Effects of variable density for film evaporation on laminar mixed convection in a vertical channel

A numerical investigation was conducted to study mixed convection in a vertical parallel-plate channel with evaporation of thin liquid films on wetted walls. Air-water vapor and air-hexane vapor mixtures, assumed as ideal gases, are considered under various boundary conditions. Steady laminar, two-dimensional flows are examined in detail for large mixture density changes between the inlet and outlet sections of the channel. Comparisons with the usual problem formulations based on the Boussinesq approximation are discussed. The elliptic flow model used allow to predict flow reversal as well as recirculation cells in the entrance region. The evaporation of water and hexane into a downward laminar stream of dry air leads to various flow structures according to the interfacial mass fraction, W-v,W-w, and differences in the molecular weights of the species. For water evaporation, the thermal and solutal forces are opposing. In the entrance region, evaporation produces a significant increase in axial velocity at the core region in comparison with pure forced flow. For W-v,W-w larger than approximate to 0.2, upward velocities may be observed in the wall regions due to solutal buoyancy forces near the wetted Surfaces. For hexane evaporation, the solutal force acts downward. Mass diffusion produces both a strong flow acceleration in the boundary layers and flow recirculations at the channel center for large mass evaporation rates. (C) 2008 Elsevier Ltd. All rights reserved.