the second case of this study and for isothermal conditions, the radiative transfer is taken into account with mutual radiative heat exchanges between the surfaces and for a transparent not participating medium. The comparison results between the experimental and numerical heat fuxes along the heated plate gives a very good agreement, as well as for the mean Nusselt values for 2.28 � 10 2 �Ra m �8.22 � 10 5 . 1. Introduction Convection is the most common heat transfer mode adopted in miscellaneous practical engineering felds systems due to its simplicity, reliability, and cost effectiveness. Natural convection heat transfers and fuid fow in vertical parallel-plate channels are relevant to a wide range of heat exchange applications especially in buildings where passive solar heating and ventilation of buildings is gaining more and more impor- tance [1,2]. Natural convection in vertical parallel-plate channels may take place under laminar or turbulent fow regimes depending on the geometrical size and thermal parameters. Laminar natural convection in vertical parallel-plate channels has been studied using experimental, analytical, and numerical techniques is many research works due to its wide applicability and [3–11]. Work of Elenbaas 1942 [12] appears among the frst in this feld, it presents an experimental device of vertical plane plates heated using a density fux of constant heat (square plates of 12 cm side). It determines that the parameters of this fow are the numbers of Nusselt, Grashof and Prandtl and proposes a correlation connecting them. This one is ob- tained in an analytical way for an infnitely long channel and is compared with the experimental results, which are corrected to be valid with the assumption infnite length. These results sweep a range of modifed Rayleigh (Ra m ) going from 0.1 to 10 5 . Lastly, Elenbaas deter- mined the optimal spacing making it possible to maximize the transfer of heat in the channel. In 1980, Sparrow and Bahrami [13] presented an original experimental device to confrm the study of Elenbaas [12]. They made the analogy between mass transfer (via Sherwood num- ber) and heat transfer (Nusselt number) by using the sublimation of naphthalene. Their device is almost the same one as that of Elenbaas except that the vertical walls were covered with naphthalene, which quantity was given before and after the experiment according to the quantity of matter which evaporated. They assessed the infuence of the * Corresponding author. Laboratoire de Physique Appliqu� ee (LPA), Universit� e Libanaise, Facult� e des Sciences, Campus Fanar, Lebanon. ** Corresponding author. Univ. Artois, ULR 4515, Laboratoire de G� enie Civil et g� eo-Environnement (LGCgE), B� ethune, F-62400, France. E-mail addresses: yassine.cherif@univ-artois.fr (Y. Cherif), emilio.sassine@gmail.com (E. Sassine). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: http://www.elsevier.com/locate/ijts https://doi.org/10.1016/j.ijthermalsci.2020.106288 Received 2 May 2019; Received in revised form 4 January 2020; Accepted 23 January 2020