Gas radiation effects on opposing double-diffusive convection in a non-gray aireH 2 O mixture Siham Laouar-Meftah a, * , Mohamed Cherifi a , Denis Lemonnier b , Abderrahmane Benbrik a a Faculty of Hydrocarbon and Chemistry, M’hamed Bougara University, Boumerdes, Algeria b Institut Pprime, CNRS, ENSMA, University of Poitiers, Futuroscope, Poitiers, France article info Article history: Received 24 January 2013 Received in revised form 7 August 2013 Accepted 10 October 2013 Available online 18 November 2013 Keywords: Gas radiation Square cavity Double diffusion SLW radiative model Discrete ordinates abstract We studied numerically the effects of gas radiation on double-diffusive convection in a square enclosure filled with a non-gray aireH 2 O mixture at different concentrations. Uniform temperatures and con- centrations are imposed along the two vertical side walls of the enclosure so as to induce opposing thermal and mass buoyancy forces within the fluid. In this work, the radiative aspect of the problem is treated by the discrete ordinate method (to solve the radiative transfer equation) and the SLW spectral model (to account for the radiative properties of the non-gray mixture). Gas absorption varies with the local concentration of H 2 O, which induces a strong direct coupling between the concentration and thermal fields that otherwise would not exist. Numerical results show that radiative effects on the characteristics of streamline, temperature and concentration fields are important, and depend on the nature of the flow regime (thermal at 5% H 2 O, transitional at 10% and mass at 25%). The total heat transfer is reduced whatever the flow regime and the mass transfer is also affected, outside the thermal flow. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction In double-diffusive natural convection, the flow is induced by the simultaneous action of thermal and mass buoyancy forces. Earlier, this transport phenomenon was widely studied in different geometries by considering a transparent medium (no radiation). More recently, the problem of combined double diffusive convec- tion with volumetric radiation attracted the attention of many scientists, owing to developments in several engineering applica- tions and industrial processes (such as nuclear reactors, crystal growth, combustion chambers, and so on). Most of the studies dealing with this kind of coupling use the simple assumption of a medium with uniform absorption over space and wavelengths (gray and homogeneous medium) [1e4]. A limited number of them considered the more realistic situation of an absorption coefficient proportional to the local concentration of the absorbing species [5,6], but still under the gray gas assumption. To date, there are few investigations dealing with double diffusioneradiation in gaseous mixtures based on the real (non-gray) radiative properties of the medium (absorption varies with temperature, concentration and wavelength). In this context, we can mention the numerical studies by Meftah et al. [7,8] and Laouar-Meftah [9] who have analyzed the interaction of gas radiation and double diffusive natural convection in a non-gray participating mixture (aireCO 2 and aireH 2 O). The authors used the SLW spectral model of Denison and Webb [10] with discrete ordinate method to account for the real radiative participation of the medium. They showed that, at steady state and in aiding cases (cooperating buoyancy forces), radiation creates oblique stratifications in temperature and concentration fields, decreases the Nusselt numbers and slightly reduces the Sherwood number. Recently Ibrahim and Lemonnier [11] have considered the same problem in N 2 eCO 2 mixture at transient state. The results show that radiation accelerates the convergence to steady state in aiding case, while it favors the generation of instabilities and delays the arrival to a stable solution in opposing one. In this paper, we investigated the effects of gas radiation on double-diffusive con- vection in aireH 2 O mixture at steady state and in opposing case, when temperature and concentration gradient are set as to induce buoyancy forces in opposing directions. This configuration may yield complex flow structures, where the balance of thermal to mass forces may be significantly modified by radiative absorption within the fluid. In this paper, the average concentrations of pollutant considered correspond to mole fractions: 5%, 10% and 25% H 2 O.They basically correspond (for a transparent fluid) to thermally dominated flow, transitional flow and mass dominated flow. * Corresponding author. Fax: þ213 24 819172. E-mail address: laouarmeftah@gmail.com (S. Laouar-Meftah). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts 1290-0729/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ijthermalsci.2013.10.004 International Journal of Thermal Sciences 77 (2014) 38e46